chapter
stringlengths 1.97k
1.53M
| path
stringlengths 47
241
|
|---|---|
Eek!
Being bitten on the nose by an eel certainly qualifies as a frightening experience! The fear this man is experiencing produces the same physiological responses in most people— racing heart, rapid breathing, clammy hands. These and other fight-or-flight responses prepare the body to either defend itself or run away from danger. Why does fear elicit these changes in the body? The responses occur in large part because of hormones secreted by the adrenal glands.
Introduction to the Adrenal Glands
The adrenal glands are endocrine glands that produce a variety of hormones. Adrenal hormones include the fight-or-flight hormone adrenaline and the steroid hormone cortisol. The two adrenal glands are located on both sides of the body, just above the kidneys, as shown in Figure \(2\). The right adrenal gland (on the left in the figure) is smaller and has a pyramidal shape. The left adrenal gland (on the right in the figure) is larger and has a half-moon shape.
Each adrenal gland has two distinct parts, and each part has a different function, although both parts produce hormones. There is an outer layer, called the adrenal cortex, which produces steroid hormones including cortisol. There is also an inner layer, called the adrenal medulla, which produces non-steroid hormones including adrenaline.
Adrenal Cortex
The adrenal cortex, or the outer layer of the adrenal gland, is divided, in turn, into three additional layers, called zones (Figure \(3\)). Each zone has distinct enzymes that produce different hormones from the common precursor molecule cholesterol, which is a lipid.
1. Zona glomerulosa is the outermost layer of the adrenal cortex. It lies immediately under the outer fibrous capsule that encloses the adrenal gland.
2. Zona fasciculata is the middle layer of the adrenal cortex. It is the largest of the three zones, accounting for nearly 80 percent of the adrenal cortex.
3. Zona reticularis is the innermost layer of the adrenal cortex. It is directly adjacent to the medulla of the adrenal gland.
Types of Adrenal Cortex Hormones
Hormones produced by the adrenal cortex are called corticosteroids. As steroid hormones, corticosteroids are endocrine hormones that are made of lipids and exert their effects on target cells by crossing the plasma membrane and binding with receptors within the cytoplasm. A steroid hormone and its receptor form a complex that enters the cell nucleus and affects gene expression. There are three types of corticosteroids synthesized and secreted by the adrenal cortex. Each type is produced by a different zone of the adrenal cortex, as shown in Figure \(3\).
Mineralocorticoids
Mineralocorticoids are produced in the zona glomerulosa and include the hormone aldosterone. These hormones help control the balance of mineral salts (electrolytes) in the body. In the kidneys, aldosterone increases the reabsorption of sodium ions and the excretion of potassium ions. Aldosterone also stimulates the retention of sodium ions by cells in the colon and by the sweat glands. The amount of sodium in the body affects the volume of extracellular fluids including the blood and thereby affects blood pressure. In this way, mineralocorticoids help control blood volume and blood pressure.
Glucocorticoids
Glucocorticoids are produced in the zona fasciculata and include the hormone cortisol, which is released in response to stress and is considered the primary stress hormone. Glucocorticoids help control the rate of metabolism of proteins, fats, and sugars. In general, they increase the level of glucose and fatty acids circulating in the blood. Cells rely primarily on glucose for energy, but they can also use fatty acids for energy as an alternative to glucose. Glucocorticoids are also involved in the suppression of the immune system, having a potent anti-inflammatory effect. In addition, cortisol reduces the production of new bone and decreases the absorption of calcium from the gastrointestinal tract.
Androgens
Androgens are produced in the zona reticularis and include the hormone DHEA (dehydroepiandrosterone). Androgens are a general term for male sex hormones, although this is somewhat misleading as adrenal cortex androgens are produced by both males and females. In adult males, they are converted to more potent androgens such as testosterone in the male gonads (testes). In adult females, they are converted to female sex hormones called estrogens in the female gonads (ovaries).
Regulation of Adrenal Cortex Hormones
Steroid hormone production by the three zones of the adrenal cortex is regulated by hormones secreted by the anterior lobe of the pituitary gland as well as by other physiological stimuli. For example, the production of glucocorticoids such as cortisol is stimulated by adrenocorticotropic hormone (ACTH) from the anterior pituitary, which in turn is stimulated by corticotropin releasing hormone (CRH) from the hypothalamus. When levels of glucocorticoids start to rise too high, they provide negative feedback to the hypothalamus and pituitary gland to stop secreting CRH and ACTH, respectively. This negative feedback mechanism is illustrated in Figure \(4\). The opposite occurs when levels of glucocorticoids start to fall too low.
Adrenal Medulla
The adrenal medulla is at the center of each adrenal gland and is surrounded by the adrenal cortex. It contains a dense network of blood vessels into which it secretes its hormones. The hormones synthesized and secreted by the adrenal medulla are called catecholamines, and they include adrenaline (also called epinephrine) and noradrenaline (also called norepinephrine). These are water-soluble, non-steroid hormones are made of amino acids. As non-steroid hormones, they cannot cross the plasma membrane of target cells. Instead, they exert their effects by binding to receptors on the surface of target cells. The binding of hormones and receptors activates an enzyme in the plasma membrane that controls a second messenger. It is the second messenger that influences processes inside the cell.
Catecholamines function to produce a rapid response throughout the body in stressful situations. They bring about such changes as increased heart rate, more rapid breathing, constriction of blood vessels in certain parts of the body, and an increase in blood pressure. The release of catecholamines by the adrenal medulla is stimulated by activation of the sympathetic division of the autonomic nervous system.
Disorders of the Adrenal Glands
Disorders of the adrenal glands generally include either hypersecretion or hyposecretion of adrenal hormones. The underlying cause of the abnormal secretion may be a problem with the adrenal glands or with the pituitary gland, which controls adrenal cortex hormone production. Both adrenal and pituitary glands are subject to the formation of tumors, which may cause adrenal disorders. The adrenal gland may also be affected by infections or autoimmune diseases.
Adrenal Hypersecretion: Cushing’s Syndrome
Hypersecretion of the glucocorticoid hormone cortisol leads to a disorder named Cushing’s syndrome. The most common cause of Cushing’s syndrome is a pituitary tumor, which causes excessive production of ACTH. The disease produces a wide variety of signs and symptoms, which may include obesity, diabetes, high blood pressure (hypertension), excessive body hair, osteoporosis, and depression. A distinctive sign of Cushing’s syndrome is the appearance of stretch marks in the skin, as the skin becomes progressively thinner. Another distinctive sign is a moon face shown in the section Introduction to the Endocrine System, in which fat deposits give the face a rounded appearance. Treatment of Cushing’s syndrome depends on its cause and may include surgery to remove a tumor or medications to suppress the activity of the adrenal glands.
Adrenal Hyposecretion: Addison’s Disease
Hyposecretion of the glucocorticoid hormone cortisol leads to a disorder named Addison’s disease. There may also be hyposecretion of mineralocorticoids with this disorder. Addison’s disease is generally an autoimmune disorder, in which the immune system produces abnormal antibodies that attack cells of the adrenal cortex. Untreated infections, especially of tuberculosis, may also damage the adrenal cortex and cause Addison’s disease. A third possible cause is the decreased output of ACTH by the pituitary gland, generally due to a pituitary tumor. A distinctive sign of Addison’s disease is hyperpigmentation of the skin (Figure \(5\)). Other symptoms tend to be nonspecific and include excessive fatigue. Addison’s disease is generally treated with replacement hormones in pill form.
Feature: My Human Body
Does just looking at this photo cause you to break out in a cold sweat and experience heart palpitations? Imagine how scary it would be to fling yourself backward off a tall building like the BASE jumper in the photo. There would be very little time to use a parachute to slow your fall before you hit the ground. BASE jumping is called the most dangerous sport on Earth. In fact, it is so dangerous that it is outlawed in some places.
People who participate in such dangerous activities as BASE jumping are likely to be adrenaline “junkies.” They are addicted to the adrenaline rush and euphoria, or “high,” it causes when their fight-or-flight response is triggered by danger. Why does adrenaline have this effect? Adrenaline is closely related to dopamine, a chemical messenger in the brain that plays a major role in pleasure and addiction.
Adrenaline addicts don’t have to participate in BASE jumping or other dangerous sports to get an adrenaline rush. They might choose a dangerous occupation such as firefighting, participate in risky behaviors such as reckless driving or bank robbing, or just pick fights with other people. They might even create their own stress by always taking on too much work or delaying projects until close to their deadline.
While some excitement in one’s life is generally a good thing, always putting oneself in danger or constantly being under stress are obviously not good things. If you think you might be an adrenaline addict, note that there are healthier ways to experience a hormonal “high.” Running, biking, or participating in some other form of vigorous aerobic exercise causes the pituitary gland and hypothalamus to produce opiate-like endorphins, leading to a so-called “runner’s high.” Like the euphoric feeling adrenaline causes, a runner’s high may last for hours.
Review
1. Describe the structure and location of the adrenal glands.
2. Compare and contrast the adrenal cortex and adrenal medulla.
3. Identify the three layers of the adrenal cortex and the type of hormones each layer produces.
4. Give an example of each type of corticosteroid and state its function.
5. Explain how the production of glucocorticoids is regulated.
6. What is a catecholamine? Give an example of a catecholamine and state its function.
7. Compare and contrast Cushing’s syndrome and Addison’s disease.
8. Cortisol is a type of:
1. Corticosteroid
2. Mineralocorticoid
3. Glucocorticoid
4. A and C
9. True or False. The adrenal glands help regulate the body’s stress response and reproductive functions.
10. True or False. The left adrenal gland produces steroid hormones, while the right adrenal gland produces non-steroid hormones.
11. Would it help to give someone with Cushing’s syndrome more ACTH? Explain your answer.
12. What are two ways in which the nervous system (which includes the brain, spinal cord, and nerves) controls the adrenal gland?
13. If the level of cortisol rises too high, the amount of CRH secreted will normally:
1. not change
2. become excessively high
3. become slightly higher
4. decrease
14. Noradrenaline is also called:
1. norepinephrine
2. adrenaline
3. adrenocorticotropin
4. glucocorticoid
15. Explain why a pituitary tumor can cause either hypersecretion or hyposecretion of cortisol.
Explore More
Learn more about the effects of stress and cortisol on the brain here:
Attributions
1. Attack by Jerry Kirkhart, CC BY 2.0 via Wikimedia Commons
2. Adrenal gland by cancer.gov, public domain via Wikimedia Commons
3. Adrenal cortex labeled by Jpogi, CC0
4. ACTH Negative Feedback by DRosenbach; CC BY-SA 3.0 via Wikimedia Commons
5. A 69 Year Old with Tiredness and a Persistent; CC BY 2.5; Petros Perros via Wikimedia Commons
6. Base Jump by Kontizas Dimitrios; CC BY-SA 3.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/12%3A_Endocrine_System/12.6%3A_Adrenal_Glands.txt |
A Shot in the Arm
Giving yourself an injection can be difficult, but for someone with diabetes, it may be a matter of life or death. The person in the photo has diabetes and is injecting himself with insulin, the hormone that helps control the level of glucose in the blood. Insulin is produced by the pancreas.
Introduction to the Pancreas
The pancreas is a large gland located in the upper left abdomen behind the stomach, as shown in the figure below. The pancreas is about 15 centimeters (6 in.) long; and it has a flat, oblong shape. Structurally, the pancreas is divided into a head, body, and tail. Functionally, the pancreas serves as both an endocrine gland and an exocrine gland.
• As an endocrine gland, the pancreas is part of the endocrine system. As such, it releases hormones, such as insulin, directly into the bloodstream for transport to cells throughout the body.
• As an exocrine gland, the pancreas is part of the digestive system. As such, it releases digestive enzymes into ducts that carry the enzymes to the gastrointestinal tract where they assist with digestion. In this concept, the focus is on the pancreas as an endocrine gland. You can read about the pancreas as an exocrine gland in the chapter Digestive System.
The Pancreas as an Endocrine Gland
The tissues within the pancreas that have an endocrine role exist as clusters of cells called pancreatic islets. They are also called the islets of Langerhans. In Figure \(3\), you can see pancreatic tissue, including islets. There are approximately 3 million pancreatic islets, and they are crisscrossed by a dense network of capillaries. The capillaries are lined by layers of islet cells that have direct contact with the blood vessels, into which they secrete their endocrine hormones.
The pancreatic islets consist of four main types of cells, each of which secretes a different endocrine hormone. However, all of the hormones produced by the pancreatic islets play crucial roles in glucose metabolism and the regulation of blood glucose levels, among other functions.
1. Islet cells called alpha (α) cells secrete the hormone glucagon. The function of glucagon is to increase the level of glucose in the blood. It does this by stimulating the liver to convert stored glycogen into glucose, which is released into the bloodstream.
2. Islet cells called beta (β) cells secrete the hormone insulin. The function of insulin is to decrease the level of glucose in the blood. It does this by promoting the absorption of glucose from the blood into fat, liver, and skeletal muscle cells. In these tissues, the absorbed glucose is converted into glycogen, fats (triglycerides), or both.
3. Islet cells called delta (δ) cells secrete the hormone somatostatin. This hormone is also called the growth hormone inhibiting hormone because it inhibits the anterior lobe of the pituitary gland from producing growth hormone. Somatostatin also inhibits the secretion of pancreatic endocrine hormones and pancreatic exocrine enzymes.
4. Islet cells called gamma (γ) cells secrete the hormone pancreatic polypeptide. The function of pancreatic polypeptide is to help regulate the secretion of both endocrine and exocrine substances by the pancreas.
Disorders of the Pancreas
There are a variety of disorders that affect the pancreas. They include pancreatitis, pancreatic cancer, and diabetes mellitus.
Pancreatitis
Pancreatitis is inflammation of the pancreas. It has a variety of possible causes including gallstones, chronic alcohol use, infections such as measles or mumps, genetic causes, and certain medications. Pancreatitis occurs when digestive enzymes produced by the pancreas damage the gland’s tissues, which causes problems with fat digestion. The disorder is usually associated with intense pain in the central abdomen, and the pain may radiate to the back. Yellowing of the skin and whites of the eyes (Figure \(4\)), which is called jaundice, is a common sign of pancreatitis. People with pancreatitis may also have pale stools and dark urine. Treatment of pancreatitis includes administering drugs to manage pain and addressing the underlying cause of the disease, for example, by removing gallstones.
Pancreatic Cancer
There are several different types of pancreatic cancer that may affect either the endocrine or the exocrine tissues of the gland. Cancers affecting the endocrine tissues are all relatively rare. However, their incidence has been rising sharply. It is unclear to what extent this reflects increased detection, especially through medical imaging techniques. Unfortunately, pancreatic cancer is usually diagnosed at a relatively late stage when it is too late for surgery, which is the only way to cure the disorder. In the United States, pancreatic cancer is the fourth most common cause of death due to cancer.
Pancreatic cancer is rare before the age of 40 and occurs most often after the age of 60. Factors that increase the risk of developing pancreatic cancer include smoking, chronic pancreatitis, and diabetes. About one in four cases of pancreatic cancer are attributable to smoking. Certain rare genetic conditions are also risk factors for pancreatic cancer.
Diabetes Mellitus
By far the most common type of pancreatic disorder is diabetes mellitus, more commonly called simply diabetes. There are many different types of diabetes, but diabetes mellitus is the most common. It occurs in two major types, type 1 diabetes and type 2 diabetes. The two types have different causes and may also have different treatments, but they generally produce the same initial symptoms, which include excessive urination and thirst. These symptoms occur because the kidneys excrete more urine in an attempt to rid the blood of excess glucose, and loss of water in urine stimulates greater thirst. Other signs and symptoms of diabetes are listed in Figure \(5\).
When diabetes is not well controlled, it is likely to have several serious long-term consequences. Most of these consequences are due to damage to small blood vessels because of high blood levels of glucose. Damage to blood vessels, in turn, may lead to an increased risk of coronary artery disease and stroke. Damage to blood vessels in the retina of the eye can result in gradual vision loss and blindness. Damage to blood vessels in the kidneys can lead to chronic kidney disease, sometimes requiring dialysis or a kidney transplant. Long-term consequences of diabetes may also include damage to the nerves of the body, known as diabetic neuropathy. In fact, this is the most common complication of diabetes. Symptoms of diabetic neuropathy may include numbness, tingling, and pain in the extremities.
Type 1 Diabetes
Type 1 diabetes is a chronic autoimmune disorder in which the immune system attacks the insulin-secreting beta cells of the pancreas. As a result, people with type 1 diabetes lack the insulin needed to keep blood glucose levels within the normal range. Type 1 diabetes may develop in people of any age but is most often diagnosed before adulthood. For type 1 diabetics, insulin injections are critical for survival.
Type 2 Diabetes
Type 2 diabetes is the single most common form of diabetes. The cause of high blood glucose in this form of diabetes usually includes a combination of insulin resistance and impaired insulin secretion. Both genetic and environmental factors play roles in the development of type 2 diabetes. Management of type 2 diabetes includes changes in diet and physical activity, which may increase insulin sensitivity and help reduce blood glucose levels to normal ranges. Medications may also be used as part of the treatment, as may insulin injections.
Feature: Human Biology in the News
Some patients with type 1 diabetes have been given pancreatic islet cells transplants from other human donors. If the transplanted cells are not rejected by the recipient’s immune system, they can cure the patient of diabetes. However, only about 1,000 such surgeries have been performed over the past 10 years because of a shortage of appropriate human donors.
In June of 2016, a research team led by Dr. David K.C. Cooper at the Thomas E. Starzl Transplantation Institute in Pittsburgh, Pennsylvania, reported on their work developing pig islet cells for transplant into human diabetes patients. The researchers genetically engineered the pig islet cells to be protected from the human immune response. As a result, patients receiving transplanted cells would require only minimal suppression of their immune system after the surgery. The pig islet cells would also be less likely to transmit pathogenic agents because the animals could be raised in a controlled environment.
The researchers have successfully transplanted the pig islet cells into monkey models of type 1 diabetes. As of June 2016, the scientists were looking for funding to undertake clinical trials in humans with type 1 diabetes. Dr. Cooper predicted then that if the human trials go as well as expected, the pig islet cells could be available for curing patients in as little as two years.
Review
1. Describe the structure and location of the pancreas.
2. Distinguish between the endocrine and exocrine functions of the pancreas.
3. Identify the four types of pancreatic islet cells and the endocrine hormone each type of cell produces.
4. What is pancreatitis? What are the possible causes and effects of pancreatitis?
5. Describe the incidence, prognosis, and risk factors of cancer of the endocrine tissues of the pancreas.
6. Compare and contrast type 1 and type 2 diabetes.
7. If the alpha islet cells of the pancreas were damaged to the point that they no longer functioned, how would this affect blood glucose levels? Would the administration of insulin be more likely to help or hurt the condition? Explain your answer.
8. Explain how the pancreas is able to regulate the production of its own endocrine hormones, to some extent.
9. True or False. The pancreas is part of both the digestive system and the endocrine system.
10. Give an example of how the pancreas can regulate the production of hormones from the pituitary gland.
11. Which is the most common form of diabetes mellitus?
12. Explain why diabetes causes excessive thirst.
13. Damage to __________ is the underlying cause of many of the long-term consequences of diabetes.
1. the adrenal gland
2. gamma islet cells
3. blood vessels
4. the pituitary gland
Attributions:
1. Insulin Application by Mr. Hyde, public domain, via Czech Wikipedia
2. Pancreas anatomy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
3. Exocrine and Endocrine Pancreas by OpenStax College, CC BY 3.0, via Wikimedia Commons
4. Jaundice eye by CDC, public domain via Wikimedia Commons
5. Symptoms of diabetes; licensed CC-0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/12%3A_Endocrine_System/12.7%3A_Pancreas.txt |
Case Study Conclusion: Hormonal Havoc
Gabrielle, who you read about in the beginning of this chapter, has polycystic ovary syndrome (PCOS). PCOS is named for the multiple fluid-filled sacs, or cysts, that are present in the ovaries of women with this syndrome. You can see these cysts in the illustration above, which compares a normal ovary with a polycystic ovary. The cysts result from follicles in the ovary that did not properly produce and release an egg. Mature eggs are normally released from follicles monthly during the process of ovulation, but in PCOS this often does not occur. Ovarian cysts can be common and do not necessarily mean that a woman has PCOS, but the presence of multiple ovarian cysts plus other telltale signs and symptoms may cause her physician suspect PCOS.
Gabrielle’s symptoms of PCOS included irregular menstrual periods, weight gain, acne, and excess facial hair. There are many other symptoms of PCOS that women can experience, such as male-pattern baldness, pelvic pain, and depression, among others. As you may recall, Gabrielle also had some abnormal blood test results, such as high levels of androgens and blood glucose. These can also be indications of PCOS.
As you have learned, androgens are a term for male sex hormones, but females also normally produce androgens, albeit to a lesser extent than males. In women with PCOS, the level of androgens is abnormally high. These androgens include testosterone, which is produced by the ovaries, and DHEA, which is produced by the adrenal glands. This increase in androgens can have a “masculinizing” effect on women, including an increase in facial and body hair, male-pattern baldness, and interference with the menstrual cycle by preventing ovulation. Androgens also can cause weight gain and acne — two of the other common symptoms of PCOS.
In addition to hypersecretion of androgens, PCOS often causes high blood glucose as a result of insulin resistance. As you have learned, insulin is a hormone secreted by the pancreas that works in conjunction with other pancreatic hormones (such as glucagon) to regulate the level of blood glucose. What is another disease involving insulin resistance? If you answered type 2 diabetes, you are correct! In fact, women with PCOS are at a high risk of developing type 2 diabetes because of their resistance to insulin. More than 50 percent of women with PCOS will develop diabetes or pre-diabetes before they are 40 years old.
Besides diabetes, women with PCOS have a higher chance of developing fertility problems, heart disease, sleep apnea (briefly stopping breathing during sleep), and uterine cancer, among other diseases and disorders. There is hope, however. Lifestyle modifications and medicines not only can help women cope with the symptoms of PCOS, but may also reduce the risk of some of the possible long-term consequences by lowering blood sugar and androgen levels. For instance, eating a healthy diet and exercising regularly can help women with PCOS lose weight. This can help lower blood glucose levels, improve insulin functioning, and can even make the menstrual cycle more regular. Medications such as birth control pills and anti-androgens can help restore a more regular menstrual cycle and reduce facial and body hair and acne. The diabetes medication metformin can be used to treat several of the symptoms of PCOS, and even may prevent type 2 diabetes, by improving insulin functioning and lowering testosterone. Finally, women with PCOS who are trying to conceive may be helped with fertility medications that stimulate ovulation.
The underlying cause of PCOS is not definitively known, although it is thought that both genetic and environmental factors play a role. PCOS tends to run in families, and women with a sister with PCOS are twice as likely to also have it. Researchers think that the insulin resistance seen in PCOS may cause an increase in androgens, illustrating how hormonal systems can influence each other.
As you have seen throughout this chapter, endocrine hormones can have a wide variety of effects on the body, including the regulation of metabolism, reproductive functions, homeostasis of different ions and molecules, and mediating responses to stressful situations. Different hormones have different effects, but even a single hormone can have multiple effects. Hormones travel throughout the bloodstream and affect any cells that have the appropriate receptors for them, known as target cells. Many hormones have target cells in multiple types of organs and tissues, or they regulate molecules, such as blood glucose, that affect many organ systems. These are some of the reasons why changes in the normal level of an endocrine hormone — either hypersecretion or hyposecretion — can result in a wide variety of symptoms, such as is seen in Cushing’s syndrome, diabetes, and PCOS. By understanding what goes wrong in these disorders, you can better appreciate how important the endocrine system is for regulating the many diverse functions of the human body.
Chapter Summary
In this chapter, you learned about the glands and hormones of the endocrine system, their functions, how they are regulated, and some diseases and disorders of the endocrine system. Specifically, you learned that:
• The endocrine system is a system of glands that release chemical messenger molecules called hormones into the bloodstream. Other glands, called exocrine glands, release substances onto nearby body surfaces through ducts.
• Endocrine hormones travel more slowly than nerve impulses, which are the body’s other way of sending messages. However, the effects of endocrine hormones may be much longer lasting.
• The pituitary gland is the master gland of the endocrine system. Most of the hormones it produces control other endocrine glands. These glands include the thyroid gland, parathyroid glands, pineal gland, pancreas, adrenal glands, gonads (testes and ovaries), and thymus gland.
• Diseases of the endocrine system are relatively common. An endocrine disease usually involves hypersecretion or hyposecretion of a hormone. Hypersecretion is frequently caused by a tumor. Hyposecretion is often caused by the destruction of hormone-secreting cells by the body’s own immune system.
• Endocrine hormones travel throughout the body but affect only certain cells, called target cells, which have receptors specific to particular hormones.
• Steroid hormones such as estrogen are endocrine hormones made of lipids that cross plasma membranes and bind to receptors inside target cells. The hormone-receptor complexes then move into the nucleus where they influence gene expression.
• Non-steroid hormones such as insulin are endocrine hormones made of amino acids that bind to receptors on the surface of target cells. This activates an enzyme in the plasma membrane, and the enzyme controls a second messenger molecule, which influences cell processes.
• Most endocrine hormones are controlled by negative feedback loops in which rising levels of hormone feedback to stop its own production — and vice-versa. For example, a negative feedback loop controls the production of thyroid hormones. The loop includes the hypothalamus, pituitary gland, and thyroid gland.
• Only a few endocrine hormones are controlled by positive feedback loops in which rising levels of hormone feedback to stimulate continued production of the hormone. Prolactin, the pituitary hormone that stimulates milk production by mammary glands, is controlled by a positive feedback loop. The loop includes the nipples, hypothalamus, pituitary gland, and mammary glands.
• The pituitary gland is at the base of the brain, where it is connected to the hypothalamus by nerves and capillaries. It has an anterior (front) lobe that synthesizes and secretes pituitary hormones and a posterior (back) lobe that stores and secretes hormones from the hypothalamus.
• Hormones synthesized and secreted by the anterior pituitary include growth hormone, which stimulates cell growth throughout the body, and thyroid stimulating hormone (TSH), which stimulates the thyroid gland to secrete its hormones.
• Hypothalamic hormones stored and secreted by the posterior pituitary include vasopressin, which helps maintain homeostasis in body water; and oxytocin, which stimulates uterine contractions during birth and the letdown of milk during lactation.
• The thyroid gland is a large endocrine gland in the front of the neck. It is composed mainly of clusters of cells called follicles, which are specialized to absorb iodine and use it to make thyroid hormones. Parafollicular cells among the follicles synthesize the hormone calcitonin.
• The thyroid hormones thyroxine (T4) and triiodothyronine (T3) cross cell membranes and regulate gene expression to control the rate of metabolism in cells body-wide, among other functions. The production of T4 and T3 is regulated by thyroid stimulating hormone (TSH) from the pituitary, which is regulated, in turn, by thyrotropin-releasing hormone (TRH) from the hypothalamus.
• Calcitonin helps regulate blood calcium levels by stimulating the movement of calcium into bone. It works in conjunction with parathyroid hormone to maintain calcium homeostasis.
• Abnormal secretion of thyroid hormones may occur for a variety of reasons and may lead to the development of a goiter. The most common cause of hyperthyroidism is Graves’ disease, an autoimmune disorder. Iodine deficiency is a common cause of hypothyroidism worldwide. In the United States, the most common cause of hypothyroidism is Hashimoto’s thyroiditis, another autoimmune disorder. Hypothyroidism in pregnant women may cause permanent cognitive deficits in children.
• The adrenal glands are endocrine glands that produce a variety of hormones. The two adrenal glands are located on both sides of the body, just above the kidneys. Each gland has two layers: an outer layer called the adrenal cortex and an inner layer called the adrenal medulla.
• The adrenal cortex produces steroid hormones called by the general term corticosteroids, of which there are three types: mineralocorticoids such as aldosterone, which helps control electrolyte balance; glucocorticoids such as cortisol, which helps control the rate of metabolism and suppresses the immune system; and androgens such as DHEA, which is converted to sex hormones in the gonads.
• The adrenal medulla produces non-steroid catecholamine hormones including adrenaline and noradrenaline. These hormones stimulate the fight-or-flight response.
• Disorders of the adrenal glands generally include either hypersecretion or hyposecretion of adrenal hormones. The cause may be a problem with the adrenal glands or with the pituitary gland, which controls adrenal cortex hormone production. Examples include Cushing’s syndrome, in which there is hypersecretion of cortisol; and Addison’s disease, in which there is hyposecretion of cortisol and mineralocorticoids.
• The pancreas is a gland located in the upper left abdomen behind the stomach that functions as both an endocrine gland and an exocrine gland. As an endocrine gland, the pancreas releases hormones, such as insulin, directly into the bloodstream. As an exocrine gland, the pancreas releases digestive enzymes into ducts that carry them to the gastrointestinal tract.
• Tissues in the pancreas that have an endocrine role exist as clusters of cells called pancreatic islets. The islets consist of four main types of cells, each of which secretes a different endocrine hormone. Alpha (α) cells secrete glucagon, beta (β) cells secrete insulin, delta (δ) cells secrete somatostatin, and gamma (γ) cells secrete pancreatic polypeptide.
• The endocrine hormones secreted by the pancreatic islets all play a role, either directly or indirectly, in glucose metabolism and homeostasis of blood glucose levels. For example, insulin stimulates the uptake of glucose by cells and decreases the level of glucose in the blood, whereas glucagon stimulates the conversion of glycogen to glucose and increases the level of glucose in the blood.
• Disorders of the pancreas include pancreatitis, pancreatic cancer, and diabetes mellitus. Pancreatitis is a painful inflammation of the pancreas that has many possible causes. Pancreatic cancer of the endocrine tissues is rare but increasing in frequency. It is generally discovered too late to cure surgically. Smoking is a major risk factor for pancreatic cancer.
• Diabetes mellitus is the most common type of pancreatic disorder. In diabetes, the inadequate activity of insulin results in high blood levels of glucose. Type 1 diabetes is a chronic autoimmune disorder in which the immune system attacks the insulin-secreting beta cells of the pancreas. Type 2 diabetes is usually caused by a combination of insulin resistance and impaired insulin secretion due to a variety of environmental and genetic factors.
Chapter Summary Review
1. The pituitary gland is considered the master gland of the endocrine system because its hormones control other endocrine glands. For each of the glands below, describe one way in which it is controlled by the pituitary gland.
1. The thyroid gland
2. The adrenal gland
3. The gonads (ovaries and testes)
2. What is the name of the main brain structure that secretes hormones that control the pituitary gland?
3. Define hyposecretion and give an example of an endocrine disorder involving hyposecretion. Be sure to include the name of the hormone involved.
4. Define hypersecretion and give an example of an endocrine disorder involving hypersecretion. Be sure to include the name of the hormone involved.
5. Which hormone plays a role in regulating metabolism in some way?
1. Cortisol
2. Thyroid hormone
3. Glucagon
4. All of the above
6. Which endocrine gland plays an important role in the fight-or-flight response?
7. True or False. Sex hormones, such as androgens, are only produced by the gonads.
8. True or False. Estrogen can travel to the nucleus of a cell.
9. Explain why non-steroid hormones typically require the activation of second messenger molecules to have their effects, instead of directly affecting intracellular processes themselves.
10. Explain what it means that endocrine hormones are “chemical messengers.”
11. If you were a physician, and a patient came to you complaining of excessive thirst and urination, what endocrine disorder might you suspect the patient has?
1. In order to diagnose this disorder, what would you want to check for in the patient’s blood? Explain your answer.
12. Pancreatic islet cells all produce:
1. Insulin
2. Glucagon
3. Endocrine hormones
4. Digestive enzymes
13. Give one example of negative feedback in the endocrine system.
14. Explain the circumstances in which organs and hormones in a negative feedback loop can actually increase the level of a hormone.
15. True or False. The hormone vasopressin is synthesized by the hypothalamus.
16. True or False. Like most other hormones, prolactin is regulated by a negative feedback loop.
17. Identify the gland that secretes each of the following hormones:
1. Melatonin
2. Growth hormone
3. Thyroid stimulating hormone
4. Aldosterone
18. A goiter is an enlargement of which structure?
19. Explain why giving iodine can treat some cases of hypothyroidism, but it is not usually helpful when someone has hypothyroidism due to Hashimoto’s thyroiditis.
20. For each disease below, identify the hormone involved and whether the problem involves hyposecretion or hypersecretion of this hormone.
1. Addison’s disease
2. Graves’ disease
3. Cushing’s syndrome
4. Type 1 diabetes
21. What is an example of a disease that is due to hormone resistance?
22. True or False. Adrenaline is an exocrine hormone.
23. Steroid hormones:
1. always increase muscle mass
2. are fat soluble
3. bind to receptors on the plasma membrane
4. include insulin
24. Explain generally how autoimmune disorders can disrupt the endocrine system, and give one example.
Attributions
1. Polycystic Ovary, by U.S. Department of Health and Human Services; public domain
2. Text adapted fromHuman Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/12%3A_Endocrine_System/12.8%3A_Case_Study_Conclusion%3A__Hormonal_and_Chapter_Summary.txt |
This chapter describes the structure and functions of the epidermis and dermis, hair, and nails. In addition, the chapter outlines the types of skin cancer and risk factors for skin cancer.
• 13.1: Case Study: Skin Cancer
In this chapter, you will learn about the structure and functions of the integumentary system. Specifically, you will learn about: The functions of the organs of the integumentary system - the skin, hair, and nails - including protecting the body, helping to regulate homeostasis, and sensing and interacting with the external world. The two main layers of the skin: the thinner outer layer called the epidermis and the thicker inner layer called the dermis.
• 13.2: Introduction to the Integumentary System
In addition to the skin, the integumentary system includes the hair and nails, which are organs that grow out of the skin. Because the organs of the integumentary system are mostly external to the body, you may think of them as little more than accessories, like clothing or jewelry, but they serve vital physiological functions. They provide a protective covering for the body, sense the environment, and help the body maintain homeostasis.
• 13.3: Skin
The epidermis is the outer of the two main layers of the skin, the inner layer being the dermis. It averages about 0.10 mm thick and is much thinner than the dermis. The epidermis is thinnest on the eyelids (0.05 mm) and thickest on the palms of the hands and soles of the feet (1.50 mm). The epidermis covers almost the entire body surface. It is continuous with, but structurally distinct from, the mucous membranes that line the mouth, anus, urethra, and vagina.
• 13.4: Hair and Nails
Hair is a filament that grows from a hair follicle in the dermis of the skin. It consists mainly of tightly packed, keratin-filled cells called keratinocytes. The human body is covered with hair follicles except for a few areas, including the mucous membranes, lips, palms of the hands, and soles of the feet.
• 13.5: Case Study Conclusion: Skin Cancer and Chapter Summary
Skin cancer begins in the outer layer of skin, the epidermis. There are three common types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma.
Skin and its layers. Shown is a drawing of the layers of skin and associated glands and vessels (epidermis, dermis, fatty tissue, blood vessels, follicle, oil gland, sweat gland).(Public Domain; National Cancer Institute; NIH).
13: Integumentary System
The summer sun may feel good on your body, but its invisible UV rays wreak havoc on your skin. Exposing the skin to UV light causes photo-aging: premature wrinkling, brown discolorations, and other unattractive signs of sun exposure. Even worse, UV light increases your risk of skin cancer.
Exposure to UV radiation causes about 90% of all skin cancer cases. The connection between skin cancer and UV light is so strong that the World Health Organization has classified UV radiation (whether from tanning beds or the sun) as a Group 1 carcinogen (cancer-causing agent). Group 1 carcinogens are those carcinogens that are known with virtual certainty to cause cancer. In addition to UV light, Group 1 carcinogens include tobacco and plutonium. In terms of the number of cancers caused, UV radiation is far worse than tobacco. More people develop skin cancer because of UV light exposure than develop lung cancer because of smoking. The increase in cancer risk due to UV light is especially great if you have ever had blistering sunburns as a child or teen.
Besides UV light exposure, other risk factors for skin cancer include:
• having light-colored skin
• having a lot of moles
• being diagnosed with precancerous skin lesions
• having a family history of skin cancer
• having a personal history of skin cancer
• having a weakened immune system
• being exposed to other forms of radiation or to certain toxic substances such as arsenic
What exactly is skin cancer? Skin cancer is a disease in which skin cells grow out of control. It is caused mainly by excessive exposure to UV light, which damages DNA. Therefore, skin cancer most often develops on areas of the skin that are frequently exposed to UV light. However, it can also occur in areas that are rarely exposed to UV light. Skin cancer affects people of all skin colors, including those with dark skin. It also affects more people altogether than all other cancers combined. One in five Americans develops skin cancer in his or her lifetime.
At the end of the chapter, you will learn about the different types of skin cancer and how to identify if a growth is a mole or potentially cancerous.
Chapter Overview: Integumentary System
In this chapter, you will learn about the structure and functions of the integumentary system. Specifically, you will learn about:
• The functions of the organs of the integumentary system—the skin, hair, and nails—including protecting the body, helping to regulate homeostasis, and sensing and interacting with the external world.
• The two main layers of the skin: the thinner outer layer called the epidermis and the thicker inner layer called the dermis.
• The cells and layers of the epidermis and their functions, including synthesizing vitamin D and protecting the body against injury and pathogens, UV light exposure, and water loss.
• The composition and layers of the dermis and their functions, including cushioning other tissues, regulating body temperature, sensing the environment, and excreting wastes.
• The specialized structures in the dermis, which include sweat and sebaceous (oil) glands, hair follicles, and sensory receptors that detect touch, temperature, and pain.
• The structure and biological functions of hair, which include retaining body heat, detecting sensory stimuli, and protecting the body against UV light, pathogens, and small particles.
• The structure and functions of nails, which include protecting the fingers and toes, enhancing the detection of sensory stimuli, and acting like tools.
As you read this chapter and learn more about the skin, think about the following questions:
1. What is skin cancer and how does it form?
2. What are the similarities and differences of various types of cancer?
3. How can people decrease their risk of getting skin cancer?
Attribution
1. Stolen moment in the sun by Angie Garrett, CC BY 2.0 via Wikimedia Commons
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/13%3A_Integumentary_System/13.1%3A_Case_Study%3A__Skin_Cancer.txt |
Art for All Eras
This is Maud Stevens Wagner, a tattoo artist who is pictured in Figure \(1\). Maud was pictured in 1907. Clearly, tattoos are not just a late 20th and early 21st-century trend. They have been popular in many eras and cultures. Tattoos literally illustrate the biggest organ of the human body: the skin. The skin is very thin, but it covers a large area — about 2 m2 in adults. The skin is the major organ in the integumentary system.
What Is the Integumentary System?
In addition to the skin, the integumentary system includes the hair and nails, which are organs that grow out of the skin. Because the organs of the integumentary system are mostly external to the body, you may think of them as little more than accessories, like clothing or jewelry, but they serve vital physiological functions. They provide a protective covering for the body, sense the environment, and help the body maintain homeostasis.
The Skin
The skin is remarkable not only because it is the body’s largest organ. It is remarkable for other reasons as well. The average square inch of skin has 20 blood vessels, 650 sweat glands, and more than a thousand nerve endings. It also has an incredible 60,000 pigment-producing cells. All of these structures are packed into a stack of cells that is just 2 mm thick, or about as thick as the cover of a book. Although the skin is thin, it consists of two distinct layers, the epidermis and dermis, as shown in Figure \(2\).
Outer Layer of Skin
The outer layer of skin is the epidermis. This layer is thinner than the inner layer, the dermis. The epidermis consists mainly of epithelial cells, called keratinocytes, which produce the tough, fibrous protein keratin. The innermost cells of the epidermis are stem cells that divide continuously to form new cells. The newly formed cells move up through the epidermis toward the skin surface, while producing more and more keratin. The cells become filled with keratin and die by the time they reach the surface, where they form a protective, waterproof layer. As the dead cells are shed from the surface of the skin, they are replaced by other cells that move up from below. The epidermis also contains melanocytes, the cells that produce the brown pigment melanin, which gives skin most of its color. Although the epidermis contains some sensory receptor cells, called Merkel cells, it contains no nerves, blood vessels, or other structures.
Inner Layer of Skin
The dermis is the inner and thicker layer of skin. It consists mainly of tough connective tissue and is attached to the epidermis by collagen fibers. The dermis contains many structures, as shown in the figure above, including blood vessels, sweat glands, and hair follicles, which are structures where hairs originate. In addition, the dermis contains many sensory receptors, nerves, and oil glands.
Functions of the Skin
The skin has multiple roles in the body. Many of these roles are related to homeostasis. The skin’s main functions include preventing water loss from the body and serving as a barrier to the entry of microorganisms. Another function of the skin is synthesizing vitamin D, which occurs when the skin is exposed to ultraviolet (UV) light. Melanin in the epidermis blocks some of the UV light and protects the dermis from its damaging effects.
Another important function of the skin is helping to regulate body temperature. For example, when the body is too warm, the skin lowers body temperature by producing sweat, which cools the body when it evaporates. The skin also increases the amount of blood flowing near the body surface through vasodilation (widening of blood vessels), bringing heat from the body core to radiate out into the environment.
Hair
Hair is a fiber that is found only in mammals. It consists mainly of keratin-producing keratinocytes. Each hair grows out of a follicle in the dermis. By the time the hair reaches the surface, it consists mainly of dead cells filled with keratin. Hair serves several homeostatic functions. Head hair is important in preventing heat loss from the head and protecting its skin from UV radiation. Hairs in the nose trap dust particles and microorganisms in the air and prevent them from reaching the lungs. Hair all over the body provides sensory input when objects brush against it or it sways in moving air. Eyelashes and eyebrows protect the eyes from water, dirt, and other irritants.
Nails
Fingernails and toenails consist of dead keratinocytes that are filled with keratin. The keratin makes them hard but flexible, which is important for the functions they serve. Nails prevent injury by forming protective plates over the ends of the fingers and toes. They also enhance sensation by acting as a counterforce to the sensitive fingertips when objects are handled. In addition, fingernails can be used as tools.
Interactions with Other Organ Systems
The skin and other parts of the integumentary system work with other organ systems to maintain homeostasis.
• The skin works with the immune system to defend the body from pathogens by serving as a physical barrier to microorganisms.
• Vitamin D is needed by the digestive system to absorb calcium from food. By synthesizing vitamin D, the skin works with the digestive system to ensure that calcium can be absorbed.
• Most immune cells, such as B and T cells have Vitamin D receptors. Vitamin D levels in the body are associated with autoimmune diseases and immune deficiencies.
• To control body temperature, the skin works with the cardiovascular system to either lose body heat or conserve it through vasodilation or vasoconstriction.
• To detect certain sensations from the outside world, the nervous system depends on nerve receptors in the skin.
Review
1. Name the organs of the integumentary system.
2. Compare and contrast the epidermis and dermis.
3. Identify the functions of the skin.
4. What is the composition of hair?
5. Describe three physiological roles played by the hair.
6. What do nails consist of?
7. List two functions of nails.
8. What do the outermost surface of the skin, the nails, and hair have in common, in terms of their composition?
9. The innermost layer of the epidermis consists of _________ cells than the outermost layer of the epidermis.
A. older
B. younger
C. more sweat glands
D. more blood vessel
10. Identify two types of cells found in the epidermis of the skin and describe their functions.
11. True or False. Keratin-producing cells in the epidermis are a type of epithelial cell.
12. True or False. Vasodilation is used to warm the body.
13. Which structure and layer of skin do hair grow out of?
14. Identify three main functions of the integumentary system and give an example of each.
15. What are two ways in which the integumentary system protects the body against UV radiation?
Explore More
You already know that a trip to be beach could result in a nasty sunburn. Check out this video to learn more about the different types of sunscreen and why they should be used daily:
Attributions
1. Maud Stevens Wagner by The Plaza Gallery, public domain via Wikimedia Commons
2. Skin epidermis and dermis by National Cancer Institute, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/13%3A_Integumentary_System/13.2%3A_Introduction_to_the_Integumentary_System.txt |
Feel the Burn
The person in Figure \(1\) is no doubt feeling the burn — sunburn that is. Sunburn occurs when the outer layer of the skin is damaged by UV light from the sun or tanning lamps. Some people deliberately allow UV light to burn their skin because after the redness subsides, they are left with a tan. A tan may look healthy, but it is actually a sign of skin damage. People who experience one or more serious sunburns are significantly more likely to develop skin cancer. Natural pigment molecules in the skin help protect it from UV light damage. These pigment molecules are found in the layer of the skin called the epidermis.
The epidermis is the outer of the two main layers of the skin, the inner layer being the dermis. It averages about 0.10 mm thick and is much thinner than the dermis. The epidermis is thinnest on the eyelids (0.05 mm) and thickest on the palms of the hands and soles of the feet (1.50 mm). The epidermis covers almost the entire body surface. It is continuous with, but structurally distinct from, the mucous membranes that line the mouth, anus, urethra, and vagina.
Structure of the Epidermis
There are no blood vessels and very few nerve cells in the epidermis. Without blood to bring epidermal cells oxygen and nutrients, the cells must absorb oxygen directly from the air and obtain nutrients via diffusion of fluids from the dermis below. However, as thin as it is, the epidermis still has a complex structure. It has a variety of cell types and multiple layers.
Cells of the Epidermis
There are several different types of cells in the epidermis. All of the cells are necessary for the important functions of the epidermis.
• The epidermis consists mainly of stacks of keratin-producing epithelial cells called keratinocytes. These cells make up at least 90 percent of the epidermis. Near the top of the epidermis, these cells are also called squamous cells.
• Another 8 percent of epidermal cells are melanocytes. These cells produce the pigment melanin that protects the dermis from UV light.
• About 1 percent of epidermal cells are Langerhans cells. These are immune system cells that detect and fight pathogens entering the skin.
• Less than 1 percent of epidermal cells are Merkel cells, which respond to light touch and connect to nerve endings in the dermis.
Layers of the Epidermis
The epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs in the palms of the hands and soles of the feet, where the epidermis thicker than it is in the rest of the body. The layers of the epidermis are shown in Figure \(2\) and described in the following text.
Stratum Basale
The stratum basale is the innermost or the deepest layer of the epidermis. It is separated from the dermis by a membrane called the basement membrane. The stratum basale contains stem cells, called basal cells, which divide to form all the keratinocytes of the epidermis. When keratinocytes first form, they are cube-shaped and contain almost no keratin. As more keratinocytes are produced, previously formed cells are pushed up through the stratum basale. Melanocytes and Merkel cells are also found in the stratum basale. The Merkel cells are especially numerous in touch-sensitive areas such as the fingertips and lips.
Stratum Spinosum
Just above the stratum basale is the stratum spinosum. This is the thickest of the four epidermal layers. The keratinocytes in this layer have begun to accumulate keratin, and they have become tougher and flatter. Spiny cellular projections form between the keratinocytes and hold them together. In addition to keratinocytes, the stratum spinosum contains the immunologically active Langerhans cells.
Stratum Granulosum
The next layer above the stratum spinosum is the stratum granulosum. In this layer, keratinocytes have become nearly filled with keratin, giving their cytoplasm a granular appearance. Lipids are released by keratinocytes in this layer to form a lipid barrier in the epidermis. Cells in this layer have also started to die because they are becoming too far removed from blood vessels in the dermis to receive nutrients. Each dying cell digests its own nucleus and organelles, leaving behind only a tough, keratin-filled shell.
Stratum Lucidum
Only on the palms of the hands and soles of the feet, the next layer above the stratum granulosum is the stratum lucidum. This is a layer consisting of stacks of translucent, dead keratinocytes that provide extra protection to the underlying layers.
Stratum Corneum
The uppermost layer of the epidermis everywhere on the body is the stratum corneum. This layer is made of flat, hard, tightly packed dead keratinocytes that form a waterproof keratin barrier to protect the underlying layers of the epidermis. Dead cells from this layer are constantly shed from the surface of the body. The shed cells are continually replaced by cells moving up from the lower layers of the epidermis. It takes a period of about 48 days for newly formed keratinocytes in the stratum basale to make their way to the top of the stratum corneum to replace shed cells.
Functions of the Epidermis
The epidermis has several crucial functions in the body. These functions include protection, water retention, and vitamin D synthesis.
Protective Functions
The epidermis provides protection to underlying tissues from physical damage, pathogens, and UV light.
Protection from Physical Damage
Most of the physical protection of the epidermis is provided by its tough outer layer, the stratum corneum. Because of this layer, minor scrapes and scratches generally do not cause significant damage to the skin or underlying tissues. Sharp objects and rough surfaces have difficulty penetrating or removing the tough, dead, keratin-filled cells of the stratum corneum. If cells in this layer are pierced or scraped off, they are quickly replaced by new cells moving up to the surface from lower skin layers.
Protection from Pathogens
When pathogens such as viruses and bacteria try to enter the body, it is virtually impossible for them to enter through intact epidermal layers. Generally, pathogens can enter the skin only if the epidermis has been breached, for example by a cut, puncture, or scrape in Figure \(3\). That’s why it is important to clean and cover even a minor wound in the epidermis. This helps ensure that pathogens do not use the wound to enter the body. Protection from pathogens is also provided by conditions at or near the skin surface. These include relatively high acidity (pH of about 5.0), low amounts of water, the presence of antimicrobial substances produced by epidermal cells, and Langerhans cells, which phagocytize bacteria or other pathogens.
Protection from UV Light
The UV light that penetrates the epidermis can damage epidermal cells. In particular, it can cause mutations in DNA that lead to the development of skin cancer, in which epidermal cells grow out of control. The UV light can also destroy vitamin B9 (in forms such as folate or folic acid), which is needed for good health and successful reproduction. In a person with light skin, just an hour of exposure to intense sunlight can reduce the body’s vitamin B9 level by 50 percent.
Melanocytes in the stratum basale of the epidermis contain small organelles called melanosomes, which produce, store, and transport the dark brown pigment melanin. As melanosomes become full of melanin, they move into thin extensions of the melanocytes. From there, the melanosomes are transferred to keratinocytes in the epidermis, where they absorb UV light that strikes the skin. This prevents the light from penetrating deeper into the skin and causing damage. The more melanin there is in the skin, the more UV light that can be absorbed.
Water Retention
The ability of the skin to hold water and not lose it to the surrounding environment is due mainly to the stratum corneum. Lipids arranged in an organized way among the cells of the stratum corneum form a barrier to water loss from the epidermis. This is critical for maintaining healthy skin and preserving proper water balance in the body.
Although the skin is impermeable to water, it is not impermeable to all substances. Instead, the skin is selectively permeable, allowing certain fat-soluble substances to pass through the epidermis. The selective permeability of the epidermis is both a benefit and a risk.
• Selective permeability allows certain medications to enter the bloodstream through the capillaries in the dermis. This is the basis of medications that are delivered using topical ointments or patches that are applied to the skin. These include steroid hormones such as estrogen (for hormone replacement therapy), scopolamine (for motion sickness), nitroglycerin (for heart problems), and nicotine (for people trying to quit smoking).
• Selective permeability of the epidermis also allows certain harmful substances to enter the body through the skin. Examples include the heavy metal lead and many pesticides.
Vitamin D Synthesis
Vitamin D is a nutrient that is needed in the human body for the absorption of calcium from food. Molecules of a lipid compound named 7-dehydrocholesterol are precursors of vitamin D. These molecules are present in the stratum basale and stratum spinosum layers of the epidermis. When UV light strikes the molecules, it changes them to vitamin D3. In the kidneys, vitamin D3 is converted to calcitriol, which is the form of vitamin D that is active in the body.
What Gives Skin Its Color?
Melanin in the epidermis is the main substance that determines the color of human skin and explains most of the variation in skin color in people around the world. However, two other substances also contribute to skin color, especially in light-skinned people: carotene and hemoglobin.
• The pigment carotene is present in the epidermis and gives skin a yellowish tint, especially in the skin with low levels of melanin.
• Hemoglobin is a red pigment found in red blood cells. It is visible through the skin as a pinkish tint, again mainly in the skin with low levels of melanin. The pink color is most visible when capillaries in the underlying dermis dilate, allowing greater blood flow near the surface.
Bacteria on Skin
The surface of the human skin normally provides a home to countless numbers of bacteria. Just one square inch of skin normally has an average of about 50 million bacteria. These generally harmless bacteria represent roughly 1,000 bacterial species (Figure \(4\)) from 19 different bacterial phyla. Typical variations in the moistness and oiliness of the skin produce a variety of rich and diverse habitats for these microorganisms. For example, the skin in the armpits is warm and moist and often hairy, whereas the skin on the forearms is smooth and dry. These two areas of the human body are as diverse to microorganisms as rainforests and deserts are to larger organisms. The density of bacterial populations on the skin depends largely on the region of the skin and its ecological characteristics. For example, oily surfaces, such as the face, may contain over 500 million bacteria per square inch. Despite the huge number of individual microorganisms living on the skin, their total volume is only about the size of a pea.
In general, the normal microorganisms living on the skin keep one another in check and thereby play an important role in keeping the skin healthy. However, if the balance of microorganisms is disturbed, there may be an overgrowth of certain species, and this may result in an infection. For example, when a patient is prescribed antibiotics, it may kill off normal bacteria and allow an overgrowth of single-celled yeast. Even if the skin is disinfected, no amount of cleaning can remove all of the microorganisms it contains. Disinfected areas are also quickly recolonized by bacteria residing in deeper areas such as hair follicles and in adjacent areas of the skin.
What is Dermis?
The dermis is the inner of the two major layers that make up the skin, the outer layer being the epidermis. The dermis consists mainly of connective tissues. It also contains most skin structures such as glands and blood vessels. The dermis is anchored to the tissues below it by flexible collagen bundles that permit most areas of the skin to move freely over subcutaneous (“below-the-skin”) tissues. Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes.
Anatomy of the Dermis
The basic anatomy of the dermis is a matrix, or sort of scaffolding, composed of connective tissues. These tissues include collagen fibers, which provide toughness; and elastin fibers, which provide elasticity. Surrounding these fibers, the matrix also includes a gel-like substance made of proteins. The tissues of the matrix give the dermis both strength and flexibility.
The dermis is divided into two layers: the papillary layer and the reticular layer.
Papillary Layer
The papillary layer is the upper layer of the dermis, just below the basement membrane that connects the dermis to the epidermis above it. The papillary layer is the thinner of the two dermal layers. It is composed mainly of loosely arranged collagen fibers. The papillary layer is named for its fingerlike projections, or papillae, that extend upward into the epidermis. The papillae contain capillaries and sensory touch receptors.
The papillae give the dermis a bumpy surface that interlocks with the epidermis above it, strengthening the connection between the two layers of skin. On the palms and soles, the papillae create epidermal ridges. Epidermal ridges on the fingers are commonly called fingerprints (see the photo below). Fingerprints are genetically determined, so no two people (other than identical twins) have exactly the same fingerprint pattern. Therefore, fingerprints can be used as a means of identification, for example, at crime scenes. Fingerprints were much more commonly used forensically before DNA analysis was introduced for this purpose.
Reticular Layer
The reticular layer is the lower layer of the dermis, below the papillary layer. It is the thicker of the two dermal layers. It is composed of densely woven collagen and elastin fibers. These protein fibers give the dermis its properties of strength and elasticity. This layer of the dermis cushions subcutaneous tissues of the body from stress and strain. The reticular layer of the dermis also contains most of the structures in the dermis, such as glands and hair follicles.
Structures in the Dermis
Both papillary and reticular layers of the dermis contain numerous sensory receptors, which make the skin the body’s primary sensory organ for the sense of touch. Both dermal layers also contain blood vessels. They provide nutrients to and remove wastes from dermal cells as well as cells in the lowest layer of the epidermis, the stratum basale. The circulatory components of the dermis are shown in Figure \(7\).
Glands
Glands in the reticular layer of the dermis include sweat glands and sebaceous (oil) glands. Both are exocrine glands, which are glands that release their secretions through ducts to nearby body surfaces. The diagram below shows these glands and also several other structures in the dermis.
Sweat glands produce the fluid called sweat, which contains mainly water and salts. The glands have ducts that carry the sweat to hair follicles or to the surface of the skin. There are two different types of sweat glands: eccrine glands and apocrine glands.
• Eccrine sweat glands occur in the skin all over the body. Their ducts empty through tiny openings called pores onto the skin surface. These sweat glands are involved in temperature regulation.
• Apocrine sweat glands are larger than eccrine glands and occur only in the skin of the armpits and groin. The ducts of apocrine glands empty into hair follicles, and then the sweat travels along hairs to reach the surface. Apocrine glands are inactive until puberty, at which point they start producing an oily sweat that is consumed by bacteria living on the skin. The digestion of apocrine sweat by bacteria is the cause of body odor.
Sebaceous glands are exocrine glands that produce a thick, fatty substance called sebum. Sebum is secreted into hair follicles and makes its way to the skin surface along with hairs. It waterproofs the hair and skin and helps prevent them from drying out. Sebum also has antibacterial properties, so it inhibits the growth of microorganisms on the skin. Sebaceous glands are found in every part of the skin except for the palms of the hands and soles of the feet where hair does not grow.
Hair Follicles
Hair follicles are the structures where hairs originate (Figure \(8\)). Hairs grow out of follicles, pass through the epidermis, and exit at the surface of the skin. Associated with each hair follicle is a sebaceous gland, which secretes sebum that coats and waterproofs the hair. Each follicle also has a bed of capillaries, a nerve ending, and a tiny muscle called arrector pili.
Functions of the Dermis
The main functions of the dermis are regulating body temperature, enabling the sense of touch, and eliminating wastes from the body.
Temperature Regulation
Several structures in the reticular layer of the dermis are involved in regulating body temperature. For example, when the body temperature rises, the hypothalamus of the brain sends nerve signals to sweat glands, causing them to release sweat. An adult can sweat up to four liters an hour. As the sweat evaporates from the surface of the body, it uses energy in the form of body heat, thus cooling the body. The hypothalamus also causes dilation of blood vessels in the dermis when the body temperature rises. This allows more blood to flow through the skin, bringing body heat to the surface, where it can radiate into the environment.
When the body is too cool, sweat glands stop producing sweat, and blood vessels in the skin constrict, thus conserving body heat. The arrector pili muscles also contract, moving hair follicles and lifting hair shafts. This results in more air being trapped under the hairs to insulate the surface of the skin. These contractions of arrector pili muscles are the cause of goosebumps.
Sensing the Environment
Sensory receptors in the dermis are mainly responsible for the body’s tactile senses. The receptors detect such tactile stimuli as warm or cold temperature, shape, texture, pressure, vibration, and pain. They send nerve impulses to the brain which interprets and responds to the sensory information. Sensory receptors in the dermis can be classified on the basis of the type of touch stimulus they sense. Mechanoreceptors sense mechanical forces such as pressure, roughness, vibration, and stretching. Thermoreceptors sense variations in temperature that are above or below body temperature. Nociceptors sense painful stimuli. Figure \(9\) shows several specific kinds of tactile receptors in the dermis. Each kind of receptor senses one or more types of touch stimuli.
• Free nerve endings sense pain and temperature variations.
• Merkel cells sense light touch, shapes, and textures.
• Meissner’s corpuscles sense light touch.
• Pacinian corpuscles sense pressure and vibration.
• Ruffini corpuscles sense stretching and sustained pressure.
Excreting Wastes
The sweat released by eccrine sweat glands is one way the body excretes waste products. Sweat contains excess water, salts (electrolytes), and other waste products that the body must get rid of to maintain homeostasis. The most common electrolytes in sweat are sodium and chloride. Potassium, calcium, and magnesium electrolytes may be excreted in sweat as well. When these electrolytes reach high levels in the blood, extra electrolytes are excreted in sweat. This helps to bring their blood levels back into balance. Besides electrolytes, sweat contains small amounts of waste products from metabolism including ammonia and urea. Sweat may also contain alcohol in someone who has been drinking alcoholic beverages.
Feature: My Human Body
Acne is the most common skin disorder in the United States. At least 40 million Americans have acne at any given time. Acne occurs most commonly in teens and young adults, but it can occur at any age. Even newborn babies can get acne.
The main sign of acne is the appearance of pimples (pustules) on the skin, like those in the photo above. Other signs of acne may include whiteheads, blackheads, nodules, and other lesions. Besides the face, acne can appear on the back, chest, neck, shoulders, upper arms, and buttocks. Acne can permanently scar the skin, especially if it isn’t treated appropriately. Besides its physical effects on the skin, acne can also lead to low self-esteem and depression.
Acne is caused by clogged, sebum-filled pores that provide a perfect environment for the growth of bacteria. The bacteria cause infection, and the immune system responds with inflammation. Inflammation, in turn, causes swelling and redness and may be associated with the formation of pus. If the inflammation goes deep into the skin, it may form an acne nodule.
Mild acne often responds well to treatment with over-the-counter (OTC) products containing benzoyl peroxide or salicylic acid. Treatment with these products may take a month or two to clear up the acne. Once the skin clears, treatment generally needs to be continued for some time to prevent future breakouts.
If acne fails to respond to OTC products, nodules develop, or acne is affecting self-esteem, a visit to a dermatologist is in order. A dermatologist can determine which treatment is best for a given patient. A dermatologist can also prescribe prescription medications (which are likely to be more effective than OTC products) and provide other medical treatments such as laser light therapies or chemical peels.
What can you do to maintain healthy skin and prevent or reduce acne? Dermatologists recommend the following tips:
• Wash affected or acne-prone skin (such as the face) twice a day and after sweating.
• Use your fingertips to apply a gentle, non-abrasive cleanser. Avoid scrubbing, which can make acne worse.
• Use only alcohol-free products and avoid any products that irritate the skin, such as harsh astringents or exfoliants.
• Rinse with lukewarm water, and avoid using very hot or cold water.
• Shampoo your hair regularly.
• Do not pick, pop, or squeeze acne. If you do, it will take longer to heal and is more likely to scar.
• Keep your hands off your face. Avoid touching your skin throughout the day.
• Stay out of the sun and tanning beds. Some acne medications make your skin very sensitive to UV light.
Review
1. What is the dermis?
2. Describe the basic anatomy of the dermis.
3. Compare and contrast the papillary and reticular layers of the dermis.
4. What causes epidermal ridges, and why can they be used to identify individuals?
5. Name the two types of sweat glands in the dermis and state how they differ.
6. What is the function of sebaceous glands?
7. Describe structures associated with hair follicles.
8. Explain how the dermis helps regulate body temperature.
9. Identify three specific kinds of tactile receptors in the dermis and the type of stimuli they sense.
10. How does the dermis excrete wastes, and what waste products does it excrete?
11. What are the subcutaneous tissues? Which layer of the dermis provides cushioning for subcutaneous tissues and why does this layer provide most of the cushioning instead of the other layer?
12. For each of the following functions, describe which structure within the dermis carries it out.
1. Brings nutrients to and removes wastes from dermal and lower epidermal cells
2. Causes hairs to move
3. Detects painful stimuli on the skin
13. What is the epidermis?
14. Identify the types of cells in the epidermis.
15. Describe the layers of the epidermis.
16. State one function of each of the four epidermal layers found all over the body.
17. Explain three ways the epidermis protects the body.
18. What makes the skin waterproof?
19. Why is the selective permeability of the epidermis both a benefit and risk?
20. How is vitamin D synthesized in the epidermis?
21. Identify three pigments that impart color to the skin.
22. Describe bacteria that normally reside on the skin, and explain why they do not usually cause infections.
23. Explain why the keratinocytes at the surface of the epidermis are dead, while keratinocytes located deeper in the epidermis are still alive.
24. Which layer of the epidermis contains keratinocytes that have begun to die?
25. True or False. The extra layer of epidermis found on the palms of the hands and soles of the feet is located on the very outer surface of the skin.
26. True or False. Melanin can be found in keratinocytes.
27. Explain why our skin is not permanently damaged if we rub off some of the surface layers by using a rough washcloth.
Attributions
1. Sunburn by QuinnHK, public domain via Wikimedia Commons
2. Structure epidermis by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
3. Scraped knee by Remux, CC0 via Wikimedia Commons
4. Staphylococcus epidermidis by Janice Carr, CDC, public domain via Wikimedia Commons
5. Epidermis and dermis slide by Kilbad, public domain via Wikimedia Commons
6. Fingerprint detail by Frettie, CC BY 3.0 via Wikimedia Commons
7. Dermal circulation by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
8. Anatomy of the skin by Don Bliss, National Cancer Institute, public domain via Wikimedia Commons
9. Skin tactile receptors by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
10. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/13%3A_Integumentary_System/13.3%3A_Skin.txt |
Fashion Statement
This pink hairstyle makes quite a fashion statement. Many people spend a lot of time and money on their hair, even if they don’t have such an exceptional hairstyle as this one. Besides its display value, hair actually has important physiological functions.
Hair is a filament that grows from a hair follicle in the dermis of the skin. It consists mainly of tightly packed, keratin-filled cells called keratinocytes. The human body is covered with hair follicles except for a few areas, including the mucous membranes, lips, palms of the hands, and soles of the feet.
Structure of Hair
The part of the hair that is located within the follicle is called the hair root. The root is the only living part of the hair. The part of the hair that is visible above the surface of the skin is the hair shaft. The shaft of the hair has no biochemical activity and is considered dead.
Follicle and Root
Hair growth begins inside a follicle (Figure \(2\):). Each hair follicle contains stem cells that can keep dividing and allow hair to grow. The stem cells can also regrow new hair after one falls out. Another structure associated with a hair follicle is a sebaceous gland that produces oily sebum, which lubricates and helps to waterproof the hair. A tiny arrector pili muscle is also attached to the follicle. When it contracts, the follicle moves and the hair in the follicle stands up.
Functions of Hair
In humans, one function of head hair is to provide insulation and help the head retain heat. Head hair also protects the skin on the head from damage by UV light. The function of hair in other locations on the body is debated. One idea is that body hair helps to keep us warm in cold weather. When the body is too cold, the arrector pili muscles contract and cause hairs to stand up, trapping a layer of warm air above the epidermis. However, this is more effective in mammals that have thick hair or fur than it is in relatively hairless human beings.
Human hair has an important sensory function as well. Sensory receptors in the hair follicles can sense when the hair moves, whether it moves because of a breeze or the touch of a physical object. The receptors may also provide sensory awareness of the presence of parasites on the skin. Some hairs, such as eyelashes, are especially sensitive to the presence of potentially harmful matter. The eyebrows protect the eyes from dirt, sweat, and rain. In addition, the eyebrows play a key role in nonverbal communication (Figure \(3\)). They help express emotions such as sadness, anger, surprise, and excitement.
What Are Nails?
Nails are accessory organs of the skin. They are made of sheets of dead keratinocytes and are found on the far, or distal, ends of the fingers and toes. The keratin in nails makes them hard but flexible. Nails serve a number of purposes, including protecting the digits, enhancing sensations, and acting like tools.
Nail Anatomy
A nail has three main parts: the root, plate, and free margin. Other structures around or under the nail include the nail bed, cuticle, and nail fold. Nails grow from a deep layer of living epidermal tissue, known as the nail matrix, at the proximal end of the nail. The nail matrix surrounds the nail root. It contains stem cells that divide to form keratinocytes, which are cells that produce keratin and make up the nail. These structures are shown in Figure \(4\).
• The nail root is the portion of the nail found under the surface of the skin at the near, or proximal, end of the nail. It is where the nail begins.
• The nail plate (or body) is the portion of the nail that is external to the skin. It is the visible part of the nail.
• The free margin is the portion of the nail that protrudes beyond the distal end of the finger or toe. This is the part that is cut or filed to keep the nail trimmed.
• The nail bed is the area of skin under the nail plate. It is pink in color due to the presence of capillaries in the dermis.
• The cuticle is a layer of dead epithelial cells that overlaps and covers the edge of the nail plate. It helps to seal the edges of the nail to prevent infection of the underlying tissues.
• The nail fold is a groove in the skin in which the side edges of the nail plate are embedded.
Nails and Health
Healthcare providers, particularly EMTs, often examine the fingernail beds as a quick and easy indicator of oxygen saturation of the blood or the amount of blood reaching the extremities. If the nail beds are bluish or purple, it is generally a sign of low oxygen saturation. To see if blood flow to the extremities is adequate, a blanch test may be done. In this test, a fingernail is briefly depressed to turn the nail bed white by forcing the blood out of its capillaries. When the pressure is released, the pink color of the nail bed should return within a second or two if there is normal blood flow. If the return to a pink color is delayed, then it can be an indicator of low blood volume due to dehydration or shock. Nails — especially toenails — are common sites of fungal infections, causing nails to become thickened and yellowish in color. Toenails are more often infected than fingernails because they are often confined in shoes. This provides a dark, warm, moist environment where fungi can thrive. Toes also tend to have less blood flow than fingers, making it harder for the immune system to detect and stop infections in toenails. Although nails are harder and tougher than the skin, they are more permeable than the skin. Harmful substances, such as herbicides may be absorbed through the nails and cause health problems.
Feature: Reliable Sources
Do you get regular manicures or pedicures from a nail technician? If so, there is a chance that you are putting your health at risk. Nail tools that are not properly disinfected between clients may transmit infections from one person to another. Cutting the cuticles with scissors may create breaks in the skin that let infective agents enter the body. Products such as acrylics, adhesives, and UV gels that are applied to the nails may be harmful, especially if they penetrate the nails and enter the skin.
Use the Internet and find several reliable sources that address the health risks of professional manicures or pedicures. Try to find answers to the following questions:
1. What training and certification are required for professional nail technicians?
2. What licenses and inspections are required for nail salons?
3. What hygienic practices should be followed in nail salons to reduce the risk of infections being transmitted to clients?
4. Which professional nail products are potentially harmful to the human body and which are safer?
5. How likely is it to have an adverse health consequence when you get a professional manicure or pedicure?
6. What steps can you take to ensure that a professional manicure or pedicure is safe?
Review
1. Compare and contrast the hair root and hair shaft.
2. Describe hair follicles.
3. Identify the three zones of a hair shaft.
4. Describe two functions of human hair.
5. True or False. Eyelashes can have a sensory function.
6. Hair consists mainly of:
• A. Melanocytes
• B. Keratinocytes
• C. Epidermocytes
• D. Hirocytes
7. What are the nails?
8. Describe three parts of the nail.
9. Explain why most of the nail plate looks pink.
10. Describe a lunula.
11. Explain how a nail grows.
12. Identify three functions of nails.
13. Give several examples of how nails are related to health.
14. True or False. Nails grow from the distal end to the proximal end of your fingers and toes.
15. True or False. The nail bed refers to the middle portion of the hard nail plate
16. Nails are composed mainly of a protein called:
A. Elastin
B. Collagen
C. Keratin
D. Melanin
17. a. What is the cuticle of the nail composed of?
b. What is the function of the cuticle?
c. Why is it a bad idea to cut the cuticle during a manicure?
18. What is the name of the part of the nail that you trim?
19. Is the nail plate composed of living or dead cells?
Explore More
Do you wonder what causes male pattern baldness? Watch this short video to find out.
Nails are not just for decoration, they can actually tell us a lot about our health. Learn more here:
Attributions
1. Tangle portrait by Disabled And Here, licensed CC BY 4.0
2. Skin layers by Madhero88 and M.Komorniczak, CC BY-SA 3.0 via Wikimedia Commons
3. Omer's scowl by Jon Eben Field, CC BY 2.0 via Wikimedia Commons
4. Fingernail anatomy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/13%3A_Integumentary_System/13.4%3A_Hair_and_Nails.txt |
Case Study Conclusion
Skin cancer begins in the outer layer of skin, the epidermis. There are three common types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma.
Basal Cell Carcinoma
Basal cell carcinoma occurs in basal cells of the epidermis. Basal cells are stem cells in the stratum basale layer that divide to form all the keratinocytes of the epidermis. Basal cell carcinoma is the most common form of skin cancer. More than four million cases occur in the United States each year. A basal cell carcinoma may appear as a pearly or waxy bump, like the one shown in Figure \(2\). Basal cell carcinomas rarely spread (or undergo metastasis), so they can generally be cured with a biopsy, in which the lesion is cut out of the skin and analyzed in a medical lab.
Squamous Cell Carcinoma
Squamous cell carcinoma occurs in squamous cells of the epidermis. Squamous cells are flattened, keratin-filled cells in the upper layers of the epidermis. Squamous cell carcinoma is the second most common form of skin cancer. More than two million cases occur in the United States each year. A squamous cell carcinoma may appear as a firm, red nodule, or as a flat lesion with a scaly or crusty surface, like the one pictured in Figure \(3\). Squamous cell carcinomas are generally localized and unlikely to metastasize, so they are usually curable surgically.
Melanoma
Melanoma occurs in the melanocytes of the epidermis. Melanocytes are the melanin-producing cells in the stratum basale of the epidermis. Melanoma is the rarest type of skin cancer, accounting for less than one percent of all skin cancer cases. Melanoma, however, is the most deadly type of skin cancer. It causes the vast majority of skin cancer deaths because melanoma is malignant. If not treated, it will metastasize and spread to other parts of the body. If melanoma is detected early and while it is still localized in the skin, most patients survive for at least five years. If melanoma is discovered only after it has already metastasized to distant organs, there is only a 17 percent chance of patients surviving for five years. You can see an example of melanoma in Figure \(4\).
Melanoma can develop anywhere on the body. It may develop in otherwise normal skin, or an existing mole may become cancerous. Signs of melanoma may include a:
• mole that changes in size, feel, or color
• mole that bleeds
• large brown spot on the skin sprinkled with darker specks
• small lesion with an irregular border and parts that appear red, white, blue, or blue-black
• dark lesion on the palms, soles, fingertips, toes, or mucous membranes
As with most types of cancer, skin cancer is the easiest to treat and most likely to be cured the earlier it is detected. The skin is one of the few organs that you can monitor for cancer yourself, as long as you know what to look for. A brown spot on the skin is likely to be a harmless mole, but it could be a sign of skin cancer. As shown in Figure \(5\), unlike moles, skin cancers may be asymmetrical, have irregular borders, be very dark in color, and may have a relatively great diameter. These characteristics can be remembered with the mnemonic ABCD.
With the help of mirrors, you should check all of your skin regularly. Look for new skin growths or changes in any existing moles, freckles, bumps, or birthmarks. Report anything suspicious or different to your doctor.
If you have risk factors for skin cancer, it’s a good idea to have an annual skin check by a dermatologist. This helps ensure that cancerous or precancerous lesions will be detected before they grow too large and become difficult to cure—or, in the case of melanoma, before they metastasize.
Chapter Review
In this chapter, you learned about the structures and functions of the organs of the integumentary system. Specifically, you learned that:
• The integumentary system consists of the skin, hair, and nails. Functions of the integumentary system include providing a protective covering for the body, sensing the environment, and helping the body maintain homeostasis.
• The skin’s main functions include preventing water loss from the body, serving as a barrier to the entry of microorganisms, synthesizing vitamin D, blocking UV light, and helping to regulate body temperature.
• The skin consists of two distinct layers: a thinner outer layer called the epidermis and a thicker inner layer called the dermis.
• The epidermis consists mainly of epithelial cells called keratinocytes, which produce keratin. New keratinocytes form at the bottom of the epidermis. They become filled with keratin and die as they move upward toward the surface of the skin, where they form a protective, waterproof layer.
• The dermis consists mainly of tough connective tissues that provide strength and stretch; and almost all skin structures, including blood vessels, sensory receptors, hair follicles, and oil and sweat glands.
• Cell types in the epidermis include keratinocytes which make up 90 percent of epidermal cells; melanocytes that produce melanin; Langerhans cells that fight pathogens in the skin; and Merkel cells that respond to light touch.
• The epidermis in most parts of the body consists of four distinct layers. A fifth layer occurs only in the epidermis of the palms of the hands and soles of the feet.
• The innermost layer of the epidermis is the stratum basale, which contains stem cells that divide to form new keratinocytes. The next layer is the stratum spinosum, which is the thickest layer and contains Langerhans cells and spiny keratinocytes. This is followed by the stratum granulosum, in which keratinocytes are filling with keratin and starting to die. The stratum lucidum is next, but only on the palms and soles. It consists of translucent dead keratinocytes. The outermost layer is the stratum corneum, which consists of flat, dead, tightly packed keratinocytes that form a tough, waterproof barrier for the rest of the epidermis.
• Functions of the epidermis include protecting underlying tissues from physical damage and pathogens. Melanin in the epidermis absorbs and protects underlying tissues from UV light. The epidermis also prevents the loss of water from the body and synthesizes vitamin D.
• Melanin is the main pigment that determines the color of human skin. However, the pigments of carotene and hemoglobin also contribute to skin color, especially in the skin with low levels of melanin.
• The surface of healthy skin normally is covered by vast numbers of bacteria representing about 1,000 species from 19 phyla. Different areas of the body provide diverse habitats for skin microorganisms. Usually, microorganisms on the skin keep each other in check unless their balance is disturbed.
• The thicker inner layer of the skin, the dermis, has two layers. The upper papillary layer has papillae extending upward into the epidermis and loose connective tissues. The lower reticular layer has denser connective tissues and structures such as glands and hair follicles. Glands in the dermis include eccrine and apocrine sweat glands and sebaceous glands. Hair follicles are structures where hairs originate.
• Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes. The dense connective tissues of the dermis provide cushioning. The dermis regulates body temperature mainly by sweating and by vasodilation or vasoconstriction. The many tactile sensory receptors in the dermis make it the main organ for the sense of touch. Wastes excreted in sweat include excess water, electrolytes, and certain metabolic wastes.
• Hair is a filament that grows from a hair follicle in the dermis of the skin. It consists mainly of tightly packed, dead keratinocytes that are filled with keratin. The human body is almost completely covered with hair follicles.
• Hair helps prevent heat loss from the head and protects its skin from UV light. Hair in the nose filters the incoming air, and the eyelashes and eyebrows keep harmful substances out of the eyes. Hair all over the body provides tactile sensory input. The eyebrows also play a role in nonverbal communication.
• The part of the hair that is within the follicle is the hair root. This is the only living part of a hair. The part of the hair that is visible above the skin surface is the hair shaft. It consists of dead cells.
• Hair growth begins inside a follicle when stem cells within the follicle divide to produce new keratinocytes.
• A hair shaft has three zones: the outermost zone called the cuticle; the middle zone called the cortex, and the innermost zone called the medulla.
• Genetically controlled, visible characteristics of hair include hair color, hair texture, and the extent of balding in adult males. Melanin (eumelanin and/or pheomelanin) is the pigment that gives hair its color. Aspects of hair texture include curl pattern, thickness, and consistency.
• Among mammals, humans are nearly unique in having undergone a significant loss of body hair during their evolution, probably because sweat evaporates more quickly from the less hairy skin. Curly hair also is thought to have evolved at some point during human evolution, perhaps because it provided better protection from UV light.
• Hair has social significance for human beings, being an indicator of biological sex, age, and ethnic ancestry. Human hair also has cultural significance. For example, hairstyle may be an indicator of a social group membership.
• Nails consist of sheets of dead, keratin-filled keratinocytes. The keratin in nails makes them hard but flexible. They help protect the ends of the fingers and toes, enhance the sense of touch in the fingertips, and may be used as tools.
• A nail has three main parts: the nail root, which is under the epidermis; the nail plate, which is the visible part of the nail; and the free margin, which is the distal edge of the nail. Other structures under or around a nail include the nail bed, cuticle, and nail fold.
• A nail grows from a deep layer of living epidermal tissues, called the nail matrix, at the proximal end of the nail. Stem cells in the nail matrix keep dividing to allow nail growth, forming first the nail root and then the nail plate as the nail continues to grow longer and emerges from the epidermis.
• Fingernails grow faster than toenails. Actual rates of growth depend on many factors, such as age, sex, and season.
• The color of the nail bed can be used to quickly assess oxygen and blood flow in a patient. How the nail plate grows out can reflect recent health problems, such as illness or nutrient deficiency. Nails — and especially toenails — are prone to fungus infections. Nails are more permeable than skin and can absorb several harmful substances such as herbicides.
• Skin cancer is a disease in which skin cells grow out of control. It is caused mainly by excessive exposure to UV light, which damages DNA. Skin cancer affects more Americans than all other cancers combined.
• There are three common types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma. Carcinomas are more common and unlikely to metastasize. Melanoma is rare and likely to metastasize. It causes the most skin cancer deaths.
• Besides exposure to UV light, risk factors for skin cancer include having light-colored skin, many moles, and a family history of skin cancer, among several others.
Now that you have learned about the organs on the surface of the body, read the next chapter to travel inside and learn about the skeletal system, which protects and supports us internally, among other functions.
Chapter Summary Review
1. What is skin cancer?
2. How common is skin cancer?
3. Compare and contrast the three common types of skin cancer.
4. Identify factors that increase the risk of skin cancer.
5. How does exposure to UV light cause skin cancer?
6. In which layer of the skin does skin cancer normally start?
7. Which two skin cancers described in this section start in the same sublayer? Include the name of the sublayer and the cells affected in each of these cancers in your answer.
8. If a type of skin cancer spreads to other organs, which type is it most likely to be? Explain your answer.
9. True or False. A mole is a form of cancer.
10. True or False. Exposure to UV light can contribute to wrinkles.
11. True or False. Skin cancers are always dark in color.
12. Which form of skin cancer is the most deadly?
13. What are some ways people can reduce their risk of getting skin cancer? Explain your answer.
14. True or False. UV radiation causes more cancers than tobacco use.
15. Describe one way in which the integumentary system works with another organ system to carry out a particular function.
16. Put the following layers of skin in order, from the deepest layer to the layer closest to the surface:
1. papillary layer
2. stratum basale
3. reticular layer
4. stratum spinosum
17. The basement membrane is between the:
1. Dermis and epidermis
2. Dermis and the subcutaneous tissues beneath it
3. Dermis and the hair in the follicle
4. Nail matrix and the nail bed
18. For each of the descriptions below (A-D), match it to the protein that is best described by it (protein choices: keratin, collagen, melanin, elastin).
1. Helps provide strength and elasticity in the lower layer of the dermis
2. Makes up the loosely arranged fibers in the upper layer of the dermis
3. The predominant protein in hair, skin, and nails
4. Protects against damage from UV light.
19. Keratinocytes are found in:
1. Skin
2. Hair
3. Nails
4. All of the above
20. Papillae extend from the :
1. sebaceous glands to the surface of the skin
2. sweat glands to the surface of the skin
3. epidermis down into the dermis
4. dermis up into the epidermis
21. Describe two types of waterproofing used in the integumentary system. Include the types of molecules and where they are located in your answer.
22. Explain why nails enhance touch sensations.
23. Why do you think light-colored skin is a risk factor for skin cancer?
24. Which vitamin is synthesized by the skin?
1. Vitamin A
2. Vitamin D
3. Vitamin B9
4. Vitamin E
25. Describe the similarities between how the epidermis, hair, and nails all grow.
26. True or False. The inside of the mouth is considered to be epidermal tissue.
27. True or False. Epidermal cells are filled with an increasing amount of keratin as they go from the lowest layer to the outermost layer.
28. True or False. Cells in the stratum corneum of the skin do not have a nucleus or organelles.
29. What does the whitish crescent-shaped area at the base of your nails (towards your hands) represent? What is its function?
30. What is one difference between human hair and the hair of non-human primates?
31. True or False. Blood vessels extend through the entire thickness of the skin.
32. True or False. Cells that produce melanin are located in the dermis of the skin.
33. Describe the relationship between skin and hair.
34. What kind of skin cancer is a cancer of a type of stem cell?
35. For the skin and hair, describe one way in which they each protect the body against pathogens.
36. If sweat glands are in the dermis, how is sweat released to the surface of the body?
37. Explain why you think that physicians usually insist that patients remove any nail polish before having surgery.
38. True or False. Langerhans cells are immune cells located in the epidermis.
39. True or False. Fingerprints are due to structures on the surface of the epidermis.
40. Describe generally how the brain gets touch information from the skin.
Attributions
1. Skin cancer prevention in NZ by Sarang, public domain via Wikimedia Commons
2. Basal cell carcinoma by National Cancer Institute, public domain via Wikimedia Commons
3. Squamous cell carcinoma by National Cancer Institute, public domain via Wikimedia Commons
4. Melanoma by National Cancer Institute, public domain via Wikimedia Commons
5. ABCD melanoma by CK-12 licensed CC BY-NC 3.0
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/13%3A_Integumentary_System/13.5%3A_Case_Study_Conclusion%3A_Skin_Cancer_and_Chapter_Summary.txt |
This chapter describes the structure and functions of the skeletal system and its two major divisions, the axial skeleton and the appendicular skeleton. It details the structure of bone, how bones grow, and how they are remodeled and repaired. The chapter also explains how joints work and how they are classified as well as the causes and effects of major skeletal system disorders.
• 14.1: Case Study: Your Support System
Amari loves wearing high heels when they go out at night, especially stilettos. They know high heels are not the most practical shoes, but Amari likes how they look. Lately Amari has been experiencing pain in the balls of thier feet—the area just behind the toes. Even when they trades heels for comfortable sneakers, it still hurts when Amari stands or walks.
• 14.2: Introduction to the Skeletal System
The skull and cross-bones symbol has been used for a very long time to represent death, perhaps because after death and decomposition, bones are all that remain. Many people think of bones as being dead, dry, and brittle. These adjectives may correctly describe the bones of a preserved skeleton, but the bones of a living human being are very much alive. Living bones are also strong and flexible. Bones are the major organs of the skeletal system.
• 14.3: Divisions of the Skeletal System
This somewhat macabre display can be viewed at the Slovak National Museum in Bratislava, Slovakia. The skulls are meant to represent normal human skeletal anatomy. The skull is part of the axial skeleton, which is one of the two major divisions of the human skeleton. The other division is the appendicular skeleton.
• 14.4: Structure of Bone
Do you recognize the food item in the top left of this photo? It's roasted bone marrow, still inside the bones. It's considered a delicacy in some cuisines. Marrow is a type of tissue found inside many animal bones, including our own. It's a soft tissue that in adults may be mostly fat. You'll learn more about bone marrow and other tissues that make up bones when you read this concept.
• 14.5: Bone Growth, Remodeling, and Repair
Did you ever break a leg or other bone, like the man looking longingly at the water in this swimming pool? Having a broken bone can really restrict your activity. Bones are very hard, but they will break, or fracture, if enough force is applied to them. Fortunately, bones are highly active organs that can repair themselves if they break. Bones can also remodel themselves and grow. You'll learn how bones can do all of these things in this concept.
• 14.6: Joints
Joints are locations at which bones of the skeleton connect with one another. A joint is also called an articulation. The majority of joints are structured in such a way that they allow movement. However, not all joints allow movement. Of joints that do allow movement, the extent and direction of the movements they allow also vary.
• 14.7: Disorders of the Skeletal System
The woman on the right in this image has a deformity in her back commonly called dowager's (widow's) hump, because it occurs most often in elderly women. Its medical name is kyphosis, and it is defined as excessive curvature of the spinal column in the thoracic region. The curvature generally results from fractures of thoracic vertebrae. As the inset drawings suggest, these fractures may occur due to a significant decrease in bone mass, which is called osteoporosis. Osteoporosis is one of the mo
• 14.8: Case Study Conclusion: Heels and Chapter Summary
You may have seen signs indicating that high-heeled shoes are not allowed on certain walking surfaces because of the risk of injury. High heels affect a person's balance, and wearers can easily twist their ankle on uneven or slippery surfaces, causing a sprain or even a fracture. Besides twisting an ankle, wearing high heels on a regular basis can cause a variety of other negative health consequences—some of which may be long-lasting.
14: Skeletal System
Case Study: A Pain in the Foot
Amari loves wearing high heels when they go out at night, like the stiletto heels shown in Figure \(1\). Amari uses gender-neutral pronouns, such as they, them, and their. They know high heels are not the most practical shoes, but they like how they look. Lately, Amari has been experiencing pain in the balls of their feet—the area just behind the toes. Even when they trade heels for comfortable sneakers, it still hurts when they stand or walk.
What could be going on? Amari searches online to try to find some answers. They find a reputable source for foot pain information—a website from a professional organization of physicians that peer reviews the content by experts in the field. There, Amari reads about a condition called metatarsalgia, which produces pain in the ball of the foot that sounds very similar to what they are experiencing.
Amari learns that a common cause of metatarsalgia is the wearing of high heels because they push the foot into an abnormal position. This results in excessive pressure being placed onto the ball of the foot. Looking at the photograph above, you can imagine how much of the body weight is focused on the ball of the foot because of the shape of the high heels. If they were not wearing high heels, the weight would be more evenly distributed across the foot.
As they read more about the hazards of high heels, Amari learns that heels can also cause foot deformities such as hammertoes and bunions, small cracks in the bone called stress fractures, and may even contribute to the development of osteoarthritis of the knees at an early age.
These conditions caused by high heels are all problems of the skeletal system, which includes bones and connective tissues that hold bones together and cushion them at joints such as the knee. The skeletal system supports the body’s weight and protects internal organs, but as you will learn as you read this chapter, it also carries out a variety of other important physiological functions.
At the end of the chapter, you will find out why high heels can cause these skeletal system problems and the steps Amari takes to recover from their foot pain and prevent long-term injury.
Chapter Overview: Skeletal System
In this chapter, you will learn about the structure, functions, growth, repair, and disorders of the skeletal system. Specifically, you will learn about:
• The components of the skeletal system, which include bones, ligaments, and cartilage.
• The functions of the skeletal system, which include supporting and giving shape to the body, protecting internal organs, facilitating movement, producing blood cells, helping maintain homeostasis, and producing endocrine hormones.
• The organization and functions of the two main divisions of the skeletal system: the axial skeletal system, which includes the skull, spine, and rib cage; and the appendicular skeletal system, which includes the limbs and girdles that attach the limbs to the axial skeleton.
• The tissues and cells that make up bones and their specific functions, including making new bone, breaking down bone, producing blood cells, and regulating mineral homeostasis.
• The different types of bones in the skeletal system, based on shape and location.
• How bones grow, remodel, and repair themselves.
• The different types of joints between bones, where they are located, and the ways in which they allow different types of movement depending on their structure.
• The causes, risk factors, and treatments for the two most common disorders of the skeletal system: osteoporosis and osteoarthritis.
As you read this chapter, think about the following questions:
1. Amari suspects they have a condition called metatarsalgia. This term is related to the term “metatarsals.” What are metatarsals, where are they located, and how do you think they are related to metatarsalgia?
2. High heels can cause stress fractures, which are small cracks in the bone that usually appear after repeated mechanical stress, instead of after a significant acute injury. What other condition described in this chapter involves a similar process?
3. What are bunions and osteoarthritis of the knee? Why do you think they can be caused by wearing high heels?
Attributions
1. High heels by Agnali via Pixabay license
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/14%3A_Skeletal_System/14.1%3A_Case_Study%3A_Your_Support_System.txt |
Skull and Cross-Bones
The skull and cross-bones symbol has been used for a very long time to represent death, perhaps because after death and decomposition, bones are all that remain. Many people think of bones as being dead, dry, and brittle. These adjectives may correctly describe the bones of a preserved skeleton, but the bones of a living human being are very much alive. Living bones are also strong and flexible. Bones are the major organs of the skeletal system.
The skeletal system is the organ system that provides an internal framework for the human body. Why do you need a skeletal system? Try to imagine what you would look like without it. You would be a soft, wobbly pile of skin containing muscles and internal organs but no bones. You might look something like a very large slug. Not that you would be able to see yourself — folds of skin would droop down over your eyes and block your vision because of your lack of skull bones. You could push the skin out of the way if you could only move your arms, but you need bones for that as well!
Components of the Skeletal System
In adults, the skeletal system includes 206 bones, many of which are shown in Figure \(2\). Bones are organs made of dense connective tissues, mainly the tough protein collagen. Bones contain blood vessels, nerves, and other tissues. Bones are hard and rigid due to deposits of calcium and other mineral salts within their living tissues. Locations, where two or more bones meet, are called joints. Many joints allow bones to move like levers. For example, your elbow is a joint that allows you to bend and straighten your arm.
Besides bones, the skeletal system includes cartilage and ligaments.
• Cartilage is a type of dense connective tissue, made of tough protein fibers. It is strong but flexible and very smooth. It covers the ends of bones at joints, providing a smooth surface for bones to move over.
• Ligaments are bands of fibrous connective tissue that hold bones together. They keep the bones of the skeleton in place.
Axial and Appendicular Skeletons
The skeleton is traditionally divided into two major parts: the axial skeleton and the appendicular skeleton, both of which are pictured in Figure \(3\).
• The axial skeleton forms the axis of the body. It includes the skull, vertebral column (spine), and rib cage. The bones of the axial skeleton, along with ligaments and muscles, allow the human body to maintain its upright posture. The axial skeleton also transmits weight from the head, trunk, and upper extremities down the back to the lower extremities. In addition, the bones protect the brain and organs in the chest.
• The appendicular skeleton forms the appendages and their attachments to the axial skeleton. It includes the bones of the arms and legs, hands and feet, and shoulder and pelvic girdles. The bones of the appendicular skeleton make possible locomotion and other movements of the appendages. They also protect the major organs of digestion, excretion, and reproduction.
Functions of the Skeletal System
The skeletal system has many different functions that are necessary for human survival. Some of the functions, such as supporting the body, are relatively obvious. Other functions are less obvious but no less important. For example, three tiny bones (hammer, anvil, and stirrup) inside the middle ear transfer sound waves into the inner ear.
Support, Shape, and Protection
The skeleton supports the body and gives it shape. Without the rigid bones of the skeletal system, the human body would be just a bag of soft tissues, as described above. The bones of the skeleton are very hard and provide protection to the delicate tissues of internal organs. For example, the skull encloses and protects the soft tissues of the brain, and the vertebral column protects the nervous tissues of the spinal cord. The vertebral column, ribs, and sternum (breast bone) protect the heart, lungs, and major blood vessels. Providing protection to these latter internal organs requires the bones to be able to expand and contract. The ribs and the cartilage that connects them to the sternum and vertebrae are capable of small shifts that allow breathing and other internal organ movements.
Movement
The bones of the skeleton provide attachment surfaces for skeletal muscles. When the muscles contract, they pull on and move the bones. The figure below, for example, shows the muscles attached to the bones at the knee. They help stabilize the joint and allow the leg to bend at the knee. The bones at joints act like levers moving at a fulcrum point, and the muscles attached to the bones apply the force needed for movement.
Hematopoiesis
Hematopoiesis is the process in which blood cells are produced. This process occurs in a tissue called red marrow, which is found inside some bones, including the pelvis, ribs, and vertebrae. Red marrow synthesizes red blood cells, white blood cells, and platelets. Billions of these blood cells are produced inside the bones every day.
Mineral Storage and Homeostasis
Another function of the skeletal system is storing minerals, especially calcium and phosphorus. This storage function is related to the role of bones in maintaining mineral homeostasis. Just the right levels of calcium and other minerals are needed in the blood for the normal functioning of the body. When mineral levels in the blood are too high, bones absorb some of the minerals and store them as mineral salts, which is why bones are so hard. When blood levels of minerals are too low, bones release some of the minerals back into the blood. Bone minerals are alkaline (basic), so their release into the blood buffers the blood against excessive acidity (low pH), whereas their absorption back into bones buffers the blood against excessive alkalinity (high pH). In this way, bones help maintain acid-base homeostasis in the blood.
Another way bones help to maintain homeostasis is by acting as an endocrine organ. One endocrine hormone secreted by bone cells is osteocalcin, which helps regulate blood glucose and fat deposition. It increases insulin secretion and also the sensitivity of cells to insulin. In addition, it boosts the number of insulin-producing cells and reduces fat stores.
Review
1. What is the skeletal system? How many bones are there in the adult skeleton?
2. Describe the composition of bones.
3. Besides bones, what other organs are included in the skeletal system?
4. Identify the two major divisions of the skeleton.
5. List several functions of the skeletal system.
6. Discuss sexual dimorphism in the human skeleton.
7. Bones, cartilage, and ligaments are all made of types of ____________ tissue.
8. True or False. Bones contain living tissue and can affect processes in other parts of the body.
9. True or False. Bone cells contract to pull on muscles in order to initiate a movement.
10. If a person has a problem with blood cell production, what type of bone tissue is most likely involved? Explain your answer.
11. Are the pelvic girdles part of the axial or appendicular skeleton?
12. What are three forms of homeostasis that the skeletal system regulates? Briefly explain how each one is regulated by the skeletal system.
13. What do you think would happen to us if we did not have ligaments? Explain your answer.
14. a. Define a joint in the skeletal system.
b. How is cartilage related to joints?
c. Identify one joint in the human body and describe its function.
Attributions
1. Fighter squadron 84 by US Navy, public domain via Wikimedia Commons
2. Human skeleton front by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
3. Axial skeleton by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
4. Appendicular skeleton by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
5. Knee anatomy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/14%3A_Skeletal_System/14.2%3A_Introduction_to_the_Skeletal_System.txt |
Skulls on Display
This somewhat macabre display can be viewed at the Slovak National Museum in Bratislava, Slovakia. The skulls are meant to represent normal human skeletal anatomy. The skull is part of the axial skeleton, which is one of the two major divisions of the human skeleton. The other division is the appendicular skeleton.
Axial Skeleton
The axial skeleton, shown in blue in Figure \(2\), consists of a total of 80 bones. Besides the skull, it includes the rib cage and vertebral column. It also includes the three tiny ossicles (hammer, anvil, and stirrup) in the middle ear and the hyoid bone in the throat, to which the tongue and some other soft tissues are attached.
Skull
The skull is the part of the human skeleton that provides a bony framework for the head. It consists of 22 different bones. There are 8 bones in the cranium, which encloses the brain, and 14 bones in the face.
Cranium
The cranium, sometimes called the braincase, forms the entire upper portion of the skull. As shown in Figure \(3\), it consists of eight bones: one frontal bone, two parietal bones, two temporal bones, one occipital bone, one sphenoid bone, and one ethmoid bone. The ethmoid bone separates the nasal cavity from the brain. The sphenoid bone is one of several bones, including the frontal bone, that helps form the eye sockets. The other bones of the cranium are large and plate-like. They cover and protect the brain. The bottom of the skull has openings for major blood vessels and nerves. A large opening, called the foramen magnum, allows the spinal cord and brain to connect.
Facial Bones
The 14 facial bones of the skull are located below the frontal bone of the cranium. They are depicted in Figure \(4\). Large bones in the face include the upper jawbones, or maxillae (singular, maxilla), which form the middle part of the face and the bottom of the two eye sockets. The maxillae are fused together except for an opening between them for the nose. The lower edge of the maxillae contains sockets for the upper teeth. The lower jaw bone, or mandible, is also large. The top edge of the mandible contains sockets for the lower teeth. The mandible opens and closes to chew food and is controlled by strong muscles. There are two zygomatic or cheekbones and two nasal bones. The nasal region also contains seven smaller bones, as indicated in the figure.
Vertebral Column
The vertebral column, also called the spine or backbone, is the flexible column of vertebrae (singular, vertebra) that connects the trunk with the skull and encloses the spinal cord. It consists of 33 vertebrae that are divided into five regions, as shown in Figure \(5\): the cervical, thoracic, lumbar, sacral, and coccygeal regions. From the neck down, the first 24 vertebrae (cervical, thoracic, and lumbar) are individual bones. The five sacral vertebrae are fused together, as are the four coccygeal vertebrae.
The human vertebral column reflects adaptations for upright bipedal locomotion. For example, the vertebral column is less like a rigid column than an S-shaped spring (see a profile view in the figure above). Although newborn infants have a relatively straight spine, the curves develop as the backbone starts taking on its support functions, such as keeping the trunk erect, holding up the head, and helping to anchor the limbs. The S shape of the vertebral column allows it to act as a shock absorber, absorbing much of the jarring of walking and running so the forces are not transmitted directly from the pelvis to the skull. The S shape also helps protect the spine from breaking, which would be more likely with a straight, more rigid vertebral column. In addition, the S shape helps to distribute the weight of the body, and particularly of the internal organs, so the weight load is not all at the bottom, as would occur with a straight spine.
Rib Cage
The rib cage (also called thoracic cage) is aptly named because it forms a sort of cage that holds within it the organs of the upper part of the trunk, including the heart and lungs (Figure \(6\)). The rib cage includes the 12 thoracic vertebrae and the breastbone (or sternum) as well as 12 pairs of ribs, which are attached at joints to the vertebrae. The ribs are divided into three groups, called true ribs, false ribs, and floating ribs. The top seven pairs of ribs are true ribs. They are attached by cartilage directly to the sternum. The next three pairs of ribs are false ribs. They are attached by cartilage to the ribs above them, rather than directly to the sternum. The lowest two pairs of ribs are floating ribs. They are attached by cartilage to muscles in the abdominal wall. The attachments of false and floating ribs let the lower part of the rib cage expand to accommodate the internal movements of breathing.
Appendicular Skeleton
The appendicular skeleton, shown in red in Figure \(7\), consists of a total of 126 bones. It includes all the bones of the limbs (arms, legs, hands, and feet) as well as the bones of the shoulder (shoulder girdle) and pelvis (pelvic girdle).
Upper Limbs
Each upper limb consists of 30 bones. As shown in Figure \(8\), there is one bone, called the humerus, in each of the upper arms, and there are two bones, called the ulna and radius, in each of the lower arms.
The remaining bones of the upper limb are shown in Figure \(9\). Each wrist contains eight carpal bones, which are arranged in two rows of four bones each; and each hand contains five metacarpal bones. The bones in the fingers of each hand include 14 phalanges (three in each finger except the thumb, which has two phalanges). The thumb has the unique ability to move into opposition with the palm of the hand and with each of the fingers when they are slightly bent. This allows the hand to handle and manipulate objects such as tools.
Lower Limbs
Each lower limb consists of 30 bones. As shown in Figure \(10\), there is one bone, called the femur, in each of the upper legs, and there are two bones, called the tibia and fibula, in each of the lower legs. The knee cap, or patella, is an additional leg bone at the front of each knee, which is the largest joint in the human body.
The remaining bones of the lower limbs are shown in Figure \(11\). Each ankle contains seven tarsal bones (including the talus and calcaneus), and each foot contains five metatarsal bones. The tarsals and metatarsals form the ankle, heel, and arch of the foot. They give the foot strength while allowing flexibility. The bones in the toes of each foot consist of 14 phalanges (three in each toe except the big toe, which has two phalanges)
The pectoral girdle (also called shoulder girdle) attaches the upper limbs to the trunk of the body. Its connection with the axial skeleton is by muscles alone. This allows a considerable range of motion in the upper limbs. The shoulder girdle consists of just two pairs of bones, with one of each pair on opposite sides of the body (Figure \(12\)). There is a right and left clavicles (collarbone) and right and left scapulae (shoulder blade). The scapula is a pear-shaped flat bone that helps to form the shoulder joint. The clavicle is a long bone that serves as a strut between the shoulder blade and the sternum.
Pelvic Girdle
The pelvic girdle attaches the legs to the trunk of the body and also provides a basin to contain and support the organs of the abdomen. It is connected to the vertebral column of the axial skeleton by ligaments. The pelvic girdle consists of two halves, one half for each leg, but the halves are fused with each other in adults at a joint called the pubic symphysis. Each half of the pelvic girdle includes three bones, as shown in the figure below: the ilium (flaring upper part of the pelvic girdle), pubis (lower front), and ischium (lower back). Each of these bones helps form the acetabulum, which is a depression into which the top of the femur (thigh bone) fits. When the body is in a seated position, it rests on protrusions (called tuberosities) of the two ischial bones.
Review
1. What bones are included in the axial skeleton?
2. Identify the two main parts of the skull. How many bones does each part contain?
3. Describe the vertebral column.
4. What are the advantages of an S-shaped vertebral column?
5. What is the rib cage, and what is its function?
6. What bones are included in the appendicular skeleton?
7. How many bones are found in each upper limb? What are they?
8. Identify the bones in each of the lower limbs.
9. What is the shoulder girdle, and why does it allow considerable upper limb mobility?
10. Describe the pelvic girdle and the bones it contains.
11. True or False. False ribs are made of cartilage and are not true rib bones.
12. True or False. The jaw contains two maxillae and one mandible.
13. Describe some of the similarities between the upper limbs and the lower limbs.
14. Explain the advantage of having some ribs that are not attached directly to the sternum.
15. Put the following vertebral regions in order, from the closest to the head to the farthest from the head:
sacral; lumbar; cervical; coccygeal; thoracic
Attributions
1. Human skulls on display by KiwiEV, CC0 via Wikimedia Commons
2. Axial skeleton diagram by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
3. Cranial bones, original by Edoarado, adapted text by Was a bee, CC0 via Wikimedia Commons
4. Facial bones, public domain via Wikimedia Commons
5. Vertebral column by OpenStax College, CC BY 3.0 via Wikimedia Commons
6. Thoracic cage, public domain via Wikimedia Commons
7. Appendicular skeleton diagram by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
8. Arm bones by BruceBlaus, CC BY 4.0 via Wikimedia Commons
9. Bones of the wrist and hand by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
10. Leg bones by Jecowa, public domain via Wikimedia Commons
11. Foot bones by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
12. Shoulder bones by LadyofHats Mariana Ruiz Villarreal, public domain via Wikimedia Commons
13. Pelvis diagram by Je at uwo, public domain via Wikimedia Commons
14. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/14%3A_Skeletal_System/14.3%3A_Divisions_of_the_Skeletal_System.txt |
Roasted Bone Marrow
Do you recognize the food item in the top left of this photo in Figure \(1\)? It’s roasted bone marrow, still inside the bones. It’s considered a delicacy in some cuisines. Marrow is a type of tissue found inside many animal bones, including our own. It’s a soft tissue that in adults may be mostly fat. You’ll learn more about bone marrow and other tissues that make up bones when you read this concept.
Bones are organs that consist primarily of bone tissue, also called osseous tissue. Bone tissue is a type of connective tissue consisting mainly of a collagen matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and hard mineral crystals makes bone tissue hard without making it brittle.
Bone Anatomy
There are several different types of tissues in bones, including two types of osseous tissues.
Types of Osseous Tissue
The two different types of osseous tissue are compact bone tissue (also called hard or cortical bone) tissue and spongy bone tissue (also called cancellous or trabecular bone).
Compact bone tissue forms the extremely hard outside layer of bones. Cortical bone tissue gives bone its smooth, dense, solid appearance. It accounts for about 80 percent of the total bone mass of the adult skeleton. Spongy bone tissue fills part or all of the interior of many bones. As its name suggests, spongy bone is porous like a sponge, containing an irregular network of spaces. This makes spongy bone much less dense than compact bone. Spongy bone has a greater surface area than cortical bone but makes up only 20 percent of bone mass.
Both compact and spongy bone tissues have the same types of cells, but they differ in how the cells are arranged. The cells in the compact bone are arranged in multiple microscopic columns, whereas the cells in the spongy bone are arranged in a looser, more open network. These cellular differences explain why cortical and spongy bone tissues have such different structures.
Other Tissues in Bones
Besides cortical and spongy bone tissues, bones contain several other tissues, including blood vessels and nerves. In addition, bones contain bone marrow and periosteum. You can see these tissues in Figure \(2\).
• Bone marrow is a soft connective tissue that is found inside a cavity, called the marrow cavity. There are two types of marrow in adults, yellow bone marrow, which consists mostly of fat, and red bone marrow. All marrow is red in newborns, but by adulthood, much of the red marrow has changed to yellow marrow. In adults, red marrow is found mainly in the femur, ribs, vertebrae, and pelvic bones. Red bone marrow contains hematopoietic stem cells that give rise to red blood cells, white blood cells, and platelets in the process of hematopoiesis.
• Periosteum is a tough, fibrous membrane that covers the outer surface of bones. It provides a protective covering for cortical bone tissue. It is also the source of new bone cells.
Bone Cells
As shown in Figure \(3\), bone tissues are composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells.
• Osteoblasts are bone cells with a single nucleus that make and mineralize bone matrix. They make a protein mixture that is composed primarily of collagen and creates the organic part of the matrix. They also release calcium and phosphate ions that form mineral crystals within the matrix. In addition, they produce hormones that also play a role in the mineralization of the matrix.
• Osteocytes are mainly inactive bone cells that form from osteoblasts that have become entrapped within their own bone matrix. Osteocytes help regulate the formation and breakdown of bone tissue. They have multiple cell projections that are thought to be involved in communication with other bone cells.
• Osteoclasts are bone cells with multiple nuclei that resorb bone tissue and break down bone. They dissolve the minerals in bone and release them into the blood.
• Osteogenic cells are undifferentiated stem cells. They are the only bone cells that can divide. When they do, they differentiate and develop into osteoblasts.
Bone is a very active tissue. It is constantly remodeled by the work of osteoblasts and osteoclasts. Osteoblasts continuously make new bone, and osteoclasts keep breaking down bone. This allows for minor repair of bones as well as homeostasis of mineral ions in the blood.
Microscopic Anatomy of The Compact Bone
The basic microscopic unit of bone is an osteon (or Haversian system). Osteons are roughly cylindrical structures that can measure several millimeters long and around 0.2 mm in diameter. Each osteon consists of lamellae of compact bone tissue that surround a central canal (Haversian canal). The Haversian canal contains the bone's blood supplies. The boundary of an osteon is called the cement line. Osteons can be arranged into woven bone or lamellar bone. Osteoblasts make the matrix of bone which calcifies hardens. This entraps the mature bone cells, osteocytes, in a little chamber called lacunae. The osteocytes receive their nutrition from the central (Haversian) canal via little canals called canaliculi. All of these structures plus more are visible in Figure \(4\).
Types of Bones
There are six types of bones in the human body based on their shape or location: long, short, flat, sesamoid, sutural, and irregular bones. You can see an example of each type of bone in Figure \(5\).
• Long bones are characterized by a shaft that is much longer than it is wide and by a rounded head at each end of the shaft. Long bones are made mostly of compact bone, with lesser amounts of spongy bone and marrow. Most bones of the limbs, including those of the fingers and toes, are long bones.
• Short bones are roughly cube-shaped and have only a thin layer of cortical bone surrounding a spongy bone interior. The bones of the wrists and ankles are short bones.
• Flat bones are thin and generally curved, with two parallel layers of compact bone sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum (breast bone).
• Sesamoid bones are embedded in tendons, the connective tissues that bind muscles to bones. Sesamoid bones hold tendons farther away from joints so the angle of the tendons is increased, thus increasing the leverage of muscles. The patella (knee cap) is an example of a sesamoid bone.
• Sutural bones are very small bones that are located between the major bones of the skull, within the joints (sutures) between the larger bones. They are not always present.
• Irregular bones are those that do not fit into any of the above categories. They generally consist of thin layers of cortical bone surrounding a spongy bone interior. Their shapes are irregular and complicated. Examples of irregular bones include the vertebrae and the bones of the pelvis.
Feature: Reliable Sources
Diseased or damaged bone marrow can be replaced by donated bone marrow cells, which help treat and often cure many life-threatening conditions, including leukemia, lymphoma, sickle cell anemia, and thalassemia. If a bone marrow transplant is successful, the new bone marrow will start making healthy blood cells and improve the patient’s condition.
Learn more about bone marrow donation, and consider whether you might want to do it yourself. Find reliable sources to answer the following questions:
1. How does one become a potential bone marrow donor?
2. Who can and who cannot donate bone marrow?
3. How is a bone marrow donation made?
4. What risks are there in donating bone marrow?
Review
1. Describe osseous tissue.
2. Why are bones hard but not brittle?
3. Compare and contrast the two main types of osseous tissue.
4. What non-osseous tissues are found in bones?
5. List four types of bone cells and their functions.
6. Identify six types of bones, and give an example of each type.
7. True or False. Spongy bone tissue is another name for bone marrow.
8. True or False. Periosteum covers osseous tissue.
9. Compare and contrast yellow bone marrow and red bone marrow.
10. Which bone is mostly made of cortical bone tissue?
A. Pelvis
B. Vertebrae
C. Femur
D. Carpal
11. a. Which type of bone cell divides to produce new bone cells?
b. Where is this cell type located?
12. Where do osteoblasts and osteocytes come from, and how are they related to each other?
13. Which type of bone is embedded in tendons?
14. True or False. Calcium is the only mineral in bones.
Explore More
Watch this entertaining and fast-paced Crash Course video to further explore bone structure:
Check out this video to learn more about bone remodeling:
Attributions
1. Roast Bone Marrow by Simon Doggett, CC BY 2.0 via Wikimedia Commons
2. Bone structure by Christopher Auyeung via CK-12 licensed CC BY-NC 3.0
3. Bone cells by OpenStax College, CC BY 3.0 via Wikimedia Commons
4. Compact bone by OpenStax Anatomy and Physiology, CC BY 3.0 via Wikimedia Commons
5. Types of bone by BruceBlaus, CC BY 3.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/14%3A_Skeletal_System/14.4%3A_Structure_of_Bone.txt |
Break a Leg
Did you ever break a leg or other bone, like the man looking longingly at the water in this swimming pool? Having a broken bone can really restrict your activity. Bones are very hard, but they will break, or fracture if enough force is applied to them. Fortunately, bones are highly active organs that can repair themselves if they break. Bones can also remodel themselves and grow. You’ll learn how bones can do all of these things in this concept.
Bone Growth
Early in the development of a human fetus, the skeleton is made almost entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification. Ossification is a process in which bone tissue is created from cartilage. The steps in which bones of the skeleton form from cartilage are illustrated in Figure \(2\). The steps include the following:
1. Cartilage “model” of bone forms; this model continues to grow as ossification takes place.
2. Ossification begins at a primary ossification center in the middle of the bone.
3. Ossification then starts to occur at secondary ossification centers at the ends of the bone.
4. The medullary cavity forms and will contain red bone marrow.
5. Areas of ossification meet at epiphyseal plates, and articular cartilage forms. Bone growth ends.
Primary and Secondary Ossification Centers
When bone forms from cartilage, ossification begins with a point in the cartilage called the primary ossification center. This generally appears during fetal development, although a few short bones begin their primary ossification after birth. Ossification occurs toward both ends of the bone from the primary ossification center, and it eventually forms the shaft of the bone in the case of long bones.
Secondary ossification centers form after birth. Ossification from secondary centers eventually forms the ends of the bones. The shaft and ends of the bone are separated by a growing zone of cartilage until the individual reaches skeletal maturity.
Skeletal Maturity
Throughout childhood, the cartilage remaining in the skeleton keeps growing and allows for bones to grow in size. However, once all of the cartilage has been replaced by bone and fusion has taken place at epiphyseal plates, bones can no longer keep growing in length. This is the point at which skeletal maturity has been reached. It generally takes place by age 18 to 25.
The use of anabolic steroids by teens can speed up the process of skeletal maturity, resulting in a shorter period of cartilage growth before fusion takes place. This means that teens who use steroids are likely to end up shorter as adults than they would otherwise have been.
Bone Remodeling
Even after skeletal maturity has been attained, bone is constantly being resorbed and replaced with new bone in a process known as bone remodeling. In this lifelong process, mature bone tissue is continually turned over, with about 10 percent of the skeletal mass of an adult being remodeled each year. Bone remodeling is carried out through the work of osteoclasts, which are bone cells that resorb bone and dissolve its minerals; and osteoblasts, which are bone cells that make the new bone matrix.
Bones remodeling serves several functions. It shapes the bones of the skeleton as a child grows, and it repairs tiny flaws in the bone that result from everyday movements. Remodeling also makes bones thicker at points where muscles place the most stress on them. In addition, remodeling helps regulate mineral homeostasis because it either releases minerals from bones into the blood or absorbs minerals from the blood into bones. The figure below shows how osteoclasts in bones are involved in calcium regulation.
The action of osteoblasts and osteoclasts in bone remodeling and calcium homeostasis is controlled by a number of enzymes, hormones, and other substances that either promote or inhibit the activity of the cells. In this way, these substances control the rate at which bone is made, destroyed, and changed in shape. For example, the rate at which osteoclasts resorb bone and release calcium into the blood is promoted by parathyroid hormone (PTH) and inhibited by calcitonin, which is produced by the thyroid gland (Figure \(3\)). The rate at which osteoblasts create new bone is stimulated by growth hormone, which is produced by the anterior lobe of the pituitary gland. Thyroid hormone and sex hormones (estrogens and androgens) also stimulate osteoblasts to create new bone.
Bone Repair
Bone repair, or healing, is the process in which a bone repairs itself following a bone fracture. You can see an X-ray of bone fracture in Figure \(4\). In this fracture, the humerus in the upper arm has been completely broken through its shaft. Before this fracture heals, a physician must push the displaced bone parts back into their correct positions. Then the bone must be stabilized — for example, with a cast and/or pins surgically inserted into the bone — until the bone’s natural healing process is completed. This process may take several weeks.
The process of bone repair is mainly determined by the periosteum, which is the connective tissue membrane covering the bone. The periosteum is the primary source of precursor cells that develop into osteoblasts, which are essential to the healing process. Bones heal as osteoblasts form new bone tissue.
Although bone repair is a natural physiological process, it may be promoted or inhibited by several factors. For example, fracture repair is likely to be more successful with adequate nutrient intake. Age, bone type, drug therapy, and pre-existing bone disease are additional factors that may affect healing. Bones that are weakened by diseases, such as osteoporosis or bone cancer, are not only likely to heal more slowly but are also more likely to fracture in the first place.
Feature: Myth vs. Reality
Bone fractures are fairly common, and there are many myths about them. Knowing the facts is important because fractures generally require emergency medical treatment.
Myth: A bone fracture is a milder injury than a broken bone.
Reality: A bone fracture is the same thing as a broken bone.
Myth: If you still have a full range of motion in a limb, then it must not be fractured.
Reality: Even if a bone is fractured, the muscles and tendons attached to it may still be able to move the bone normally. This is especially likely if the bone is cracked but not broken into two pieces. Even if a bone is broken all the way through, the range of motion may not be much affected if the bones on either side of the fracture remain properly aligned.
Myth: A fracture always produces a bruise.
Reality: Many but not all fractures produce a bruise. If a fracture does produce a bruise, it may take several hours or even a day or more for the bruise to appear.
Myth: Fractures are so painful that you will immediately know if you break a bone.
Reality: Ligament sprains and muscle strains are also very painful, sometimes more painful than fractures. Additionally, every person has a different pain tolerance. People with high pain tolerance may continue using a broken bone in spite of the pain.
Myth: You can tell when a bone is fractured because there will be very localized pain over the break.
Reality: A broken bone is often accompanied by injuries to surrounding muscles or ligaments. As a result, the pain may extend far beyond the location of the fracture. The pain may be greater directly over the fracture, but the intensity of the pain may make it difficult to pinpoint exactly where the pain originates.
Review
1. Outline how bone develops from early in the fetal stage through the age of skeletal maturity.
2. Describe the process of bone remodeling. When does it occur?
3. What purposes does bone remodeling serve?
4. Define bone repair. How long does this process take?
5. Explain how bone repair occurs.
6. Identify factors that may affect bone repair.
7. Parts of bone that have not yet become ossified are made of _________.
8. If there is a large region between the primary and secondary ossification centers in a bone, is the person young or old? Explain your answer.
9. The region where the primary and secondary ossification centers meet is called the ________________.
10. True or False. Most bones are made entirely of cartilage at birth.
11. True or False. A broken bone is the same as a bone fracture.
12. If bones can repair themselves, why are casts and pins sometimes needed?
13. Which bone cell type causes the release of calcium to the bloodstream when calcium levels are low?
14. Which tissue and bone cell type are mainly involved in bone repair after a fracture?
15. Describe one way in which hormones are involved in bone remodeling.
Attributions
1. Orthopedic cast by 4x4king10, CC BY 2.0 via Wikimedia Commons
2. Ossification by OpenStax Biology, CC BY 4.0 via Wikimedia Commons
3. Calcium homeostasis by OpenStax College, CC BY 3.0 via Wikimedia Commons
4. Communitive midshaft humeral fracture with callus formation by Bill Rhodes, CC BY 2.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/14%3A_Skeletal_System/14.5%3A_Bone_Growth_Remodeling_and_Repair.txt |
Double Jointed?
Is this person double jointed? No; there is no such thing, at least as far as humans are concerned. However, some people, like the individual pictured here, are much more flexible than others, generally because they have looser ligaments. Physicians call the condition joint hypermobility. Regardless of what it’s called, the feats of people with highly mobile joints can be quite impressive.
What Are Joints?
Joints are locations at which bones of the skeleton connect with one another. A joint is also called an articulation. The majority of joints are structured in such a way that they allow movement. However, not all joints allow movement. Of joints that do allow movement, the extent, and direction of the movements they allow also vary.
Classification of Joints
Joints can be classified as structurally or functionally. The structural classification of joints depends on the manner in which the bones connect to each other. The functional classification of joints depends on the nature of the movement the joints allow. There is significant overlap between the two types of classifications because function depends largely on the structure.
Structural Classification of Joints
The structural classification of joints is based on the type of tissue that binds the bones to each other at the joint. There are three types of joints in the structural classification: fibrous, cartilaginous, and synovial joints.
1. Fibrous joints are joints in which bones are joined by dense connective tissue that is rich in collagen fibers. These joints are also called sutures. The joints between bones of the cranium are fibrous joints.
2. Cartilaginous joints are joints in which bones are joined by cartilage. The joints between most of the vertebrae in the spine are cartilaginous joints.
3. Synovial joints are characterized by a fluid-filled space, called a synovial cavity, between the bones of the joints. You can see a drawing of a typical synovial joint in Figure \(2\). The cavity is enclosed by a membrane and filled with a fluid, called the synovial fluid, which provides extra cushioning to the ends of the bones. Cartilage covers the articulating surfaces of the two bones, but the bones are actually held together by ligaments. The knee is a synovial joint.
Functional Classification of Joints
The functional classification of joints is based on the type and degree of movement that they allow. There are three types of joints in the functional classification: immovable, partly movable, and movable joints.
1. Immovable joints allow little or no movement at the joint. Most immovable joints are fibrous joints. Besides the bones of the cranium, immovable joints include joints between the tibia and fibula in the lower leg and between the radius and ulna in the lower arm.
2. Partly movable joints permit slight movement. Most partly movable joints are cartilaginous joints. Besides the joints between vertebrae, they include the joints between the ribs and sternum (breast bone).
3. Movable joints allow bones to move freely. All movable joints are synovial joints. Besides the knee, they include the shoulder, hip, and elbow. Movable joints are the most common type of joints in the body.
Types of Movable Joints
Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: pivot, hinge, saddle, plane, condyloid, and ball-and-socket joints. An example of each class, as well as the type of movement it allows, is shown in Figure \(3\).
• A pivot joint allows one bone to rotate around another. An example of a pivot joint is the joint between the first two vertebrae in the spine. This joint allows the head to rotate from left to right and back again.
• A hinge joint allows back and forth movement like the hinge of a door. An example of a hinge joint is the elbow. This joint allows the arm to bend back and forth.
• A saddle joint allows two different types of movement. An example of a saddle joint is the joint between the first metacarpal bone in the hand and one of the carpal bones in the wrist. This joint allows the thumb to move toward and away from the index finger and also to cross over the palm toward the little finger.
• A plane joint also called a gliding joint, allows two bones that glide over one another. The joints between the tarsals in the ankles and between the carpals in the wrists are mainly gliding joints. In the wrist, this type of joint allows the hand to bend upward at the wrist and also to wave from side to side while the lower arm is held steady.
• A condyloid joint is one in which an oval-shaped head on one bone moves in an elliptical cavity in another bone, allowing movement in all directions except rotation around an axis. The joint between the radius in the lower arm and carpal bones of the wrist is a condyloid joint as is the joint at the base of the index finger.
• A ball-and-socket joint allows the greatest range of movement of any movable joint. It allows forward and backward as well as upward and downward motions. It also allows rotation in a circle. The hip and shoulder are the only two ball-and-socket joints in the human body.
Feature: My Human Body
Of all the parts of the skeletal system, the joints are generally the most fragile and subject to damage. If the cartilage that cushions bones at joints wears away, it does not grow back. Eventually, all of the cartilage may wear away. This is the cause of osteoarthritis, which can be both painful and debilitating. In serious cases, people may lose the ability to climb stairs, walk long distances, perform routine daily activities, or participate in activities they love such as gardening or playing sports. If you protect your joints, you can reduce your chances of joint damage, pain, and disability. If you already have joint damage, it is equally important to protect your joints and limit further damage. Follow these five tips:
1. Maintain a normal, healthy weight. The higher your weight is, the more force you exert on your joints. When you walk, each knee has to bear a force equal to as much as six times your body weight. If a person weighs 200 pounds, each knee bears more than half a ton of weight with every step. Seven in ten knee replacement surgeries for osteoarthritis can be attributed to obesity.
2. Avoid too much high-impact exercise. Examples of high-impact activities include volleyball, basketball, and tennis. These activities generally involve running or jumping on hard surfaces, which puts tremendous stress on weight-bearing joints, especially the knees. Replace some or all of your high-impact activities with low-impact activities, such as biking, swimming, yoga, or lifting light weights.
3. Reduce your risk of injury. Don’t be a weekend warrior, sitting at a desk all week and then crowding all your physical activity into two days. Get involved in a regular, daily exercise routine that keeps your body fit and your muscles toned. Building up muscles will make your joints more stable and spread stress across them. Be sure to do some stretching every day to keep the muscles around joints flexible and less prone to injury.
4. Distribute work over your body, and use your largest, strongest joints. Use your shoulder, elbow, and wrist to lift heavy objects, not just your fingers. Hold small items in the palm of your hand, rather than by the fingers. Carry heavy items in a backpack rather than in your hands. Hold weighty objects close to your body rather than at arms’ length. Lift with your hips and knees, not your back.
5. Respect pain. If it hurts, stop doing it. Take a break from the activity at least until the pain stops. Try to use joints only to the point of mild fatigue, not pain.
Review
1. What are the joints?
2. What are the two ways that joints are commonly classified?
3. How are joints classified structurally?
4. Describe the functional classification of joints.
5. How are movable joints classified?
6. Name the six classes of movable joints, and describe how they move.
7. Give an example of a joint in each of the classes of movable joints.
8. True or False. The skull is one smooth bone and has no joints.
9. True or False. A plane joint is a type of synovial joint.
10. Which specific type of moveable joint do you think your knee joint is? Explain your reasoning.
11. Explain the difference between cartilage in a cartilaginous joint and cartilage in a synovial joint.
12. Why are fibrous joints immovable?
13. Which type of joint has ligaments?
1. Ball-and-socket
2. Fibrous
3. Cartilaginous
4. None of the above
14. Which type of joint allows for the greatest range of motion?
15. What is the function of synovial fluid?
Explore More
Ehlers-Danlos syndrome is a group of inherited disorders that affect connective tissues. A relatively common form of the syndrome involves mainly the joints. People with this form of Ehlers-Danlos have overly flexible joints or joint hypermobility. This makes their joints prone to excessive wear and tear, dislocations, and early osteoarthritis. You can learn more about this disorder by watching these compelling videos:
Attributions
1. Yoga by YogawithAmit,Pixabay license
2. Synovial joint byOpenStax College,CC BY 3.0 via Wikimedia Commons
3. Types of joints by OpenStax College,CC BY 3.0 via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/14%3A_Skeletal_System/14.6%3A_Joints.txt |
Dowager's Hump
The individual on the right in Figure \(1\) has a deformity in her back commonly called dowager’s hump because it occurs most often in elderly women. Its medical name is kyphosis, and it is defined as excessive curvature of the spinal column in the thoracic region. The curvature generally results from fractures of thoracic vertebrae. As the inset drawings suggest, these fractures may occur due to a significant decrease in bone mass, which is called osteoporosis. Osteoporosis is one of the most prevalent disorders of the skeletal system.
Common Skeletal System Disorders
A number of disorders affect the skeletal system, including bone fractures and bone cancers. However, the two most common disorders of the skeletal system are osteoporosis and osteoarthritis. At least ten million people in the United States have osteoporosis, and more than 8 million of them are women. Osteoarthritis is even more common, affecting almost 30 million people in the United States. Because osteoporosis and osteoarthritis are so common, they are the focus of this concept. These two disorders are also good examples to illustrate the structure and function of the skeletal system.
Osteoporosis
Osteoporosis is an age-related disorder in which bones lose mass, weaken, and break more easily than normal bones. Bones may weaken so much that a fracture can occur with minor stress — or even spontaneously, without any stress at all. Osteoporosis is the most common cause of broken bones in the elderly, but until a bone fracture occurs, it typically causes no symptoms. The bones that break most often include those in the wrist, hip, shoulder, and spine. When the thoracic vertebrae are affected, there can be a gradual collapse of the vertebrae due to compression fractures, as shown in Figure \(2\). This is what causes kyphosis, as pictured in Figure \(1\).
Changes in Bone Mass with Age
As shown in the graph below, bone mass in both males and females generally peaks when people are in their thirties. Bone mass usually decreases after that, and this tends to occur more rapidly in individuals with XX chromosomes, especially after menopause. This is generally attributable to low levels of estrogen in the post-menopausal years.
What Causes Osteoporosis?
Osteoporosis is due to an imbalance between bone formation by osteoblasts and bone resorption by osteoclasts. Normally, bones are constantly being remodeled by these two processes, with up to ten percent of all bone mass undergoing remodeling at any point in time. If these two processes are in balance, no net loss of bone occurs. There are three main ways that an imbalance between bone formation and bone resorption can occur and lead to a net loss of bone.
1. An individual never develops normal peak bone mass during the young adult years: If the peak level is lower than normal, then there is less bone mass, to begin with, making osteoporosis more likely to develop.
2. There is greater than normal bone resorption: Bone resorption normally increases after peak bone mass is reached, but age-related bone resorption may be greater than normal for a variety of reasons. One possible reason is calcium or vitamin D deficiency, which causes the parathyroid gland to release PTH, the hormone that promotes resorption by osteoclasts.
3. There is the inadequate formation of new bone by osteoblasts during remodeling: Lack of estrogen may decrease the normal deposition of new bone. Inadequate levels of calcium and vitamin D also lead to the impaired bone formation by osteoblasts.
An imbalance between bone building and bone destruction leading to bone loss may also occur as a side effect of other disorders. For example, people with alcoholism, anorexia nervosa, or hyperthyroidism have an increased rate of bone loss. Some medications — including anti-seizure medications, chemotherapy drugs, steroid medications, and some antidepressants — also increase the rate of bone loss.
Osteoporotic Fractures
Fractures are the most dangerous aspect of osteoporosis, and osteoporosis is responsible for millions of fractures annually. Debilitating pain among the elderly is often caused by fractures from osteoporosis, and it can lead to further disability and early mortality. Fractures of the long bones (such as the femur) can impair mobility and may require surgery. A hip fracture usually requires immediate surgery, as well. The immobility associated with fractures — especially of the hip — increases the risk of deep vein thrombosis, pulmonary embolism, and pneumonia. Osteoporosis is rarely fatal, but these complications of fractures often are. Older people tend to have more falls than younger people, due to such factors as poor eyesight and balance problems, increasing their risk of fractures even more. The likelihood of falls can be reduced by removing obstacles and loose carpets or rugs in the living environment.
Risk Factors for Osteoporosis
There are a number of factors that increase the risk of osteoporosis. Eleven of them are listed below. The first five factors cannot be controlled, but the remaining factors generally can be controlled by changing behaviors.
1. older age
2. XX chromosome
3. European or Asian ancestry
4. family history of osteoporosis
5. short stature and small bones
6. smoking
7. alcohol consumption
8. lack of exercise
9. vitamin D deficiency
10. poor nutrition
11. consumption of soft drinks
Treatment and Prevention of Osteoporosis
Osteoporosis is often treated with medications that may slow or even reverse bone loss. Medications called bisphosphonates, for example, are commonly prescribed. Bisphosphonates slow down the breakdown of bone, allowing bone rebuilding during remodeling to keep pace. This helps maintain bone density and decreases the risk of fractures. The medications may be more effective in patients who have already broken bones than in those who have not, significantly reducing their risk of another fracture. Generally, patients are not recommended to stay on bisphosphonates for more than three or four years. There is no evidence for continued benefit after this time — in fact, there is a potential for adverse side effects.
Preventing osteoporosis includes eliminating any risk factors that can be controlled through changes of behavior. If you smoke, stop. If you drink, reduce your alcohol consumption — or cut it out altogether. Eat a nutritious diet and make sure you are getting adequate amounts of vitamin D. You should also avoid drinking carbonated beverages.
If you’re a couch potato, get involved in regular exercise. Aerobic, weight-bearing and resistance exercises can all help maintain or increase bone mineral density. Exercise puts stress on bones, which stimulates bone building. Good weight-bearing exercises for bone-building include weight training, dancing, stair climbing, running, and hiking (Figure \(4\)). Biking and swimming are less beneficial because they don’t stress the bones. Ideally, you should exercise for at least 30 minutes a day on most days of the week.
Osteoarthritis
Osteoarthritis (OA) is a joint disease that results from the breakdown of joint cartilage and bone. The most common symptoms are joint pain and stiffness. Other symptoms may include joint swelling and decreased range of motion. Initially, symptoms may occur only after exercise or prolonged activity, but over time, they may become constant, negatively affecting work and normal daily activities. As shown in Figure \(5\), the most commonly involved joints are those near the ends of the fingers, at the bases of the thumbs, and in the neck, lower back, hips, and knees. Often, joints on one side of the body are affected more than those on the other side.
What Causes Osteoarthritis?
OA is thought to be caused by mechanical stress on the joints with insufficient self-repair of cartilage. The stress may be exacerbated by low-grade inflammation of the joints, as cells lining the joint attempt to remove breakdown products from cartilage in the synovial space. OA develops over decades as stress and inflammation cause an increasing loss of articular cartilage. Eventually, bones may have no cartilage to separate them, so bones rub against one another at joints. This damages the articular surfaces of the bones and contributes to the pain and other symptoms of OA. Because of the pain, movement may be curtailed, leading to loss of muscle, as well.
Diagnosing Osteoarthritis
Diagnosis of OA is typically made on the basis of signs and symptoms. Signs include joint deformities, such as bony nodules on the finger joints or bunions on the feet (Figure \(6\)). Symptoms include joint pain and stiffness. The pain is usually described as a sharp ache or burning sensation, which may be in the muscles and tendons around the affected joints, as well as in the joints themselves. The pain is usually made worse by prolonged activity, and it typically improves with rest. Stiffness is most common when first arising in the morning, and it usually improves quickly as daily activities are undertaken.
X-rays or other tests are sometimes used to either support the diagnosis of OA or to rule out other disorders. Blood tests might be done, for example, to look for factors that indicate rheumatoid arthritis (RA), an autoimmune disease in which the immune system attacks the body’s joints. If these factors are not present in the blood, then RA is unlikely, and a diagnosis of OA is more likely to be correct.
Risk Factors for Osteoarthritis
Age is the chief risk factor for osteoarthritis. By age 65, as many as 80 percent of all people have evidence of osteoarthritis. However, people are more likely to develop OA — especially at younger ages — if they have had a joint injury. A high school football player might have a bad knee injury that damages the joint, leading to OA in the knee by the time he is in his thirties. If people have joints that are misaligned due to congenital malformations or disease, they are also more likely to develop OA. Excess body weight is another factor that increases the risk of OA, because of the added stress it places on weight-bearing joints.
Researchers have found that people with a family history of OA have a heightened risk of developing the disorder, which suggests that genetic factors are also involved in OA. It is likely that many different genes are needed for normal cartilage and cartilage repair. If such genes are defective and cartilage is abnormal or not normally repaired, OA is more likely to result.
Treatment and Prevention of Osteoarthritis
OA cannot be cured, but the symptoms — especially the pain — can often be treated successfully to maintain a good quality of life for people with OA. Treatments include exercise, efforts to decrease stress on joints, pain medications, and surgery.
Destressing Joints
Efforts to decrease stress on joints include resting and using mobility devices such as canes, which reduce the weight placed on weight-bearing joints and also improve stability. In people who are overweight, losing weight may also reduce joint stress.
Exercise
Exercise helps maintain joint mobility and also increases muscle strength. Stronger muscles may help keep the bones in joints correctly aligned, and this can reduce joint stress. Good exercises for OA include swimming, water aerobics (see Figure \(7\)), and biking. These activities are recommended for OA because they put relatively little stress on the joints.
Pain Medications
The first type of pain medication likely to be prescribed for OA is acetaminophen (e.g., Tylenol). When taken as prescribed, it has a relatively low risk of serious side effects. If this medication is inadequate to relieve the pain, non-steroidal anti-inflammatory drugs (NSAIDs, such as ibuprofen) may be prescribed. NSAIDs, however, are more likely to cause serious side effects, such as gastrointestinal bleeding, elevated blood pressure, and increased risk of stroke. Opioids usually are reserved for patients who have suffered serious side effects or for whom other medications have failed to relieve pain. Due to the risk of addiction, the short-term use of opioids is generally recommended.
Surgery
Joint replacement surgery is the most common treatment for serious OA in the knee or hip. In fact, knee and hip replacement surgeries are among the most common of all surgeries. Although they require a long period of healing and physical rehabilitation, the results are usually worth it. The replacement “parts” are usually pain-free and fully functional for at least a couple of decades. Quality, durability, and customization of artificial joints are constantly improving.
Feature: Myth vs. Reality
About one out of every two Americans will develop osteoarthritis in his or her lifetime. The more you know about this disease, the more you can do to avoid it or slow its progression. That means knowing the facts, rather than believing the myths about osteoarthritis.
Myth: Cracking my knuckles will cause osteoarthritis.
Reality: Cracking your knuckles may lead to inflammation of your tendons, but it will not cause osteoarthritis.
Myth: My diet has no effect on my joints.
Reality: What and how much you eat does affect your body weight, and every pound you gain translates into an additional four pounds (or more!) of stress on your knees. Being overweight, therefore, increases the chances of developing osteoarthritis — and also the rate at which it progresses.
Myth: Exercise causes osteoarthritis or makes it worse, so I should avoid it.
Reality: This is one of the biggest myths about osteoarthritis. Low-impact exercise can actually lessen the pain and improve other symptoms of osteoarthritis. If you don’t have osteoarthritis, exercise can reduce your risk of developing it. Low-impact exercise helps keep the muscles around joints strong and flexible, so they can help stabilize and protect the joints.
Myth: If my mom or dad has osteoarthritis, I will also develop it.
Reality: It is true that you are more likely to develop osteoarthritis if a parent has it, but it isn’t a sure thing. There are several things you can do to decrease your risk, such as getting regular exercise and maintaining a healthy weight.
Myth: Bad weather causes osteoarthritis.
Reality: Weather conditions do not cause osteoarthritis, although, in some people who already have osteoarthritis, bad weather seems to make the symptoms worse. It is primarily low barometric pressure that increases osteoarthritis pain, probably because it leads to greater pressure inside the joints relative to the outside air pressure. Some people think their osteoarthritis pain is worse in cold weather, but systematic studies have not found convincing evidence for this.
Myth: Joint pain is unavoidable as you get older, so there is no need to see a doctor for it.
Reality: Many people with osteoarthritis think there is nothing that can be done for the pain of osteoarthritis, or that surgery is the only treatment option. In reality, osteoarthritis symptoms often can be improved with a combination of exercise, weight loss, pain management techniques, and pain medications. If osteoarthritis pain interferes with daily life and lasts more than a few days, you should see your doctor.
Myth: Osteoarthritis is inevitable in seniors.
Reality: Although many people over 65 develop osteoarthritis, there are many people who never develop it, no matter how old they live to be. You can reduce your risk of developing osteoarthritis in later life by protecting your joints throughout life.
Review
1. Name the two most common disorders of the skeletal system.
2. What is osteoporosis? What causes it?
3. How is osteoporosis diagnosed?
4. Why is osteoporosis dangerous?
5. Identify risk factors for osteoporosis.
6. How is osteoporosis treated? What can be done to prevent it?
7. What is OA? What are its chief symptoms?
8. What causes OA?
9. Describe how OA is diagnosed.
10. Identify risk factors for OA.
11. How is OA treated?
12. Why is it important to build sufficient bone mass in your young adult years?
13. Explain the difference in the cause of rheumatoid arthritis and osteoarthritis.
14. True or False: Osteoarthritis is caused by physical activity, so people who are equally active are equally susceptible to it.
15. True or False: Estrogen generally promotes the production of new bone.
Explore More
Osteoarthritis grinds down millions of joints. Many people find relief from hip or knee pain and disability by having one or more joints replaced with artificial joints made of metal and plastic. In the U.S. alone, more than a million knee and hip joint replacements are performed each year. However, the best remedy for worn out, painful joints is replacement with real biological tissue from a tissue donor rather than replacement with artificial joints. Unfortunately, using human donor tissues to repair joints is very costly. There is also a severe shortage of donor tissues. Orthopedic surgeon and researcher Kevin Stone is developing a treatment that could avoid these drawbacks of human tissue transplants by using specially developed animal tissues. Watch his TED talk to learn more:
Check out this video to learn about Primordial Dwarfism here:
Attributions
1. Osteoporosis by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
2. Osteoporosis of spine by OpenStax College, CC BY 3.0 via Wikimedia Commons
3. Age and bone mass by OpenStax College, CC BY 3.0 via Wikimedia Commons
4. Hikers enjoying a wild trail by Hillebrand Steve, U.S. Fish and Wildlife Service, public domain via Wikimedia Commons
5. Areas affected by osteoarthritis by US Federal Government, public domain via Wikimedia Commons
6. Hallux valgus by Malmstajn, CC BY 3.0 via Wikimedia Commons
7. Water aerobics by Tim Ross, public domain via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/14%3A_Skeletal_System/14.7%3A_Disorders_of_the_Skeletal_System.txt |
Case Study Conclusion: A Pain in the Foot
You may have seen signs such as the one in Figure \(1\) indicating that high-heeled shoes are not allowed on certain walking surfaces because of the risk of injury. High heels affect a person’s balance, and wearers can easily twist their ankle on uneven or slippery surfaces, causing a sprain or even a fracture. Besides twisting an ankle, wearing high heels on a regular basis can cause a variety of other negative health consequences—some of which may be long-lasting.
As Amari discovered at the beginning of the chapter, wearing high heels can result in a condition called metatarsalgia. Metatarsalgia is named for the metatarsal bones, which are the five bones that run through the ball of the foot just behind the toes (highlighted in Figure \(2\) ) Wearing high heels causes excessive pressure on the ball of the foot, as described at the beginning of this chapter. Additionally, the toes are forced to pull upwards in high heels, which moves the fleshy padding away from the ball of the foot, adding to the overall pressure placed on this region. Over time, this can cause inflammation and direct stress on the bones, resulting in pain in the ball of the foot known as metatarsalgia. The pain particularly occurs in weight-bearing positions such as standing, walking, or running—which is what Amari was experiencing. There may also be pain, numbness, or tingling in the toes associated with metatarsalgia.
Wearing high heels can also cause stress fractures in the feet, which are tiny breaks in the bone that occur due to repeated mechanical stress. This is due to the excessive pressure that high heels put on some of the bones of the feet. These fractures are somewhat similar to what occurs in osteoporosis when the bone mass decreases to the point where bones can fracture easily as people go about their daily activities. In both cases, a major, noticeable injury is not necessary to create tiny fractures. As you have learned, tiny fractures that accrue over time are the cause of dowager’s hump, or kyphosis, which is often seen in women with osteoporosis.
Don’t think you are immune to stress fractures just because you don’t wear high heels! This injury also commonly occurs in people who participate in sports that involve repetitive striking of the foot on the ground, such as running, tennis, basketball, or gymnastics. Stress fractures may be avoided by taking preventative measures such as ramping up any increase in activity slowly, cross-training by engaging in a variety of different sports or activities, resting if you experience pain, and wearing well-cushioned and supportive running shoes.
Amari learned through their online research that wearing high heels can also lead to foot deformities such as bunions and hammertoes. As you saw in the section Disorders of the Skeletal System, a bunion is a protrusion on the side of the foot, most often at the base of the big toe. It can be caused by wearing shoes with a narrow, pointed toe box — a common shape for high heels (see Figure \(3\)). The pressure of the shoes on the side of the foot causes an enlargement of bone or inflammation of other tissues in the region, which pushes the big toe towards the other toes.
Hammertoes are abnormal bend in the middle joint of the second, third, or fourth toe (with the big toe being the first toe), causing the toe to be shaped similar to a hammer as seen in Figure \(4\). The narrow, pointed toe box of many high heels, combined with the way the toes are squished into the front of the shoe as a result of the height of the heel, can cause the toes to become deformed in this manner. Treatments for bunions and hammertoe include wearing shoes with a roomy toe box, padding or taping the toes, and toe exercises and stretches. If the bunion or hammertoe does not respond to these treatments, surgery may be necessary to correct the deformity.
Because the bones of the skeleton are connected and work together with other systems to support the body, wearing high heels can also cause physical problems in areas other than the feet. Wearing high heels shifts a person’s posture and alignment and can put a strain on tendons, muscles, and other joints in the body. For instance, research from a team at Stanford University published in 2014 suggests that wearing high heels, particularly if the person is overweight or the heels are very high, may increase the risk of osteoarthritis (OA) in the knee due to added stress on the knee joint as the person walks. As you have learned, OA results from the breakdown of cartilage and bone at the joint. Because it can only be treated to minimize symptoms, not cured, OA could be an unfortunate long-term consequence of wearing high heels.
Amari has decided that wearing high heels regularly is not worth the pain and potential long-term damage to their body. After consulting with their doctor, who confirmed they have metatarsalgia, Amari was able to successfully treat it with ice, rest, and wearing comfortable, supportive shoes instead of heels.
High heels are not the only kind of shoes that can cause problems. Flip-flops, worn-out sneakers, and shoes that are too tight can all cause foot issues. To prevent future problems from shoe choices, Amari is following guidelines recommended by medical experts, which include:
• Wearing shoes that fit well, have plenty of room in the toe box, are supportive, and are comfortable right away. There should be no “break-in” period needed for shoes.
• Avoiding shoes that have high heels, especially ones over two inches in height; narrow, pointed toe boxes; or very thin heels. The shoes in Figure \(3\) are an example of a type that should be avoided!
• If high heels must be worn, they should be worn for only a limited period of time.
As you have learned in this chapter, your skeletal system carries out a variety of important functions in your body, including physical support. But even though it is strong, your skeletal system can become damaged and deformed—even through such a seemingly innocuous act as wearing a certain type of shoe. Taking good care of your skeletal system is necessary to help it continue to take good care of the rest of you.
Chapter Summary
In this chapter, you learned about the skeletal system. Specifically, you learned that:
• The skeletal system is the organ system that provides an internal framework for the human body. In adults, the skeletal system contains 206 bones.
• Bones are organs made of dense connective tissues, mainly the tough protein collagen. Bones also contain blood vessels, nerves, and other tissues. Bones are hard and rigid due to deposits of calcium and other mineral salts within their living tissues. Besides bones, the skeletal system includes cartilage and ligaments.
• The skeletal system has many different functions, including supporting the body and giving it shape, protecting internal organs, providing attachment surfaces for skeletal muscles, allowing body movements, producing blood cells, storing minerals, helping to maintain mineral homeostasis, and producing endocrine hormones.
• The skeleton is traditionally divided into two major parts: the axial skeleton and the appendicular skeleton.
• The axial skeleton consists of a total of 80 bones. It includes the skull, vertebral column, and rib cage. It also includes the three tiny ossicles in the middle ear and the hyoid bone in the throat.
• The skull provides a bony framework for the head. It consists of 22 different bones: eight in the cranium, which encloses the brain, and 14 in the face, which includes the upper and lower jaw.
• The vertebral column is a flexible, S-shaped column of 33 vertebrae that connects the trunk with the skull and encloses the spinal cord. The vertebrae are divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal regions. The S shape of the vertebral column allows it to absorb shocks and distribute the weight of the body.
• The rib cage holds and protects the organs of the upper part of the trunk, including the heart and lungs. It includes the 12 thoracic vertebrae, the sternum, and 12 pairs of ribs.
• The appendicular skeleton consists of a total of 126 bones. It includes the bones of the four limbs, shoulder girdle, and pelvic girdle. The girdles attach the appendages to the axial skeleton.
• Each upper limb consists of 30 bones. There is one bone, called the humerus, in the upper arm, and two bones, called the ulna and radius, in the lower arm. The wrist contains eight carpal bones, the hand contains five metacarpals, and the fingers consist of 14 phalanges. The thumb is opposable to the palm and fingers of the same hand.
• Each lower limb also consists of 30 bones. There is one bone, called the femur, in the upper leg, and two bones, called the tibia and fibula, in the lower leg. The patella covers the knee joint. The ankle contains seven tarsal bones, and the foot contains five metatarsals. The tarsals and metatarsals form the heel and arch of the foot. The bones in the toes consist of 14 phalanges.
• The shoulder girdle attaches the upper limbs to the trunk of the body. It is connected to the axial skeleton only by muscles, allowing mobility of the upper limbs. Bones of the shoulder girdle include a right and left clavicle and a right and left scapula.
• The pelvic girdle attaches the legs to the trunk of the body and supports the organs of the abdomen. It is connected to the axial skeleton by ligaments. The pelvic girdle consists of two halves that are fused together in adults. Each half consists of three bones: the ilium, pubis, and ischium.
• Bones are organs that consist mainly of bone, or osseous, tissue. Osseous tissue is a type of connective tissue consisting of a collagen matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and minerals makes bone hard without making it brittle.
• There are two types of osseous tissues: cortical bone tissue and spongy bone tissue. Cortical bone tissue is smooth and dense. It forms the outer layer of bones. Spongy bone tissue is porous and light. It is found inside many bones.
• Besides osseous tissues, bones also contain nerves, blood vessels, bone marrow, and periosteum.
• Bone tissue is composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells. Osteoblasts form new collagen matrix and mineralize it, osteoclasts break down bone, osteocytes regulate the formation and breakdown of bone, and osteogenic cells divide and differentiate to form new osteoblasts. Bone is a very active tissue, constantly being remodeled by the work of osteoblasts and osteoclasts.
• There are six types of bones in the human body: long bones such as the limb bones, short bones such as the wrist bones, sesamoid bones such as the patella, sutural bones in the skull, and irregular bones such as the vertebrae.
• Early in the development of a human fetus, the skeleton is made almost entirely of cartilage. The relatively soft cartilage gradually turns into hard bone. This is called ossification. It begins at a primary ossification center in the middle of the bone and later also occurs at secondary ossification centers in the ends of the bone. The bone can no longer grow in length after the areas of ossification meet and fuse at the time of skeletal maturity.
• Throughout life, bone is constantly being replaced in the process of bone remodeling. In this process, osteoclasts resorb bone and osteoblasts make new bone to replace it. Bone remodeling shapes the skeleton, repairs tiny flaws in bones, and helps maintain mineral homeostasis in the blood.
• Bone repair is the natural process in which a bone repairs itself following a bone fracture. This process may take several weeks. In the process, periosteum produces cells that develop into osteoblasts, and the osteoblasts form a new bone matrix to heal the fracture. Bone repair may be affected by diet, age, pre-existing bone disease, or other factors.
• Joints are locations at which bones of the skeleton connect with one another.
• Joints can be classified structurally or functionally, and there is significant overlap between the two types of classifications.
• The structural classification of joints depends on the type of tissue that binds the bones to each other at the joint. There are three types of joints in the structural classification: fibrous, cartilaginous, and synovial joints.
• The functional classification of joints is based on the type and degree of movement that they allow. There are three types of joints in the functional classification: immovable, partly movable, and movable joints.
• Movable joints can be classified further according to the type of movement they allow. There are six classes of movable joints: pivot, hinge, saddle, plane, condyloid, and ball-and-socket joints.
• A number of disorders affect the skeletal system, including bone fractures and bone cancers. The two most common disorders of the skeletal system are osteoporosis and osteoarthritis.
• Osteoporosis is an age-related disorder in which bones lose mass, weaken, and break more easily than normal bones. The underlying mechanism in all cases of osteoporosis is an imbalance between bone formation and bone resorption in bone remodeling. Osteoporosis may also occur as a side effect of other disorders or certain medications.
• Osteoporosis is diagnosed by measuring a patient’s bone density and comparing it with the normal level of peak bone density. Fractures are the most dangerous aspect of osteoporosis. Osteoporosis is rarely fatal, but complications of fractures often are.
• Risk factors for osteoporosis include older age, female sex, European or Asian ancestry, family history of osteoporosis, short stature and small bones, smoking, alcohol consumption, lack of exercise, vitamin D deficiency, poor nutrition, and consumption of soft drinks.
• Osteoporosis is often treated with medications such as bisphosphonates that may slow or even reverse bone loss. Preventing osteoporosis includes eliminating any risk factors that can be controlled through changes of behavior, such as undertaking weight-bearing exercise.
• Osteoarthritis (OA) is a joint disease that results from the breakdown of joint cartilage and bone. The most common symptoms are joint pain and stiffness. OA is thought to be caused by mechanical stress on the joints with insufficient self-repair of cartilage, coupled with low-grade inflammation of the joints.
• Diagnosis of OA is typically made on the basis of signs and symptoms, such as joint deformities, pain, and stiffness. X-rays or other tests are sometimes used to either support the diagnosis or rule out other disorders. Age is the chief risk factor for OA. Other risk factors include joint injury, excess body weight, and a family history of OA.
• OA cannot be cured, but the symptoms can often be treated successfully. Treatments may include exercise, efforts to decrease stress on joints, pain medications, and surgery to replace affected hip or knee joints.
As you have learned in this chapter, one of the important functions of the skeletal system is to allow movement of the body. But it doesn’t do it alone. Movement is caused by the contraction of muscles, which pull on the bones, causing them to move. Read the next chapter to learn about this and other important functions of the muscular system.
Chapter Summary Review
1. Hematopoiesis is carried out by:
1. spongy bone tissue
2. periosteum
3. yellow bone marrow
4. red bone marrow
2. True or False. Osteocalcin is a hormone produced by bone cells.
3. True or False. Vertebrae make up part of the rib cage.
4. For each of the following bones, indicate whether they are part of the axial or appendicular skeleton.
1. The ossicles of the middle ear
2. The femur
3. The phalanges
4. The bones of the cranium
5. The ilium
5. Why does the rib cage need to be flexible and why is it able to be flexible?
6. In general, what do “girdles” in the skeletal system do?
7. Which protein does bone mainly consist of?
1. Keratin
2. Collagen
3. Cellulose
4. Elastin
8. For each of the descriptions below, identify which process best fits the description. Use each process only once. Processes: bone growth; bone repair; bone remodeling
1. New osteoblasts form from the periosteum and produce new bone tissue.
2. Cartilage grows, and the primary and secondary ossification centers move towards each other.
3. Osteoclasts break down bone tissue and osteoblasts build new bone tissue.
9. For each of the following processes, describe when it occurs.
1. Bone growth
2. Bone repair
3. Bone remodeling
10. Would swimming likely be more effective as an exercise for preventing osteoporosis or as a treatment for osteoarthritis? Explain your answer.
11. True or False. Use of anabolic steroids in the teenage years generally makes people taller.
12. True or False. The largest joint in the human body is the knee joint.
13. How much of an adult’s skeletal mass is broken down and rebuilt each year?
1. None
2. 5 percent
3. 10 percent
4. 30 percent
14. Explain why some of the vertebrae become misshapen in the condition called dowager’s hump, or kyphosis.
15. Explain why osteoarthritis often involves inflammation in the joints.
16. Osteoporosis can involve both excess bone resorption as well as insufficient production of new bone tissue. What are the two main bone cell types that carry out these processes, respectively?
17. True or False. Bone mass does not decrease as men age.
18. True or False. Ideally, a person’s spine would be perfectly straight and rigid.
19. Compare and contrast a tendon and a ligament.
20. Describe two roles that calcium plays in the bones of the body.
21. How many bones are in the adult human skeleton?
1. 80
2. 126
3. 206
4. 270
Attributions
1. No Heels by Sam Howzit, CC BY 4.0 via Wikimedia Commons
2. Mratatarsus by Henry Gray, public domain via Wikimedia Commons
3. Stiletto heels by berthovanrhee, CC BY 2.0 via Wikimedia Commons
4. Hammer Toe by Mikael Häggström, M.D., CC0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/14%3A_Skeletal_System/14.8%3A_Case_Study_Conclusion%3A__Heels_and_Chapter_Summary.txt |
This chapter describes the structure and functions of the muscular system. It compares and contrasts the three major types of muscle tissue and explains in detail how muscles contract according to the sliding filament theory. The chapter also relates physical exercise to fitness and health and describes several musculoskeletal and neuromuscular disorders.
• 15.1: Case Study: Muscles and Movement
Forty-three-year-old Nasir has a rare condition called cervical dystonia, which is also called spasmodic torticollis. In this condition, the muscles in the neck contract involuntarily, often causing the person’s head to twist to one side. Fortunately for Nasir and other cervical dystonia sufferers, there is a treatment that can significantly reduce symptoms in many people—and it might surprise you!
• 15.2: Introduction to the Muscular System
The largest percentage of muscles in the muscular system consists of skeletal muscles, which are attached to bones and enable voluntary body movements. There are almost 650 skeletal muscles in the human body, many of them shown in the figure below. Besides skeletal muscles, the muscular system also includes cardiac muscle - which makes up the walls of the heart - and smooth muscles, which control movement in other internal organs and structures.
• 15.3: Types of Muscle Tissue
Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles are connective tissues, such as tendons that attach skeletal muscles to bones and sheaths of connective tissues that cover or line muscle tissues. Only muscle tissue per se, however, has cells with the ability to contract.
• 15.4: Muscle Contraction
A muscle contraction is an increase in the tension or a decrease in the length of a muscle. Muscle tension is the force exerted by the muscle on a bone or other object. A muscle contraction is isometric if muscle tension changes, but muscle length remains the same. An example of isometric muscle contraction is holding a book in the same position. A muscle contraction is isotonic if muscle length changes, but muscle tension remains the same.
• 15.5: Physical Exercise
Physical exercise is any bodily activity that enhances or maintains physical fitness and overall health and wellness. We generally think of physical exercise as activities that are undertaken for the main purpose of improving physical fitness and health. However, physical activities that are undertaken for other purposes may also count as physical exercise. Scrubbing a floor, raking a lawn, or playing active games with young children or a dog are all activities that can have health benefits.
• 15.6: Disorders of the Muscular System
Musculoskeletal disorders are injuries that occur in muscles or associated tissues (such as tendons) because of biomechanical stresses. They may be caused by sudden exertion, over-exertion, repetitive motions, or long periods maintaining awkward positions. Musculoskeletal disorders are often work- or sports-related, and generally just one or a few muscles are affected. They can often be treated successfully, and full recovery can be very likely.
• 15.7: Case Study Conclusion: Needing to Relax and Chapter Summary
As you learned in the beginning of this chapter, botulinum toxin—one form of which is sold under the brand name Botox - does much more than smooth out wrinkles. It can be used to treat a number of disorders involving excessive muscle contraction, including cervical dystonia. You also learned that cervical dystonia, which Nasir suffers from, causes abnormal, involuntary muscle contractions of the neck. This results in jerky movements of the head and neck.
15: Muscular System
Case Study: Needing to Relax
The dog in Figure \(3\) is expressing his interest in something—perhaps a piece of food—by using the neck muscles to tilt its head in an adorable fashion. Humans also sometimes tilt their heads to express interest. But imagine how disturbing and painful it would be if your neck tilted involuntarily, without you being able to control it! Forty-three-year-old Nasir, unfortunately, knows just how debilitating this can be. Nasir uses they, them, and their pronouns.
Nasir has a rare condition called cervical dystonia, which is also called spasmodic torticollis. In this condition, the muscles in the neck contract involuntarily, often causing the person’s head to twist to one side. The illustration in Figure \(2\) shows one type of abnormal head positioning that can be caused by cervical dystonia. The muscles may contract in a sustained fashion, holding the head and neck in one position, or they may spasm repeatedly, causing jerky movements of the head and neck.
Cervical dystonia is painful and can significantly interfere with individuals' ability to carry out their usual daily activities. In Nasir’s case, they can no longer drive a car, because their uncontrollable head and neck movements and abnormal head positioning prevent them from navigating the road safely. Nasir also has severe neck and shoulder pain much of the time.
Although it can be caused by an injury, there is no known cause of cervical dystonia—and there is also no cure. Fortunately for Nasir and other cervical dystonia sufferers, though, there is a treatment that can significantly reduce symptoms in many people. You may be surprised to learn that this treatment is the same substance that, when injected into the face, is used for cosmetic purposes to reduce wrinkles!
The substance is botulinum toxin, one preparation of which may be familiar to you by its brand name: Botox. It is a neurotoxin produced by the bacterium Clostridium botulinum, and can cause a life-threatening illness called botulism. However, when injected in very small amounts by a skilled medical professional, botulinum toxins have some safe and effective uses. In addition to cervical dystonia, botulinum toxins can be used to treat other disorders involving the muscular system, such as strabismus (misalignment of the eyes), eye twitches, excessive muscle contraction due to neurological conditions like cerebral palsy; and even overactive bladder.
Botulinum toxin has its effect on the muscular system by inhibiting muscle contractions. When used to treat wrinkles, it relaxes the muscles of the face, lessening the appearance of wrinkles. When used to treat cervical dystonia and other disorders involving excessive muscle contraction, it reduces abnormal contractions.
In this chapter, you will learn about the muscles of the body, how they contract to produce movements and carry out their functions, and some disorders that affect the muscular system. At the end of the chapter, you will find out if botulinum toxin helped relieve Nasir’s cervical dystonia, and how this toxin works to inhibit muscle contraction.
Chapter Overview: Muscular System
In this chapter, you will learn about the muscular system, which carries out both voluntary body movements and involuntary contractions of internal organs and structures. Specifically, you will learn about:
• The different types of muscle tissue—skeletal, cardiac, and smooth muscle—and their different characteristics and functions
• How muscle cells are specialized to contract and cause voluntary and involuntary movements
• The ways in which muscle contraction is controlled
• How skeletal muscles can grow or shrink, causing changes in strength
• The structure and organization of skeletal muscles (including the different types of muscle fibers) and how actin and myosin filaments move across each other, according to the sliding filament theory, to cause muscle contraction
• How cardiac muscle tissue in the heart contracts to pump blood through the body
• Smooth muscle tissue that makes up internal organs and structures, such as the digestive system, blood vessels, and uterus
• The physical and mental health benefits of aerobic and anaerobic exercise, such as running and weight lifting
• How individuals vary in their response to exercise
• Disorders of the muscular system, including musculoskeletal disorders (such as strains and carpal tunnel syndrome) and neuromuscular disorders (such as muscular dystrophy, myasthenia gravis, and Parkinson’s disease)
As you read the chapter, think about the following questions:
1. How is the contraction of skeletal muscles controlled?
2. Botulinum toxin works on the cellular and molecular levels to inhibit muscle contraction. Based on what you learn about how muscle contraction works, can you think of some ways it could potentially be inhibited?
3. What is one disorder involving a lack of sufficient muscle contraction? Why does it occur?
Attributions
1. Whisky's 2nd Birthday by Kelly Hunter, CC BY 2.0 via Flickr
2. Gray 1194 by Henry Gray, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/15%3A_Muscular_System/15.1%3A_Case_Study%3A_Muscles_and_Movement.txt |
Marvelous Muscles
Does the word muscle make you think of the well-developed muscles of a weightlifter, like a woman in Figure \(1\)? Her name is Natalia Zabolotnaya, and she’s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren’t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can’t control internal muscles like these. It’s a good thing that these internal muscles work without any conscious effort on your part because the movement of these muscles is essential for survival. Muscles are the organs of the muscular system.
What Is the Muscular System?
The muscular system consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles, which are attached to bones and enable voluntary body movements. There are almost 650 skeletal muscles in the human body, many of them shown in Figure \(2\). Besides skeletal muscles, the muscular system also includes cardiac muscle — which makes up the walls of the heart — and smooth muscles, which control movement in other internal organs and structures.
Muscle Structure and Function
Muscles are organs composed mainly of muscle cells, which are also called muscle fibers (mainly in skeletal and cardiac muscle) or myocytes (mainly in smooth muscle). Muscle cells are long and thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP. The sliding filaments increase the tension in — or shorten the length of — muscle cells, causing a contraction. Muscle contractions are responsible for virtually all the movements of the body, both inside and out.
Skeletal muscles are attached to the bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help keep balance.
Smooth muscles in the walls of blood vessels contract to cause vasoconstriction, which may help conserve body heat. Relaxation of these muscles causes vasodilation, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called peristalsis. Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.
Cardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heartbeat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.
Muscle Hypertrophy and Atrophy
Muscles can grow larger, or hypertrophy. This generally occurs through increased use, although hormonal or other influences can also play a role. The increase in testosterone during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight-bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of a muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.
Muscles can also grow smaller, or atrophy, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time — for example, because of a broken bone or surgery — lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just “skin and bones.” Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (Figure \(3\)).
Many diseases, including cancer and AIDS, are often associated with muscle atrophy. Atrophy of muscles also happens with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as sarcopenia. The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines the strength, muscle atrophy causes a corresponding decline in muscle strength.
In both hypertrophy and atrophy, the number of muscle fibers does not change. What changes the size of the muscle fibers? When muscle hypertrophy happens, the individual fibers become wider. When muscle atrophy happens, the fibers become narrower.
Interactions with Other Body Systems
Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a neuromuscular junction. Let’s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibers in your arm and shoulder to contract, causing your arm to rise.
Involuntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the autonomic nervous system (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.
Muscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the musculoskeletal system. Skeletal muscles are attached to the skeleton by tough connective tissues called tendons. Many skeletal muscles are attached to the ends of bones that meet at a joint. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.
Review
1. What is the muscular system?
2. Describe muscle cells and their function.
3. Identify three types of muscle tissue and where each type is found.
4. Define muscle hypertrophy and muscle atrophy.
5. What are the possible causes of muscle hypertrophy?
6. Give three reasons that muscle atrophy may occur.
7. How do muscles change when they increase or decrease in size?
8. How do changes in muscle size affect strength?
9. Explain why astronauts can easily lose muscle mass in space.
10. Describe how the terms muscle cells, muscle fibers, and myocytes relate to each other.
11. Muscle tissue in the stomach is considered ___________________.
A. cardiac muscle
B. skeletal muscle
C. smooth muscle
D. voluntary muscle
12. Muscle contraction is the __________ of muscle fibers.
A. hypertrophy
B. atrophy
C. lengthening
D. shortening
13. True or False: Smooth muscle does not contract.
14. Name two systems in the body that work together with the muscular system to carry out movements.
15. Describe one way in which the muscular system is involved in regulating body temperature.
Explore More
Check out this video to learn about peristalsis of the large intestine:
Attributions
1. Natalia Zabolotnaya by Simon Q, CC BY 2.0 via Wikimedia Commons
2. Bougle whole2 retouched by Bouglé, Julien, public domain via Wikimedia Commons
3. Daniel Tani by NASA, public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/15%3A_Muscular_System/15.2%3A_Introduction_to_the_Muscular_System.txt |
Work Those Eye Muscles!
Turn your eyes—a tiny movement, considering the conspicuously large and strong external eye muscles that control eyeball movements. These muscles have been called the strongest muscles in the human body relative to the work they do. However, the external eye muscles actually do a surprising amount of work. Eye movements occur almost constantly during waking hours, especially when we are scanning faces or reading. Eye muscles are also exercised nightly during the phase of sleep called rapid eye movement sleep. External eye muscles can move the eyes because they are made mainly of muscle tissue.
What is Muscle Tissue?
Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles are connective tissues, such as tendons that attach skeletal muscles to bones and sheaths of connective tissues that cover or line muscle tissues. Only muscle tissue per se, however, has cells with the ability to contract.
There are three major types of muscle tissues in the human body: skeletal, smooth, and cardiac muscle tissues. Figure \(2\) shows how the three types of muscle tissues appear under a microscope. When you read about each type below, you will learn why the three types appear as they do.
Skeletal Muscle Tissue
Skeletal muscle is muscle tissue attached to bones by tendons, which are bundles of collagen fibers. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are voluntary or under the conscious control of the central nervous system via the somatic nervous system. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42 percent skeletal muscles, and the average adult female is about 36 percent skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figures \(3\) and Figure \(4\) and listed in Table \(1\).
Table \(1\): Skeletal muscles. Some muscles are visible from both anterior and posterior views.
Muscles visible in Figure \(3\) Muscles visible in Figure \(4\)
rotator cuff (multiple muscles are part of this group) levator scapulae
biceps brachii rhomboids
brachialis rotator cuff
pronator teres triceps brachii
brachioradialis gluteus maximus
adductor muscles tibialis posterior
tibialis anterior peroneus longus
deltoid peroneus brevis
pectoralis major trapezius
rectus abdominis deltoid
abdominal external oblique brachioradialis
iliopsoas latissimus dorsi
quadriceps femoris biceps femoris
peroneus longus semitendinosus
peroneus bravis semimembranousus
gastrocnemius
soleus
Skeletal Muscle Pairs
To move bones in opposite directions, skeletal muscles often consist of muscle pairs that work in opposition to one another. For example, when the biceps muscle (on the front of the upper arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in Figure \(5\). When the triceps muscle (on the back of the upper arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles are examples of a muscle pair where the muscles work in opposition to each other.
Skeletal Muscle Structure
Each skeletal muscle consists of hundreds — or even thousands — of skeletal muscle fibers, which are long, string-like cells. As shown in Figure \(6\), skeletal muscle fibers are individually wrapped in connective tissue called endomysium. The skeletal muscle fibers are bundled together in units called muscle fascicles, surrounded by sheaths of connective tissue called perimysium. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibers. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called epimysium. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibers, allowing them to withstand contraction forces by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. Also, the epimysium anchors the muscles to tendons.
The same bundles-within-bundles structure is replicated within each muscle fiber. As shown in Figure \(7\), a muscle fiber consists of a bundle of myofibrils, which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein actin, anchored to structures called Z discs — and thick filaments of the protein myosin. The filaments are arranged together within a myofibril in repeating units called sarcomeres, which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal (and cardiac) muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibers. Other components of a skeletal muscle fiber include multiple nuclei and mitochondria.
Slow- and Fast-Twitch Skeletal Muscle Fibers
Skeletal muscle fibers can be divided into two types, called slow-twitch (or type I) muscle fibers and fast-twitch (or type II) muscle fibers.
• Slow-twitch muscle fibers are dense with capillaries and rich in mitochondria and myoglobin, a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibers, slow-twitch fibers can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibers can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.
• Fast-twitch muscle fibers contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fiber can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibers can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibers, fast-twitch fibers contribute more to muscle strength and have a greater potential for increasing mass. They are relied upon primarily in short, strenuous events, such as sprinting or weight lifting.
Proportions of fiber types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fiber than the other. Generally, an individual who has more slow-twitch fibers is better suited for activities requiring endurance. In contrast, an individual who has more fast-twitch fibers is better suited for activities requiring short bursts of power.
Smooth Muscle
Smooth muscle is muscle tissue in the walls of internal organs and other internal structures such as blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. When smooth muscles in the stomach wall contract, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are involuntary, so they are not under conscious control. Instead, they are controlled by the autonomic nervous system, hormones, neurotransmitters, and other physiological factors.
Structure of Smooth Muscle
The cells that make up smooth muscle are generally called myocytes. Unlike the muscle fibers of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in sarcomeres. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle contain myofibrils, which contain bundles of myosin and actin filaments. The filaments cause contractions when they slide over each other, as shown in Figure \(8\).
Functions of Smooth Muscle
Unlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. An extracellular matrix secreted by myocytes enhances the elasticity of smooth muscle. The matrix consists of elastin, collagen, and other stretchy fibers. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (Figure \(9\)), both of which must stretch considerably as they perform their normal functions.
The following list indicates where many smooth muscles are found, along with some of their specific functions.
• Walls of the gastrointestinal tract (such as the esophagus, stomach, and intestines), moving food through the tract by peristalsis.
• Walls of air passages of the respiratory tract (such as the bronchi), controlling the diameter of the passages and the volume of air that can pass through them
• Walls of organs of the male and female reproductive tracts; in the uterus, for example, pushing a baby out of the uterus and into the birth canal
• Walls of the urinary system structures, including the urinary bladder, allow the bladder to expand so it can hold more urine and then contract as urine is released.
• Walls of blood vessels, controlling the diameter of the vessels and thereby affecting blood flow and blood pressure
• Walls of lymphatic vessels, squeezing the fluid called lymph through the vessels.
• Iris of the eyes, controlling the size of the pupils and thereby the amount of light entering the eyes
• Arrector pili in the skin, raising hairs in hair follicles in the dermis.
Cardiac Muscle
Cardiac muscle is found only in the wall of the heart. It is also called myocardium. As shown in Figure \(10\), the myocardium is enclosed within connective tissues, including the endocardium on the inside of the heart and pericardium on the outside of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac muscle cells in the heart muscle area called the sinoatrial node.
Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres inside the muscle fibers. However, in cardiac muscle, the myofibrils are branched at irregular angles rather than arranged in parallel rows (as they are in skeletal muscle). This explains why cardiac and skeletal muscle tissues look different from one another.
The cells of cardiac muscle tissue are arranged in interconnected networks. This arrangement allows rapid transmission of electrical impulses, which stimulate virtually simultaneous contractions of the cells. This enables the cells to coordinate contractions of the heart muscle.
The heart is the muscle that performs the greatest amount of physical work in a lifetime. Although the heart's power output is much less than the maximum power output of some other muscles in the human body, the heart does its work continuously over an entire lifetime without rest. The cardiac muscle contains many mitochondria, which produce ATP for energy and help the heart resist fatigue.
Feature: Human Body in the News
The human heart develops in a sequence of events that are controlled by communication among different types of cells, including cells that will become myocardium (the cardiac muscle that forms the wall of the heart) and cells that will become endocardium (the connective tissue that covers the inside surface of the myocardium). If communication among the cells is abnormal, it can lead to various heart defects, such as cardiac hypertrophy or abnormal enlargement of the heart muscle. Cardiac hypertrophy causes the heart to thicken and weaken over time, so it is less able to pump blood. Eventually, heart failure may develop, causing fluid to build up in the lungs and extremities.
Abnormal cell communication is the mechanism by which a mutation called PTPN11 leads to cardiac hypertrophy in disorder referred to as NSML (Noonan Syndrome with Multiple Lentigines). New research by scientists at Beth Israel Deaconess Medical Center in Boston has determined which type of cell abnormalities occur that lead to NSML. In the research, the scientists engineered mouse models to express the PTPN11 mutation as they developed. The researchers manipulated the mouse models so that the mutation was expressed only in cells that would develop into the myocardium in some of the mice. In contrast, in other mice, the mutation was expressed only in cells that would develop into endocardium. Unexpectedly, the heart's hypertrophy occurred only in the mice that expressed the mutation in endocardial cells, not in myocardial cells, which had long been assumed to be the cells affected. The results of the research suggest potential targets for the treatment of NSML. They may also help scientists understand the causes of other cardiac disorders that are much more common than NSML.
Review
1. What is muscle tissue?
2. Where is the skeletal muscle found, and what is its general function?
3. Why do many skeletal muscles work in pairs?
4. Describe the structure of a skeletal muscle.
5. Relate muscle fiber structure to the functional units of muscles.
6. Why is skeletal muscle tissue striated?
7. Compare and contrast slow-twitch and fast-twitch skeletal muscle fibers.
8. Where is the smooth muscle found? What controls the contraction of smooth muscle?
9. Compare and contrast smooth muscle and striated muscle (such as skeletal muscle).
10. Where is the cardiac muscle found? What controls its contractions?
11. Both cardiac and skeletal muscle tissues are striated, but they look different from one another. Why?
12. The heart muscle is smaller and less powerful than some other muscles in the body. Why is the heart the muscle that performs the greatest amount of physical work in a lifetime? How does the heart resist fatigue?
13. Arrange the following units within a skeletal muscle in order, from smallest to largest: fascicle; sarcomere; muscle fiber; myofibril
14. Give one example of connective tissue that is found in muscles. Describe one of its functions.
15. True or False: skeletal muscle fibers are cells with multiple nuclei.
Explore More
You can learn more about the three types of muscle tissues by watching this Khan Academy video:
Attributions
1. Eyes by Nappy; public domain
2. Muscle tissue by Mdunning13, CC BY 3.0 via Wikimedia Commons
3. Muscles anterior labeled by Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain. via Wikimedia Commons
4. Muscles posterior labeled by Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain. via Wikimedia Commons
5. Muscle movement by CK-12 licensed CC BY-NC 3.0
6. Muscle structure by National Cancer Institute, public domain via Wikimedia Commons
7. Muscle fibers by OpenStax, CC BY 4.0 via Wikimedia Commons
8. Actin-myosin filament by Boumphreyfr, CC BY 3.0 via Wikimedia Commons
9. Placenta by Gray38, public domain via Wikimedia Commons
10. Heart Wall by OpenStax College, CC BY 3.0 via Wikimedia Commons
11. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/15%3A_Muscular_System/15.3%3A_Types_of_Muscle_Tissue.txt |
Arm Wrestling
A sport like arm-wrestling depends on muscle contractions. Arm wrestlers must contract muscles in their hands and arms and keep them contracted to resist their opponent's opposing force. The wrestler whose muscles can contract with greater force wins the match.
Muscle Contraction
How a Skeletal Muscle Contraction Begins
Excluding reflexes, all skeletal muscle contractions occur as a result of conscious effort originating in the brain. The brain sends electrochemical signals through the somatic nervous system to motor neurons that innervate muscle fibers (to review how the brain and neurons function, see the chapter Nervous System). A single motor neuron with multiple axon terminals can innervate multiple muscle fibers, thereby causing them to contract at the same time. The connection between a motor neuron axon terminal and a muscle fiber occurs at a neuromuscular junction site. This is a chemical synapse where a motor neuron transmits a signal to muscle fiber to initiate a muscle contraction.
The process by which a signal is transmitted at a neuromuscular junction is illustrated in Figure $2$. The sequence of events begins when an action potential is initiated in the cell body of a motor neuron, and the action potential is propagated along the neuron’s axon to the neuromuscular junction. Once the action potential reaches the end of the axon terminal, it causes the neurotransmitter acetylcholine (ACh) from synaptic vesicles in the axon terminal. The ACh molecules diffuse across the synaptic cleft and bind to the muscle fiber receptors, thereby initiating a muscle contraction. Muscle contraction is initiated with the depolarization of the sarcolemma caused by the sodium ions' entrance through the sodium channels associated with the ACh receptors.
Things happen very quickly in the world of excitable membranes (think about how quickly you can snap your fingers as soon as you decide to do it). Immediately following depolarization of the membrane, it repolarizes, re-establishing the negative membrane potential. Meanwhile, the ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase (AChE). The ACh cannot rebind to a receptor and reopen its channel, which would cause unwanted extended muscle excitation and contraction.
Propagation of an action potential along the sarcolemma enters the T-tubules. For the action potential to reach the membrane of the Sarcoplasmic Reticulum (SR), there are periodic invaginations in the sarcolemma, called T-tubules (“T” stands for “transverse”). The arrangement of a T-tubule with the membranes of SR on either side is called a triad (Figure $3$). The triad surrounds the cylindrical structure called a myofibril, which contains actin and myosin. The T-tubules carry the action potential into the interior of the cell, which triggers the opening of calcium channels in the membrane of the adjacent SR, causing $\text{Ca}^{++}$ to diffuse out of the SR and into the sarcoplasm. It is the arrival of $\text{Ca}^{++}$ in the sarcoplasm that initiates contraction of the muscle fiber by its contractile units, or sarcomeres.
Excitation-contraction coupling
Although the term excitation-contraction coupling confuses or scares some students, it comes down to this: for a skeletal muscle fiber to contract, its membrane must first be “excited”—in other words, it must be stimulated to fire an action potential. The muscle fiber action potential, which sweeps along the sarcolemma as a wave, is “coupled” to the actual contraction through the release of calcium ions ($\text{Ca}^{++}$) from the SR. Once released, the $\text{Ca}^{++}$ interacts with the shielding proteins, troponin and tropomyosin complex, forcing them to move aside so that the actin-binding sites are available for attachment by myosin heads. The myosin then pulls the actin filaments toward the center, shortening the muscle fiber.
In skeletal muscle, this sequence begins with signals from the somatic motor division of the nervous system. In other words, the “excitation” step in skeletal muscles is always triggered by signaling from the nervous system.
Sliding Filament Theory of Muscle Contraction
Once the muscle fiber is stimulated by the motor neuron, actin, and myosin protein filaments within the skeletal muscle fiber slide past each other to produce a contraction. The sliding filament theory is the most widely accepted explanation for how this occurs. According to this theory, muscle contraction is a cycle of molecular events in which thick myosin filaments repeatedly attach to and pull on thin actin filaments, so they slide over one another. The actin filaments are attached to Z discs, each of which marks the end of a sarcomere. The sliding of the filaments pulls the Z discs of a sarcomere closer together, thus shortening the sarcomere. As this occurs, the muscle contracts.
Crossbridge Cycling
Crossbridge cycling is a sequence of molecular events that underlies the sliding filament theory. There are many projections from the thick myosin filaments, each of which consists of two myosin heads (you can see the projections and heads in Figures $5$ and $3$). Each myosin head has binding sites for ATP (or ATP hydrolysis products: ADP and Pi) and actin. The thin actin filaments also have binding sites for the myosin heads—a cross-bridge forms when a myosin head binds with an actin filament.
The process of cross-bridge cycling is shown in Figure $6$. A cross-bridge cycle begins when the myosin head binds to an actin filament. ADP and Pi are also bound to the myosin head at this stage. Next, a power stroke moves the actin filament inward toward the sarcomere center, thereby shortening the sarcomere. At the end of the power stroke, ADP and Pi are released from the myosin head, leaving the myosin head attached to the thin filament until another ATP binds to the myosin head. When ATP binds to the myosin head, it causes the myosin head to detach from the actin filament. ATP is again split into ADP and Pi and the energy released is used to move the myosin head into a "cocked" position. Once in this position, the myosin head can bind to the actin filament again, and another cross-bridge cycle begins.
Feature: Human Biology in the News
Interesting and hopeful basic research on muscle contraction is often in the news because muscle contractions are involved in so many different body processes and disorders, including heart failure and stroke.
• Heart failure is a chronic condition in which cardiac muscle cells cannot contract forcefully enough to keep body cells adequately supplied with oxygen. In 2016, researchers at the University of Texas Southwestern Medical Center identified a potential new target for developing drugs to increase the strength of cardiac muscle contractions in patients with heart failure. The UT researchers found a previously unidentified protein involved in muscle contraction. The minimal protein turns off the “brake” on the heart, so it pumps blood more vigorously. At the molecular level, the protein affects the calcium-ion pump that controls muscle contraction. This result is likely to lead to searches for additional such proteins.
• A stroke occurs when a blood clot lodges in an artery in the brain and cuts off blood flow to part of the brain. Damage from the clot would be reduced if the smooth muscles lining brain arteries relaxed following a stroke because the arteries would dilate and allow greater blood flow to the brain. In a recent study undertaken at the Yale University School of Medicine, researchers determined that the muscles lining blood vessels in the brain actually contract after a stroke. This constricts the vessels, reduces blood flow to the brain, and appears to contribute to permanent brain damage. The hopeful takeaway of this finding is that it suggests a new target for stroke therapy.
Review
1. What is skeletal muscle contraction?
2. Distinguish between isometric and isotonic contractions of skeletal muscle.
3. How does a motor neuron stimulate a skeletal muscle contraction?
4. What is the sliding filament theory?
5. Describe cross-bridge cycling.
6. Where does the ATP needed for a muscle contraction come from?
7. Explain why an action potential in a single motor neuron can cause multiple muscle fibers to contract.
8. The name of the synapse between a motor neuron and a muscle fiber is the _______________ _________.
9. If a drug blocks the acetylcholine receptors on muscle fibers, what do you think this would do to muscle contraction? Explain your answer.
10. True or False: According to the sliding filament theory, actin filaments actively attach to and pull on myosin filaments.
11. True or False: When a motor neuron produces an action potential, the sarcomeres in the muscle fiber that it innervates become shorter as a result.
12. Explain how cross-bridge cycling and sliding filament theory are related to each other.
13. When does anaerobic respiration typically occur in human muscle cells?
14. If there were no ATP available in a muscle, how would this affect cross-bridge cycling? What would this do to muscle contraction?
Attributions
1. Arm wrestling by U.S. Navy photo by Lt. Kenneth Honek, public domain via Wikimedia Commons
2. Motor End Plate and Innervation by OpenStax, CC BY 4.0 via Wikimedia Commons
3. Skeletal muscle by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
4. Actin-tropomyosin-troponin by Daniel Walsh and Alan Sved, CC BY 4.0 via Wikimedia Commons
5. Sliding filament model by OpenStax, CC BY 4.0 via Wikimedia Commons
6. Crossbridge cycling by OpenStax, CC BY 4.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/15%3A_Muscular_System/15.4%3A_Muscle_Contraction.txt |
Strollers
These caregivers are setting a great example for their children by engaging in physical exercise. Adopting a regular physical exercise habit is one of the most important ways to maintain fitness and good health. From higher self-esteem to a healthier heart, physical exercise can positively affect virtually all aspects of health, including physical, mental, and emotional health.
What Is Physical Exercise?
Physical exercise is any bodily activity that enhances or maintains physical fitness and overall health and wellness. We generally think of physical exercise as activities that are undertaken for the main purpose of improving physical fitness and health. However, physical activities that are undertaken for other purposes may also count as physical exercise. Scrubbing a floor, raking a lawn, or playing active games with young children or a dog are all activities that can have fitness and health benefits, even though they generally are not done mainly for this purpose.
How much physical exercise should people get? In the United States, both the Centers for Disease Control and Prevention and the Surgeon General have recommended that every adult participates in moderate exercise for a minimum of 30 minutes a day. This might include walking, swimming, and/or household or yard work.
Types of Physical Exercise
Physical exercise can be classified into three types, depending on the effects it has on the body: aerobic exercise, anaerobic exercise, and flexibility exercise. Many specific physical exercise examples (including playing soccer and rock climbing) can be classified as more than one type.
Aerobic Exercise
Aerobic exercise is any physical activity in which muscles are used below their maximum contraction strength, but for long periods of time. Aerobic exercise uses a relatively high percentage of slow-twitch muscle fibers that consume a large amount of oxygen. The main goal of aerobic exercise is to increase cardiovascular endurance, although it can have many other benefits, including muscle toning. Examples of aerobic exercise include cycling, swimming, brisk walking, jumping rope, rowing, hiking, and tennis.
Anaerobic Exercise
Anaerobic exercise is any physical activity in which muscles are used close to their maximum contraction strength but for relatively short periods of time. Anaerobic exercise uses a relatively high percentage of fast-twitch muscle fibers that consume a small amount of oxygen. The goals of anaerobic exercise include building and strengthening muscles and improving bone strength, balance, and coordination. Examples of anaerobic exercise include push-ups, lunges, sprinting, interval training, resistance training, and weight training (such as biceps curls with a dumbbell, as pictured Figure \(2\)).
Flexibility Exercise
Flexibility exercise is any physical activity that stretches and lengthens muscles. The goals of flexibility exercise include increasing joint flexibility, keeping muscles limber, and improving the range of motion, all of which can reduce the risk of injury. Examples of flexibility exercises include stretching, yoga, and tai chi.
Health Benefits of Physical Exercise
Many studies have shown that physical exercise is positively correlated with a diversity of health benefits. Some of these benefits include maintaining physical fitness, losing weight and maintaining a healthy weight, regulating digestive health, building and maintaining healthy bone density, increasing muscle strength, improving joint mobility, strengthening the immune system, boosting cognitive ability, and promoting psychological well-being. Some studies have also found a significant positive correlation between exercise and quality of life and life expectancy. People who participate in moderate to high levels of physical activity have been shown to have lower mortality rates than people of the same ages who are not physically active. The years of life gained with different amounts of physical activity are shown in the graph in Figure \(3\).
The underlying physiological mechanisms explaining why exercise has these positive health benefits are not completely understood. However, developing research suggests that many of the benefits of exercise may come about because of skeletal muscles' role as endocrine organs. Contracting muscles release hormones called myokines, which promote tissue repair and the growth of new tissue. Myokines also have anti-inflammatory effects, which, in turn, reduce the risk of developing inflammatory diseases. Exercise also reduces cortisol levels, the adrenal cortex stress hormone that may cause many health problems — both physical and mental — at sustained high levels.
Cardiovascular Benefits of Physical Exercise
The beneficial effects of exercise on the cardiovascular system are well documented. Physical inactivity has been identified as a risk factor for the development of coronary artery disease. There is also a direct correlation between physical inactivity and cardiovascular disease mortality. Physical exercise, in contrast, has been demonstrated to reduce several risk factors for cardiovascular disease, including hypertension (high blood pressure), “bad” cholesterol (low-density lipoproteins), high total cholesterol, and excess body weight. Physical exercise has also been shown to increase “good” cholesterol (high-density lipoproteins), insulin sensitivity, the mechanical efficiency of the heart, and exercise tolerance, which can perform physical activity without undue stress and fatigue.
Cognitive Benefits of Physical Exercise
Physical exercise has been shown to help protect people from developing neurodegenerative disorders, such as dementia. A 30-year study of almost 2,400 men found that those who exercised regularly had a 59 percent reduction in dementia when compared with those who did not exercise. Similarly, a review of cognitive enrichment therapies for the elderly found that physical activity — in particular, aerobic exercise — can enhance the cognitive function of older adults. Anecdotal evidence suggests that frequent exercise may even help reverse alcohol-induced brain damage. There are several possible reasons why exercise is so beneficial for the brain. Physical exercise:
• increases blood flow and oxygen availability to the brain
• increases growth factors that promote new brain cells and new neuronal pathways in the brain
• increases levels of neurotransmitters (such as serotonin), which increase memory retention, information processing, and cognition
Mental Health Benefits of Physical Exercise
Numerous studies suggest that regular aerobic exercise works as well as pharmaceutical antidepressants in treating mild-to-moderate depression. A possible reason for this effect is that exercise increases the biosynthesis of at least three neurochemicals that may act as euphoriants. The euphoric effect of exercise is well known. Distance runners may refer to it as “runner’s high,” and people who participate in the crew (Figure \(4\)) may refer to it as “rower’s high.” Because of these effects, health care providers often promote aerobic exercise as a treatment for depression.
Additional mental health benefits of physical exercise include reducing stress, improving body image, and promoting positive self-esteem. Conversely, there is evidence to suggest that being sedentary is associated with an increased risk of anxiety.
Sleep Benefits of Physical Exercise
A recent review of published scientific research suggests that exercise generally improves sleep for most people and helps sleep disorders, such as insomnia. Exercise is the most recommended alternative to sleeping pills for people with insomnia. For sleep benefits, the optimum time to exercise is four to eight hours before bedtime, although exercise at any time of day seems to be beneficial. The only possible exception is a heavy exercise undertaken shortly before bedtime, which may actually interfere with sleep.
Other Benefits of Physical Exercise
Some studies suggest that physical activity may benefit the immune system. For example, moderate excise is associated with a decreased incidence of upper respiratory tract infections. Evidence from many studies has found a correlation between physical exercise and reduced death rates from cancer, specifically breast cancer and colon cancer. Physical exercise has also been shown to reduce the risk of type 2 diabetes and obesity.
Variation in Responses to Physical Exercise
Not everyone benefits equally from physical exercise. When participating in aerobic exercise, most people will have a moderate increase in their endurance, but some will double their endurance. On the other hand, some people will show little or no increase in endurance from aerobic exercise. Genetic differences in slow-twitch and fast-twitch skeletal muscle fibers may play a role in these different results. People with more slow-twitch fibers may develop greater endurance because these muscle fibers have more capillaries, mitochondria, and myoglobin than fast-twitch fibers. As a result, slow-twitch fibers can carry more oxygen and sustain aerobic activity for a longer period of time than fast-twitch fibers. Studies show that endurance athletes (like the marathoner in Figure \(5\)) generally tend to have a higher proportion of slow-twitch fibers than other people.
There is also great variation in individual responses to muscle building as a result of anaerobic exercise. Some people have a much greater capacity to increase muscle size and strength, whereas other people never develop large muscles, no matter how much they exercise them. People who have more fast-twitch than slow-twitch muscle fibers may develop bigger, stronger muscles because fast-twitch muscle fibers contribute more to muscle strength and have greater potential to increase in mass. Evidence suggests that athletes who excel at power activities (such as throwing and jumping) tend to have a higher proportion of fast-twitch fibers than endurance athletes.
Can You “Overdose” on Physical Exercise?
Is it possible to exercise too much? Can too much exercise be harmful? Evidence suggests that some adverse effects may occur if exercise is extremely intense and the body is not given proper rest between exercise sessions. Athletes who train for multiple marathons have been shown to develop scarring of the heart and heart rhythm abnormalities. Doing too much exercise without prior conditioning also increases the risk of injuries to muscles and joints. Damage to muscles due to overexertion is often seen in new military recruits (Figure \(6\)). Too much exercise in females may cause amenorrhea, which is a cessation of menstrual periods. When this occurs, it generally indicates that a woman is pushing her body too hard.
Many people develop delayed onset muscle soreness (DOMS), which is pain or discomfort in muscles felt one to three days after exercising and generally subsides two or three days later. DOMS was once thought to be caused by the buildup of lactic acid in the muscles. Lactic acid is a product of anaerobic respiration in muscle tissues. However, lactic acid disperses fairly rapidly, so it is unlikely to explain pain experienced several days after exercise. The current theory is that DOMS is caused by tiny tears in muscle fibers, which occur when muscles are used at too high a level of intensity.
Feature: My Human Body
Most people know that exercise is important for good health, and it’s easy to find endless advice about exercise programs and fitness plans. What is not so easy to find is the motivation to start exercising and stick with it. This is the main reason why so many people fail to get regular exercise. Practical concerns like a busy schedule and bad weather can certainly make exercising more of a challenge, but the biggest barriers to adopting a regular exercise routine are mental. If you want to exercise but find yourself making excuses or getting discouraged and giving up, here are some tips that may help you get started and stay moving:
• Avoid an all-or-nothing point of view. Don’t think you need to spend hours sweating at the gym or training for a marathon to get healthy. Even a little bit of exercise is better than nothing at all. Start with ten or 15 minutes of moderate activity each day. Taking a walk around your neighborhood is a great way to begin! From there, gradually increase the amount of time until you exercise to at least 30 minutes a day, five days a week.
• Be kind to yourself, and reinforce positive behaviors with rewards. Don’t be down on yourself because you are overweight or out of shape. Don’t beat yourself up because of a supposed lack of willpower. Instead, look at any past failures as opportunities to learn and do better. When you do achieve even small exercise goals, treat yourself to something special. Did you complete your first workout? Reward yourself with a relaxing bath or other treats.
• Don’t make excuses for not exercising. Common complaints include being too busy or tired or not athletic enough. Such excuses are not valid reasons to avoid exercising, and they will sabotage any plans to improve your fitness. If you can’t find a 30-minute period to work out, try to find ten minutes, three times a day. If you’re feeling tired, know that exercise can actually reduce fatigue and boost your energy level. If you feel clumsy and uncoordinated, remind yourself that you don’t need to be athletic to take a walk or engage in vigorous house or yard work.
• Find an activity that you truly enjoy doing. Don’t think you have to lift weights or run on a treadmill to exercise your muscles. If you find such activities boring or unpleasant, you won’t stick with them. Any activity that increases your heart rate and uses large muscles can provide a workout, especially if you’re not in the habit of exercising, so find something you like to do. Do you like to dance? Put on some music and dance up a sweat! Do you enjoy gardening? Get out in the yard and dig up some dirt! Still not interested? Try an activity-based video game, such as Wii or Kinect. You may find it so much fun that it doesn’t seem like exercise until you realize you’ve worked up a sweat.
• Make yourself accountable. Tell friends and family members that you’re going to start exercising. You’ll be letting them — as well as yourself — down if you don’t follow through. Some people find that keeping an exercise log to track their progress is a good way to be accountable and stick to an exercise program. Perhaps the best way to keep at it is to find an exercise partner. If you’ve got someone waiting to exercise with you, you will be less likely to make excuses for not exercising.
• Add more physical activity to your daily life. You don’t need to follow a structured exercise program to increase your activity level. Do your house or yard work briskly for a workout. Park your car further than necessary from work or the mall, and walk the extra distance. If you live close enough, leave the car at home and walk to and from your destination. Rather than taking elevators or escalators, walk up and downstairs. When you take breaks at work, take a walk instead of sitting. Every time a commercial comes on while you’re watching TV, take a quick exercise break — run in place or do some curls with hand weights.
Review
1. How is physical exercise defined?
2. What are the current recommendations for physical exercise for adults?
3. Describe aerobic exercise, and give examples of aerobic exercises.
4. How does anaerobic exercise differ from aerobic exercise, and what are some examples of anaerobic exercises?
5. Define flexibility exercise, and state its benefits. What are two examples of flexibility exercises?
6. In general, how does physical exercise affect health, quality of life, and longevity?
7. What mechanism may underlie many of the general health benefits of physical exercise?
8. Relate physical exercise to cardiovascular disease risk.
9. What may explain the positive benefits of physical exercise on cognition?
10. How does physical exercise compare with antidepressant drugs in the treatment of depression?
11. Identify several other health benefits of physical exercise.
12. Explain how genetics may influence the way individuals respond to physical exercise.
13. Can too much physical exercise be harmful?
14. Lifting hefty weights for a short period of time is likely to:
A. use a relatively high percentage of fast-twitch muscle fibers
B. use a relatively high percentage of slow-twitch muscle fibers
C. be an aerobic exercise
D. use a large amount of oxygen
15. Walking quickly for an extended period of time is likely to:
A. use a relatively high percentage of fast-twitch muscle fibers
B. use a relatively high percentage of slow-twitch muscle fibers
C. use muscles at close to their maximum contraction
D. cause the muscles to use only a small amount of oxygen
Explore More
Watch this fascinating TED talk to learn why some people find it more difficult to exercise than others and what they can do to make it easier for them to adopt an exercise routine.
Attributions
1. Stroller Moms by Serge Melki, CC BY 2.0 via Wikimedia Commons
2. Man lifting weights by Spirit-Fire, CC BY 2.0 via Flickr
3. Life expectancy gains from physical exercise by NIH, public domain
4. Rowing Team by Carlie Horigan Via Pixy license
5. Gashaw Asfaw by Walknboston, CC BY 2.0 via Wikimedia Commons
6. Drill Instructors by US Marines, public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/15%3A_Muscular_System/15.5%3A_Physical_Exercise.txt |
Pain in the Neck
Spending hours each day looking down at hand-held devices is a pain in the neck — literally. The weight of the head bending forward can put a lot of strain on neck muscles, and muscle injuries can be excruciating. Neck pain is one of the most common complaints that bring people to the doctor’s office. In any given year, about one in five adults will suffer from neck pain. That’s a lot of pain in the neck! Not all of them are due to muscular disorders, but many of them are. Muscular disorders, in turn, generally fall into two general categories: musculoskeletal disorders and neuromuscular disorders.
Musculoskeletal Disorders
Musculoskeletal disorders are injuries in muscles or associated tissues (such as tendons) because of biomechanical stresses. They may be caused by sudden exertion, over-exertion, repetitive motions, or long periods of maintaining awkward positions. Musculoskeletal disorders often work- or sports-related, and generally, just one or a few muscles are affected. They can often be treated successfully, and full recovery can be very likely. The disorders include muscle strains, tendonitis, and carpal tunnel syndrome.
Muscle Strain
A muscle strain is an injury in which muscle fibers tear as a result of overstretching. A muscle strain is also commonly called a pulled muscle or torn muscle. (Strains are often confused with sprains, which are similar injuries to ligaments.) Depending on the degree of injury to muscle fibers, a muscle strain can range from mildly to extremely painful. Besides pain, typical symptoms include stiffness and bruising in the area of the strained muscle. Figure \(2\) shows a large bruise caused by a hamstring muscle strain. Hamstring strains are prevalent in track and field athletes. In sprinters, for example, about one-third of injuries are hamstring injuries. Having a previous hamstring injury puts an athlete at increased risk of having another one.
Proper first aid for a muscle strain includes five steps, which are represented by the acronym PRICE. The PRICE steps should be followed for several days after the injury. The five steps are:
1. Protection: Apply soft padding to the strained muscle to minimize impact with objects that might cause further damage.
2. Rest: Rest the muscle to accelerate healing and reduce the potential for re-injury.
3. Ice: Apply ice for 20 minutes at a time every two hours to reduce swelling and pain.
4. Compression: Apply a stretchy bandage to the strained muscle to reduce swelling.
5. Elevation: Keep the strained muscle elevated to reduce the chance of blood pooling in the muscle.
Non-steroidal anti-inflammatory drugs (NSAIDs, such as ibuprofen) can help reduce inflammation and relieve pain. However, because such drugs interfere with blood clotting, they should be taken only after bleeding in the muscle has stopped — not immediately after the injury occurs. For severe muscle strains, professional medical care may be needed.
Tendinitis
Tendinitis is inflammation of a tendon that occurs when it is over-extended or worked too hard without rest. Tendons that are commonly affected include those in the ankle, knee, shoulder, and elbow. The affected tendon depends on the type of use that causes inflammation. Rock climbers tend to develop tendinitis in their fingers, while basketball players are more likely to develop tendonitis in the knees, to name a few examples.
Symptoms of tendinitis may include aching, sharp pain, a burning sensation, or joint stiffness. In some cases, swelling occurs around the inflamed tendon, and the area feels hot and looks red. Treatment includes the PRICE guidelines listed above and the use of NSAIDs to reduce inflammation and pain further. Although symptoms should show improvement within a few days of treatment, full recovery may take several months. A gradual return to exercise or other use of the affected tendon is recommended. Physical or occupational therapy may speed the return to normal activity levels.
Carpal Tunnel Syndrome
Carpal tunnel syndrome is a common biomechanical problem in the wrist when the median nerve becomes compressed between carpal bones (Figure \(3\)). This may occur due to repetitive use of the wrist, a tumor, or trauma to the wrist. Two-thirds of the cases are work-related. Computer work, work with vibrating tools and work requiring a strong grip all increase one's risk of developing this problem. Carpal tunnel syndrome occurs more often in women than in men. Other risk factors include obesity, pregnancy, and arthritis. Genetics may also play a role.
Compression of the median nerve results in the muscles' inadequate nervous stimulation in the thumb and first two fingers of the hand. The main symptoms are pain, numbness, and tingling in these digits. Sometimes, symptoms can be improved by wearing a wrist splint or receiving corticosteroid injections. Surgery to cut the carpal ligament reduces pressure on the median nerve and is generally more effective than nonsurgical treatment. Recurrence of carpal tunnel syndrome after surgery is rare. On the other hand, without treatment, the lack of nervous stimulation by the median nerve may eventually cause the affected muscles of the hand to weaken and waste away.
Neuromuscular Disorders
Neuromuscular disorders are systemic disorders that occur because of problems with the nervous control of muscle contractions or muscle cells themselves. These disorders are often due to faulty genes and not due to biomechanical stresses. Other system-wide problems, such as abnormal immune system responses, may also be involved in neuromuscular disorders.
Unlike musculoskeletal disorders, neuromuscular disorders generally affect most or all of the muscles in the body. The disorders also tend to be progressive and incurable. However, in most cases, treatment is available to slow the disease progression or lessen symptoms. Examples of neuromuscular disorders include muscular dystrophy, myasthenia gravis, and Parkinson’s disease.
Muscular Dystrophy
Muscular dystrophy is a genetic disorder caused by defective proteins in muscle cells. It is characterized by progressive skeletal muscle weakness and death of muscle cells and tissues. Muscles become increasingly unable to contract in response to nervous stimulation.
There are at least nine major types of muscular dystrophy caused by different gene mutations. Some of the mutations cause autosomal recessive or autosomal dominant disorders, and some cause X-linked disorders. The most common type of childhood muscular dystrophy is Duchenne muscular dystrophy (DMD) due to a mutation in a recessive gene on the X chromosome. As an X-linked recessive disorder, Duchenne muscular dystrophy occurs almost exclusively in males.
Different types of muscular dystrophy affect different major muscle groups. In Duchenne muscular dystrophy, the lower limbs are affected. Signs of the disorder usually first become apparent when a child starts walking. Difficulty walking becomes progressively worse through childhood. By the time a child is ten, braces may be needed for walking — and walking may no longer even be possible by age 12. The lifespan of someone with muscular dystrophy is likely to be shorter than normal because of the disease, ranging from 15 to 45 years.
Duchenne muscular dystrophy (DMD) is a progressive weakening of the skeletal muscles. It is one of several diseases collectively referred to as “muscular dystrophy.” DMD is caused by a lack of the protein dystrophin, which helps the thin filaments of myofibrils bind to the sarcolemma. Without sufficient dystrophin, muscle contractions cause the sarcolemma to tear, causing an influx of Ca++, leading to cellular damage and muscle fiber degradation. Over time, as muscle damage accumulates, muscle mass is lost, and greater functional impairments develop.
In some cases, physical therapy, drug therapy, or orthopedic surgery may improve muscular dystrophy signs and symptoms. However, at present, there is no known cure for the disorder. Research is ongoing to find a cure, with financial support from such sources as the Muscular Dystrophy Association (MDA) (see photo below). MDA is a non-profit organization dedicated to curing muscular dystrophy by funding worldwide research.
Myasthenia Gravis
Myasthenia gravis is an autoimmune disorder in which circulating antibodies block the nicotinic acetylcholine receptors on the neuromuscular junction's motor endplate. This blockage of acetylcholine receptors causes muscle weakness, often first exhibiting drooping eyelids and expanding to include overall muscle weakness and fatigue. It occurs more commonly in women and generally begins between the ages of 20 and 40. The initial symptom of myasthenia gravis is painless muscle weakness, generally in muscles around the eye (Figure \(5\)). The disease then progresses to muscles elsewhere in the body, eventually involving most of the muscles. Swallowing and chewing may become difficult as the disease progresses, and speech may become slow and slur. In more advanced cases, myasthenia crises may occur, during which the muscles that control breathing may be affected. Emergency medical care to provide assisted ventilation is required to sustain life. A myasthenia gravis crisis may be triggered by various stressors, such as infection, fever, or stress. Treatment of myasthenia gravis may include medications to counter the mutant gene's effects or suppress the immune system.
Parkinson’s Disease
Parkinson’s disease is a degenerative disorder of the central nervous system that mainly affects the muscular system and movement. Four motor signs and symptoms are considered defining Parkinson’s disease: muscle tremor (shaking), muscle rigidity, slowness of movement, and postural instability. Tremor is the most common and obvious symptom, and it most often occurs in a limb at rest, so it disappears during sleep or when the patient moves the limb voluntarily. Difficulty walking eventually develops, and dementia is common in the advanced stages of the disease. Depression is common, as well.
Parkinson’s disease is more common in older people, with most cases being diagnosed after 50. Often, the disease occurs for no known reason. Cases like this are called primary Parkinson’s disease. Sometimes, the disease has a known or suspected cause, such as exposure to toxins in pesticides or repeated head trauma. In this case, it is called secondary Parkinson’s disease. Regardless of the cause, the disease's motor symptoms result from the death of neurons in the midbrain. The cause of cell death is not fully understood, but it appears to involve the buildup in the brain of protein structures called Lewy bodies. Early in the course of the illness, medications can be prescribed to help reduce the motor disturbances. As the disease progresses, however, the medications become ineffective. They also cause a negative side effect of involuntary writhing movements.
Feature: Human Biology in the News
On June 3, 2016, media worldwide exploded with news of the death of Muhammad Ali at the age of 74. The world champion boxer and Olympic gold medalist died that day of a respiratory infection complication, but the underlying cause was Parkinson’s disease. Ali was diagnosed with Parkinson’s in 1984 when he was only 42 years old. Doctors attributed his disease to repeated head trauma from boxing.
In the days following Ali’s death, the news was full of stories and images from milestones in the athlete’s life, both before and after his diagnosis with Parkinson’s disease. Sadly, the news coverage also provided an overview of his gradual decline as the disease progressed. Ali was pictured in 1996, lighting the flame at the Summer Olympics in Atlanta. In 2012, Ali had to be helped to his feet by his wife to stand before the flag he was supposed to carry into the stadium. He was unable to carry it because of the ravages of Parkinson’s disease.
Muhammad Ali retired from boxing in 1981 at 39, but he didn’t retire from fighting. Until the final year of his life, Ali was a passionate activist for peace and justice and against war and racism. In 1998, he joined Michael J. Fox, who also has Parkinson’s disease, to raise awareness and fund research on Parkinson’s disease. In 2002, Fox and Ali made a joint appearance before Congress to present their case. In 2005, Ali received the Presidential Medal of Freedom from George W. Bush (Figure \(7\)) for the many achievements and contributions he made throughout his amazing life, despite Parkinson’s disease.
Review
1. What are musculoskeletal disorders? What causes them?
2. How does a muscle strain occur?
3. Define tendinitis. Why does it occur?
4. Identify first-aid steps for treating musculoskeletal disorders such as muscle strains and tendinitis.
5. Describe carpal tunnel syndrome and how it may be treated.
6. Define neuromuscular disorders.
7. Identify the cause and symptoms of muscular dystrophy.
8. Outline the cause and progression of myasthenia gravis.
9. What is Parkinson’s disease? List four characteristic signs of the disorder.
10. What are the main differences between musculoskeletal disorders and neuromuscular disorders?
11. Why is the padding of a strained muscle part of the typical treatment?
12. Which disorder would be the most likely to be caused by repeated use of a jackhammer?
A. Parkinson’s disease
B. Muscular dystrophy
C. Carpal tunnel syndrome
D. A neuromuscular disorder
13. True or False. Participation in some sports may cause Parkinson’s disease.
14. True or False. Myasthenia gravis occurs because the body stops making acetylcholine.
15. What are two tissues, other than muscle tissue, that can experience problems that result in muscular system disorders?
Attributions
1. Meet and Tweet by Alliance Internationale, CC BY 2.0 via Wikimedia Commons
2. Pulled Hamstring by Daniel.Cardenas, CC BY 3.0 via Wikimedia Commons
3. Carpal Tunnel Syndrome by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
4. Fill the boot by USMC, public domain via Wikimedia Commons
5. Myasthenia gravis by Mohankumar Kurukumbi, Roger L Weir, Janaki Kalyanam, Mansoor Nasim, Annapurni Jayam-Trouth. CC BY 2.0 via Wikimedia Commons
6. Paralysis agitans by Albert Londe, public domain via Wikimedia Commons
7. Muhammad Ali and President Bush, White House photo by Paul Morse, public domain via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/15%3A_Muscular_System/15.6%3A_Disorders_of_the_Muscular_System.txt |
Case Study Conclusion: Needing to Relax
As you learned at the beginning of this chapter, botulinum toxin—one form sold under the brand name Botox—does much more than smooth out wrinkles. It can be used to treat several disorders involving excessive muscle contraction, including cervical dystonia. You also learned that cervical dystonia, from which Nasir suffers, causes abnormal, involuntary muscle contractions of the neck. This results in the head and neck's jerky movements and/or a sustained abnormal tilt to the head. It is often painful and can significantly interfere with a person’s life.
How could a toxin actually help treat a muscular disorder? The soil bacterium Clostridium botulinum produces the botulinum toxin, and it is the cause of the potentially deadly disease called botulism. Botulism is often a foodborne illness, commonly caused by improperly canned foods. Other forms of botulism are caused by wound infections or occur when infants consume the bacteria's spores from soil or honey.
Botulism can be life-threatening because it paralyzes muscles throughout the body, including those involved in breathing. When a minimal amount of botulinum toxin is injected carefully into specific muscles by a trained medical professional, however, it can inhibit unwanted muscle contractions.
For cosmetic purposes, botulinum toxin injected into the facial muscles relaxes them to reduce wrinkles' appearance. When used to treat cervical dystonia, it is injected into the neck muscles to inhibit excessive muscle contractions. For many patients, this helps relieve the abnormal positioning, movements, and pain associated with the disorder. The effect is temporary, so the injections must be repeated every three to four months to keep the symptoms under control.
How does botulinum toxin inhibit muscle contraction? First, recall how skeletal muscle contraction works. A motor neuron instructs skeletal muscle fibers to contract at a synapse between them called the neuromuscular junction. A nerve impulse called an action potential travels down to the motor neuron's axon terminal, where it causes the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles. The ACh travels across the synaptic cleft and binds to ACh receptors on the muscle fiber, signaling the muscle fiber to contract. According to the sliding filament theory, muscle fibers' contraction occurs due to the sliding of myosin and actin filaments across each other. This causes the Z discs of the sarcomeres to close together, shortening the sarcomeres and causing the muscle fiber to contract.
If you wanted to inhibit muscle contraction, at what points could you theoretically interfere with this process? Inhibiting the action potential in the motor neuron, the release of ACh, the activity of ACh receptors, or the sliding filament process in the muscle fiber would all theoretically impair this process and inhibit muscle contraction. For example, in the disease myasthenia gravis, the ACh receptors' function is impaired, causing a lack of sufficient muscle contraction. As you have learned, this results in muscle weakness that can eventually become life-threatening. Botulinum toxin works by inhibiting the release of ACh from the motor neurons, thereby removing the signal instructing the muscles to contract (Figure \(3\)).
Fortunately, Nasir’s excessive muscle contractions and associated pain improved significantly, thanks to botulinum toxin injections. Although cervical dystonia cannot currently be cured, botulinum toxin injections have improved many patients' quality of life with this and other disorders involving excessive involuntary muscle contractions.
As you have learned in this chapter, our muscular system allows us to make voluntary movements, digest our food, and pump blood through our bodies. Whether in your arm, heart, stomach, or blood vessels, muscle tissue works by contracting. But as you have seen here, too much contraction can be a terrible thing. Fortunately, scientists and physicians have found a way to put a potentially deadly toxin—and wrinkle-reducing treatment—to excellent use as a medical treatment for some muscular system disorders.
Chapter Summary
In this chapter, you learned about the muscular system. Specifically, you learned that:
• The muscular system consists of all the muscles of the body. There are three types of muscle: skeletal muscle (which is attached to bones by tendons and enables voluntary body movements), cardiac muscle (which makes up the walls of the heart and makes it beat), and smooth muscle (which is found in the walls of internal organs and other internal structures and controls their movements).
• Muscles are organs composed mainly of muscle cells, which may also be called muscle fibers or myocytes. Muscle cells are specialized for contracting, which occurs when protein filaments inside the cells slide over one another using energy from ATP. Muscle tissue is the only type of tissue that has cells with the ability to contract.
• Muscles can grow larger or hypertrophy. This generally occurs through increased use, although hormonal or other influences can also play a role. Muscles can also grow smaller or atrophy. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size—but not the number—of muscle fibers changes. The size of the muscles is the main determinant of muscle strength.
• Skeletal muscles need the stimulus of motor neurons to contract and to move the body; they need the skeletal system to act upon.
• Skeletal muscle is the most common type of muscle tissue in the human body. To move bones in opposite directions, skeletal muscles often consist of pairs of muscles that work in opposition to move bones in different directions at joints.
• Skeletal muscle fibers are bundled together in muscle fascicles, which are bundled together to form individual skeletal muscles. Skeletal muscles also have connective tissue supporting and protecting the muscle tissue.
• Each skeletal muscle fiber consists of a bundle of myofibrils, which are bundles of protein filaments. The filaments are arranged in repeating units called sarcomeres, which are the skeletal muscles' basic functional units. Skeletal muscle tissue is striated because of the pattern of sarcomeres in its fibers.
• Skeletal muscle fibers can be divided into two types, called slow-twitch and fast-twitch fibers. Slow-twitch fibers are used mainly in aerobic endurance activities (such as long-distance running). Fast-twitch fibers are used mainly for non-aerobic, strenuous activities (such as sprinting). Proportions of the two types of fibers vary from muscle to muscle and person to person.
• Smooth muscle tissue is found in the walls of internal organs and vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. Contractions of smooth muscles are involuntary and controlled by the autonomic nervous system, hormones, and other substances.
• Cells of smooth muscle tissue are not striated because they lack sarcomeres, but the cells contract in the same basic way as striated muscle cells. Unlike striated muscle, smooth muscle can sustain very long-term contractions and maintain its contractile function, even when stretched.
• Cardiac muscle tissue is found only in the wall of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. Electrical impulses from specialized cardiac cells control them.
• Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres. However, the exact arrangement differs, making cardiac and skeletal muscle tissues look different from one another.
• The heart is the muscle that performs the greatest amount of physical work in a lifetime. Its cells contain many mitochondria to produce ATP for energy and help the heart resist fatigue.
• A muscle contraction is an increase in tension or a decrease in the length of a muscle. A muscle contraction is isometric if muscle tension changes, but muscle length remains the same. It is isotonic if muscle length changes, but muscle tension remains the same.
• A skeletal muscle contraction begins with the electrochemical stimulation of a muscle fiber by a motor neuron. This occurs at a chemical synapse called a neuromuscular junction. The neurotransmitter acetylcholine diffuses across the synaptic cleft and binds to receptors on the muscle fiber. This initiates a muscle contraction.
• Once stimulated, the skeletal muscle fibers' protein filaments slide past each other to produce a contraction. The sliding filament theory is the most widely accepted explanation for how this occurs. According to this theory, thick myosin filaments repeatedly attach to and pull on thin actin filaments, thus shortening sarcomeres.
• Crossbridge cycling is a cycle of molecular events that underlies the sliding filament theory. Using energy in ATP, myosin heads repeatedly bind with and pull on actin filaments. This moves the actin filaments toward the center of a sarcomere, shortening the sarcomere and causing a muscle contraction.
• The ATP needed for a muscle contraction comes first from ATP already available in the cell, and more is generated from creatine phosphate. These sources are quickly used up. Glucose and glycogen can be broken down to form ATP and pyruvate. Pyruvate can then be used to produce ATP in aerobic respiration if oxygen is available or used in anaerobic respiration if oxygen is not available.
• Physical exercise is defined as any bodily activity that enhances or maintains physical fitness and overall health. Activities such as household chores may even count as physical exercise! Current recommendations for adults are 30 minutes of moderate exercise a day.
• Aerobic exercise is any physical activity that uses muscles at less than their maximum contraction strength but for long periods of time. This type of exercise uses a relatively high percentage of slow-twitch muscle fibers that consume large amounts of oxygen. Aerobic exercises increase cardiovascular endurance and include cycling and brisk walking.
• Anaerobic exercise is any physical activity that uses muscles at close to their maximum contraction strength, but for short periods of time. This type of exercise uses a relatively high percentage of fast-twitch muscle fibers that consume small amounts of oxygen. Anaerobic exercises increase muscle and bone mass and strength, and they include push-ups and sprinting.
• Flexibility exercise is any physical activity that stretches and lengthens muscles, thereby improving the range of motion and reducing injury risk. Examples include stretching and yoga.
• Many studies have shown that physical exercise is positively correlated with a diversity of physical, mental, and emotional health benefits. Physical exercise also increases the quality of life and life expectancy.
• Many of the exercise benefits may come about because contracting muscles release hormones called myokines, which promote tissue repair and growth and have anti-inflammatory effects.
• Physical exercise can reduce risk factors for cardiovascular disease, including hypertension and excess body weight. Physical exercise can also increase cardiovascular health factors, such as the mechanical efficiency of the heart.
• Physical exercise has been shown to offer protection from dementia and other cognitive problems, perhaps because it increases blood flow or neurotransmitters in the brain, among other potential effects.
• Numerous studies suggest that regular aerobic exercise works and pharmaceutical antidepressants in treating mild-to-moderate depression, possibly because it increases the synthesis of natural euphoriants in the brain.
• Research shows that physical exercise generally improves sleep for most people and helps sleep disorders, such as insomnia. Other health benefits of physical exercise include better immune system function and reduced risk of type 2 diabetes and obesity.
• There is great variation in individual responses to exercise, partly due to genetic differences in proportions of slow-twitch and fast-twitch muscle fibers. People with more slow-twitch fibers may be able to develop greater endurance from aerobic exercise. In contrast, people with more fast-twitch fibers may develop greater muscle size and strength from anaerobic exercise.
• Some adverse effects may occur if exercise is extremely intense, and the body is not given proper rest between exercise sessions. Many people who overwork their muscles develop delayed onset muscle soreness (DOMS), caused by tiny tears in muscle fibers.
• Musculoskeletal disorders are injuries in muscles or associated tissues (such as tendons) because of biomechanical stresses. The disorders may be caused by sudden exertion, over-exertion, repetitive motions, and similar stresses.
• A muscle strain is an injury in which muscle fibers tear as a result of overstretching. First aid for a muscle strain includes the five steps represented by the acronym PRICE (protection, rest, ice, compression, and elevation). Medications for inflammation and pain (such as NSAIDs) may also be used.
• Tendinitis is inflammation of a tendon that occurs when it is over-extended or worked too hard without rest. Tendinitis may also be treated with PRICE and NSAIDs.
• Carpal tunnel syndrome is a biomechanical problem in the wrist when the median nerve becomes compressed between carpal bones. It may occur with repetitive use, a tumor, or trauma to the wrist. It may cause pain, numbness, and eventually—if untreated—muscle wasting in the thumb and first two fingers of the hand.
• Neuromuscular disorders are systemic disorders that occur because of problems with the nervous control of muscle contractions or muscle cells themselves.
• Muscular dystrophy is a genetic disorder caused by defective proteins in muscle cells. It is characterized by progressive skeletal muscle weakness and death of muscle tissues.
• Myasthenia gravis is a genetic neuromuscular disorder characterized by fluctuating muscle weakness and fatigue. More muscles are affected, and muscles become increasingly weakened, as the disorder progresses. Myasthenia gravis most often occurs because immune system antibodies block acetylcholine receptors on muscle cells because of the actual loss of acetylcholine receptors.
• Parkinson’s disease is a degenerative disorder of the central nervous system that mainly affects the muscular system and movement. It occurs because of the death of neurons in the midbrain. Characteristic signs of the disorder are muscle tremor, muscle rigidity, slowness of movement, and postural instability. Dementia and depression also often characterize advanced stages of the disease.
As you saw in this chapter, muscles need oxygen to provide enough ATP for most of their activities. In fact, all of the body’s systems require oxygen and remove waste products, such as carbon dioxide. In the next chapter, you will learn about how the respiratory system obtains and distributes oxygen throughout the body and how it removes wastes, such as carbon dioxide.
Chapter Summary Review
1. True or False. Each motor neuron controls one muscle fiber.
2. True or False. Peristalsis is a pattern of muscle contraction in smooth muscle tissue.
3. When muscles atrophy:
1. muscle fibers become narrower
2. muscle fibers turn into fat cells
3. muscle fibers are lost
4. muscle fibers become shorter
4. What are tendons?
5. What is a muscular system disorder involving tendons?
6. Which of the main types of muscle tissue is used when you make a voluntary movement of one of your limbs?
7. Which main types of muscle tissue function independently of conscious control by the brain?
8. Describe the relationship between muscles, muscle fibers, and fascicles.
9. Choose one. The (autonomic; somatic) nervous system controls the skeletal muscles.
10. True or False. Sarcomeres are the cells of the muscular system.
11. True or False. Muscles contain connective tissue as well as muscle tissue.
12. The biceps and triceps muscles are shown in Figure \(4\). Answer the following questions about these arm muscles.
1. When the biceps contract and become shorter, as in the illustration, what kind of motion does this produce in the arm?
2. Is the situation described in part more likely to be an isometric or isotonic contraction? Explain your answer.
3. If the triceps were to then contract, which way would help the arm move?
13. Put the following events in order of when they occur during the process of skeletal muscle contraction, from earliest to latest:
1. Acetylcholine binds to receptors on the muscle fiber
2. Actin filaments slide, shortening the sarcomere
3. An action potential is initiated in a motor neuron
4. Acetylcholine is released from synaptic vesicles
14. What are Z discs, and what happens to them during muscle contraction?
15. True or False. Synapses only exist between neurons.
16. True or False. Muscles can produce hormones.
17. Which have been called the strongest muscles in the human body, relative to their work?
1. The heart muscles
2. The hamstring muscles
3. The external eye muscles
4. The stomach muscles
18. What is the function of mitochondria in muscle cells? Which type of muscle fiber has more mitochondria—slow-twitch or fast-twitch?
19. Fast-twitch and slow-twitch are types of which kind of muscle fibers?
1. skeletal
2. smooth
3. cardiac
4. B and C
20. Myoglobin:
1. Stores oxygen for anaerobic respiration
2. Stores oxygen for aerobic respiration
3. Is present in higher amounts in fast-twitch fibers than slow-twitch fibers
4. Is where ATP is produced
21. True or False. All people have the same proportion of slow-twitch to fast-twitch muscle fibers.
22. True or False. A sprain is a tear in the muscle fibers.
23. Which condition directly damages neurons, not muscles?
1. Myasthenia gravis
2. Muscular dystrophy
3. Musculoskeletal disorder
4. Parkinson’s disease
24. What is the difference between primary and secondary Parkinson’s disease?
25. Why can carpal tunnel syndrome cause muscle weakness in the hands?
26. True or False. The heart consists of smooth muscle tissue.
27. True or False. Sprinting is considered an anaerobic exercise
Attribution
1. Botox comic by Michael Reuter, CC BY 2.0 via Flickr
2. Botulism by Jason Wilson, CC BY 2.0 via Flickr
3. Botulinum Toxin Mechanism by Rysin, public domain via Wikimedia Commons
4. Biceps by Pearson Scott Foresman, public domain via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/15%3A_Muscular_System/15.7%3A_Case_Study_Conclusion%3A_Needing_to_Relax_and_Chapter_Summary.txt |
This chapter describes the structure and function of the respiratory system, including how breathing occurs and what controls it, as well as how the process of gas exchange takes place in the lungs. The chapter also describes several disorders of the respiratory system and details the adverse health effects of smoking.
• 16.1: Case Study: Respiratory System and Gas Exchange
Three weeks ago, 20-year-old Sacheen had a runny nose, fatigue, and a mild cough. Her symptoms had been starting to improve, but recently her cough has been getting worse. Her doctor diagnoses her with acute bronchitis, which you will better understand as you read this chapter on the respiratory system, along with the treatment recommendations for this disease.
• 16.2: Structure and Function of the Respiratory System
Respiration is the life-sustaining process in which gases are exchanged between the body and the outside atmosphere. Specifically, oxygen moves from the outside air into the body; and water vapor, carbon dioxide, and other waste gases move from inside the body into the outside air. Respiration is carried out mainly by the respiratory system. Respiration by the respiratory system is not the same process as cellular respiration that occurs inside cells, although they are closely connected.
• 16.3: Breathing
The swimmer in this photo is doing the butterfly stroke. This swimming style requires the swimmer to carefully control his breathing so it is coordinated with his swimming movements. Breathing is the process of moving air into and out of the lungs, which are the organs in which gas exchange takes place between the atmosphere and the body. Breathing is also called ventilation, and it is one of two parts of the life-sustaining process of respiration, the other part being gas exchange.
• 16.4: Disorders of the Respiratory System
Asthma is a chronic inflammatory disease of the airways in the lungs, in which the airways periodically become inflamed. Another common inflammatory disease of the respiratory tract is pneumonia. Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by chronic poor airflow. Lung cancer is a malignant tumor characterized by uncontrolled cell growth in tissues of the lung.
• 16.5: Smoking and Health
tobacco smoking has adverse effects on just about every bodily system and organ. The detrimental health effects of smoking depend on the number of years that a person smokes and how much the person smokes. Contrary to popular belief, all forms of tobacco smoke - including smoke from cigars and tobacco pipes - have similar health risks as those of cigarette smoke. Smokeless tobacco may be less of a danger to the lungs and heart, but it too has serious health effects.
• 16.6: Case Study Conclusion: Bronchitis and Chapter Summary
As you have learned in this chapter, the respiratory system is critical to carry out the gas exchange necessary for life’s functions and to protect the body from pathogens and other potentially harmful substances in the air. But this ability to interface with the outside air has a cost. The respiratory system is prone to infections, as well as damage and other negative effects from allergens, mold, air pollution, and cigarette smoke.
16: Respiratory System
Case Study: Cough That Won't Quit
Three weeks ago, 20-year-old Sacheen came down with symptoms typical of the common cold. She had a runny nose, fatigue, and a mild cough. Her symptoms had been starting to improve, but recently her cough has been getting worse. She coughs up a lot of thick mucus, her throat is sore from frequent coughing, and her chest feels very congested. According to her wife, Sacheen has a “chest cold.” Sacheen is a smoker and wonders if her habit is making her cough worse. She decides that it is time to see a doctor.
Dr. Tsosie examines Sacheen and asks about her symptoms and health history. She checks the level of oxygen in Sacheen’s blood by attaching a device called a pulse oximeter to Sacheen’s finger (Figure \(2\)). Dr. Tsosie concludes that Sacheen has bronchitis, an infection that commonly occurs after a person has a cold or flu. Bronchitis is sometimes referred to as a “chest cold,” so Sacheen’s wife was right! Bronchitis causes inflammation and a build-up of mucus in the bronchial tubes in the chest.
Because viruses, and not bacteria, usually cause bronchitis, Dr. Tsosie tells Sacheen that antibiotics are not likely to help. Instead, she recommends that Sacheen try to thin and remove the mucus by drinking plenty of fluids and using a humidifier, or spending time in a steamy shower. She also recommends that Sacheen get plenty of rest.
Dr. Tsosie also tells Sacheen some things not to do—most importantly, not to smoke while she is sick and to try to quit smoking in the long term. She explains that smoking can make people more susceptible to bronchitis and can hinder recovery. She also advises Sacheen not to take over-the-counter cough suppressant medication.
As you read this chapter on the respiratory system, you will better understand what bronchitis is and why Dr. Tsosie made the treatment recommendations that she did. At the end of the chapter, you will learn more about acute bronchitis, which is the type that Sacheen has. This information may come in handy to you personally because the chances are high that you will get this common infection at some point in your life—there are millions of bronchitis cases every year!
Chapter Overview: Respiratory System
In this chapter, you will learn about the respiratory system, the system that exchanges gases such as oxygen and carbon dioxide between the body and the outside air. Specifically, you will learn about:
• The process of respiration, in which oxygen moves from the outside air into the body and carbon dioxide and other waste gases move from inside the body into the outside air.
• The organs of the respiratory system, including the lungs, bronchial tubes, and the rest of the respiratory tract.
• How the respiratory tract protects itself from pathogens and other potentially harmful substances in the air.
• How the rate of breathing is regulated to maintain homeostasis of blood gases and pH.
• How ventilation, or breathing, allows us to inhale air into the body and exhale air out of the body.
• The conscious and unconscious control of breathing.
• Nasal breathing compared to mouth breathing.
• What happens when a person is drowning.
• How gas exchange occurs between the air and blood in the alveoli of the lungs, and between the blood and cells throughout the body.
• Disorders of the respiratory system, including asthma, pneumonia, chronic obstructive pulmonary disease (COPD), and lung cancer.
• The negative health effects of smoking.
As you read the chapter, think about the following questions:
1. Where are the bronchial tubes, and what is their function?
2. What is the function of mucus, and why can too much mucus be a bad thing?
3. Why did Dr. Tsosie check Sacheen’s blood oxygen level?
4. Why do you think Dr. Tsosie warned Sacheen not to take cough suppressant medications?
5. How does acute bronchitis compare to chronic bronchitis, and how do they both relate to smoking?
Attributions
1. Coughing by GabboT, CC BY-SA 2.0, via Wikimedia Commons
2. Wrist oximeter by UusiAjaja, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/16%3A_Respiratory_System/16.1%3A_Case_Study%3A_Respiratory_System_and_Gas_Exchange.txt |
Seeing Your Breath
Why can you “see your breath” on a cold day? The air you exhale through your nose and mouth is warm, like the inside of your body. Exhaled air also contains a lot of water vapor because it passes over moist surfaces from the lungs to the nose or mouth. The water vapor in your breath cools suddenly when it reaches the much colder outside air. This causes the water vapor to condense into a fog of tiny droplets of liquid water. You release water vapor and other gases from your body through the process of respiration.
What is Respiration?
Respiration is the life-sustaining process in which gases are exchanged between the body and the outside atmosphere. Specifically, oxygen moves from the outside air into the body; and water vapor, carbon dioxide, and other waste gases move from inside the body into the outside air. Respiration is carried out mainly by the respiratory system. It is important to note that respiration by the respiratory system is not the same process as cellular respiration that occurs inside cells, although the two processes are closely connected. Cellular respiration is the metabolic process in which cells obtain energy, usually by “burning” glucose in the presence of oxygen. When cellular respiration is aerobic, it uses oxygen and releases carbon dioxide as a waste product. Respiration by the respiratory system supplies the oxygen needed by cells for aerobic cellular respiration and removes the carbon dioxide produced by cells during cellular respiration.
Respiration by the respiratory system actually involves two subsidiary processes. One process is ventilation or breathing. This is the physical process of conducting air to and from the lungs. The other process is gas exchange. This is the biochemical process in which oxygen diffuses out of the air and into the blood while carbon dioxide and other waste gases diffuse out of the blood and into the air. All of the organs of the respiratory system are involved in breathing, but only the lungs are involved in gas exchange.
Respiratory Organs
The organs of the respiratory system form a continuous system of passages called the respiratory tract, through which air flows into and out of the body. The respiratory tract has two major divisions: the upper respiratory tract and the lower respiratory tract. The organs in each division are shown in Figure \(2\). In addition to these organs, certain muscles of the thorax (the body cavity that fills the chest) are also involved in respiration by enabling breathing. Most important is a large muscle called the diaphragm, which lies below the lungs and separates the thorax from the abdomen. Smaller muscles between the ribs also play a role in breathing. You can learn more about breathing muscles in the concept of Breathing.
Upper Respiratory Tract
All of the organs and other structures of the upper respiratory tract are involved in the conduction or the movement of air into and out of the body. Upper respiratory tract organs provide a route for air to move between the outside atmosphere and the lungs. They also clean, humidity, and warm the incoming air. However, no gas exchange occurs in these organs.
Nasal Cavity
The nasal cavity is a large, air-filled space in the skull above and behind the nose in the middle of the face. It is a continuation of the two nostrils. As inhaled air flows through the nasal cavity, it is warmed and humidified. Hairs in the nose help trap larger foreign particles in the air before they go deeper into the respiratory tract. In addition to its respiratory functions, the nasal cavity also contains chemoreceptors that are needed for the sense of smell and that contribute importantly to the sense of taste.
Pharynx
The pharynx is a tube-like structure that connects the nasal cavity and the back of the mouth to other structures lower in the throat, including the larynx. The pharynx has dual functions: both air and food (or other swallowed substances) pass through it, so it is part of both the respiratory and digestive systems. Air passes from the nasal cavity through the pharynx to the larynx (as well as in the opposite direction). Food passes from the mouth through the pharynx to the esophagus.
Larynx
The larynx connects the pharynx and trachea and helps to conduct air through the respiratory tract. The larynx is also called the voice box because it contains the vocal cords, which vibrate when air flows over them, thereby producing sound. You can see the vocal cords in the larynx in Figure \(3\). Certain muscles in the larynx move the vocal cords apart to allow breathing. Other muscles in the larynx move the vocal cords together to allow the production of vocal sounds. The latter muscles also control the pitch of sounds and help control their volume.
A very important function of the larynx is protecting the trachea from aspirated food. When swallowing occurs, the backward motion of the tongue forces a flap called the epiglottis to close over the entrance to the larynx. You can see the epiglottis in Figure \(3\). This prevents swallowed material from entering the larynx and moving deeper into the respiratory tract. If swallowed material does start to enter the larynx, it irritates the larynx and stimulates a strong cough reflex. This generally expels the material out of the larynx and into the throat.
Lower Respiratory Tract
The trachea and other passages of the lower respiratory tract conduct air between the upper respiratory tract and the lungs. These passages form an inverted tree-like shape (Figure \(4\)), with repeated branching as they move deeper into the lungs. All told, there are an astonishing 1,500 miles of airways conducting air through the human respiratory tract! It is only in the lungs, however, that gas exchange occurs between the air and the bloodstream.
Trachea
The trachea, or windpipe, is the widest passageway in the respiratory tract. It is about 2.5 cm (1 in.) wide and 10-15 cm (4-6 in.) long. It is formed by rings of cartilage, which make it relatively strong and resilient. The trachea connects the larynx to the lungs for the passage of air through the respiratory tract. The trachea branches at the bottom to form two bronchial tubes.
Bronchi and Bronchioles
There are two main bronchial tubes, or bronchi (singular, bronchus), called the right and left bronchi. The bronchi carry air between the trachea and lungs. Each bronchus branches into smaller, secondary bronchi; and secondary bronchi branch into still smaller tertiary bronchi. The smallest bronchi branch into very small tubules called bronchioles. The tiniest bronchioles end in alveolar ducts, which terminate in clusters of minuscule air sacs, called alveoli (singular, alveolus), in the lungs.
Lungs
The lungs are the largest organs of the respiratory tract. They are suspended within the pleural cavity of the thorax. In Figure \(5\), you can see that each of the two lungs is divided into sections. These are called lobes, and they are separated from each other by connective tissues. The right lung is larger and contains three lobes. The left lung is smaller and contains only two lobes. The smaller left lung allows room for the heart, which is just left of the center of the chest.
Lung tissue consists mainly of alveoli (Figure \(6\)). These tiny air sacs are the functional units of the lungs where gas exchange takes place. The two lungs may contain as many as 700 million alveoli, providing a huge total surface area for gas exchange to take place. In fact, alveoli in the two lungs provide as much surface area as half a tennis court! Each time you breathe in, the alveoli fill with air, making the lungs expand. Oxygen in the air inside the alveoli is absorbed by the blood in the mesh-like network of tiny capillaries that surrounds each alveolus. The blood in these capillaries also releases carbon dioxide into the air inside the alveoli. Each time you breathe out, air leaves the alveoli and rushes into the outside atmosphere, carrying waste gases with it.
The lungs receive blood from two major sources. They receive deoxygenated blood from the heart. This blood absorbs oxygen in the lungs and carries it back to the heart to be pumped to cells throughout the body. The lungs also receive oxygenated blood from the heart that provides oxygen to the cells of the lungs for cellular respiration.
Protecting the Respiratory System
You may be able to survive for weeks without food and for days without water, but you can survive without oxygen for only a matter of minutes except under exceptional circumstances. Therefore, protecting the respiratory system is vital. That’s why making sure a patient has an open airway is the first step in treating many medical emergencies. Fortunately, the respiratory system is well protected by the ribcage of the skeletal system. However, the extensive surface area of the respiratory system is directly exposed to the outside world and all its potential dangers in inhaled air. Therefore, it should come as no surprise that the respiratory system has a variety of ways to protect itself from harmful substances such as dust and pathogens in the air.
The main way the respiratory system protects itself is called the mucociliary escalator. From the nose through the bronchi, the respiratory tract is covered in the epithelium that contains mucus-secreting goblet cells. The mucus traps particles and pathogens in the incoming air. The epithelium of the respiratory tract is also covered with tiny cell projections called cilia (singular, cilium), as shown in Figure \(7\). The cilia constantly move in a sweeping motion upward toward the throat, moving the mucus and trapped particles and pathogens away from the lungs and toward the outside of the body.
What happens to the material that moves up the mucociliary escalator to the throat? It is generally removed from the respiratory tract by clearing the throat or coughing. Coughing is a largely involuntary response of the respiratory system that occurs when nerves lining the airways are irritated. The response causes air to be expelled forcefully from the trachea, helping to remove mucus and any debris it contains (called phlegm) from the upper respiratory tract to the mouth. The phlegm may spit out (expectorated), or it may be swallowed and destroyed by stomach acids.
Sneezing is a similar involuntary response that occurs when nerves lining the nasal passage are irritated. It results in forceful expulsion of air from the mouth, which sprays millions of tiny droplets of mucus and other debris out of the mouth and into the air, as shown in Figure \(8\). This explains why it is so important to sneeze into a sleeve rather than the air to help prevent the transmission of respiratory pathogens.
How the Respiratory System Works with Other Organ Systems
The amount of oxygen and carbon dioxide in the blood must be maintained within a limited range for the survival of the organism. Cells cannot survive for long without oxygen, and if there is too much carbon dioxide in the blood, the blood becomes dangerously acidic (pH is too low). Conversely, if there is too little carbon dioxide in the blood, the blood becomes too basic (pH is too high). The respiratory system works hand-in-hand with the nervous and cardiovascular systems to maintain homeostasis in blood gases and pH.
It is the level of carbon dioxide rather than the level of oxygen that is most closely monitored to maintain blood gas and pH homeostasis. The level of carbon dioxide in the blood is detected by cells in the brain, which speed up or slow down the rate of breathing through the autonomic nervous system as needed to bring the carbon dioxide level within the normal range. Faster breathing lowers the carbon dioxide level (and raises the oxygen level and pH); slower breathing has the opposite effects. In this way, the levels of carbon dioxide and oxygen, as well as pH, are maintained within normal limits.
The respiratory system also works closely with the cardiovascular system to maintain homeostasis. The respiratory system exchanges gases between the blood and the outside air, but it needs the cardiovascular system to carry them to and from body cells. Oxygen is absorbed by the blood in the lungs and then transported through a vast network of blood vessels to cells throughout the body where it is needed for aerobic cellular respiration. The same system absorbs carbon dioxide from cells and carries it to the respiratory system for removal from the body.
Feature: My Human Body
Choking is the mechanical obstruction of the flow of air from the atmosphere into the lungs. It prevents breathing and may be partial or complete. Partial choking allows some though inadequate airflow into the lung—prolonged or complete choking results in asphyxia, or suffocation, which is potentially fatal.
Obstruction of the airway typically occurs in the pharynx or trachea. Young children are more prone to choking than are older people, in part because they often put small objects in their mouths and do not appreciate the risk of choking that they pose. Young children may choke on small toys or parts of toys or on household objects in addition to food. Foods that can adapt their shape to that of the pharynx, such as bananas and marshmallows, are especially dangerous and may cause choking in adults as well as children.
How can you tell if a loved one is choking? The person cannot speak or cry out or has great difficulty doing so. Breathing, if possible, is labored, producing gasping or wheezing. The person may desperately clutch at his or her throat or mouth. If breathing is not soon restored, the person’s face will start to turn blue from lack of oxygen. This will be followed by unconsciousness if oxygen deprivation continues beyond a few minutes.
If an infant is choking, turning the baby upside down and slapping on the back may dislodge the obstructing object. To help an older person who is choking, first, encourage the person to cough. Give them a few hardback slaps to help force the lodged object out of the airway. If these steps fail, perform the Heimlich maneuver on the person. You can easily find instructional videos online to learn how to do it. If the Heimlich maneuver also fails, call for emergency medical care immediately.
Review
1. What is respiration, as carried out by the respiratory system? Name the two subsidiary processes it involves.
2. Describe the respiratory tract.
3. Identify the organs of the upper respiratory tract, and state their functions.
4. List the organs of the lower respiratory tract. Which organs are involved only in conduction?
5. Where does gas exchange take place?
6. How does the respiratory system protect itself from potentially harmful substances in the air?
7. Explain how the rate of breathing is controlled.
8. Why does the respiratory system need the cardiovascular system to help it perform its main function of gas exchange?
9. Place the following organs or structures of the respiratory system in order of when they are encountered by air entering the body — from earliest to latest.
trachea; nasal cavity; alveoli; bronchioles; larynx; bronchi; pharynx
10. Which organ is part of both the digestive and respiratory systems?
A. Larynx
B. Trachea
C. Pharynx
D. Bronchus
11. Describe two ways in which the body prevents food from entering the lungs.
12. True or False. The lungs receive some oxygenated blood.
13. True or False. Gas exchange occurs in both the upper and lower respiratory tracts.
14. Coughing can expel ___________ from the body.
A. mucus
B. food particles
C. phlegm
D. All of the above
15. What is the relationship between respiration and cellular respiration?
Attributions
1. Snowboarders breath on a cold day by Alain Wong via Unsplash License
2. Conducting Passages by Lord Akryl, Jmarchn, public domain via Wikimedia Commons
3. Larynx by Alan Hoofring, National Cancer Institute, public domain via Wikimedia Commons
4. Lung Diagram by Patrick J. Lynch; CC BY 2.5 via Wikimedia Commons
5. Lung Structure by National Heart Lung and Blood Institute, public domain via Wikimedia Commons
6. Alveoli by helix84 licensed CC BY 2.5, via Wikimedia Commons
7. Ciliated Epithelium by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
8. Sneeze by James Gathany, CDC, public domain via Wikimedia Commons
9. Abdominal Thrusts by Amanda M. Woodhead, public domain via Wikimedia Commons
10. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/16%3A_Respiratory_System/16.2%3A_Structure_and_Function_of_the_Respiratory_System.txt |
Doing the ‘Fly
The swimmer in this photo is doing the butterfly stroke. This swimming style requires the swimmer to carefully control his breathing so it is coordinated with his swimming movements. Breathing is the process of moving air into and out of the lungs, which are the organs in which gas exchange takes place between the atmosphere and the body. Breathing is also called ventilation, and it is one of two parts of the life-sustaining process of respiration, the other part being gas exchange. Before you can understand how breathing is controlled, you need to know how breathing occurs.
How Breathing Occurs
Breathing is a two-step process that includes drawing air into the lungs, or inhaling, and letting the air out of the lungs, or exhaling. Both processes are illustrated in Figure \(2\).
Inhaling
Inhaling is an active process that results mainly from the contraction of a muscle called the diaphragm, shown in Figure \(2\). The diaphragm is a large, dome-shaped muscle below the lungs that separates the thoracic (chest) and abdominal cavities. When the diaphragm contracts, the thoracic cavity expands and the contents of the abdomen are pushed downward. Other muscles, such as external intercostal muscles between the ribs, also contribute to the process of inhalation, especially when inhalation is forced, as when taking a deep breath. These muscles help increase thoracic volume by expanding the ribs outward. With the chest expanded, there is lower air pressure inside the lungs than outside the body, so outside air flows into the lungs via the respiratory tract.
Exhaling
Exhaling involves the opposite series of events. The diaphragm relaxes, so it moves upward and decreases the volume of the thorax ( Figure \(2\). Air pressure inside the lungs increases so it is higher than the air pressure outside the lungs. Exhaling, unlike inhaling, is typically a passive process that occurs mainly due to the elasticity of the lungs. With the change in air pressure, the lungs contract to their pre-inflated size, forcing out the air they contain in the process. Air flows out of the lungs, similar to the way air rushes out of a balloon when it is released. If exhalation is forced, internal intercostal and abdominal muscles may help move the air out of the lungs.
Control of Breathing
Breathing is one of the few vital bodily functions that can be controlled consciously as well as unconsciously. Think about using your breath to blow up a balloon. You take a long, deep breath, and then you exhale the air as forcibly as you can into the balloon. Both the inhalation and exhalation are consciously controlled.
Conscious Control of Breathing
You can control your breathing by holding your breath, slowing your breathing, or hyperventilating, which is breathing more quickly and shallowly than necessary. You can also exhale or inhale more forcefully or deeply than usual. Conscious control of breathing is common in many activities besides blowing up balloons, including swimming, speech training, singing, playing many different musical instruments ( Figure \(3\)), and doing yoga, to name just a few.
There are limits on the conscious control of breathing. For example, it is not possible for a healthy person to voluntarily stop breathing indefinitely. Before long, there is an irrepressible urge to breathe. If you were able to stop breathing for a long enough time, you would lose consciousness. The same thing would happen if you were to hyperventilate for too long. Once you lose consciousness so you can no longer exert conscious control over your breathing, involuntary control of breathing takes over.
Unconscious Control of Breathing
Unconscious breathing is controlled by respiratory centers in the medulla and pons of the brainstem ( Figure \(4\)). The respiratory centers automatically and continuously regulate the rate of breathing depending on the body’s needs. These are determined mainly by blood acidity or pH. When you exercise, for example, carbon dioxide levels increase in the blood because of increased cellular respiration by muscle cells. The carbon dioxide reacts with water in the blood to produce carbonic acid, making the blood more acidic, so pH falls. The drop in pH is detected by chemoreceptors in the medulla. Blood levels of oxygen and carbon dioxide, in addition to pH, are also detected by chemoreceptors in major arteries, which send the “data” to the respiratory centers. The respiratory center responds by sending nerve impulses to the diaphragm, “telling” it to contract more quickly so the rate of breathing speeds up. With faster breathing, more carbon dioxide is released into the air from the blood, and blood pH returns to the normal range.
The opposite events occur when the level of carbon dioxide in the blood becomes too low and blood pH rises. This may occur with involuntary hyperventilation, which can happen in panic attacks, episodes of severe pain, asthma attacks, and many other situations. When you hyperventilate, you blow off a lot of carbon dioxide, leading to a drop in blood levels of carbon dioxide. The blood becomes more basic (alkaline), causing its pH to rise.
Nasal vs. Mouth Breathing
Nasal breathing is breathing through the nose rather than the mouth, and it is generally considered to be superior to mouth breathing. The hair-lined nasal passages do a better job of filtering particles out of the air before it moves deeper into the respiratory tract. The nasal passages are also better at warning and moistening the air, so nasal breathing is especially advantageous in the winter when the air is cold and dry. In addition, the smaller diameter of the nasal passages creates greater pressure in the lungs during exhalation. This slows the emptying of the lungs, giving them more time to extract oxygen from the air.
Feature: Myth vs. Reality
Drowning is defined as respiratory impairment from being in or under a liquid. It is further classified according to its outcome into death, ongoing health problems, or no ongoing health problems (full recovery). In the United States, accidental drowning is the second leading cause of death (after motor vehicle crashes) in children aged 12 years and younger. There are some potentially dangerous myths about drowning. Knowing what they are might save your life or the life of a loved one, especially a child.
Myth: People drown when they aspirate water into their lungs.
Reality: Generally, in the early stages of drowning, very little water enters the lungs. A small amount of water entering the trachea causes a muscular spasm in the larynx that seals the airway and prevents the passage of water into the lungs. This spasm is likely to last until unconsciousness occurs.
Myth: You can tell when someone is drowning because they will shout for help and wave their arms to attract attention.
Reality: The muscular spasm that seals the airway prevents the passage of air as well as water, so a person who is drowning is unable to shout or call for help. In addition, instinctive reactions that occur in the final minute or so before a drowning person sinks under the water may look similar to calm, safe behavior. The head is likely to be low in the water, tilted back with the mouth open. The person may have uncontrolled movements of the arms and legs, but they are unlikely to be visible above the water.
Myth: It is too late to save a person who is unconscious in the water.
Reality: An unconscious person rescued with an airway still sealed from the muscular spasm of the larynx stands a good chance of full recovery if they start receiving CPR within minutes. Without water in the lungs, CPR is much more effective. Even if the cardiac arrest has occurred so the heart is no longer beating, there is still a chance of recovery. However, the longer the brain goes without oxygen, the more likely brain cells will die. Brain death is likely after about six minutes without oxygen, except in exceptional circumstances, such as young people drowning in very cold water. There are examples of children surviving, apparently without lasting ill effects, for as long as an hour in cold water (see Explore More below for an example). Therefore, rescuers retrieving a child from cold water should attempt resuscitation even after a protracted period of immersion.
Myth: If someone is drowning, you should start administering CPR immediately, even before you try to get the person out of the water.
Reality: Removing a drowning person from the water is the first priority because CPR is ineffective in the water. The goal should be to bring the person to stable ground as quickly as possible and then to start CPR.
Myth: You are unlikely to drown unless you are in water over your head.
Reality: Depending on circumstances, people have drowned in as little as 30 mm (about 1 ½ in.) of water. For example, inebriated people or those under the influence of drugs have been known to have drowned in puddles. Hundreds of children have drowned in the water in toilets, bathtubs, basins, showers, pails, and buckets (see figure below).
Review
1. Define breathing.
2. What is another term for the process of breathing?
3. What is the main difference between the processes of inhaling and exhaling?
4. Give examples of activities in which breathing is consciously controlled.
5. Young children sometimes threaten to hold their breath until they get something they want. Why is this an idle threat?
6. Explain how unconscious breathing is controlled.
7. Why is nasal breathing generally considered to be superior to mouth breathing?
8. For each of the following, indicate whether it occurs during the process of inhalation (I) or exhalation (E).
1. The diaphragm moves downward.
2. The diaphragm relaxes.
3. The thoracic cavity becomes smaller.
4. The air pressure in the lungs is lower than outside the body.
9. Give one example of a situation that would cause blood pH to rise excessively and explain why this occurs.
10. Blood levels of oxygen and carbon dioxide and pH are detected by which of these:
1. Mechanoreceptors
2. Chemoreceptors
3. Lung receptors
4. Carbon receptors
11. True or False. The diaphragm can contract due to conscious control.
12. True or False. Hypoventilating is breathing that is fast and shallow.
Explore More
You may have heard of “miracles” in which young people survived for extended periods of time without breathing underwater and made a full recovery. How does this happen? Read the amazing story of an Italian boy who survived for 42 minutes underwater. The article explains the physiology behind the “miracle.”
Magician and stuntman extraordinaire David Blaine reportedly can hold his breath for 17 minutes underwater. In this TED talk, he explains how he manages to perform this feat:
Attributions
1. Butterfly stroke by Cpl. Jasper Schwartz, public domain via Wikimedia Commons
2. Breathing by Zachary Wilson from CK-12 licensed CC BY-NC 3.0
3. Ivan Podyomov by Alexei Zoubov, public domain via Wikimedia Commons
4. Respiratory Centers of the Brain by OpenStax College, CC BY 3.0, via Wikimedia Commons
5. Drowning Situations by U.S. Consumer Product Safety Commission, public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/16%3A_Respiratory_System/16.3%3A_Breathing.txt |
A “Mitey” Monster
The scary beast in Figure \(1\) is likely to be lurking in your own home, where it feeds on organic debris, including human skin. What is it? It’s the common dust mite, a close relative of spiders. The dust mite is so small that it is barely visible with the unaided eye, so it’s obviously shown above greatly enlarged. If you think you can get rid of dust mites in your home by frequent and thorough cleaning, think again. There may be thousands of dust mites in just one gram of dust! Regardless of how clean you keep your house, you can't eliminate dust mites entirely. So why even bother trying? The feces of dust mites contain proteins that are a common trigger of asthma attacks.
Asthma
Asthma is a chronic inflammatory disease of the airways in the lungs, in which the airways periodically become inflamed. As you can see in Figure \(2\), this causes swelling and narrowing of the airways, often accompanied by excessive mucus production. Symptoms of asthma include difficulty breathing, coughing, wheezing, shortness of breath, and chest tightness. Some people with asthma rarely experience symptoms, and then usually only in response to certain triggers in the environment. Other people may have symptoms almost all of the time.
Asthma is thought to be caused by a combination of genetic and environmental factors. A person with a family history of asthma is more likely to develop the disease. Dozens of genes have been found to be associated with asthma, many of which are related to the immune system. Additional risk factors include obesity and sleep apnea (see the feature My Human Body below). Environmental factors trigger asthma attacks in people who have a genetic predisposition to the disease. Besides dust mite feces, triggers may include other allergens (such as pet dander, cockroaches, and mold), certain medications including aspirin, air pollution, and stress, among other possible factors. Symptoms tend to be worse at night and early in the morning. They may also worsen during upper respiratory tract infections, strenuous exercise, or when the airways are exposed to cold air.
There is no cure for asthma at present, but the symptoms of asthma attacks usually can be reversed with the use of inhaled medications called bronchodilators. These medications soothe the constricted air passages and help to re-expand them, making breathing easier. The medications usually start to take effect almost immediately. Other medications can be taken for long-term control of the disease. These medications help prevent asthma attacks from occurring. Corticosteroids are generally considered the most effective treatment for long-term control. Another way to prevent asthma attacks is by avoiding triggers whenever possible.
Pneumonia
Another common inflammatory disease of the respiratory tract is pneumonia. In pneumonia, the inflammation affects primarily the alveoli, which are the tiny air sacs of the lungs. Inflammation causes some of the alveoli to become filled with fluid so that gas exchange cannot occur. This is illustrated in Figure \(3\). Symptoms of pneumonia typically include coughing, chest pain, difficulty breathing, and fever.
Pneumonia often develops as a consequence of an upper respiratory tract infection such as the common cold or flu, especially in the very young and the elderly. It is usually caused by bacteria or viruses, although some cases may be caused by other microorganisms such as fungi. The majority of cases are caused by just a few pathogens, the most common being the bacterium Streptococcus pneumoniae. Pneumonia is more likely to develop in people who have other lung diseases such as asthma, a history of smoking, heart failure, or a weakened immune system.
Vaccines are available to prevent certain types of bacterial and viral pneumonia, including pneumonia caused by Streptococcus pneumoniae. Treatment of pneumonia depends on the cause. For example, if it is caused by bacteria, antibiotics are generally prescribed. In cases of severe pneumonia, hospitalization and supplemental oxygen may be required.
Chronic Obstructive Pulmonary Disease
Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by chronic poor airflow. The main symptoms include shortness of breath and a cough that produces phlegm. These symptoms are usually present for a long period of time and typically become worse over time. Eventually, walking upstairs and similar activities become difficult because of shortness of breath.
COPD formerly was referred to as chronic bronchitis or emphysema. Now, the term chronic bronchitis is used to refer to the symptoms of COPD, and the term emphysema is used to refer to the lung changes that occur with COPD. Some of these lung changes are shown in Figure \(4\). They include a breakdown of connective tissues that reduces the number and elasticity of alveoli. As a result, the patient can no longer fully exhale air from the lungs, so the air becomes trapped in the lungs. Gas exchange is hampered and may lead to low oxygen levels and too much carbon dioxide in the blood.
Tobacco smoking is the major cause of COPD, with a number of other factors such as air pollution and genetics playing smaller roles. Of people who are life-long smokers, about half will eventually develop COPD. Exposure to secondhand smoke in nonsmokers also increases the risk of COPD and accounts for about 20 percent of cases. Most cases of COPD could have been prevented by never smoking. In people who have already been diagnosed with COPD, cessation of smoking can slow down the rate at which COPD worsens. People with COPD may be treated with supplemental oxygen and inhaled bronchodilators. These treatments may reduce the symptoms but there is no cure for COPD except, in very severe cases, lung transplantation (see the Explore More video below).
Lung Cancer
Lung cancer is a malignant tumor characterized by uncontrolled cell growth in tissues of the lung. The tumor may arise directly from lung tissue (primary lung cancer) or as a result of metastasis from cancer in another part of the body (secondary lung cancer). Primary lung cancer may also metastasize and spread to other parts of the body. Lung cancer develops following genetic damage to DNA that affects the normal functions of the cell. As more damage accumulates, the risk of cancer increases. The most common symptoms of lung cancer include coughing (especially coughing up blood), wheezing, shortness of breath, chest pain, and weight loss.
The major cause of primary lung cancer is tobacco smoking, which accounts for about 85 percent of cases. Cigarette smoke contains numerous cancer-causing chemicals. Besides smoking, other potential causes of lung cancer include exposure to radon gas, asbestos, secondhand smoke, or other air pollutants. When tobacco smoking is combined with other risk factors such as exposure to radon or asbestos, the risk of lung cancer is heightened. People who have close biological relatives with lung cancer are also at increased risk of developing the disease.
Most cases of lung cancer cannot be cured. In many people, cancer has already spread beyond the original site by the time they have symptoms and seek medical attention. About 10 percent of people with lung cancer do not have symptoms when they are diagnosed, and the cancers are found when they have a chest X-ray for another problem. In part because of its typically late diagnosis, lung cancer is the most common cause of cancer-related death in men and the second most common cause in women (after breast cancer). Common treatments for lung cancer include surgical removal of the tumor, radiation therapy, chemotherapy, or some combination of these three types of treatment.
Feature: My Human Body
Do you — or someone you love — snore? Snoring may be more than just an annoyance. It may also be a sign of a potentially dangerous and common disorder known as sleep apnea. Sleep apnea is characterized by pauses in breathing that occur most often because of physical blockage to airflow during sleep. When breathing is paused, carbon dioxide builds up in the bloodstream. The higher-than-normal level of carbon dioxide in the blood causes the respiratory centers in the brain to wake the person enough to start breathing normally. This reduces the carbon dioxide level, and the person falls back asleep. This occurs repeatedly throughout the night, causing serious disruption in sleep. Most people with sleep apnea are unaware that they have the disorder because they don’t awake fully enough to remember the repeated awakenings throughout the night. Instead, sleep apnea is more commonly recognized by other people who witness the episodes.
Figure \(5\)shows how sleep apnea typically occurs. The muscle tone of the body normally relaxes during sleep, allowing the soft tissues in the throat to collapse and block the airway. The relaxation of muscles may be exacerbated by the use of alcohol, tranquilizers, or muscle relaxants. The risk of sleep apnea is greater in people who are overweight, smoke tobacco, or have diabetes. The disorder is also more likely to occur in older people and males. Common symptoms of sleep apnea include loud snoring, restless sleep, and daytime sleepiness and fatigue. Daytime sleepiness, in turn, increases the risk of driving and work-related accidents. Continued sleep deprivation may cause moodiness and belligerence. Lack of adequate oxygen to the body because of sleep apnea may also lead to other health problems including fatty liver diseases and high blood pressure. Symptoms of sleep apnea may be present for years or even decades until (and if) a diagnosis is finally made.
Treatment of sleep apnea may include avoiding alcohol, quitting smoking, or losing weight. Elevating the upper body during sleep or sleeping on one’s side may help prevent airway collapse in many people with sleep apnea. Another type of treatment is the use of an oral device during sleep that shifts the lower jaw forward to help keep the airway open. The most common treatment for moderate to severe sleep apnea is the use during sleep of CPAP (continuous positive airway pressure), which keeps the airway open by means of pressurized air. In this treatment, the person typically wears a plastic facial mask that is connected by a flexible tube to a small bedside CPAP machine. Although CPAP is effective, long-term compliance is often poor because patients find the mask uncomfortable or they experience unpleasant side effects such as dry mouth and nose. A more extreme form of treatment is surgery to remove some of the tissues — such as the tonsils or part of the soft palate — that tend to collapse and block the airway in people with sleep apnea.
Review
1. What is asthma, and what are its symptoms and causes?
2. Identify common risk factors and triggers of asthma attacks.
3. How can asthma attacks be prevented or controlled?
4. What are the causes and symptoms of pneumonia?
5. How can pneumonia be prevented? How is it treated?
6. Define COPD. How is it related to chronic bronchitis and emphysema?
7. Relate COPD to tobacco smoking.
8. What is the difference between primary and secondary lung cancer?
9. What is the major cause of primary lung cancer?
10. Discuss lung cancer as a cause of death.
11. How is lung cancer treated?
12. Define sleep apnea.
13. What is the difference between how COPD and pneumonia affect the alveoli?
Attributions
1. Dust mite by FDA, public domain via Wikimedia Commons
2. Asthma attach by United States-National Institute of Health: National Heart, Lung, Blood Institute, public domain via Wikimedia Commons
3. New Pneumonia cartoon public domain via Wikimedia Commons
4. COPD by National Heart Lung and Blood Institute, public domain via Wikimedia Commons
5. Obstruction of ventilation by Habib M’henni via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/16%3A_Respiratory_System/16.4%3A_Disorders_of_the_Respiratory_System.txt |
Sure Death
The anti-smoking cartoon in Figure \(1\) clearly makes the point that smoking causes death. The cartoon is not using hyperbole, because smoking actually is deadly. It causes about six million deaths each year and is the single greatest cause of preventable death worldwide. As many as half of all people who smoke tobacco die from it. As a result of smoking’s deadly effects, the life expectancy of long-term smokers is significantly less than that of non-smokers. In fact, long-term smokers can expect their lifespan to be reduced by as much as 18 years, and they are three times as likely to die before the age of 70 as non-smokers.
Why Is Smoking Deadly?
As shown in Figure \(2\), tobacco smoking has adverse effects on just about every bodily system and organ. The detrimental health effects of smoking depend on the number of years that a person smokes and how much the person smokes. Contrary to popular belief, all forms of tobacco smoke — including smoke from cigars and tobacco pipes — have similar health risks as those of cigarette smoke. Smokeless tobacco may be less of a danger to the lungs and heart, but it too has serious health effects. It significantly increases the risk of cancers of the mouth and throat, among other health problems.
Even non-smokers may not be spared the deadly risks of tobacco smoke. If you spend time around smokers either at home or on the job, then you are at risk of the dangers of secondhand smoke. Secondhand smoke enters the air directly from burning cigarettes (and cigars and pipes) and indirectly from the lungs of smokers. This smoke may linger in indoor air for hours and increase the risk of a wide range of adverse health effects. For example, non-smokers who are exposed to secondhand smoke may have as much as a 30 percent increase in their risk of lung cancer and heart disease. The 2014 U.S. Surgeon General’s Report concluded that there is no established risk-free level of exposure to secondhand smoke.
Tobacco contains nicotine, which is a psychoactive drug. Although nicotine in tobacco smoke does not directly cause cancer or most of the other health risks of smoking, it is a highly addictive drug. In fact, nicotine is even more addictive than cocaine or heroin. The addictive nature of nicotine explains why it is so difficult for smokers to quit the habit even when they know the health risks and really want to stop smoking. The good news is that if someone does stop smoking, his or her risks of smoking-related diseases and death soon start to fall. For example, by one year after quitting, the risk of heart disease drops to only half of that of a continuing smoker.
Smoking and Cancer
One of the main health risks of smoking is cancer, particularly cancer of the lung. Because of the increased risk of lung cancer with smoking, the risk of dying from lung cancer before age 85 is more than 20 times higher for a male smoker than for a male non-smoker. As the rate of smoking increases, so does the rate of lung cancer deaths, although the effects of smoking on lung cancer deaths can take up to 20 years to manifest themselves, as shown in Figure \(3\).
Besides lung cancer, several other forms of cancer are also significantly more likely in smokers than non-smokers, including cancers of the kidney, larynx, mouth, lip, tongue, throat, bladder, esophagus, pancreas, and stomach. Unfortunately, many of these cancers have extremely low cure rates.
When you consider the composition of tobacco smoke, it’s not surprising that it increases the risk of cancer. Tobacco smoke contains dozens of chemicals that have been proven to be carcinogens or causes of cancer. Many of these chemicals bind to DNA in a smoker’s cells and may either kill the cells or cause mutations. If the mutations inhibit programmed cell death, the cells can survive to become cancer cells. Some of the most potent carcinogens in tobacco smoke include benzopyrene, acrolein, and nitrosamines. Other carcinogens in tobacco smoke are radioactive isotopes, including lead-210 and polonium-210.
Respiratory Effects of Smoking
Long-term exposure to the compounds found in cigarette smoke, such as carbon monoxide and cyanide, is thought to be responsible for much of the lung damage caused by smoking. These chemicals reduce the elasticity of alveoli, leading to chronic obstructive pulmonary disease (COPD). COPD is a permanent, incurable, and often fatal reduction in the capacity of the lungs, reducing the ability of the lungs to fully exhale air. The chronic inflammation that is also present in COPD is exacerbated by the tobacco smoke carcinogen acrolein and its derivatives. COPD is almost completely preventable simply by not smoking and by also avoiding secondhand smoke.
Cardiovascular Effects of Smoking
Inhalation of tobacco smoke causes several immediate responses in the heart and blood vessels. Within one minute of inhalation of smoke, the heart rate begins to rise, increasing by as much as 30 percent during the first 10 minutes of smoking. Carbon monoxide in tobacco smoke binds with hemoglobin in red blood cells, thereby reducing the blood’s ability to carry oxygen. Hemoglobin bound to carbon monoxide forms such a stable complex that it may result in a permanent loss of red blood cell function. Several other chemicals in tobacco smoke lead to the narrowing and weakening of blood vessels and an increase in substances that contribute to blood clotting. These changes increase blood pressure and the chances of a blood clot forming and blocking a vessel, thereby elevating the risk of heart attack and stroke. A recent study found that smokers are five times more likely than non-smokers to have a heart attack before the age of 40.
Smoking has also been shown to have a negative impact on the levels of blood lipids. Total cholesterol levels tend to be higher in smokers than non-smokers. Ratios of “good” cholesterol to “bad” cholesterol tend to be lower in smokers than in non-smokers.
Additional Adverse Health Effects of Smoking
A wide diversity of additional adverse health effects are attributable to smoking. Here are just a few of them:
• Smokers are at a significantly increased risk of developing chronic kidney disease (in addition to kidney cancer). For example, smoking hastens the progression of kidney damage in people with diabetes.
• People who smoke, especially the elderly, have a greater risk of influenza and other infectious diseases than non-smokers. Smoking more than 20 cigarettes a day has been found to increase the risk of infectious diseases by as much as four times the risk in non-smokers. These effects occur because of damage to both the respiratory system and the immune system.
• In addition to oral cancer, smoking causes other oral problems including periodontitis (gum disease). Roughly half of the cases of gum inflammation are attributable to current or former smoking. Such inflammation increases the risk of tooth loss, which is also higher in smokers than non-smokers. In addition, smoking stains the teeth and causes halitosis (bad breath).
• Smoking is a key cause of erectile dysfunction (ED), probably because it leads to narrowing of arteries in the penis as it does elsewhere in the body. The incidence of ED is about 85 percent higher in males who smoke than it is in non-smokers.
• Smoking also has adverse effects on the female reproductive system, potentially causing infertility, in part because it interferes with the body’s ability to produce estrogen. Female smokers are about 60 percent more likely to be infertile than non-smokers. Pregnant women who smoke or are exposed to secondhand smoke have a higher risk of miscarriages and low-birth-weight infants.
• Certain therapeutic drugs, including some antidepressants and anticonvulsants, are less effective in smokers than in non-smokers. This occurs because smoking increases levels of liver enzymes that break down the drugs.
• Smoking causes an estimated 10 percent of all deaths due to fires worldwide. Smokers are also at greater risk of dying in motor vehicle crashes and other accidents.
• Smoking leads to an increased risk of bone fractures, especially of the hip. It also leads to slower wound healing after surgery and an increased rate of postoperative complications.
Feature: Human Biology in the News
The item in Figure \(4\) looks like a regular cigarette, but it’s actually an electronic cigarette or e-cigarette. E-cigarettes are battery-powered devices that change flavored liquids and nicotine into a vapor that is inhaled by the user. E-cigarettes are often promoted as being safer than traditional tobacco products and their use is touted as a good way to quit smoking. They are often not banned in smoke-free areas where it is illegal to smoke tobacco cigarettes.
A study completed in 2015 by researchers at the Harvard School of Public Health and widely reported in the mass media found that e-cigarettes may in fact be very harmful to the user’s health. E-cigarettes contain nicotine and cancer-causing chemicals such as formaldehyde. According to the study, about three-quarters of flavored e-cigarettes also contain a chemical named diacetyl that causes an incurable and potentially fatal disorder of the lungs, commonly called “popcorn lung” (bronchiolitis obliterans). In this disorder, the bronchioles compress and narrow due to the formation of scar tissue. This greatly diminishes the breathing capacity of people with the disorder. Popcorn lung gained its common name in 2004 when it was diagnosed in workers at popcorn factories. The buttery flavoring used in the factories contained diacetyl.
Some manufacturers of e-cigarettes and flavorings advertise that their products are now free of diacetyl. However, because e-cigarettes are not currently regulated by the FDA, there is no way of knowing for sure whether the products are actually safe. Equally disturbing is the appeal of flavored e-cigarettes to teens and the attempts of producers to specifically market their products to this age group. Flavors such as “cotton candy,” “Katy Perry’s cherry,” and “alien blood” are obviously marketed to youth. Not surprisingly, the use of e-cigarettes is on the rise in middle and high school students, who are more likely to use them than regular cigarettes. Public health officials fear that e-cigarettes will be a gateway for teens to move on to smoking tobacco cigarettes. Some states have recently passed laws prohibiting minors from buying e-cigarettes. As more questions are raised about their potential negative health effects, it is likely that more laws will be passed to regulate them. Watch the news for updates on this issue.
Review
1. What percentage of people who smoke are likely to die from it?
2. Contrast the life expectancy of long-term smokers and non-smokers.
3. What factors related to smoking determine how smoking affects a smoker’s health?
4. What are the two sources of secondhand cigarette smoke? How does exposure to secondhand smoke affect non-smokers?
5. Why is it so difficult for smokers to quit the habit? How is their health likely to be affected by quitting?
6. List five types of cancer that are significantly more likely in smokers than non-smokers.
7. Why does smoking cause cancer?
8. Explain how smoking causes COPD.
9. Identify some of the adverse effects of smoking on the cardiovascular system.
10. Give three examples of additional adverse health effects that are more likely with smoking.
11. Do you think e-cigarettes can be addictive? Explain your reasoning.
12. People who smoke are more likely to get which of these than people who do not smoke.
1. lung cancer
2. influenza
3. kidney disease
4. All of the above
13. Name three toxic chemicals present in tobacco smoke.
14. True or False. Nicotine is more addictive than heroin.
15. True or False. Smoking has many negative effects on the respiratory and cardiovascular systems, but not on other systems of the body.
Attributions
1. Have another by Wellcome Images, CC BY 4.0 via Wikimedia Commons
2. Risks from smoking by CDC, public domain via Wikimedia Commons
3. Smoking lung cancer correlation by Sakurambo, public domain via Wikimedia Commons
4. e-cigarette, public domain via pixy.org
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/16%3A_Respiratory_System/16.5%3A_Smoking_and_Health.txt |
Case Study Conclusion: Cough That Won't Quit
The little child shown in Figure \(1\) seems to be enjoying the air coming out of a humidifier. Inhaling the moist air from a humidifier or steamy shower can feel particularly good if you have a respiratory system infection, such as bronchitis. The moist air helps to loosen and thin mucus in the respiratory system, allowing you to breathe easier.
At the beginning of this chapter, you learned about Sacheen, who developed acute bronchitis after getting a cold. She had a worsening cough, sore throat due to coughing, and chest congestion. She was also coughing up thick mucus.
Acute bronchitis usually occurs after a cold or flu, usually due to the same viruses that cause cold or flu. Because bronchitis is not usually caused by bacteria (although it can be), antibiotics are not an effective treatment in most cases.
Bronchitis affects the bronchial tubes, which, as you have learned, are air passages in the lower respiratory tract. The main bronchi branch off of the trachea and then branch into smaller bronchi and then bronchioles. In bronchitis, the walls of the bronchi become inflamed, which makes them narrower. Also, there is excessive production of mucus in the bronchi, which further narrows the pathway through which can flow. Figure \(2\) shows how bronchitis affects the bronchial tubes.
The function of mucus is to trap pathogens and other potentially dangerous particles that enter the respiratory system from the air. However, when too much mucus is produced in response to an infection (as in the case of bronchitis), it can interfere with normal airflow. The body responds by coughing as it tries to rid itself of the pathogen-laden mucus.
The treatment for most cases of bronchitis involves thinning and loosening the mucus so that it can be effectively coughed out of the airways. This can be done by drinking plenty of fluids, using humidifiers or steam, and in some cases, using over-the-counter medications such as expectorants that are found in some cough medicines. This is why Dr. Tsosie recommended some of these treatments to Sacheen and also warned against using cough suppressants. Cough suppressants work on the nervous system to suppress the cough reflex. When a patient has a “productive” cough—i.e. they are coughing up mucus—doctors generally advise them to not take cough suppressants so that they can cough the mucus out of their bodies.
When Dr. Tsosie was examining Sacheen, she used a pulse oximeter to measure the oxygen level in her blood. Why did she do this? As you have learned, the bronchial tubes branch into bronchioles, which ultimately branch into the alveoli of the lungs. The alveoli are where gas exchange occurs between the air and the blood to take in oxygen and remove carbon dioxide and other wastes. By checking Sacheen’s blood oxygen level, Dr. Tsosie was making sure that her clogged airways were not impacting her level of much-needed oxygen.
Sacheen has acute bronchitis, but you may recall that chronic bronchitis was discussed earlier in this chapter as a term that describes the symptoms of chronic obstructive pulmonary disease (COPD). COPD is often due to tobacco smoking and causes damage to the walls of the alveoli, whereas acute bronchitis typically occurs after a cold or flu and involves inflammation and mucus build-up in the bronchial tubes. As implied by the difference in their names, chronic bronchitis is an ongoing, long-term condition, while acute bronchitis is likely to resolve relatively quickly with proper rest and treatment.
However, Sacheen smokes cigarettes, so she is more likely to develop chronic respiratory conditions such as COPD. As you have learned, smoking damages the respiratory system as well as many other systems of the body. Smoking increases the risk of respiratory infections, including bronchitis and flu, due to its damaging effects on the respiratory and immune systems. Dr. Tsosie strongly encouraged Sacheen to quit smoking, not only so that her acute bronchitis resolves, but so that she can avoid future infections and other negative health outcomes associated with smoking, including COPD and lung cancer.
As you have learned in this chapter, the respiratory system is critical to carry out the gas exchange necessary for life’s functions and to protect the body from pathogens and other potentially harmful substances in the air. But this ability to interface with the outside air has a cost. The respiratory system is prone to infections, as well as damage and other negative effects from allergens, mold, air pollution, and cigarette smoke. Although exposure to most of these things cannot be avoided, not smoking is an important step you can take to protect this organ system—as well as many other systems of your body.
Chapter Summary
In this chapter, you learned about the respiratory system. Specifically, you learned that:
• Respiration is the process in which oxygen moves from the outside air into the body and carbon dioxide and other waste gases move from inside the body into the outside air. It involves two subsidiary processes: ventilation and gas exchange.
• The organs of the respiratory system form a continuous system of passages called the respiratory tract. It has two major divisions: the upper respiratory tract and the lower respiratory tract.
• The upper respiratory tract includes the nasal cavity, pharynx, and larynx. All of these organs are involved in conduction or the movement of air into and out of the body. Incoming air is also cleaned, humidified, and warmed as it passes through the upper respiratory tract. The larynx is also called the voice box because it contains the vocal cords, which are needed to produce vocal sounds.
• The lower respiratory tract includes the trachea, bronchi and bronchioles, and the lungs. The trachea, bronchi, and bronchioles are involved in conduction. Gas exchange takes place only in the lungs, which are the largest organs of the respiratory tract. Lung tissue consists mainly of tiny air sacs called alveoli, which is where gas exchange takes place between the air in the alveoli and the blood in capillaries surrounding them.
• The respiratory system protects itself from potentially harmful substances in the air by the mucociliary escalator. This includes mucus-producing cells, which trap particles and pathogens in the incoming air. It also includes tiny hair-like cilia that continually move to sweep the mucus and trapped debris away from the lungs and toward the outside of the body.
• The level of carbon dioxide in the blood is monitored by cells in the brain. If the level becomes too high, it triggers a faster rate of breathing, which lowers the level to the normal range. The opposite occurs if the level becomes too low. The respiratory system exchanges gases with the outside air, but it needs the cardiovascular system to carry the gases to and from cells throughout the body.
• Breathing, or ventilation, is the two-step process of drawing air into the lungs (inhaling) and letting the air out of the lungs (exhaling). Inhaling is an active process that results mainly from the contraction of a muscle called the diaphragm. Exhaling is typically a passive process that occurs mainly due to the elasticity of the lungs when the diaphragm relaxes.
• Breathing is one of the few vital bodily functions that can be controlled consciously as well as unconsciously. Conscious control of breathing is common in many activities, including swimming and singing. However, there are limits on the conscious control of breathing. If you try to hold your breath, for example, you will soon have an irrepressible urge to breathe.
• Unconscious breathing is controlled by respiratory centers in the medulla and pons of the brainstem. They respond to variations in blood pH by either increasing or decreasing the rate of breathing as needed to return the pH level to the normal range.
• Nasal breathing is generally considered to be superior to mouth breathing because it does a better job of filtering, warming, and moistening incoming air. It also results in slower emptying of the lungs, which allows more oxygen to be extracted from the air.
• Gas exchange is the biological process through which gases are transferred across cell membranes to either enter or leave the blood. Gas exchange takes place continuously between the blood and cells throughout the body and also between the blood and the air inside the lungs.
• Gas exchange in the lungs takes place in alveoli. The pulmonary artery carries deoxygenated blood from the heart to the lungs, where it travels through pulmonary capillaries, picking up oxygen, and releasing carbon dioxide. The oxygenated blood then leaves the lungs through pulmonary veins.
• Gas exchange occurs by diffusion across cell membranes. Gas molecules naturally move down a concentration gradient from an area of higher concentration to an area of lower concentration. This is a passive process that requires no energy.
• Gas exchange by diffusion depends on the large surface area provided by the hundreds of millions of alveoli in the lungs. It also depends on a steep concentration gradient for oxygen and carbon dioxide. This gradient is maintained by continuous blood flow and constant breathing.
• Asthma is a chronic inflammatory disease of the airways in the lungs, in which the airways periodically become inflamed. This causes swelling and narrowing of the airways, often with excessive mucus production, leading to difficulty breathing and other symptoms. Asthma is thought to be caused by a combination of genetic and environmental factors. Asthma attacks are triggered by allergens, air pollution, or other factors.
• Pneumonia is a common inflammatory disease of the respiratory tract in which inflammation affects primarily the alveoli, which become filled with fluid that inhibits gas exchange. Most cases of pneumonia are caused by viral or bacterial infections. Vaccines are available to prevent pneumonia; treatment often includes prescription antibiotics.
• Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by chronic poor airflow, which causes shortness of breath and a productive cough. It is caused most often by tobacco smoking, which leads to the breakdown of connective tissues in the lungs. Alveoli are reduced in number and elasticity, making it impossible to fully exhale air from the lungs. There is no cure for COPD, but stopping smoking may reduce the rate at which COPD worsens.
• Lung cancer is a malignant tumor characterized by uncontrolled cell growth in tissues of the lung. It results from accumulated DNA damage, most often caused by tobacco smoking. Lung cancer is typically diagnosed late, so most cases cannot be cured. It may be treated with surgery, chemotherapy, and/or radiation therapy.
• Smoking is the single greatest cause of preventable death worldwide. It has adverse effects on just about every body system and organ. Tobacco smoke affects not only smokers but also non-smokers who are exposed to secondhand smoke. The nicotine in tobacco is highly addictive, making it very difficult to quit smoking.
• The major health risk of smoking is cancer of the lungs. Smoking also increases the risk of many other types of cancer. Tobacco smoke contains dozens of chemicals that are known carcinogens.
• Smoking is the primary cause of COPD. Chemicals such as carbon monoxide and cyanide in tobacco smoke reduce the elasticity of alveoli so the lungs can no longer fully exhale air.
• Smoking damages the cardiovascular system and increases the risk of high blood pressure, blood clots, heart attack, and stroke. Smoking also has a negative impact on levels of blood lipids.
• A wide diversity of additional adverse health effects are attributable to smoking, such as erectile dysfunction, female infertility, and slow wound healing.
Chapter Summary Review
1. Describe the relationship between the bronchi, secondary bronchi, tertiary bronchi, and bronchioles.
2. What is the uppermost structure in the lower respiratory tract?
1. Bronchus
2. Lung
3. Alveolus
4. Trachea
3. Deoxygenated and oxygenated blood both travel to the lungs. Describe what happens to each there.
4. True or False. There are radioactive isotopes in cigarette smoke.
5. True or False. The right and left lungs are identical in structure.
6. Explain the difference between ventilation and gas exchange.
7. Which way do oxygen and carbon dioxide flow during a gas exchange in the lungs?
1. Why does this happen?
2. Which way do oxygen and carbon dioxide flow during the gas exchange between the blood and the body’s cells?
3. Why does this happen?
8. Why does the body require oxygen and give off carbon dioxide as a waste product?
9. True or False. Conduction refers to the movement of gases across cell membranes.
10. True or False. Gas exchange does not require energy.
11. What do coughing and sneezing have in common?
12. What is the name of the escalator that protects the respiratory system?
1. phlegmociliary
2. mucociliary
3. mucoflagellar
4. surfactociliary
13. COPD can lead to too much carbon dioxide in the blood. Answer the following questions about this.
1. Why can COPD cause there to be too much carbon dioxide in the blood?
2. What does this do to the blood pH?
3. How does the body respond to this change in blood pH?
14. From the following list of diseases, choose which one best fits each description. Each disease is used only once. Diseases: asthma, pneumonia, COPD, lung cancer
1. Alveoli become inflamed and fill with fluid
2. Can be caused by exposure to inhaled carcinogens
3. There is a reduction in the number of alveoli
4. Airways periodically narrow and fill with mucus
15. True or False. Pneumonia can be caused by fungi.
16. True or False. The diaphragm contracts during exhalation.
17. What are three different types of things that can enter the respiratory system and cause illness or injury? Describe the negative health effects of each in your answer.
18. Where are the respiratory centers of the brain located? What is the main function of the respiratory centers of the brain?
19. Smoking increases the risk of getting influenza, commonly known as the flu. Explain why this could lead to a greater risk of getting pneumonia.
20. If people had a gene that caused them to get asthma, could changes to their environment (such as more frequent cleaning) help their asthma? Why or why not?
21. What does the term bronchodilator refer to?
1. The largest bronchial tube
2. An area of the brain that increases breathing rate
3. A medication that opens constricted airways
4. A medication that clears the nasal cavity
22. Explain why nasal breathing generally stops particles from entering the body at an earlier stage than mouth breathing.
Attributions
1. Enjoying the Humidifier by Eden, Janine and Jim, CC BY 2.0 via Flickr.com
2. Acute Bronchitis by National Heart Lung and Blood Institute, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/16%3A_Respiratory_System/16.6%3A_Case_Study_Conclusion%3A__Bronchitis_and_Chapter_Summary.txt |
This chapter provides a detailed description of the heart, blood vessels, and blood. It explains how they function together to transport substances throughout the body and maintain homeostasis. The chapter also describes several diseases of the cardiovascular system and lifestyle choices that can help prevent most of them.
• 17.1: Case Study: Your Body's Transportation System
Nineteen-year-old Antônio is on his first plane flight when his seatmate, 60-year-old Ahaya, begins pacing the aisles and doing leg and foot exercises at regular intervals. Ahaya explains that he has chronic heart failure, which, although well-managed, puts him at greater risk for certain complications of flying, like deep vein thrombosis (DVT). In this chapter, you will learn about the heart, blood vessels, and blood that make up the cardiovascular system, as well as its potential disorders.
• 17.2: Introduction to the Cardiovascular System
The cardiovascular system, also called the circulatory system, is the organ system that transports materials to and from all the cells of the body. The materials carried by the cardiovascular system include oxygen from the lungs, nutrients from the digestive system, hormones from glands of the endocrine system, and waste materials from cells throughout the body. Transport of these and many other materials is necessary to maintain homeostasis of the body.
• 17.3: Heart
The heart is a muscular organ behind the sternum (breastbone), slightly to the left of the center of the chest. A normal adult heart is about the size of a fist. The function of the heart is to pump blood through blood vessels of the cardiovascular system. The continuous flow of blood through the system is necessary to provide all the cells of the body with oxygen and nutrients and to remove their metabolic wastes.
• 17.4: Blood Vessels
Blood vessels are the part of the cardiovascular system that transports blood throughout the human body. There are three major types of blood vessels. Besides veins, they include arteries and capillaries.
• 17.5: Blood
Blood is a fluid connective tissue that circulates throughout the body through blood vessels of the cardiovascular system. What makes blood so special that it features in widespread myths? Although blood accounts for less than 10 percent of human body weight, it is quite literally the elixir of life. As blood travels through the vessels of the cardiovascular system, it delivers vital substances such as nutrients and oxygen to all of the cells and carries away their metabolic wastes.
• 17.6: Blood Types
Blood type (or blood group) is a genetic characteristic associated with the presence or absence of certain molecules, called antigens, on the surface of red blood cells. These molecules may help maintain the integrity of the cell membrane, act as receptors, or have other biological functions. A blood group system refers to all of the gene(s), alleles, and possible genotypes and phenotypes that exist for a particular set of blood type antigens.
• 17.7: Cardiovascular Disease
Cardiovascular disease is a class of diseases that involve the cardiovascular system. They include diseases of the coronary arteries that supply the heart muscle with oxygen and nutrients; diseases of arteries such as the carotid artery that provide blood flow to the brain; and diseases of the peripheral arteries that carry blood throughout the body. Worldwide, cardiovascular disease is the leading cause of death, causing about a third of all deaths each year.
• 17.8: Case Study Conclusion: Flight and Chapter Summary
At the beginning of this chapter, you learned about Antônio and Ahaya, who met while sitting next to each other on a plane. During the flight, Ahaya got up to take frequent walks and was doing leg exercises to try to avoid the medical condition DVT. DVT occurs when a blood clot forms in a deep vein, usually in the leg. It can be very dangerous—even deadly.
Thumbnail: Flow of blood through the cardiac chambers. (Public Domain; Josinho8).
17: Cardiovascular System
Case Study: Flight Risk
Nineteen-year-old Antônio is about to take his first plane flight. Shortly after he boards the plane and sits down, a man in his late sixties sits next to him in the aisle seat. About half an hour after the plane takes off, the pilot announces that she is turning the seat belt light off and that “it is now safe to move about the cabin.”
The man in the aisle seat, who has introduced himself to Antônio as Ahaya, immediately unbuckles his seat belt and paces up and down the aisle a few times before returning to his seat. After about forty-five minutes, Ahaya gets up again, walks some more, then sits back down and does some foot and leg exercises. After the third time, Ahaya gets up and paces the aisles, Antônio asks him whether he is walking so much to accumulate steps on a pedometer or fitness tracking device. Ahaya laughs and says no, he is trying to do something even more important for his health—prevent a blood clot from forming in his legs.
Ahaya explains that he has a chronic condition called heart failure. Although it sounds scary, his condition is currently well-managed and he is able to lead a relatively normal lifestyle. However, it does put him at risk of developing other serious health conditions such as deep vein thrombosis (DVT), which is when a blood clot occurs in the deep veins, usually in the legs. Air travel, or other situations where a person has to sit for a long period of time, increases the risk of DVT. Ahaya’s doctor said that he was healthy enough to fly, but that he should walk frequently and do leg exercises to help avoid a blood clot.
As you read this chapter, you will learn about the heart, blood vessels, and blood that make up the cardiovascular system, as well as disorders of the cardiovascular system such as heart failure. At the end of the chapter, you will learn more about why DVT occurs, why Ahaya has to take extra precautions when he flies, and what can be done to lower the risk of DVT and its potentially deadly consequences.
Chapter Overview: Cardiovascular System
In this chapter, you will learn about the cardiovascular system, which transports substances throughout the body. Specifically, you will learn about:
• The major components of the cardiovascular system: the heart, blood vessels, and blood.
• The functions of the cardiovascular system, including transporting needed substances such as oxygen and nutrients to the cells of the body and picking up waste products.
• How blood is oxygenated through the pulmonary circulation, which transports blood between the heart and lungs.
• How blood is circulated throughout the body through the systemic circulation.
• The components of blood, including plasma, red blood cells, white blood cells, and platelets, and their specific functions.
• Types of blood vessels, including arteries, veins, and capillaries, and their functions, similarities, and differences.
• The structure of the heart, how it pumps blood, and how contractions of the heart are controlled.
• What blood pressure is and how it is regulated.
• Blood types: A, B, AB, and O
• Blood disorders, including anemia, HIV, and leukemia.
• Cardiovascular diseases including heart attack, stroke, and angina, and the risk factors and precursors, such as high blood pressure and atherosclerosis, which contribute to them.
As you read the chapter, think about the following questions:
1. What is heart failure? How do you think it increases the risk of DVT?
2. What is a blood clot? What are the possible health consequences of blood clots?
3. Why do you think sitting for long periods of time increases the risk of DVT and why does walking and exercising the legs help reduce this risk?
Attributions
1. Aisle by David Day, CC BY 2.0 via Flickr.com
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/17%3A_Cardiovascular_System/17.1%3A_Case_Study%3A__Your_Body%27s_Transportation_System.txt |
Ant Hill or Plumbing System?
What do you think Figure \(1\) shows? Does it show a maze of underground passageways in an anthill? A network of interconnected pipes in a complex plumbing system? The picture actually shows something that, like ant tunnels and plumbing pipes, functions as a transportation system. It shows a network of blood vessels. Blood vessels are part of the cardiovascular system.
What is the Cardiovascular System?
The cardiovascular system, also called the circulatory system, is the organ system that transports materials to and from all the cells of the body. The materials carried by the cardiovascular system include oxygen from the lungs, nutrients from the digestive system, hormones from glands of the endocrine system, and waste materials from cells throughout the body. Transport of these and many other materials is necessary to maintain homeostasis of the body. The main components of the cardiovascular system are the heart, blood vessels, and blood. Each of these components is shown in Figure \(2\) and introduced in the text.
Heart
The heart is a muscular organ in the chest. It consists mainly of cardiac muscle tissue and pumps blood through blood vessels by repeated, rhythmic contractions. As shown in Figure \(3\), the heart has four inner chambers: a right atrium and ventricle and a left atrium and ventricle. On each side of the heart, blood is pumped from the atrium to the ventricle below it and from the ventricle out of the heart. The heart also contains several valves that allow blood to flow only in the proper direction through the heart.
Unlike skeletal muscle, cardiac muscle routinely contracts without stimulation by the nervous system. Specialized cardiac muscle cells send out electrical impulses that stimulate the contractions. As a result, the atria and ventricles normally contract with just the right timing to keep blood pumping efficiently through the heart.
Blood Vessels
The blood vessels of the cardiovascular system are like a network of interconnected, one-way roads that range from superhighways to back alleys. Like a network of roads, the blood vessels have the job of allowing the transport of materials from one place to another. There are three major types of blood vessels: arteries, veins, and capillaries. They are illustrated in Figure \(4\).
• Arteries are blood vessels that carry blood away from the heart (except for the arteries that actually supply blood to the heart muscle). Most arteries carry oxygen-rich blood, and one of their main functions is distributing oxygen to tissues throughout the body. The smallest arteries are called arterioles.
• Veins are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood. The smallest veins are called venules.
• Capillaries are the smallest blood vessels. They connect arterioles and venules. As they pass through tissues, they exchange substances including oxygen with cells.
Two Circulations
Cells throughout the body need a constant supply of oxygen. They get oxygen from capillaries in the systemic circulation. The systemic circulation is just one of two interconnected circulations that make up the human cardiovascular system. The other circulation is the pulmonary system. This is where the blood picks up oxygen to carry to cells. It takes blood about 20 seconds to make one complete transit through both circulations.
Pulmonary Circulation
The pulmonary circulation involves only the heart and lungs and the major blood vessels that connect them. It is illustrated in Figure \(5\). Blood moves through the pulmonary circulation from the heart to the lungs, and back to the heart again, becoming oxygenated in the process. Specifically, the right ventricle of the heart pumps deoxygenated blood into the right and left pulmonary arteries. These arteries carry the blood to the right and left lungs, respectively. Oxygenated blood then returns from the right and left lungs through the two right and two left pulmonary veins. All four pulmonary veins enter the left atrium of the heart.
What happens to the blood while it is in the lungs? It passes through increasingly smaller arteries and finally through capillary networks surrounding the alveoli (Figure \(6\)). This is where gas exchange takes place. The deoxygenated blood in the capillaries picks up oxygen from the alveoli and gives up carbon dioxide to the alveoli. As a result, the blood returning to the heart in the pulmonary veins is almost completely saturated with oxygen.
Systemic Circulation
The oxygenated blood that enters the left atrium of the heart in the pulmonary circulation then passes into the systemic circulation. This is the part of the cardiovascular system that transports blood to and from all of the tissues of the body to provide oxygen and nutrients and pick up wastes. It consists of the heart and blood vessels that supply the metabolic needs of all the cells in the body, including those of the heart and lungs.
As shown in Figure \(7\), in the systemic circulation, the left atrium pumps oxygenated blood to the left ventricle, which pumps the blood directly into the aorta, the body’s largest artery. Major arteries branching off the aorta carry the blood to the head and upper extremities. The aorta continues down through the abdomen and carries blood to the abdomen and lower extremities. The blood then returns to the heart through the network of increasingly larger veins of the systemic circulation. All of the returning blood eventually collects in the superior vena cava (upper body) and inferior vena cava (lower body), which empty directly into the right atrium of the heart.
Blood
Blood is a fluid connective tissue that circulates throughout the body in blood vessels by the pumping action of the heart. Blood carries oxygen and nutrients to all the body’s cells, and it carries carbon dioxide and other wastes away from the cells to be excreted. Blood also transports many other substances, defends the body against infection, repairs body tissues, and controls the body’s pH, among other functions.
The fluid part of blood is called plasma. It is a yellowish, watery liquid that contains many dissolved substances and blood cells. Types of blood cells in plasma include red blood cells, white blood cells, and platelets, all of which are illustrated in Figure \(8\) and explained in the text.
• Red blood cells have the main function of carrying oxygen in the blood. Red blood cells consist mostly of hemoglobin, a protein containing iron that binds with oxygen.
• White blood cells are far fewer in number than red blood cells. They defend the body in various ways. For example, white blood cells called phagocytes swallow and destroy pathogens, dead cells, and other debris in the blood.
• Platelets are cell fragments involved in blood clotting. They stick to tears in blood vessels and to each other, forming a plug at the site of injury. They also release chemicals that are needed for clotting to occur.
Review
1. What is the cardiovascular system? What are its main components?
2. Describe the heart and how it functions.
3. List the three major types of blood vessels and their basic functions.
4. Compare and contrast the pulmonary and systemic circulations.
5. What is blood? What are its chief constituents?
6. True or False. The circulatory system brings blood to and from the body, while the cardiovascular system brings blood to and from the lungs only.
7. True or False. Arteries carry mainly oxygenated blood.
8. Name three different types of substances that are transported by the cardiovascular system.
9. Describe where and how the pulmonary and systemic circulation systems meet.
10. Which of the following carries blood to the lungs? Choose all that apply.
A. Left pulmonary artery
B. Left pulmonary vein
C. Right pulmonary artery
D. Right pulmonary vein
11. Put the following structures in order of how blood flows from the heart out to the body and back again.
capillaries; venules; aorta; veins; arteries
12. Explain why the heart and lungs need blood from the systemic circulation.
13. Choose one. Blood vessels carrying deoxygenated blood from the body back to the heart get increasingly (larger/smaller).
14. Blood becomes oxygenated in the lungs through gas exchange into:
A. Arterioles
B. Capillaries
C. Venules
D. Bronchioles
15. Which type of blood cell carries oxygen?
Explore More
Watch this fun and fast-paced CrashCourse video to explore how the cardiovascular and respiratory systems work together to deliver oxygen and remove carbon dioxide from cells.
Check out this video to learn more about how the heart pumps blood:
Attributions
1. Blood Vessels by Jiulin Du from CK-12 licensed CC BY-NC 3.0
2. Circulatory System by Mariana Ruiz Villarreal (LadyofHats), public domain via Wikimedia Commons
3. Heart Anatomy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
4. Blood Vessels by Rupali Raju from CK-12 licensed CC BY-NC 3.0
5. Pulmonary circuit by Arcadian public domain via Wikimedia Commons
6. Pulmonary blood circulation by Holly Fisher, CC BY 3.0 via Wikimedia Commons
7. Systemic Circuit by US Government, public domain via Wikimedia Commons
8. Red White Blood Cells by Electron Microscopy Facility at The National Cancer Institute at Frederick (NCI-Frederick), public domain via Wikimedia Commons
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/17%3A_Cardiovascular_System/17.2%3A_Introduction_to_the_Cardiovascular_System.txt |
Lub Dub
Lub dub, lub dub, lub dub... That’s how the sound of a beating heart is typically described. In a normal, healthy heart, those are the only two sounds that should be audible when listening to the heart through a stethoscope. If a physician assistant hears something different from the normal lub dub sounds, it’s a sign of a possible heart abnormality. What causes the heart to produce the characteristic lub dub sounds? Read on to find out.
The heart is a muscular organ behind the sternum (breastbone), slightly to the left of the center of the chest. A normal adult heart is about the size of a fist. The function of the heart is to pump blood through the blood vessels of the cardiovascular system. The continuous flow of blood through the system is necessary to provide all the cells of the body with oxygen and nutrients and to remove their metabolic wastes.
Structure of the Heart
The heart has a thick muscular wall that consists of several layers of tissue. Internally, the heart is divided into four chambers through which blood flows. Blood flows in just one direction through the chambers due to heart valves.
Heart Wall
As shown in Figure \(2\), the wall of the heart is made up of three layers, called the endocardium, myocardium, and pericardium.
• The endocardium is the innermost layer of the heart wall. It is made up primarily of simple epithelial cells. It covers the heart chambers and valves. A thin layer of connective tissue joins the endocardium to the myocardium.
• The myocardium is the middle and thickest layer of the heart wall. It consists of cardiac muscle surrounded by a framework of collagen. There are two types of cardiac muscle cells in the myocardium: pacemaker cells, which have the ability to contract easily; and pacemaker cells, which conduct electrical impulses that cause the cardiomyocytes to contract. About 99 percent of cardiac muscle cells are cardiomyocytes, and the remaining 1 percent are pacemaker cells. The myocardium is supplied with blood vessels and nerve fibers via the pericardium.
• The epicardium is the third layer which is a part of the pericardium, a protective sac that encloses and protects the heart. The pericardium consists of two membranes (visceral pericardium called epicardium and parietal pericardium), between which there is a fluid-filled cavity. The fluid helps to cushion the heart and also lubricates its outer surface.
Heart Chambers
As shown in Figure \(3\), the four chambers of the heart include two upper chambers called atria (singular, atrium) and two lower chambers called ventricles. The atria are also referred to as receiving chambers because blood coming into the heart first enters these two chambers. The right atrium receives blood from the upper and lower body through the superior vena cava and inferior vena cava, respectively; and the left atrium receives blood from the lungs through the pulmonary veins. The ventricles are also referred to as discharging chambers because the blood leaving the heart passes out through these two chambers. The right ventricle discharges blood to the lungs through the pulmonary artery, and the left ventricle discharges blood to the rest of the body through the aorta. The four chambers are separated from each other by dense connective tissue consisting mainly of collagen.
Heart Valves
Figure \(3\) also shows the location of the four valves of the heart. The heart valves allow blood to flow from the atria to the ventricles and from the ventricles to the pulmonary artery and aorta. The valves are constructed in such a way that blood can flow through them in only one direction, thus preventing the backflow of blood. The four valves are the:
1. tricuspid valve, which allows blood to flow from the right atrium to the right ventricle.
2. the mitral valve, which allows blood to flow from the left atrium to the left ventricle.
3. pulmonary valve, which allows blood to flow from the right ventricle to the pulmonary artery.
4. the aortic valve, which allows blood to flow from the left ventricle to the aorta.
The tricuspid and mitral valves are also called atrioventricular (or AV) valves because they are found between the atrium and the ventricle. The pulmonary and aortic valves are also called semilunar valves because they are shaped like half-moons.
Coronary Circulation
The cardiomyocytes of the muscular walls of the heart are very active cells because they are responsible for the constant beating of the heart. These cells need a continuous supply of oxygen and nutrients. The carbon dioxide and waste products they produce also must be continuously removed. The blood vessels that carry blood to and from the heart muscle cells make up the coronary circulation. Note that the blood vessels of the coronary circulation supply heart tissues with blood and are different from the blood vessels that carry blood to and from the chambers of the heart as part of the general circulation. Coronary arteries supply oxygen-rich blood to the heart muscle cells. Coronary veins remove deoxygenated blood from the heart muscle cells.
• There are two coronary arteries: a right coronary artery that supplies the right side of the heart and a left coronary artery that supplies the left side of the heart. These arteries branch repeatedly into smaller and smaller arteries and finally into capillaries, which exchange gases, nutrients, and waste products with cardiomyocytes.
• At the back of the heart, small cardiac veins drain into larger veins and finally into the great cardiac vein, which empties into the right atrium. At the front of the heart, small cardiac veins drain directly into the right atrium.
Blood Circulation Through the Heart
Figure \(4\) shows how blood circulates through the chambers of the heart. The right atrium collects blood from two large veins, the superior vena cava (from the upper body) and the inferior vena cava (from the lower body). The blood that collects in the right atrium is pumped through the tricuspid valve into the right ventricle. From the right ventricle, the blood is pumped through the pulmonary valve into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it enters the pulmonary circulation, gives up carbon dioxide, and picks up oxygen. The oxygenated blood travels back from the lungs through the pulmonary veins (of which there are four) and enters the left atrium of the heart. From the left atrium, the blood is pumped through the mitral valve into the left ventricle. From the left ventricle, the blood is pumped through the aortic valve into the aorta, which subsequently branches into smaller arteries that carry the blood throughout the rest of the body. After passing through capillaries and exchanging substances with cells, the blood returns to the right atrium via the superior vena cava and inferior vena cava, and the process begins anew.
Cardiac Cycle
The cardiac cycle refers to a single complete heartbeat, which includes one iteration of the lub and dub sounds heard through a stethoscope. During the cardiac cycle, the atria and ventricles work in a coordinated fashion so that blood is pumped efficiently through and out of the heart. The cardiac cycle includes two parts, called diastole and systole, which are illustrated in Figure \(5\).
• During diastole, the atria contract and pump blood into the ventricles, while the ventricles relax and fill with blood from the atria.
• During systole, the atria relax and collect blood from the lungs and body, while the ventricles contract and pump blood out of the heart.
Electrical Stimulation of the Heart
The normal, rhythmical beating of the heart is called sinus rhythm. It is established by the heart’s pacemaker cells, which are located in an area of the heart called the sinoatrial node (Figure \(6\)). The pacemaker cells create electrical signals by the movement of electrolytes (sodium, potassium, and calcium ions) into and out of the cells. For each cardiac cycle, an electrical signal rapidly travels first from the sinoatrial node to the right and left atria so they contract together. Then the signal travels to another node, called the atrioventricular node (also shown in Figure \(6\)), and from there to the right and left ventricles, which also contract together, just a split second after the atria contract.
The normal sinus rhythm of the heart is influenced by the autonomic nervous system through sympathetic and parasympathetic nerves. These nerves arise from two paired cardiovascular centers in the medulla of the brainstem. The parasympathetic nerves act to decrease the heart rate, and the sympathetic nerves act to increase the heart rate. Parasympathetic input normally predominates. Without it, the pacemaker cells of the heart would generate a resting heart rate of about 100 beats per minute, instead of a normal resting heart rate of about 72 beats per minute. The cardiovascular centers receive input from receptors throughout the body and act through the sympathetic nerves to increase the heart rate as needed. For example, increased physical activity is detected by receptors in muscles, joints, and tendons. These receptors send nerve impulses to the cardiovascular centers, causing sympathetic nerves to increase the heart rate. This allows more blood to flow to the muscles.
Besides the autonomic nervous system, other factors can also affect the heart rate. For example, thyroid hormones and adrenal hormones such as epinephrine can stimulate the heart to beat faster. The heart rate also increases when blood pressure drops or the body is dehydrated or overheated. On the other hand, cooling of the body and relaxation, among other factors, can contribute to a decrease in the heart rate.
Feature: Human Biology in the News
When a patient’s heart is too diseased or damaged to sustain life, a heart transplant is likely to be the only long-term solution. The first successful heart transplant was undertaken in South Africa in 1967. For the past two decades in the United States, about 2,400 hearts were transplanted each year. The problem is that far too few hearts are available for transplant, and many patients die each year waiting for a life-saving heart to become available.
Hearts for transplant have to be used within four hours of the death of the donor. In addition, hearts can only come from brain-dead individuals whose hearts are removed while they are still healthy. Then the hearts are placed on ice inside picnic coolers to be transported to a waiting recipient. The four-hour window means that traffic jams, bad weather, or other unforeseen delays often result in a heart being in less than optimal condition by the time it arrives at its destination. Unfortunately, there is no way to know if the heart will start up again after it is transplanted until it is actually placed in the recipient’s body. In up to seven percent of cases, a transplanted heart does not work and has to be removed.
A medical device company in Massachusetts named TransMedic was featured in many news stories when it developed the Organ Care System, commonly referred to as “heart in a box.” The system takes a new approach to maintain donated hearts until they are transplanted. The box is heated and contains a device that pumps oxygenated blood through the heart while it is being transported to the recipient. This extends the time up to 12 hours that the heart can remain healthy and usable. It also allows the heart to be monitored so it is kept in optimal condition while it is on the route. The end result, ideally, is that the recipient gets a healthier heart with less chance of failure of the new organ and a lower risk of death.
As of mid-2016, the heart-in-a-box system had already been used for several successful heart transplants in other countries. At that time, the system was also undergoing clinical trials in the United States to assess its effectiveness in promoting positive recipient outcomes. Developers of the heart-in-a-box predict that the system could increase the number of usable donor hearts by as much as 30 percent, thus greatly increasing the number of patients who are saved from death due to heart failure.
Review
1. What is the heart, where is located, and what is its function?
2. Outline the structure of the heart.
3. Describe the coronary circulation.
4. Summarize how blood flows into, through, and out of the heart.
5. Define the cardiac cycle, and identify its two parts.
6. Explain what controls the beating of the heart.
7. a. What are the two types of cardiac muscle cells in the myocardium?
b. What are the differences between these two types of cells?
8. Match each of the three layers of the walls of the heart (endocardium, myocardium, and pericardium) with the description that best matches it below.
a. Protects the heart
b. Covers the heart valves
c. Responsible for the beating of the heart
9. Is the blood flowing through the mitral valve oxygenated or deoxygenated? Explain your reasoning.
10. True or False. The coronary arteries carry blood to the heart.
11. True or False. Systole is when the heart is contracting, diastole is when the heart is fully relaxed.
12. Explain why the blood from the cardiac veins empties into the right atrium of the heart. Focus on function rather than anatomy in your answer.
Explore More
More women than men die of heart disease, but heart research has long focused on men. In the following TED talk, pioneering doctor C. Noel Bairey Merz shares what we know and don't know about women's heart health, including the very different heart attack symptoms women experience and why doctors often miss them.
Attributions
1. "MEDCAP - Natural Fire 10 - Palabek Kal Health Clinic - US Army Africa - AFRICOM - 091018-F-8314S-229" by US Army Africa is licensed under CC BY 2.0 via Flickr
2. Heart Wall by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons.
3. Heart by CK-12 licensed CC BY-NC 3.0
4. Circulation of Blood Through the Heart by Emibitch, public domain via Wikimedia Commons
5. Human healthy pumping heart by Mariana Ruiz Villarreal (LadyofHats), public domain via Wikimedia Commons
6. Heart conduction system by J. Heuser, CC BY 2.5 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/17%3A_Cardiovascular_System/17.3%3A_Heart.txt |
Bulging Veins
Why do bodybuilders have such prominent veins? Bulging muscles push surface veins closer to the skin. Couple that with a virtual lack of subcutaneous fat, and you have bulging veins as well as bulging muscles. Veins are one of three major types of blood vessels in the cardiovascular system.
Types of Blood Vessels
Blood vessels are part of the cardiovascular system that transports blood throughout the human body. There are three major types of blood vessels: veins, arteries, and capillaries.
Arteries are defined as blood vessels that carry blood away from the heart. Blood flows through arteries largely because it is under pressure from the pumping action of the heart. It should be noted that coronary arteries, which supply heart muscle cells with blood, travel toward the heart but not as part of the blood flow that travels through the chambers of the heart. Most arteries, including coronary arteries, carry oxygenated blood, but there are a few exceptions, most notably the pulmonary artery. This artery carries deoxygenated blood from the heart to the lungs, where it picks up oxygen and releases carbon dioxide. In virtually all other arteries, the hemoglobin in red blood cells is highly saturated with oxygen (95-100 percent). These arteries distribute oxygenated blood to tissues throughout the body.
The largest artery in the body is the aorta, which is connected to the heart and extends down into the abdomen (Figure \(2\)). The aorta has high-pressure, oxygenated blood pumped directly into it from the left ventricle of the heart. The aorta has many branches, and the branches subdivide repeatedly, with the subdivisions growing smaller and smaller in diameter. The smallest arteries are called arterioles.
Veins
Veins are defined as blood vessels that carry blood toward the heart. Blood traveling through veins is not under pressure from the beating heart. It gets help moving along by the squeezing action of skeletal muscles, for example, when you walk or breathe. It is also prevented from flowing backward by valves in the larger veins, as illustrated in Figure \(3\). Veins are called capacitance blood vessels because the majority (about 60 percent) of the body’s total volume of blood is contained within veins.
Most veins carry deoxygenated blood, but there are a few exceptions, including the four pulmonary veins. These veins carry oxygenated blood from the lungs to the heart, which then pumps the blood to the rest of the body. In virtually all other veins, hemoglobin is relatively unsaturated with oxygen (about 75 percent).
The two largest veins in the body are the superior vena cava, which carries blood from the upper body directly to the right atrium of the heart, and the inferior vena cava, which carries blood from the lower body directly to the right atrium. The inferior vena cava is labeled in the figure below. The superior vena cava is not labeled in Figure \(4\) but is clearly visible entering the right atrium of the heart. Like arteries, veins form a complex, branching system of larger and smaller vessels. The smallest veins are called venules. They receive blood from capillaries and transport it to larger veins. Each venule receives blood from multiple capillaries.
Capillaries
Capillaries are the smallest blood vessels in the cardiovascular system. They are so small that only one red blood cell at a time can squeeze through a capillary, and then only if the red blood cell deforms. Capillaries connect arterioles and venules, as shown in Figure \(5\). Capillaries generally form a branching network of vessels, called a capillary bed, that provides a large surface area for the exchange of substances between the blood and surrounding tissues.
Structure of Blood Vessels
All blood vessels are basically hollow tubes with an internal space, called a lumen, through which blood flows. The lumen of an artery is shown in cross-section in the photomicrograph below. The width of blood vessels varies, but they all have a lumen. The walls of blood vessels differ depending on the type of vessel. In general, arteries and veins are more similar to one another than capillaries in the structure of their walls.
Walls of Arteries and Veins
The walls of both arteries and veins have three layers: the tunica intima, tunica media, and tunica adventitia. You can see the three layers for an artery in Figure \(7\).
1. The tunica intima is the inner layer of arteries and veins. It is also the thinnest layer, consisting of a single layer of endothelial cells surrounded by a thin layer of connective tissues. It reduces friction between the blood and the inside of the blood vessel walls.
2. The tunica media is the middle layer of arteries and veins. In arteries, this is the thickest layer. It consists mainly of elastic fibers and connective tissues. In arteries, this is the thickest layer because it also contains smooth muscle tissues, which control the diameter of the vessels.
3. The tunica externa (also called tunica adventitia) is the outer layer of arteries and veins. It consists of connective tissue and also contains nerves. In veins, this is the thickest layer. In general, the tunica externa protects and strengthens vessels and attaches them to surrounding structures.
Capillary Walls
The walls of capillaries consist of little more than a single layer of epithelial cells. Being just one cell thick, the walls are well suited for the exchange of substances between the blood inside them and the cells of surrounding tissues. Substances including water, oxygen, glucose, and other nutrients as well as waste products such as carbon dioxide can pass quickly and easily through the extremely thin walls of capillaries.
Blood Pressure
The blood in arteries is normally under pressure because of the beating of the heart. The pressure is highest when the heart contracts and pumps out blood, and lowest when the heart relaxes and refills with blood. (You can feel this variation in pressure in your wrist or neck when you count your pulse.) Blood pressure is a measure of the force that blood exerts on the walls of arteries. It is generally measured in millimeters of mercury (mm Hg) and expressed as a double number: a higher number for systolic pressure when the ventricles contract; and a lower number for diastolic pressure when the ventricles relax. Normal blood pressure is generally defined as less than 120 mm Hg (systolic)/80 mm Hg (diastolic) when measured in the arm at the level of the heart. It decreases as blood flows farther away from the heart and into smaller arteries.
As arteries grow smaller, there is increasing resistance to blood flow through them because of the friction of the blood against the arterial walls. This resistance restricts blood flow so less blood reaches smaller, downstream vessels, thus reducing blood pressure before the blood flows into the tiniest vessels, the capillaries. Without this reduction in blood pressure, capillaries would not be able to withstand the pressure of the blood without bursting. By the time blood flows through the veins, it is under very little pressure. The pressure of blood against the walls of veins is always about the same and normally no more than 10 mm Hg.
Vasoconstriction and Vasodilation
Smooth muscles in the walls of arteries can contract or relax to cause vasoconstriction (narrowing of the lumen of blood vessels) or vasodilation (widening of the lumen of blood vessels). This allows the arteries — especially the arterioles — to contract or relax as needed to help regulate blood pressure. In this regard, the arterioles act like an adjustable nozzle on a garden hose. When they narrow, the increased friction with the arterial walls causes less blood to flow downstream from the narrowing, resulting in a drop in blood pressure. These actions are controlled by the autonomic nervous system in response to pressure-sensitive sensory receptors in the walls of larger arteries.
Arteries can also dilate or constrict to help regulate body temperature by allowing more or less blood to flow from the warm body core to the body’s surface. In addition, vasoconstriction and vasodilation play roles in the fight-or-flight response, under the control of the sympathetic nervous system. For example, vasodilation allows more blood to flow to skeletal muscles and vasoconstriction reduces blood flow to digestive organs.
Feature: My Human Body
The lumpy appearance of this man’s leg is caused by varicose veins. Do you have varicose veins? If you do, you may wonder whether they are a sign of a significant health problem. You may also wonder whether you should have them treated, and if so, what treatments are available. As is usually the case, when it comes to your health, “knowledge is power.”
First, the “back story:” varicose veins are veins that have become enlarged and twisted because their valves have become ineffective (see Figure \(8\)). As a consequence, blood pools in the veins and stretches them out. Varicose veins occur most frequently in the superficial veins of the legs, but they may also occur in other parts of the body. They are most common in older adults, females, and people who have a family history of the condition. Obesity and pregnancy also increase the risk of developing varicose veins. A job that requires standing for long periods of time, chronic constipation, and long-term alcohol consumption are additional risk factors.
Varicose veins usually are not serious. In many people, they are only a cosmetic issue. However, in severe cases, varicose veins may cause pain and other problems. For example, the affected leg(s) may feel heavy and achy, especially after long periods of standing. Ankles may become swollen by the end of the day. Minor injuries may bleed more than normal. The skin over varicosity may become red, dry, and itchy. In very severe cases, skin ulcers may develop.
If you are concerned about varicose veins, call them to the attention of your doctor, who can determine the best course of action for your case. There are many potential treatments for varicose veins. Some of the treatments have potential adverse side effects; and with many of the treatments, varicose veins may return. Which treatment is best for a given patient depends in part on the severity of the condition.
• If varicose veins are not serious, then conservative treatment options may be recommended. These include avoiding standing or sitting for long periods, frequently elevating the legs, and wearing graduated compression stockings.
• For more serious cases, less conservative but non-surgical options may be advised. These include sclerotherapy, in which medicine is injected into the veins to make them shrink. Another non-surgical approach is endovenous thermal ablation. In this type of treatment, laser light, radio-frequency energy, or steam is used to heat the walls of the veins, causing them to shrink and collapse.
• For the most serious cases, surgery may be the best option. The most invasive surgery is vein stripping, in which all or part of the main trunk of a vein is tied off and removed from the leg while the patient is under general anesthesia. In a less invasive surgery, called ambulatory phlebectomy, short segments of a vein are removed through tiny incisions under local anesthesia.
Review
1. What are the blood vessels? Name the three major types of blood vessels.
2. Describe arteries. Identify the largest artery in the body.
3. How are veins defined? What are the two largest veins in the body?
4. Compare and contrast how blood moves through arteries and veins.
5. What are capillaries, and what is their function?
6. Compare and contrast the structure of the walls of arteries, veins, and capillaries.
7. What is blood pressure, and how is it expressed? What blood pressure is considered normal?
8. Identify the functions of vasoconstriction and vasodilation of arteries.
9. Does the blood in most veins have any oxygen at all? Explain your answer.
10. True or False. Only one red blood cell can pass through the lumen of a capillary at a given time.
11. True or False. The pulmonary artery carries oxygenated blood.
12. Which tissue in blood vessels is responsible for vasodilation and vasoconstriction? Where is it located?
13. The blood pressure at the arterioles is generally _________ the blood pressure at the aorta.
A. lower than
B. higher than
C. the same as
D. not related to
14. Explain why it is important that the walls of capillaries are very thin.
15. Most of the blood in the body is in the:
A. Capillaries
B. Arteries
C. Heart
D. Veins
Attributions
1. Fist Pump by istolethetv licensed CC BY 2.0 via Wikimedia Commons
2. Arterial System by LadyofHats; public domain via Wikimedia Commons
3. Venous valve by Was a bee; Vectorized by ZooFari; Public domain via Wikimedia Commons
4. Venous system by LadyofHats, Mariana Ruiz Villarreal; Public domain via Wikimedia Commons
5. Capillaries by National Cancer Institute, National Institutes of Health; Public domain via Wikimedia Commons
6. Artery by Lord of Konrad licensed CC0 via Wikimedia Commons
7. Structure of artery wall by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
8. Leg before; public domain via Wikimedia Commons
9. Varicose Veins by National Heart Lung and Blood Institute; Public domain via Wikimedia Commons
10. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/17%3A_Cardiovascular_System/17.4%3A_Blood_Vessels.txt |
Vampires
From Bram Stoker’s famous novel about Count Dracula to the campy TV series Buffy the Vampire Slayer, fantasies featuring vampires, like the one in Figure \(1\), have been popular for decades. In fact, vampires are found in centuries-old myths from many cultures. In such myths, vampires are generally described as creatures that drink blood, preferably of the human variety, for sustenance. Dracula, for example, is based on Eastern European folklore about a human who attains immortality (and eternal damnation) by drinking the blood of others.
Functions of Blood
Blood performs many important functions in the body. Major functions of blood include:
• supplying tissues with oxygen, which is needed by all cells for aerobic cellular respiration.
• supplying cells with nutrients, including glucose, amino acids, and fatty acids.
• removing metabolic wastes from cells, including carbon dioxide, urea, and lactic acid.
• helping to defend the body from pathogens and other foreign substances.
• forming clots to seal broken blood vessels and stop bleeding.
• transporting hormones and other messenger molecules.
• regulating the pH of the body, which must be kept within a narrow range (7.35 to 7.45).
• helping to regulate body temperature (through vasoconstriction and vasodilation).
What Is Blood?
Blood is a fluid connective tissue that circulates throughout the body through blood vessels of the cardiovascular system. What makes blood so special that it features in widespread myths? Although blood accounts for less than 10 percent of human body weight, it is quite literally the elixir of life. As blood travels through the vessels of the cardiovascular system, it delivers vital substances such as nutrients and oxygen to all of the cells and carries away their metabolic wastes. It is no exaggeration to say that without blood, cells could not survive. Indeed, without the oxygen carried in the blood, cells of the brain start to die within just a matter of minutes.
The average adult body contains between 4.7 and 5.7 liters (5-6 quarts) of blood. More than half of that amount is fluid. Most of the rest of that amount consists of cells. The relative amounts of the various components in the blood are illustrated in Figure \(2\). The components are also described in the text.
Blood Plasma
Plasma is the liquid component of human blood and makes up about 55 percent of blood by volume. It is about 92 percent water and contains many dissolved substances. Most of these substances are proteins, but plasma also contains trace amounts of glucose, mineral ions, hormones, carbon dioxide, and other substances.
Formed Elements
The formed elements in the blood include red blood cells, white blood cells, and platelets. These different types of elements are pictured in Figure \(3\) and described in the sections that follow.
Red Blood Cells
The most numerous cells in the blood are red blood cells, also called erythrocytes. One microliter of blood contains between 4.2 and 6.1 million red blood cells, and red blood cells make up about a quarter of all the cells in the human body. The cytoplasm of a mature red blood cell is almost completely filled with hemoglobin, the iron-containing protein that binds with oxygen and gives the cell its red color. Mature red blood cells lack a cell nucleus and most organelles in order to provide maximum space for hemoglobin. They are little more than sacks of hemoglobin.
Red blood cells also carry proteins called antigens that determine blood type. Blood type is a genetic characteristic. The best known human blood type systems are the ABO and Rhesus systems. These are described in the next section.
White Blood Cells (WBC)
White blood cells are cells in the blood that defend the body against invading microorganisms and other threats. There are far fewer white blood cells (also called leukocytes) than red blood cells in the blood. There are normally only about 1,000 to 11,000 white blood cells per microliter of blood. Unlike red blood cells, white blood cells have a nucleus. White blood cells are part of the body’s immune system. They destroy and remove old or abnormal cells and cellular debris, as well as attack pathogens and foreign substances. There are two categories of WBCs, granulocytes (contain visible granules in the cytoplasm) and agranulocytes (do not contain granules). The Granulocytes include neutrophils, eosinophils, and basophils. The agranulocytes include lymphocytes and monocytes. The five types differ in their specific immune functions. The relative percent and functions of WBCs are summarized in Table \(1\):
Table \(1\): Major Types of White Blood Cells
Type of Leukocyte Percent of All Leukocytes Main Function(s)
Neutrophil 62 Phagocytize (engulf and destroy) bacteria and fungi in the blood
Eosinophil 2 Attack and kill large parasites; carry out allergic responses
Basophil <1 Release histamines in inflammatory responses
Lymphocyte 30 Attack and destroy virus-infected and tumor cells; create lasting immunity to specific pathogens
Monocyte 5 Phagocytize pathogens and debris in tissues
Platelets
Platelets, also called thrombocytes, are actually cell fragments. Like red blood cells, they lack a nucleus and are more numerous than white blood cells. There are about 150,000 to 400,000 platelets per microliter of blood.
The main function of platelets is blood clotting or coagulation. This is the process by which blood changes from a liquid to a gel, forming a plug in a damaged blood vessel. If blood clotting is successful, it results in hemostasis, which is the cessation of blood loss from the damaged vessel. A blood clot consists of both platelets and proteins, especially the protein fibrin. You can see a scanning electron microscope micrograph of a blood clot in Figure \(4\).
Coagulation begins almost instantly after an injury to the endothelium of a blood vessel occurs. Platelets become activated and change their shape from spherical to star-shaped, as shown in Figure \(5\). This helps them aggregate with one another at the site of injury to start forming a plug in the vessel wall. Activated platelets also release substances into the blood that activate additional platelets and start a sequence of reactions leading to fibrin formation. Strands of fibrin crisscross the platelet plug and strengthen it, much as rebar strengthens concrete.
Formation and Degradation of Blood Cells
Blood is considered to be a connective tissue because blood cells form inside bones. All three types of blood cells are made in the red marrow of bones in a process called hematopoiesis. The formation of blood cells occurs by the proliferation of stem cells in the marrow. These stem cells are self-renewing: when they divide, some of the daughter cells remain stem cells, so the pool of stem cells is not used up. Other daughter cells follow various pathways to differentiate into a variety of types of blood cells. Once the cells have differentiated, they cannot divide to form copies of themselves.
Eventually, blood cells die and must be replaced through the formation of new blood cells from proliferating stem cells. After blood cells die, the dead cells are phagocytized (engulfed and destroyed) by white blood cells and removed from the circulation. This most often takes place in the spleen and liver.
Blood Disorders
Many human disorders primarily affect the blood. They include cancers, genetic disorders, poisoning by toxins, infections, and nutritional deficiencies.
• Leukemia is a group of cancers of the blood-forming tissues in the bone marrow. It is the most common type of cancer in children, although most cases occur in adults. Leukemia is generally characterized by large numbers of abnormal white blood cells. Symptoms may include excessive bleeding and bruising, fatigue, fever, and an increased risk of infections. Leukemia is thought to be caused by a combination of genetic and environmental factors.
• Hemophilia refers to any of several genetic disorders that cause dysfunction in the blood clotting process. People with hemophilia are prone to potentially uncontrollable bleeding even with otherwise inconsequential injuries. They also commonly suffer bleeding into the spaces between joints, which can cause crippling.
• Carbon monoxide poisoning occurs when inhaled carbon monoxide (for example, in fumes from a faulty home furnace) binds irreversibly to the hemoglobin in red blood cells. As a result, oxygen cannot bind to the red cells for transport throughout the body, and this can quickly lead to suffocation. Carbon monoxide is extremely dangerous because it is colorless and odorless so it cannot be detected in the air by human senses.
• HIV is a virus that infects certain types of white blood cells and interferes with the body’s ability to defend itself from pathogens and other causes of illness. HIV infection may eventually lead to AIDS or acquired immunodeficiency syndrome. AIDS is characterized by rare infections and cancers that people with a healthy immune system almost never acquire.
• Anemia is a disorder in which the blood has an inadequate volume of red blood cells. This reduces the amount of oxygen that the blood can carry and may cause weakness and fatigue. These and other signs and symptoms of anemia are shown in the figure below. Anemia has many possible causes, including excessive bleeding, inherited disorders such as sickle cell hemoglobin, or nutritional deficiencies (iron, folate, or B12). Severe anemia may require transfusions of donated blood.
Feature: Myth vs. Reality
Donating blood saves lives. In fact, with each blood donation, as many as three lives may be saved. The feeling that comes from knowing you have saved lives is well worth the short amount of time it takes to make a blood donation. Nonetheless, only a minority of potential donors actually donate blood. There are many myths about blood donation that contribute to the small percentage of donors. Knowing the facts may reaffirm your decision to donate if you are already a donor. If you aren’t a donor already, getting the facts may help you decide to become one.
Myth: Your blood might become contaminated with an infection during the donation.
Reality: There is no risk of contamination because only single-use, disposable catheters, tubing, and other equipment are used to collect blood for a donation.
Myth: You are too old (or too young) to donate blood.
Reality: There is no upper age limit on donating blood as long as you are healthy. The lower age limit is 16 years.
Myth: You can’t donate blood if you have high blood pressure.
Reality: As long as your blood pressure is below 180/100 at the time of donation, you can give blood. Even if you take blood pressure medication to keep your blood pressure below this level, you can donate.
Myth: You can’t give blood if you have high cholesterol.
Reality: Having high cholesterol does not affect your ability to donate blood. Taking cholesterol-lowering medication also does not disqualify you.
Myth: You can’t donate blood if you have had a flu shot.
Reality: Having a flu shot has no effect on your ability to donate blood. You can even donate on the same day that you receive a flu shot.
Myth: You can’t donate blood if you take medication.
Reality: As long as you are healthy, in most cases taking medication does not preclude you from donating blood.
Myth: Your blood isn’t needed if it’s a common blood type.
Reality: All types of blood are in constant demand.
Review
1. What is blood? Why is blood considered to be a connective tissue?
2. Identify four physiological roles of blood in the body.
3. Describe plasma and its components.
4. Identify red blood cells and their major function.
5. What are white blood cells? Which body system besides the cardiovascular system includes white blood cells?
6. Explain how platelets cause coagulation.
7. Summarize the formation and degradation of blood cells.
8. Identify three disorders of the blood.
9. For each of the descriptions below, choose the blood cell that best fits the description. Blood cells: red blood cells; white blood cells; platelets
1. Has a nucleus
2. Responsible for blood clotting
3. Carbon monoxide binds to a protein in these cells
10. What is another name for erythrocytes? What is another name for leukocytes?
11. True or False. Plasma refers to the cytoplasm within blood cells.
12. True or False. Platelets are cell fragments.
Explore More
https://bio.libretexts.org/link?16827#Explore_More
In this TED talk, inventor Joe Landolina talks about his potentially life-saving invention: a medical gel that can instantly stop bleeding.
Attributions
1. Dracula by Clifton Chu; public domain via Pixy.org
2. Blood Centrifugation by KnuteKnudsen licensed CC BY 3.0 via Wikimedia Commons
3. Formed elements by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
4. Blood clot in scanning electron microscopy by Janice Carr; CDC; public domain via Wikimedia Commons
5. Platelets by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.licensed CC BY 3.0 via Wikimedia Commons
6. Symptoms of anemia by Mikael Häggström; Public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/17%3A_Cardiovascular_System/17.5%3A_Blood.txt |
Giving the Gift of Life
Did you ever donate blood as the individual in Figure \(1\) is doing? If you did, then you probably know that your blood type is an important factor in blood transfusions. People vary in the type of blood they inherit, and this determines which type(s) of blood they can safely receive in a transfusion. Do you know your blood type?
What Are Blood Types?
Blood type (or blood group) is a genetic characteristic associated with the presence or absence of certain molecules, called antigens, on the surface of red blood cells. These molecules may help maintain the integrity of the cell membrane, act as receptors, or have other biological functions. A blood group system refers to all of the gene(s), alleles, and possible genotypes and phenotypes that exist for a particular set of blood type antigens. Human blood group systems include the well-known ABO and Rhesus (Rh) systems, as well as at least 33 others that are less well known.
Antigens and Antibodies
Antigens such as those on the red blood cells are molecules that the immune system identifies as either self (produced by your own body) or non-self (not produced by your own body). Blood group antigens may be proteins, carbohydrates, glycoproteins (proteins attached to chains of sugars), or glycolipids (lipids attached to chains of sugars), depending on the particular blood group system. If antigens are identified as nonself, the immune system responds by forming antibodies that are specific to the non-self antigens. Antibodies are large, Y-shaped proteins produced by the immune system that recognize and bind to non-self antigens. The analogy of a lock and key is often used to represent how an antibody and antigen fit together, as shown in Figure \(2\). When antibodies bind to antigens, it marks them for destruction by other immune system cells. Nonself antigens may enter your body on pathogens such as bacteria or viruses, on foods, or on red blood cells in a blood transfusion from someone with a different blood type than your own. The last way is virtually impossible nowadays because of effective blood typing and screening protocols.
Genetics of Blood Type
An individual’s blood type depends on which alleles for a blood group system were inherited from their parents. Generally, blood type is controlled by alleles for a single gene or for two or more very closely linked genes. Closely linked genes are almost always inherited together because there is little or no recombination between them. Like other genetic traits, a person’s blood type is generally fixed for life, but there are rare instances in which blood type can change. This could happen, for example, if an individual receives a bone marrow transplant to treat a disease such as leukemia. If the bone marrow comes from a donor who has a different blood type, the patient’s blood type may eventually convert to the donor’s blood type because red blood cells are produced in the bone marrow.
ABO Blood Group System
The ABO blood group system is the best known human blood group system. Antigens in this system are glycoproteins. These antigen compounds are shown in Figure \(3\). There are four common blood types for the ABO system:
1. Type A, in which only the A antigen is present
2. Type B, in which only the B antigen is present
3. Type AB, in which both the A and B antigens are present
4. Type O, in which neither the A nor the B antigen is present
Genetics of the ABO System
The ABO blood group system is controlled by a single gene on chromosome 9. There are three common alleles for the gene, often represented by the letters IA (or A), IB (or B), and i (or O). With three alleles, there are six possible genotypes for the ABO blood group. However, alleles IA and IB are both dominant to allele i and codominant to each other. This results in just four possible phenotypes (blood types) for the ABO system. These genotypes and phenotypes are shown in Table \(1\).
Table \(1\): ABO Blood Group System
Genotype Phenotype (Blood Type, or Group)
\(I^Ai\) A
\(I^AI^A\) A
\(I^AI^B\) AB
\(I^Bi\) B
\(I^BI^B\) B
\(ii\) O
The diagram in Figure \(4\) shows an example of how ABO blood type is inherited. In this particular example, the father has blood type A (genotype AO) and the mother has blood type B (genotype BO). This mating type can produce children with each of the four possible ABO phenotypes, although in any given family not all phenotypes may be present in the children.
Medical Significance of ABO Blood Type
The ABO system is the most important blood group system in blood transfusions. If red blood cells containing a particular ABO antigen are transfused into a person who lacks that antigen, the person’s immune system will recognize the antigen on the red blood cells as non-self. Antibodies specific to that antigen will attack the red blood cells, causing them to agglutinate, or clump and break apart. If a unit of incompatible blood were to be accidentally transfused into a patient, a severe reaction (called acute hemolytic transfusion reaction) is likely to occur in which many red blood cells are destroyed. This may result in kidney failure, shock, and even death. Fortunately, such medical accidents virtually never occur today.
ABO antibodies are likely to already be present in a recipient’s blood for antigens that the person lacks. These antibodies are produced in the first years of life by sensitization to similar antigens commonly occurring in the environment. Anti-A antibodies are thought to originate from an immune response to an antigen on the influenza virus, and anti-B antibodies are thought to originate from an immune response to an antigen found on bacteria such as E. coli. Once the antibodies have been produced, they circulate in the plasma. The relationship between ABO red blood cell antigens and plasma antibodies is shown in Table \(2\).
Table \(2\): The antibodies that circulate in the plasma are for different antigens than those on red blood cells, which are recognized as self-antigens.
Blood group A B AB O
Antibodies in plasma Anti-B Anti-A None Anti-A and Anti-B
Antigens on the red blood cell A antigen B antigen A and B antigen None
Which blood types are compatible and which are not? Type O blood contains both anti-A and anti-B antibodies, so people with type O blood can only receive type O blood. However, they can donate blood to people of any ABO blood type. That’s why individuals with type O blood are called universal donors. Type AB blood contains neither anti-A nor anti-B antibodies, so people with type AB blood can receive blood from people of any ABO blood type. That’s why individuals with type AB blood are called universal recipients. However, they can donate blood only to people who also have type AB blood. These and other relationships between blood types of donors and recipients are summarized in Figure \(5\).
ABO blood type antigens are found not only on red blood cells but also on platelets, in other body fluids such as tears and urine, and on cells of other types of tissues. Blood type compatibility is important to consider for successful organ transplantation. If a transplanted organ has nonself antigens for ABO, it may be attacked by antibodies and rejected by the body.
Rhesus Blood Group System
Another well-known blood group system is the Rhesus (Rh) blood group system. The Rhesus system has dozens of different antigens but only five main antigens (named D, C, c, E, and e). The major Rhesus antigen is the D antigen. People with the D antigen are called Rh-positive (Rh+), and people who lack the D antigen are called Rh-negative (Rh-). Rhesus antigens are thought to play a role in transporting ions across cell membranes by acting as channel proteins.
The Rhesus blood group system is controlled by two linked genes on chromosome 1. One gene, called RHD, produces a single antigen, antigen D. The other gene, called RHCE, produces the other four relatively common Rhesus antigens (C, c, E, and e), depending on which alleles for this gene are inherited.
Rhesus Blood Group and Transfusions
After the ABO system, the Rhesus system is the second most important blood group system in blood transfusions. The D antigen is the one most likely to provoke an immune response in people who lack the antigen. People who have the D antigen (Rh+) can be safely transfused with either Rh+ or Rh- blood, whereas people who lack the D antigen (Rh-) can be safely transfused only with Rh- blood.
Unlike anti-A and anti-B antibodies to ABO antigens, anti-D antibodies for the Rhesus system are not usually produced by sensitization to environmental substances. However, people who lack the D antigen (Rh-) may produce anti-D antibodies if exposed to Rh+ blood. This may happen accidentally in a blood transfusion, although this is extremely unlikely today. It may also happen during pregnancy with an Rh+ fetus if some of the fetal blood cells pass into the mother’s blood circulation.
Hemolytic Disease of the Newborn
If a woman who is Rh- is carrying an Rh+ fetus, the fetus may be at risk. This is especially likely if the mother has formed anti-D antibodies during a prior pregnancy because of a mixing of maternal and fetal blood during childbirth. Unlike antibodies against ABO antigens, antibodies against the Rhesus D antigen can cross the placenta and enter the blood of the fetus. This may cause hemolytic disease of the newborn (HDN), also called erythroblastosis fetalis, an illness in which fetal red blood cells are destroyed by maternal antibodies, causing anemia. This illness may range from mild to severe. If it is severe, it may cause brain damage and is sometimes fatal for the fetus or newborn. Fortunately, HDN can be prevented by preventing the formation of anti-D antibodies in the Rh- mother. This is achieved through an injection into the mother of a medication called Rho(D) immune globulin.
Feature: Myth vs. Reality
Myth: Your nutritional needs can be determined by your ABO blood type. Knowing your blood type allows you to choose the appropriate foods that will help you lose weight, increase your energy, and live a longer, healthier life.
Reality: This idea was proposed in 1996 in a New York Times bestseller Eat Right for Your Type, by Peter D’Adamo, a naturopath. Naturopathy is a method of treating disorders that involve the use of herbs, sunlight, fresh air, and other natural substances. Some medical doctors consider naturopathy a pseudoscience. A major scientific review of the blood type diet could find no evidence to support it. In one study, adults eating the diet designed for blood type A showed improved health, but this occurred in everyone regardless of their blood type. Because the blood type diet is based solely on blood type, it fails to account for other factors that might require dietary adjustments or restrictions. For example, people with diabetes but different blood types would follow different diets, and one or both of the diets might conflict with standard diabetes dietary recommendations and be dangerous.
Myth: ABO blood type is associated with certain personality traits. For example, people with blood type A are patient and responsible but may also be stubborn and tense, whereas people with blood type B are energetic and creative but may also be irresponsible and unforgiving. In selecting a spouse, both your own and your potential mate’s blood type should be taken into account to ensure the compatibility of your personality.
Reality: The belief that blood type is correlated with personality is widely held in Japan and other East Asian countries (the Japanese booth pictured below offers fortunes based on blood type). The idea was originally introduced in the 1920s in a study commissioned by the Japanese government but later shown to have no scientific support. The idea was revived in the 1970s by a Japanese broadcaster who wrote popular books about it. There is no scientific basis for the idea, and it is generally dismissed as pseudoscience by the scientific community. Nonetheless, it remains popular in East Asian countries, like astrology in many other countries.
Review
1. Define blood type and blood group system.
2. Explain the relationship between antigens and antibodies.
3. Identify the alleles, genotypes, and phenotypes in the ABO blood group system.
4. Discuss the medical significance of the ABO blood group system.
5. Give examples of how different ABO blood types vary in their susceptibility to diseases.
6. Describe the Rhesus blood group system.
7. Relate Rhesus blood groups to blood transfusions.
8. What causes hemolytic disease of the newborn?
9. A woman is blood type O and Rh- and her husband is blood type AB and Rh+. Answer the following questions about this couple and their offspring.
a. What are the possible genotypes of their offspring in terms of ABO blood group?
b. What are the possible phenotypes of their offspring in terms of ABO blood group?
c. Can the woman donate blood to her husband? Explain your answer.
d. Can the man donate blood to his wife? Explain your answer.
10. True or False. The D antigen is part of the ABO blood group system.
11. Explain why hemolytic disease of the newborn may be more likely to occur in a second pregnancy than in a first.
Explore More
Is malaria the reason so many people have type O blood? Listen to this fascinating National Public Radio interview with Dr. Christine Cserti-Gazdewich, a blood specialist (hematologist) at the University of Toronto, who discusses an emerging theory of universal blood. If you click on the icon of the podcast, you will find the transcript on their site.
Attributions
1. Offutt blood drive by Charles Haymond; public domain via Wikimedia Commons
2. Antibody by Fvasconcellos; public domain via Wikimedia Commons
3. ABO blood group diagram by InvictaHOG; public domain via Wikimedia Commons
4. ABO system codominance by NIH; public domain via Wikimedia Commons
5. Blood compatibility by InvictaHOG; public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/17%3A_Cardiovascular_System/17.6%3A_Blood_Types.txt |
Heart Attack on a Plate
Eating this greasy cheeseburger smothered in bacon may not literally cause a heart attack. However, regularly eating high-fat, low-fiber foods such as this may increase the risk of a heart attack or other type of cardiovascular disease. In fact, unhealthy lifestyle choices such as this may account for as many as 90 percent of cases of cardiovascular disease.
What Is Cardiovascular Disease?
Cardiovascular disease is a class of diseases that involve the cardiovascular system. They include diseases of the coronary arteries that supply the heart muscle with oxygen and nutrients; diseases of arteries such as the carotid artery that provide blood flow to the brain; and diseases of the peripheral arteries that carry blood throughout the body. Worldwide, cardiovascular disease is the leading cause of death, causing about a third of all deaths each year.
Most cases of the cardiovascular disease occur in people over the age of 60, with disease onset typically being about a decade earlier in males than females. The LGBT (lesbian, gay, bisexual, and transgender) community belongs to almost every race, ethnicity, religion, age, and socioeconomic group. The LGBT youth are at a higher risk for cardiovascular diseases, obesity, anxiety, and depression as compared to the general population. LGBT youth receive poor quality of care due to stigma, lack of healthcare providers’ awareness, and insensitivity to the unique needs of this community. Young LGBT individuals find it difficult to report their sexual identity to their clinicians. Some clinicians are not well trained in addressing the concerns of members of this community.
You can’t control your age or sex, but you can control other factors that increase the risk of cardiovascular disease. Not smoking, maintaining a healthy weight, eating a healthy diet, taking medications as needed to control diabetes and cholesterol, and getting regular exercise are all ways to prevent cardiovascular disease or keep it from progressing. It should be noted that high blood lipid levels are definitely risk factors for cardiovascular disease. High levels of cholesterol in the diet do not appear to lead directly to high levels of cholesterol in the blood. Clearly, cardiovascular disease is multifactorial in terms of its causes.
Precursors of Cardiovascular Disease
There are two very common conditions that are precursors to virtually all cases of cardiovascular disease: hypertension (hypertension) and atherosclerosis (hardening of blood wall). Both conditions affect the arteries and their ability to maintain normal blood flow.
Hypertension
Hypertension is a chronic medical condition in which the blood pressure in the arteries is persistently elevated, as defined in Table \(1\). Hypertension usually does not cause symptoms, so more than half of the people with high blood pressure are unaware of their condition. Hypertension is typically diagnosed when blood pressure is routinely measured during a medical visit for some other health problem.
Table \(1\): Classification of Blood Pressure (in Adults)
Category Systolic (mm Hg) Diastolic (mm Hg)
Normal blood pressure 90-119 60-79
Prehypertension 120-139 80-89
Hypertension 140 or higher 90 or higher
High blood pressure is classified as either primary or secondary high blood pressure. At least 90% of cases are primary high blood pressure, which is caused by some combination of genetic and lifestyle factors. Numerous genes have been identified as having small effects on blood pressure. Lifestyle factors that increase the risk of high blood pressure include excess dietary salt and alcohol consumption in addition to the risk factors for cardiovascular disease stated above. Secondary high blood pressure, which makes up the remaining 10% of cases of hypertension, is attributable to chronic kidney disease or an endocrine disorder such as Cushing’s disease.
Treating hypertension is important for reducing the risk of all types of cardiovascular disease, especially stroke. These and other complications of persistent high blood pressure are shown in Figure \(2\). Lifestyle changes, such as reducing salt intake and adopting a healthier diet may be all that is needed to lower blood pressure to the normal range. In many cases, however, medications are also required.
Atherosclerosis
Atherosclerosis is a condition in which artery walls thicken and stiffen as a result of the buildup of plaques inside the arteries. Plaques consist of white blood cells, cholesterol, and other fats. Typically, there is also a proliferation of smooth muscle cells that make the plaque fibrous as well as fatty. Over time, the plaques may harden with the addition of calcium crystals. This reduces the elasticity of the artery walls. As plaques increase in size, the artery walls dilate to compensate so blood flow is not affected. Eventually, however, the lumen of the arteries is likely to become so narrowed by plaque buildup that blood flow is reduced or even blocked entirely. Figure \(3\) illustrates the formation of a plaque in a coronary artery.
In most people, plaques start to form in arteries during childhood and progress throughout life. Individuals may develop just a few plaques or dozens of them. Plaques typically remain asymptomatic for decades. Signs and symptoms appear only after there is severe narrowing (stenosis) or complete blockage of arteries. As plaques increase in size and interfere with blood flow, they commonly lead to the formation of blood clots. These may plug arteries at the site of the plaque or travel elsewhere in the circulation. Sometimes plaques rupture or become detached from an arterial wall and become lodged in a smaller, downstream artery. Blockage of arteries by plaques or clots may cause a heart attack, stroke, or other potentially life-threatening cardiovascular events. If blood flow to the kidneys is affected, it may lead to chronic kidney disease.
The process in which plaques form is not yet fully understood, but it is thought that it begins when low-density lipoproteins (LDLs) accumulate inside endothelial cells in artery walls, causing inflammation. The inflammation attracts white blood cells that start to form a plaque. Continued inflammation and a cascade of other immune responses cause the plaque to keep growing. Risk factors for the development of atherosclerosis include hypertension, high cholesterol (especially LDL cholesterol), diabetes, and smoking. The chance of developing atherosclerosis also increases with age, male sex, and a family history of cardiovascular disease.
Treatment of atherosclerosis often includes both lifestyle changes and medications to lower cholesterol, control blood pressure, and reduce the risk of blood clot formation. In extreme cases or when other treatments are inadequate, surgery may be recommended. Surgery may involve the placement of stents in arteries to keep them open and improve blood flow or the use of grafts to divert blood flow around blocked arteries.
Coronary Artery Disease
Coronary artery diseases are a group of diseases that result from atherosclerosis of coronary arteries. Treatment of the diseases mainly involves treating underlying atherosclerosis. Two of the most common coronary artery diseases are angina and myocardial infarction.
Angina
Angina is chest pain or pressure that occurs when heart muscle cells do not receive adequate blood flow and become starved of oxygen (a condition called ischemia). It is illustrated in Figure \(4\). There may also be a pain in the back, neck, shoulders, or jaw; and in some cases, the pain may be accompanied by shortness of breath, sweating, or nausea. The main goals of the treatment of angina are relieving the symptoms and slowing the progression of underlying atherosclerosis.
Angina may be classified as either stable angina or unstable angina:
• Stable angina is angina in which pain is precipitated by exertion (say, from brisk walking or running) and improves quickly with rest or the administration of nitroglycerin, which dilates coronary arteries and improves blood flow. Stable angina may develop into unstable angina.
• Unstable angina is angina in which pain occurs during rest, lasts more than 15 minutes, and is of new onset. This type of angina is more dangerous and may be a sign of an imminent heart attack. It requires urgent medical attention.
Myocardial Infarction
A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow stops to a part of the heart causing damage to the heart muscle and the death of myocardial cells. An MI usually occurs because of complete blockage of a coronary artery, often due to a blood clot or the rupture of a plaque (Figure \(5\)). An MI typically causes chest pain and pressure, among other possible symptoms, but at least one-quarter of MIs do not cause any symptoms.
In the worst case, an MI may cause sudden death. Even if the patient survives, an MI often causes permanent damage to the heart. This puts the heart at risk of heart arrhythmias, heart failure, and cardiac arrest.
• Heart arrhythmias are abnormal heart rhythms, which are potentially life-threatening. Heart arrhythmias often can be interrupted with a cardiac defibrillator, which delivers an electrical shock to the heart, in effect “rebooting” it.
• Heart failure occurs when the pumping action of the heart is impaired so tissues do not get adequate oxygen. This is a chronic condition that tends to get worse over time, although it can be managed with medications.
• Cardiac arrest occurs when the heart no longer pumps blood or pumps blood so poorly that vital organs can no longer function. This is a medical emergency requiring immediate intervention.
Other Cardiovascular Diseases
Hypertension and atherosclerosis often cause other cardiovascular diseases. These commonly include stroke and peripheral artery disease.
Stroke
A stroke, also known as a cerebrovascular accident or brain attack, occurs when blocked or broken arteries in the brain result in the death of brain cells. There are two main types of stroke: ischemic stroke and hemorrhagic stroke. Ischemic storke is illustrated in Figure \(6\).
1. An ischemic stroke occurs when an embolus (blood clot) breaks off from a plaque or forms in the heart because of arrhythmia and travels to the brain where it becomes lodged in an artery. This blocks blood flow to the part of the brain that is served by arteries downstream from the blockage. Lack of oxygen causes the death of brain cells. Treatment with a clot-busting drug within a few hours of the stroke may prevent permanent damage. Almost 90 percent of strokes are ischemic strokes.
2. A hemorrhagic stroke occurs when an artery in the brain ruptures and causes bleeding in the brain. This deprives downstream tissues of adequate blood flow and also puts pressure on brain tissue. Both factors can lead to the death of brain cells. Surgery to temporarily open the cranium may be required to relieve the pressure. Only about 10 percent of strokes are hemorrhagic strokes, but they are more likely to be fatal than ischemic strokes.
In both types of stroke, the part of the brain that is damaged loses is the ability to function normally. Signs and symptoms of stroke may include an inability to move, feel, or see on one side of the body; problems understanding speech or difficulty speaking; memory problems; confusion; and dizziness. Hemorrhagic strokes may also cause a severe headache. The symptoms of a stroke usually occur within seconds or minutes of the brain injury. Depending on the severity of the stroke and how quickly treatment is provided, the symptoms may be temporary or permanent. If the symptoms of a stroke go away on their own in less than an hour or two, the stroke is called a transient ischemic attack. Stroke is the leading cause of disability in the United States, but rehabilitation with physical, occupational, speech, or other types of therapy may significantly improve functioning.
The main risk factor for stroke is high blood pressure. Therefore, keeping blood pressure within the normal range, whether with lifestyle changes or medications, is the best way to reduce the risk of stroke. Another possible cause of stroke is the use of illicit drugs such as amphetamines or cocaine. Having had a stroke in the past greatly increases one’s risk of future strokes. Men are also more likely than women to have strokes.
Peripheral Artery Disease
Peripheral artery disease (PAD) is the narrowing of the arteries other than those that supply the heart or brain due to atherosclerosis. Figure \(7\) shows how the PAD occurs. PAD most commonly affects the legs, but other arteries may also be involved. The classic symptom is leg pain when walking, which usually resolves with rest. This symptom is known as intermittent claudication. Other symptoms may include skin ulcers, bluish skin, cold skin, or poor nail and hair growth in the affected leg(s). However, up to half of all cases of PAD do not have any symptoms.
The main risk factor for PAD is smoking. Other risk factors include diabetes, high blood pressure, and high blood cholesterol. The underlying mechanism is usually atherosclerosis. PAD is typically diagnosed when blood pressure readings taken at the ankle are lower than blood pressure readings taken at the upper arm. It is important to diagnose PAD and treat underlying atherosclerosis because people with this disorder have a four to five times higher risk of myocardial infarction or stroke. Surgery to expand the affected arteries or to graft vessels in order to bypass blockages may be recommended in some cases.
Feature: My Human Body
You read in this concept about the many dangers of hypertension. Do you know whether you have hypertension? The only way to know for sure is to have your blood pressure measured. Measuring blood pressure is quick and painless, but several measurements are needed to accurately diagnose hypertension. Some people have what is called “white coat disease.” Their blood pressure rises just because they are being examined by a physician (in a white coat). Blood pressure also fluctuates from time to time due to factors such as hydration, stress, and time of day. Repeatedly measuring and recording your own blood pressure at home can provide your doctor with valuable diagnostic data. Digital blood pressure monitors for home use, like the one pictured in Figure \(8\), are relatively inexpensive, easy to use, and available at most pharmacies.
Review
1. What is cardiovascular disease? How much mortality do cardiovascular diseases cause?
2. List risk factors for cardiovascular disease.
3. What is hypertension?
4. Define atherosclerosis.
5. What is coronary artery disease?
6. Identify two specific coronary artery diseases.
7. Explain how a stroke occurs and how it affects the patient.
8. Describe the cause of peripheral artery disease.
9. What are two cardiovascular diseases that can be caused by atherosclerosis? Explain specifically how atherosclerosis contributes to each of them.
10. True or False. A heart attack is the same thing as cardiac arrest.
11. True or False. Plaques in arteries can cause blood clots.
12. What are the similarities between angina and ischemic stroke?
13. How can kidney disease be caused by problems in the cardiovascular system?
14. In peripheral artery disease, the blood pressure at the ankle is typically ________ the blood pressure at the upper arm.
A. erratic compared to B. the same as
C. higher than D. lower than
15. Name three components of the plaque that can build up in arteries.
Attributions
1. Bacon Cheeseburger by Like_the_Grand_Canyon licensed CC-BY 2.0 via Wikimedia Commons
2. Main complications of persistent high blood pressure Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain. via Wikimedia Commons
3. Coronary heart disease-atherosclerosis by NIH: National Heart, Lung and Blood Institute. Public Domain.
4. Angina by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons.
5. Heart Attack by NIH: National Heart, Lung and Blood Institute; public domain via Wikimedia Commons.
6. Stroke Ischemic by National Heart Lung and Blood Insitute (NIH); Public domain via Wikimedia Commons
7. Peripheral Arterial Disease by National Heart Lung and Blood Institute; public domain via Wikimedia Commons
8. Wrist style blood pressure monitor by Weeksgo licensed https://creativecommons.org/publicdomain/zero/1.0/deed.enCC0 via Wikimedia commons.
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
10. Some text is adapted from Health Care Disparities Among Lesbian, Gay, Bisexual, and Transgender Youth: A Literature Review; Hudaisa Hafeez, Muhammad Zeshan, Muhammad A Tahir, Nusrat Jahan, and Sadiq Naveed; . 2017 Apr; 9(4): e1184. Published online 2017 Apr 20. doi: 10.7759/cureus.1184; CC By 4.0. | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/17%3A_Cardiovascular_System/17.7%3A_Cardiovascular_Disease.txt |
Case Study Conclusion: Flight Risk
At the beginning of this chapter, you learned about Antônio and Ahaya, who met while sitting next to each other on a plane. During the flight, Ahaya got up to take frequent walks and was doing leg exercises to try to avoid the medical condition depicted in Figure \(1\)—deep vein thrombosis (DVT). DVT occurs when a blood clot forms in a deep vein, usually in the leg. It can be very dangerous—even deadly.
As you learned in this chapter, a blood clot is an aggregation of platelets and proteins. Blood clots are helpful to prevent blood loss when a blood vessel is damaged. But in some situations, they can be extremely dangerous. For example, blood clots can cause heart attacks or strokes by blocking the flow of blood to the heart or brain, respectively.
When DVT occurs, one of the major risks is pulmonary embolism (PE). PE is when the blood clot breaks off, travels through the blood vessels, and lodges in a pulmonary artery, like the blood clot shown in Figure \(2\). Recall what the pulmonary arteries do: they carry deoxygenated blood from the heart to the lungs, where the blood picks up oxygen and releases carbon dioxide due to gas exchange between the capillaries and the alveoli of the lungs. Imagine what would happen if this flow of blood to the lungs was partially or completely blocked by a blood clot. Depending on the size of the blood clot and where it is lodged, a PE can cause a variety of serious consequences ranging from lung damage to instant death, because of the disruption of the pulmonary circulation.
Ahaya has a higher risk of DVT and its consequences because he has heart failure. As you have learned, heart failure is a chronic condition in which the pumping action of the heart is impaired. One reason that heart failure is thought to increase the risk of DVT is that the blood is not being pushed strongly enough through the cardiovascular system, allowing blood clots to form more easily.
Ahaya needs to be particularly concerned about DVT while on a long plane flight. Why do you think this is? Think about how blood flows through arteries and veins. Blood is pushed through arteries mainly due to the pumping action of the heart. Veins, on the other hand, rely on the movement of the surrounding skeletal muscles to help push blood through them. Sitting still for long periods of time in cramped quarters, such as on a plane, can cause blood to pool in the deep veins of the legs. This can lead to the formation of a blood clot.
Chapter Summary
In this chapter, you learned about the structure, functions, and disorders of the cardiovascular system. Specifically, you learned that:
• The cardiovascular system is the organ system that transports materials to and from all the cells of the body. The main components of the cardiovascular system are the heart, blood vessels, and blood.
• The cardiovascular system has two interconnected circulations. The pulmonary circulation carries blood between the heart and lungs, where blood is oxygenated. The systemic circulation carries blood between the heart and the rest of the body, where it delivers oxygen.
• The heart is a muscular organ in the chest that consists mainly of cardiac muscle and pumps blood through blood vessels by repeated, rhythmic contractions.
• The wall of the heart consists of three layers. The middle layer, the myocardium, is the thickest layer and consists mainly of cardiac muscle.
• The interior of the heart consists of four chambers, with an upper atrium and lower ventricle on each side of the heart. Blood enters the heart through the atria, which pump it to the ventricles. Then the ventricles pump blood out of the heart. Four valves in the heart keep blood flowing in the correct direction and prevent backflow.
• Deoxygenated blood flows into the right atrium through veins from the upper and lower body (superior and inferior vena cava, respectively), and oxygenated blood flows into the left atrium through four pulmonary veins from the lungs. Each atrium pumps the blood to the ventricle below it. From the right ventricle, deoxygenated blood is pumped to the lungs through the two pulmonary arteries. From the left ventricle, oxygenated blood is pumped to the rest of the body through the aorta.
• The coronary circulation consists of blood vessels that carry blood to and from the heart muscle cells. There are two coronary arteries that supply the two sides of the heart with oxygenated blood. Cardiac veins drain deoxygenated blood back into the heart.
• The cardiac cycle refers to a single complete heartbeat. It includes diastole when the atria contract; and systole when the ventricles contract.
• The normal, rhythmic beating of the heart is called sinus rhythm. It is established by the heart’s pacemaker cells in the sinoatrial node. Electrical signals from the pacemaker cells travel to the atria and cause them to contract. Then the signals travel to the atrioventricular node and from there to the ventricles, causing them to contract. Electrical stimulation from the autonomic nervous system and hormones from the endocrine system can also influence heartbeat.
• Blood vessels carry blood throughout the body. Major types of blood vessels are arteries, veins, and capillaries.
• Arteries are blood vessels that usually carry blood away from the heart (except for coronary arteries that supply the heart muscle with blood). Most arteries carry oxygenated blood. The largest artery is the aorta, which is connected to the heart and extends into the abdomen. Blood moves through arteries due to pressure from the beating of the heart.
• Veins are blood vessels that usually carry blood toward the heart. Most veins carry deoxygenated blood. The largest veins are the superior and inferior venae cavae. Blood moves through veins by the squeezing action of surrounding skeletal muscles. Valves in veins prevent the backflow of blood.
• Capillaries are the smallest blood vessels. They connect arterioles and venules. They form capillary beds where substances are exchanged between the blood and surrounding tissues.
• The walls of arteries and veins have three layers. The middle layer is thickest in arteries, in which it contains smooth muscle tissue that controls the diameter of the vessels. The outer layer is thickest in veins and consists mainly of connective tissue. The walls of capillaries consist of little more than a single layer of epithelial cells.
• Blood pressure is a measure of the force that blood exerts on the walls of arteries. It is expressed as a double number, with the higher number representing systolic pressure when the ventricles contract and the lower number representing diastolic pressure when the ventricles relax. Normal blood pressure is generally defined as a pressure of 120/80 mm Hg or less.
• Vasoconstriction (narrowing) and vasodilation (widening) of arteries can occur to help regulate blood pressure or body temperature or to change blood flow as part of the fight-or-flight response.
• Blood is a fluid connective tissue that circulates throughout the body in blood vessels. Blood supplies tissues with oxygen and nutrients and removes their metabolic wastes. Blood helps defend the body from pathogens and other threats, transports hormones and other substances, and helps to keep the body’s pH and temperature in homeostasis. Blood consists of a liquid part, called plasma, and cells, including red blood cells, white blood cells, and platelets.
• Plasma makes up more than half of blood by volume. It consists of water and many dissolved substances. Blood also contains blood cells and platelets.
• Red blood cells are the most numerous cells in the blood and consist mostly of hemoglobin, which carries oxygen. Red blood cells also carry antigens that determine blood types. White blood cells are less numerous than red blood cells and are part of the body’s immune system. They protect the body from abnormal cells, microorganisms, and other harmful substances. There are several different types of white blood cells that differ in their specific immune functions.
• Platelets are cell fragments that play important roles in blood clotting or coagulation. They stick together at breaks in blood vessels to form a clot and stimulate the production of fibrin, which strengthens the clot.
• All blood cells form by the proliferation of stem cells in red bone marrow in a process called hematopoiesis. When blood cells die, they are phagocytized by white blood cells and removed from circulation.
• Disorders of the blood include leukemia, which is a cancer of the bone-forming cells; hemophilia, which is any of several genetic blood-clotting disorders; carbon monoxide poisoning, which prevents red blood cells from binding with oxygen and causes suffocation; HIV infection, which destroys certain white blood cells and can cause AIDS; and anemia, in which there are not enough red blood cells to carry adequate oxygen to body tissues.
• Blood type (or blood group) is a genetic characteristic associated with the presence or absence of antigens on the surface of red blood cells. A blood group system refers to all of the gene(s), alleles, and possible genotypes and phenotypes that exist for a particular set of blood type antigens.
• Cardiovascular disease is a class of diseases that involve the cardiovascular system. Worldwide, it is the leading cause of death. Most cases occur in people over age 60, and onset is typically about a decade earlier in males than females. Other risk factors include smoking, obesity, diabetes, high blood cholesterol, and lack of exercise.
• Antigens are molecules that the immune system identifies as either self or nonself. If antigens are identified as nonself, the immune system responds by forming antibodies that are specific to the nonself antigens, leading to the destruction of cells bearing the antigens.
• The ABO blood group system is a system of red blood cell antigens controlled by a single gene with three common alleles on chromosome 9. There are four possible ABO blood types: A, B, AB, and O. The ABO system is the most important blood group system in blood transfusions. People with type O blood are universal donors, and people with type AB blood are universal recipients.
• The Rhesus blood group system is a system of red blood cell antigens controlled by two genes with many alleles on chromosome 1. There are five common Rhesus antigens, of which antigen D is most significant. Individuals who have antigen D are called Rh+, and individuals who lack antigen D are called Rh-. Rh- mothers of Rh+ fetuses may produce antibodies against the D antigen in the fetal blood, causing hemolytic disease of the newborn (HDN).
• Two common conditions that lead to most cases of cardiovascular disease are hypertension and atherosclerosis. Hypertension is blood pressure that is persistently at or above 140/90 mm Hg. Atherosclerosis is a buildup of fatty, fibrous plaques in arteries that may reduce or block blood flow. Treating these conditions is important for preventing cardiovascular disease.
• Coronary artery disease is a group of diseases that result from atherosclerosis of coronary arteries. Two of the most common are angina and myocardial infarction (heart attack). In angina, cardiac cells receive inadequate oxygen, which causes chest pain. In a heart attack, cardiac cells die because blood flow to part of the heart is blocked. A heart attack may cause death or lead to heart arrhythmias, heart failure, or cardiac arrest.
• A stroke occurs when blocked or broken arteries in the brain result in the death of brain cells. This may occur when an artery is blocked by a clot or plaque or when an artery ruptures and bleeds in the brain. In both cases, part of the brain is damaged and functions such as speech and controlled movements may be impaired, either temporarily or permanently.
• Peripheral artery disease occurs when atherosclerosis narrows peripheral arteries, usually in the legs, often causing pain when walking. It is important to diagnose this disease so underlying atherosclerosis can be treated before it causes a heart attack or stroke.
In this chapter, you learned that the cardiovascular system carries nutrients to the cells of the body. Read the next chapter about the Digestive System to learn about how your body transforms your meals into the nutrients that cells need to function.
Chapter Summary Review Questions
1. Alex goes to the doctor and learns that his blood pressure is 135/90 mm Hg. Answer the following questions about his blood pressure.
1. Is this normal blood pressure? Why or why not?
2. Which number refers to the systolic pressure and which number refers to the diastolic pressure?
3. Describe what the atria and ventricles of Alex’s heart are doing when the pressure is at 135 mm Hg.
4. Alex’s doctor would like him to lower his blood pressure. Why do you think he would like Alex to do this and what are some ways in which he may be able to lower his blood pressure?
2. What are the three functions of the cardiovascular system?
3. What is the watery part of the blood called?
4. Which are the chambers of the heart that receive blood? Which are the chambers of the heart that pump blood out of the heart?
5. Which chambers of the heart contain deoxygenated blood?
1. Right and left atria
2. Right and left ventricles
3. Right atrium and right ventricle
4. Left atrium and left ventricle
6. What are two different places in the cardiovascular system where valves are located?
7. Valves prevent blood from flowing backward in the cardiovascular system. Why do you think this is important?
8. Explain how pacemaker cells in the heart work together with the nervous system to regulate the beating of the heart.
9. True or False. Red marrow is the source of white blood cells.
10. True or False. Plaques that build up in arteries are made completely of fats.
11. The aorta is:
1. An artery
2. A chamber of the heart
3. A vein
4. A valve
12. Compare the coronary arteries, pulmonary arteries, and arteries elsewhere in the body in terms of their target tissues (i.e. where they bring blood to) and whether they are carrying oxygenated or deoxygenated blood.
13. The superior vena cava empties into which structure of the heart? The inferior vena cava empties into which structure of the heart?
1. right atrium; left atrium
2. right atrium; right atrium
3. right atrium; right ventricle
4. right ventricle; left ventricle
14. Venules receive blood from what?
15. Define blood type and blood group system. Explain the relationship between antigens and antibodies.
16. Identify the alleles, genotypes, and phenotypes in the ABO blood group system.
17. Discuss the medical significance of the ABO blood group system.
18. Give examples of how different ABO blood types vary in their susceptibility to diseases.
19. Describe the Rhesus blood group system.
20. Relate Rhesus blood groups to blood transfusions.
21. What causes hemolytic disease of the newborn?
22. A woman is blood type O and Rh- and her husband is blood type AB and Rh+. Answer the following questions about this couple and their offspring.
1. What are the possible genotypes of their offspring in terms of ABO blood group?
2. What are the possible phenotypes of their offspring in terms of ABO blood group?
3. Can the woman donate blood to her husband? Explain your answer.
4. Can the man donate blood to his wife? Explain your answer.
23. True or False. The D antigen is part of the ABO blood group system.
24. Explain why hemolytic disease of the newborn may be more likely to occur in a second pregnancy than in a first.
25. Anemia causes weakness and fatigue due to a reduction in the amount of oxygen that gets to the cells of the body. Explain how oxygen is transported to the cells of the body and which blood cells are affected in anemia.
26. What are the two conditions that are precursors to virtually all cases of cardiovascular disease?
27. What is another name for a heart attack?
1. Heart arrhythmia
2. Myocardial infarction
3. Angina
4. Heart failure
28. True or False. Strokes can be due to a blood clot or to a broken artery.
29. True or False. After an injury to a blood vessel, platelets respond by losing their nuclei.
30. What are the main differences between coronary circulation, pulmonary circulation, and systemic circulation?
31. Define the term sinus rhythm.
32. Which is generally a more serious and immediately life-threatening condition: heart failure or cardiac arrest? Explain your answer.
33. Match each of the following descriptions to the disease or disorder from the list below that best matches it. Use each disease or disorder only once. Diseases and disorders: hypertension; atherosclerosis; coronary artery disease; peripheral artery disease
1. Blood flow to the muscle cells of the heart is impaired.
2. Blood pressure is too high.
3. Arteries in the legs specifically become more narrow.
4. Plaque builds up in arteries (in general).
Attributions
1. Deep vein thrombosis by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons.
2. Blood clot by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.CC BY 3.0 via Wikimedia Commons.
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/17%3A_Cardiovascular_System/17.8%3A_Case_Study_Conclusion%3A__Flight_and_Chapter_Summary.txt |
This chapter outlines the structure and function of the gastrointestinal tract and accessory organs of digestion. It explains the processes of peristalsis, mechanical and chemical digestion of food, and absorption of nutrients. The chapter also describes several disorders of the gastrointestinal tract.
• 18.1: Case Study: Food Processing
Rania can't eat gluten, because she has celiac disease. For her and others with the disease, eating even very small amounts of gluten causes an autoimmune reaction that results in damage to the small, finger-like villi lining the small intestine, causing them to become inflamed and flattened. As you read this chapter and learn about how the digestive system works, you will see just how important the villi of the small intestine are to the body as a whole. At the end of the chapter, you will lear
• 18.2: Introduction to the Digestive System
The digestive system consists of organs that break down food, absorb its nutrients, and expel any remaining waste. Organs of the digestive system are shown in the following figure. Most of these organs make up the gastrointestinal (GI) tract. Food actually passes through these organs. The rest of the organs of the digestive system are called accessory organs. These organs secrete enzymes and other substances into the GI tract, but food does not actually pass through them.
• 18.3: Digestion and Absorption
Digestion of food is a form of catabolism, in which the food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs when food is moved through the digestive system. It begins in the mouth and ends in the small intestine. The final products of digestion are absorbed from the digestive tract, primarily in the small intestine. There are two different types of digestion that occur in the digestive system: mechanical digestion and chemica
• 18.4: Upper Gastrointestinal Tract
Besides the esophagus, organs of the upper gastrointestinal (GI) tract include the mouth, pharynx, and stomach. These hollow organs are all connected to form a tube through which food passes during digestion. The only role in digestion played by the pharynx and esophagus is to move food through the GI tract. The mouth and stomach, in contrast, are organs where digestion, or the breakdown of food, also occurs. In both of these organs, food is broken into smaller pieces and broken down chemically.
• 18.5: Lower Gastrointestinal Tract
Most of the bacteria that normally live in the lower gastrointestinal (GI) tract live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine. The latter is arguably the most important organ of the digest
• 18.6: Accessory Organs of Digestion
Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food but through which food does not actually pass as it is digested. Besides the liver, the major accessory organs of digestion are the gallbladder and pancreas. These organs secrete or store substances that are needed for digestion in the first part of the small intestine, the duodenum, where most chemical digestion takes place.
• 18.7: Disorders of the Gastrointestinal Tract
Inflammatory bowel disease is a collection of inflammatory conditions primarily affecting the intestines. The two principal inflammatory bowel diseases are Crohn's disease and ulcerative colitis. Unlike Crohn's disease, which may affect any part of the GI tract and the joints as well as the skin, ulcerative colitis mainly affects just the colon and rectum. Both diseases occur when the body's own immune system attacks the digestive system.
• 18.8: Case Study Conclusion: Celiac and Chapter Summary
Bread and pasta are traditionally made with wheat, which contains proteins called gluten. As you learned in the beginning of the chapter, even trace amounts of gluten can damage the digestive system of people with celiac disease. When Rania and Tui met for lunch, Rania chose a restaurant that she knew could provide her with gluten-free options. Gluten is clearly dangerous for people with celiac disease, but should people who do not have celiac disease or other diagnosed medical problems with glu
Thumbnail: Scheme of digestive tract, with esophagus marked. ( CC BY-SA 2.5; Olek Remesz).
18: Digestive System
Case Study: Please Don’t Pass the Bread
Rania and Tui are college students who met in physics class. They decide to study together for their upcoming midterm, but first, they want to grab some lunch. Rania says there is a particular restaurant she would like to go to because they are able to accommodate her dietary restrictions. Tui agrees and they head to the restaurant.
At lunch, Tui asks Rania what is special about her diet. Rania tells her that she can’t eat gluten. Tui says, “Oh yeah, my cousin did that for a while because she heard that gluten is bad for you. But it was too hard for her to not eat bread and pasta, so she gave it up.” Rania tells Tui that avoiding gluten isn’t optional for her—she has celiac disease. Eating even very small amounts of gluten could damage her digestive system.
You have probably heard of gluten—but what is it and why is it harmful to people with celiac disease? Gluten is a protein found in wheat and some other grains such as barley, rye, and oats. Therefore, it is commonly found in foods such as bread, pasta, baked goods, and many packaged foods. In people with celiac disease, eating gluten causes an autoimmune reaction that results in damage to the small, finger-like villi lining the small intestine, causing them to become inflamed and flattened. This damage interferes with the digestive process, which can result in a wide variety of symptoms including diarrhea, anemia, skin rash, bone pain, depression, and anxiety, among others. The degree of damage to the villi can vary from mild to severe, with more severe damage generally resulting in more significant symptoms and complications. Celiac disease can have serious long-term consequences, such as osteoporosis, problems in the nervous and reproductive systems, and the development of certain types of cancers.
How can celiac disease cause so many different types of symptoms and have such significant negative health consequences? As you read this chapter and learn about how the digestive system works, you will see just how important the villi of the small intestine are to the body as a whole. At the end of the chapter, you will learn more about celiac disease, why it can be so serious, and whether it is worth avoiding gluten for people who do not have a diagnosed medical issue with it.
Chapter Overview: Digestive System
In this chapter, you will learn about the digestive system, which processes food so that our bodies can obtain nutrients. Specifically, you will learn about:
• The structures and organs of the gastrointestinal (GI) tract through which food directly passes. This includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
• The functions of the GI tract, including mechanical and chemical digestion, absorption of nutrients, and the elimination of solid waste.
• The accessory organs of digestion—the liver, gallbladder, and pancreas—secrete substances needed for digestion into the GI tract, in addition to other important functions.
• Specializations of the tissues of the digestive system that allow it to carry out its functions.
• How different types of nutrients such as carbohydrates, proteins, and fats are digested and absorbed by the body.
• Beneficial bacteria that live in the GI tract and help us digest food, produce vitamins, and protect us from harmful pathogens and toxic substances.
• Disorders of the digestive system, including inflammatory bowel diseases, ulcers, diverticulitis, and gastroenteritis (commonly known as “stomach flu”).
As you read this chapter, think about the following questions related to celiac disease:
1. What are the general functions of the small intestine? What do the villi in the small intestine do?
2. Why do you think celiac disease causes so many different types of symptoms and potentially serious complications?
3. What are some other autoimmune diseases that involve the body attacking its own digestive system?
Attributions
1. Pizzaberg sign by JMacPherson CC BY 2.0 via Flickr.com | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.1%3A_Case_Study%3A__Food_Processing.txt |
Yummy!
If you’re a dessert lover, then just the sight of this flan dish may make your mouth water. The “water” in your mouth is actually saliva, a fluid released by glands that are part of the digestive system. Saliva contains digestive enzymes among other substances important for digestion. When your mouth waters at the sight of a tasty treat, it’s a sign that your digestive system is preparing to digest food.
What Is the Digestive System?
The digestive system consists of organs that break down food, absorb its nutrients, and expel any remaining waste. Organs of the digestive system are shown in Figure \(2\). Most of these organs make up the gastrointestinal (GI) tract. Food actually passes through these organs. The rest of the organs of the digestive system are called accessory organs. These organs secrete enzymes and other substances into the GI tract, but food does not actually pass through them.
Functions of the Digestive System
The digestive system has three main functions relating to food: digestion of food, absorption of nutrients from food, and elimination of solid food waste. Digestion is the process of breaking down food into components the body can absorb. It consists of two types of processes: mechanical digestion and chemical digestion. Mechanical digestion is the physical breakdown of chunks of food into smaller pieces. This type of digestion takes place mainly in the mouth and stomach. Chemical digestion is the chemical breakdown (bonds are broken) of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by body fluids (blood or lymph). This type of digestion begins in the mouth and continues in the stomach but occurs mainly in the small intestine.
After food is digested, the resulting nutrients are absorbed. Absorption is the process in which substances pass into the bloodstream or lymph system to circulate throughout the body. The absorption of nutrients occurs mainly in the small intestine. Any remaining matter from food that is not digested and absorbed passes out of the body through the anus in the process of elimination.
Gastrointestinal Tract
The gastrointestinal (GI) tract is basically a long, continuous tube that connects the mouth with the anus. If it were fully extended, it would be about 9 meters (30 feet) long in adults. It includes the mouth, pharynx, esophagus, stomach, and small and large intestines. Food enters the mouth and then passes through the other organs of the GI tract where it is digested and/or absorbed. Finally, any remaining food waste leaves the body through the anus at the end of the large intestine. It takes up to 50 hours for food or food waste to make the complete trip through the GI tract.
Tissues of the GI Tract
The walls of the organs of the GI tract consist of four different tissue layers, which are illustrated in Figure \(3\): mucosa, submucosa, muscularis externa, and serosa.
1. The mucosa is the innermost layer surrounding the lumen, or open space within the organs of the GI tract. This layer consists mainly of the epithelium with the capacity to secrete and absorb substances. For example, the epithelium can secrete digestive enzymes and mucus, and it can absorb nutrients and water.
2. The submucosa layer consists of connective tissue that contains blood and lymph vessels and also nerves. The vessels are needed to absorb and carry away nutrients after food is digested, and nerves help control the muscles of the GI tract organs.
3. The muscularis externa layer contains two types of smooth muscle: longitudinal muscle and circular muscle. The longitudinal muscle runs the length of the GI tract organs and circular muscle encircles the organs. Both types of muscles contract to keep food moving through the track by the process of peristalsis (Figure \(4\).
4. The serosa layer is the outermost layer of the walls of GI tract organs. This is a thin layer that consists of connective tissue and separates the organs from surrounding cavities and tissues.
Peristalsis in the GI Tract
The muscles in the walls of GI tract organs enable peristalsis, which is illustrated in Figure \(4\). Peristalsis is a continuous sequence of involuntary muscle contraction and relaxation that moves rapidly along an organ like a wave, similar to the way a wave moves through a spring toy. Peristalsis in organs of the GI tract propels food through the tract.
Divisions of the GI Tract
The GI tract is often divided into an upper GI tract and a lower GI tract. For medical purposes, the upper GI tract is typically considered to include all the organs from the mouth through the first part of the small intestine, called the duodenum. For instructional purposes, it makes more sense to include the mouth through the stomach in the upper GI tract and all of the small intestine as well as the large intestine in the lower GI tract. The latter approach is followed here. All organs of GI which are discussed in the text are illustrated in Figure \(2\).
Upper GI Tract
The mouth is the first digestive organ that food enters. The sight, smell, or taste of food stimulates the release of digestive enzymes and other secretions by salivary glands inside the mouth. The major salivary gland enzyme is amylase. It begins the chemical digestion of carbohydrates by breaking down starches into sugar. The mouth also begins the mechanical digestion of food. When you chew, your teeth break, crush, and grind food into increasingly smaller pieces. Your tongue helps to mix the food with saliva and also helps you swallow.
A lump of swallowed food is called a bolus. The bolus passes from the mouth into the pharynx and from the pharynx into the esophagus. The esophagus is a long, narrow tube that carries food from the pharynx to the stomach. It has no other digestive functions. Peristalsis starts at the top of the esophagus when food is swallowed and continues down the esophagus in a single wave, pushing the bolus of food ahead of it.
From the esophagus, food passes into the stomach, where both mechanical and chemical digestion continue. The muscular walls of the stomach churn and mix the food, thus completing mechanical digestion as well as mixing the food with digestive fluids secreted by the stomach. One of these fluids is hydrochloric acid. As well as killing pathogens in food, it gives the stomach the low pH needed by digestive enzymes that work in the stomach. One of these enzymes is pepsin, which chemically digests proteins. The stomach stores the partially digested food until the small intestine is ready to receive it. Food that enters the small intestine from the stomach is in the form of a thick slurry (semi-liquid) called chyme.
Lower GI Tract
The small intestine is a narrow but very long tubular organ. It may be almost 7 meters (23 feet) long in adults. It is the site of most chemical digestion and virtually all absorption of nutrients. Many digestive enzymes are active in the small intestine, some of which are produced by the small intestine itself, and some of which are produced by the pancreas, an accessory organ of the digestive system. Much of the inner lining of the small intestine is covered by tiny finger-like projections called villi, each of which in turn is covered by even tinier projections called microvilli. These projections, shown in Figure \(5\), greatly increase the surface area through which nutrients can be absorbed from the small intestine.
The small intestine is made up of three parts:
1. The duodenum is the first part of the small intestine. It is also the shortest part. This is where most chemical digestion takes place.
2. The jejunum is the second part of the small intestine. This is where most nutrients are absorbed into the blood.
3. The ileum is the last part of the small intestine. A few remaining nutrients are absorbed in the ileum. From the ileum, any remaining food waste passes into the large intestine.
From the small intestine, any remaining nutrients and food waste pass into the large intestine. The large intestine is another tubular organ like the small intestine, but it is wider and shorter than the small intestine. It connects the small intestine and the anus. Waste that enters the large intestine is in a liquid state. As it passes through the large intestine, excess water is absorbed from it. The remaining solid waste, called feces, is eventually eliminated from the body through the anus.
Accessory Organs of the Digestive System
Accessory organs of the digestive system are not part of the GI tract, so they are not sites where digestion or absorption take place. Instead, these organs secrete or store substances that are needed for the chemical digestion of food. The accessory organs include the liver, gallbladder, and pancreas. They are shown in Figure \(6\) and described in the text:
• The liver is an organ that has a multitude of functions. Its main digestive function is producing and secreting a fluid called bile, which reaches the small intestine through a duct. Bile breaks down large globules of lipids into smaller ones that are easier for enzymes to chemically digest. Bile is also needed to reduce the acidity of food entering the small intestine from the highly acidic stomach because enzymes in the small intestine require a less acidic environment in order to work.
• The gallbladder is a small sac below the liver that stores some of the bile from the liver. The gallbladder also concentrates the bile by removing some of the water from it. It then secretes the concentrated bile into the small intestine as needed for fat digestion following a meal.
• The pancreas secretes many digestive enzymes and releases them into the small intestine for the chemical digestion of carbohydrates, proteins, and lipids. The pancreas also helps to lessen the acidity of the small intestine by secreting bicarbonate, a basic substance that neutralizes the acid.
Review
1. What is the digestive system?
2. What are the three main functions of the digestive system? Define each function.
3. Describe the GI tract.
4. Distinguish between the upper and lower GI tracts.
5. Relate the tissues in the walls of GI tract organs to the functions the organs perform.
6. Identify accessory organs of digestion and their general function in digestion.
7. Identify the points in the GI tract where food becomes a bolus, chyme, and feces, respectively.
8. Does food pass through the pancreas? Why or why not?
9. True or False. Absorption mainly occurs in the stomach.
10. True or False. Some chemical digestion occurs in the mouth.
11. Most chemical digestion occurs in the _____________ .
A. Gall bladder
B. Stomach
C. Small intestine
D. Large intestine
12. a. Describe one way in which proteins are at least partially chemically digested in the digestive system.
b. Describe one way in which carbohydrates are at least partially chemically digested in the digestive system.
13. If the villi in your small intestine were damaged and could not function normally, what effect might this have on your body? Explain your reasoning.
14. The esophagus is considered:
A. An accessory organ of the digestive system
B. Part of the upper GI tract
C. Part of the lower GI tract
D. The longitudinal muscle
Explore More
Check out this 3D animation video to see peristalsis in the Large Intestine:
Why do we feel hungry? Check out this video to learn more:
Attributions
1. Caramel cream flan by RitaE via Pixabay license
2. Digestive system diagram by Mariana Ruiz, Public Domain via Wikimedia Commons
3. Mucosa by National Institute of Health, Public Domain via Wikimedia Commons
4. Peristalsis by Zachary Wilson, CC BY NC 3.0 via CK12.org
5. Small Intestines from Microbiology by Open Stax CC BY 3.0
6. Gallbladder-Liver-Pancreas Location by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.2%3A_Introduction_to_the_Digestive_System.txt |
Indigestion
The process of digestion does not always go as it should. Many people suffer from indigestion, or dyspepsia, a condition of impaired digestion. Symptoms may include upper abdominal fullness or pain, heartburn, nausea, belching, or some combination of these symptoms. The majority of cases of indigestion occur without evidence of an organic disease that is likely to explain the symptoms. Anxiety or certain foods or medications (such as aspirin) may be contributing factors in these cases. In other cases, indigestion is a symptom of an organic disease, most often gastroesophageal reflux disease (GERD) or gastritis. In a small minority of cases, indigestion is a symptom of a peptic ulcer of the stomach or duodenum, usually caused by a bacterial infection. Very rarely, indigestion is a sign of cancer.
An occasional bout of indigestion is usually nothing to worry about, especially in people less than 55 years of age. However, if you suffer frequent or chronic indigestion, it’s a good idea to see a doctor. If an underlying disorder such as GERD or an ulcer is causing indigestion, this can and should be treated. If no organic disease is discovered, the doctor can recommend lifestyle changes or treatments to help prevent or soothe the symptoms of acute indigestion. Lifestyle changes might include modifications in eating habits, such as eating more slowly, eating smaller meals, or avoiding fatty foods. You also might be advised to refrain from taking certain medications, especially on an empty stomach. The use of antacids or other medications to relieve symptoms may also be recommended
Digestion
Digestion of food is a form of catabolism, in which the food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs when food is moved through the digestive system. It begins in the mouth and ends in the small intestine. The final products of digestion are absorbed from the digestive tract, primarily in the small intestine. There are two different types of digestion that occur in the digestive system: mechanical digestion and chemical digestion. Figure \(2\) summarizes the roles played by different digestive organs in mechanical and chemical digestion, both of which are described in detail in the text.
Mechanical Digestion
Mechanical digestion is a physical process in which food is broken into smaller pieces without becoming changed chemically. It begins with your first bite of food and continues as you chew food with your teeth into smaller pieces. The process of mechanical digestion continues in the stomach. This muscular organ churns and mixes the food it contains, an action that breaks any solid food into still smaller pieces.
Although some mechanical digestion also occurs in the intestines, it is mostly completed by the time food leaves the stomach. At that stage, food in the GI tract has been changed to the thick semi-fluid called chyme. Mechanical digestion is necessary so that chemical digestion can be effective. Mechanical digestion tremendously increases the surface area of food particles so they can be acted upon more effectively by digestive enzymes.
Chemical Digestion
Chemical digestion is the biochemical process in which macromolecules in food are changed into smaller molecules that can be absorbed into body fluids and transported to cells throughout the body. Substances in food that must be chemically digested include carbohydrates, proteins, lipids, and nucleic acids. Carbohydrates must be broken down into simple sugars, proteins into amino acids, lipids into fatty acids and glycerol, and nucleic acids into nitrogen bases and sugars. Some chemical digestion takes place in the mouth and stomach, but most of it occurs in the first part of the small intestine (duodenum).
Digestive Enzymes
Chemical digestion could not occur without the help of many different digestive enzymes. Enzymes are proteins that catalyze or speed up biochemical reactions. Digestive enzymes are secreted by exocrine glands or by the mucosal layer of the epithelium lining the gastrointestinal tract. In the mouth, digestive enzymes are secreted by salivary glands. The lining of the stomach secretes enzymes, as does the lining of the small intestine. Many more digestive enzymes are secreted by exocrine cells in the pancreas and carried by ducts to the small intestine. Table \(1\) lists several important digestive enzymes, the organs and/or glands that secrete them, and the compounds they digest. You can read more about them in the text.
Table \(1\): Digestive Enzymes
Digestive Enzyme Organ, Glands That Secretes It Compound It Digests
Amylase Salivary Glands, Pancreas Amylose (Polysaccharide)
Sucrase Small Intestine Sucrose (Disaccharide)
Lactase Small Intestine Lactose (Disaccharide)
Lipase Salivary Glands, Pancreas Lipid
Pepsin Stomach Protein
Trypsin Pancreas Protein
Chymotrypsin Pancreas Protein
Deoxyribonuclease Pancreas DNA
Ribonuclease Pancreas RNA
Nuclease Small Intestine Small Nucleic Acids
Chemical Digestion of Carbohydrates
About 80 percent of digestible carbohydrates in a typical Western diet are in the form of the plant polysaccharide amylose, which consists mainly of long chains of glucose and is one of two major components of starch. Additional dietary carbohydrates include the animal polysaccharide glycogen, along with some sugars, which are mainly disaccharides.
To chemically digest amylose and glycogen, the enzyme amylase is required. The chemical digestion of these polysaccharides begins in the mouth, aided by amylase in saliva. Saliva also contains mucus, which lubricates the food, and hydrogen carbonate, which provides the ideal alkaline conditions for amylase to work. Carbohydrate digestion is completed in the small intestine, with the help of amylase secreted by the pancreas. In the digestive process, polysaccharides are reduced in length by the breaking of bonds between glucose monomers. The macromolecules are broken down to shorter polysaccharides and disaccharides, resulting in progressively shorter chains of glucose. The end result is molecules of the simple sugars glucose and maltose (which consists of two glucose molecules), both of which can be absorbed by the small intestine.
Other sugars are digested with the help of different enzymes produced by the small intestine. For example, sucrose, or table sugar, is a disaccharide that is broken down by the enzyme sucrase to form glucose and fructose, which are readily absorbed by the small intestine. Digestion of the sugar lactose, which is found in milk, requires the enzyme lactase, which breaks down lactose into glucose and galactose, which are then absorbed by the small intestine. Fewer than half of all adults produce sufficient lactase to be able to digest lactose. Those who cannot are said to be lactose intolerant.
Chemical Digestion of Proteins
Proteins consist of polypeptides, which must be broken down into their constituent amino acids before they can be absorbed. Protein digestion occurs in the stomach and small intestine through the action of three primary enzymes: pepsin, secreted by the stomach; and trypsin and chymotrypsin secreted by the pancreas. The stomach also secretes hydrochloric acid, making the contents highly acidic, which is required for pepsin to work. Trypsin and chymotrypsin in the small intestine require an alkaline environment to work. Bile from the liver and bicarbonate from the pancreas neutralize the acidic chyme as it empties into the small intestine. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, these are further broken down into amino acids by other enzymes called peptidases, also secreted by the pancreas.
Chemical Digestion of Lipids
The chemical digestion of lipids begins in the mouth. The salivary glands secrete the digestive enzyme lipase, which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.
Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas as well as bile secreted by the liver. Bile is required for the digestion of lipids because lipids are oily and do not dissolve in the watery chyme. Bile emulsifies, or breaks up, large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease. The micelles provide a great deal more surface area to be acted upon by lipase and also point the hydrophilic (“water-loving”) heads of the fatty acids outward into the watery chyme. Lipase can then access and break down the micelles into individual fatty acid molecules.
Chemical Digestion of Nucleic Acids
Nucleic acids (DNA and RNA) in foods are digested in the small intestine with the help of both pancreatic enzymes and enzymes produced by the small intestine itself. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.
Chemical Digestion by Gut Flora
The human gastrointestinal tract is normally inhabited by trillions of bacteria, some of which contribute to digestion. Here are just two of dozens of examples:
1. The most common carbohydrate in plants, which is cellulose, cannot be digested by the human digestive system. However, tiny amounts of cellulose are digested by bacteria in the large intestine.
2. Certain bacteria in the small intestine help digest lactose, which many adults cannot otherwise digest. As a byproduct of this process, the bacteria produce lactic acid, which increases the release of digestive enzymes and the absorption of minerals such as calcium and iron.
Absorption
When digestion is finished, it results in many simple nutrient molecules that must go through the process of absorption from the GI tract by blood or lymph so they can be used by cells throughout the body. A few substances are absorbed in the stomach and large intestine. For example, water is absorbed in both of these organs, and some minerals and vitamins are also absorbed in the large intestine. However, about 95 percent of nutrient molecules are absorbed in the small intestine. The absorption of the majority of these molecules takes place in the second part of the small intestine, called the jejunum. However, there are a few exceptions. For example, iron is absorbed in the duodenum, and vitamin B12 is absorbed in the last part of the small intestine, called the ileum. After being absorbed in the small intestine, nutrient molecules are transported to other parts of the body for storage or further chemical modification. For example, amino acids are transported to the liver to be used for protein synthesis.
The epithelial tissue lining the small intestine is specialized for absorption. It has many wrinkles and is covered with villi and microvilli, creating an enormous surface area for absorption. As shown in Figure \(3\), each villus also has a network of blood capillaries and fine lymphatic vessels called lacteals close to its surface. The thin surface layer of epithelial cells of the villi transports nutrients from the lumen of the small intestine into these capillaries and lacteals. Blood in the capillaries absorbs most of the molecules, including simple sugars, amino acids, glycerol, salts, and water-soluble vitamins (vitamin C and the many B vitamins). Lymph in the lacteals absorbs fatty acids and fat-soluble vitamins (vitamins A, D, E, and K).
Feature: My Human Body
The process of digestion does not always go as it should. Many people suffer from indigestion, or dyspepsia, a condition of impaired digestion. Symptoms may include upper abdominal fullness or pain, heartburn, nausea, belching, or some combination of these symptoms. The majority of cases of indigestion occur without evidence of an organic disease that is likely to explain the symptoms. Anxiety or certain foods or medications (such as aspirin) may be contributing factors in these cases. In other cases, indigestion is a symptom of an organic disease, most often gastroesophageal reflux disease (GERD) or gastritis. In a small minority of cases, indigestion is a symptom of a peptic ulcer of the stomach or duodenum, usually caused by a bacterial infection. Very rarely, indigestion is a sign of cancer.
An occasional bout of indigestion is usually nothing to worry about, especially in people less than 55 years of age. However, if you suffer frequent or chronic indigestion, it’s a good idea to see a doctor. If an underlying disorder such as GERD or an ulcer is causing indigestion, this can and should be treated. If no organic disease is discovered, the doctor can recommend lifestyle changes or treatments to help prevent or soothe the symptoms of acute indigestion. Lifestyle changes might include modifications in eating habits, such as eating more slowly, eating smaller meals, or avoiding fatty foods. You also might be advised to refrain from taking certain medications, especially on an empty stomach. The use of antacids or other medications to relieve symptoms may also be recommended.
Review
1. Define digestion. Where does it occur?
2. Identify two organ systems that control the process of digestion by the digestive system.
3. What is mechanical digestion? Where does it occur?
4. Describe chemical digestion.
5. What is the role of enzymes in chemical digestion?
6. What is absorption? When does it occur?
7. a. Where does most absorption occur in the digestive system?
b. Why does most of the absorption occur in this organ and not earlier in the GI tract?
8. Name two digestive enzymes found in saliva and identify which type of molecule they digest.
9. a. Where is bile produced?
b. What are some functions of bile?
10. True or False. Pepsin digests cellulose.
11. True or False. Glucose can be absorbed by the body without being further broken down.
12. The pH of the stomach ___________ .
A. is neutral B. is alkaline
C. is acidic D. depends only on what you eat
13. Lymph absorbs __________ .
A. fatty acids B. sugars
C. amino acids D. vitamin C
Explore More
New research shows that babies born through vaginal birth actually have healthier gut flora, learn more here:
Attributions
1. Dyspepsia wafers, Public Domain via Flickr.com
2. Mechanical and Chemical Digestion by Open Stax College, CC BY 3.0 via Wikimedia Commons
3. Intestinal villus simplified by Snow93, Public Domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.3%3A_Digestion_and_Absorption.txt |
Head Stand
Did you ever wonder what would happen if you tried to swallow food while standing on your head? Many people think that food travels down the gullet from the mouth by the force of gravity. If that were the case, then the food you swallowed would stay in your throat while you were standing on your head. In reality, your position doesn’t have much to do with your ability to swallow. Food will travel from your mouth to your stomach whether you are standing upright or upside down. That’s because the tube the food travels through — the esophagus — moves the food along by the muscular contractions is known as peristalsis. The esophagus is one of several organs that make up the upper gastrointestinal tract.
Organs of the Upper Gastrointestinal Tract
Besides the esophagus, organs of the upper gastrointestinal (GI) tract include the mouth, pharynx, and stomach. These hollow organs are all connected to form a tube through which food passes during digestion. The only role in digestion played by the pharynx and esophagus is to move food through the GI tract. The mouth and stomach, in contrast, are organs where digestion, or the breakdown of food, also occurs. In both of these organs, food is broken into smaller pieces (mechanical digestion) as well as broken down chemically (chemical digestion). It should be noted that the first part of the small intestine (duodenum) is considered in some contexts to be part of the upper GI tract, but that practice is not followed here. You can read about the small intestine (and large intestine) in the concept Lower Gastrointestinal Tract.
Mouth
The mouth is the first organ of the GI tract. Most of the oral cavity is lined with a mucous membrane. This tissue produces mucus, which helps to moisten, soften, and lubricate food. Underlying the mucous membrane is a thin layer of smooth muscle to which the mucous membrane is only loosely connected. This gives the mucous membrane considerable ability to stretch as you eat food. The roof of the mouth, called the palate, separates the oral cavity from the nasal cavity. The front part is the hard palate, consisting of a mucous membrane covering a plate of bone. The back part of the palate is softer and more pliable, consisting of mucous membrane over muscle and connective tissue. The hard surface of the front of the palate allows for the pressure needed in chewing and mixing food. The soft, pliable surface of the back of the palate can move to accommodate the passage of food while swallowing. Muscles at either side of the soft palate contract to create the swallowing action.
Several specific structures in the mouth are specialized for digestion. These include salivary glands, tongue, and teeth.
Salivary Glands
The mouth contains three pairs of major salivary glands, which are shown in Figure \(2\). These three pairs are all exocrine glands that secrete saliva into the mouth through ducts.
1. The largest of the three major pairs of salivary glands are the parotid glands, which are located on either side of the mouth in front of the ears.
2. The next largest pair is the submandibular glands, located beneath the lower jaw.
3. The third pair is the sublingual glands, located underneath the tongue.
In addition to these three pairs of major salivary glands, there are also hundreds of minor salivary glands in the oral mucosa lining the mouth and on the tongue. Along with the major glands, most of the minor glands secrete the digestive enzyme amylase, which begins the chemical digestion of starch and glycogen (polysaccharides). However, the minor salivary glands on the tongue secrete the fat-digesting enzyme lipase, which in the mouth is called lingual lipase (to distinguish it from pancreatic lipase secreted by the pancreas).
The saliva secreted by the salivary glands mainly helps digestion, but it also plays other roles. It helps to maintain dental health by cleaning the teeth, and it contains antibodies that help protect against infection. By keeping the mouth lubricated, saliva also allows the mouth movements needed for speech.
Tongue
The tongue is a fleshy, muscular organ that is attached to the floor of the mouth by a band of ligaments that gives it great mobility. This is necessary so the tongue can manipulate food for chewing and swallowing. Movements of the tongue are also necessary for speaking. The upper surface of the tongue is covered with tiny projections called papillae, which contain taste buds. The latter are collections of chemoreceptor cells. These sensory cells sense chemicals in food and send the information to the brain via cranial nerves, thus enabling the sense of taste.
Teeth
The teeth are complex structures made of a bone-like material called dentin and covered with enamel, which is the hardest tissue in the body. Adults normally have a total of 32 teeth, with 16 in each jaw. The right and left sides of each jaw are mirror images in terms of the numbers and types of teeth they contain. Teeth have different shapes to suit them for different aspects of mastication (chewing).
Pharynx
The tube-like pharynx (Figure \(3\)) plays a dual role as an organ of both respiration and digestion. As part of the respiratory system, it conducts air between the nasal cavity and larynx. As part of the digestive system, it allows swallowed food to pass from the oral cavity to the esophagus. Anything swallowed has priority over inhaled air when passing through the pharynx. During swallowing, the backward motion of the tongue causes a flap of elastic cartilage, called the epiglottis, to close over the opening to the larynx. This prevents food or drinks from entering the larynx.
Esophagus
The esophagus, which is shown in Figure \(4\), is a muscular tube through which food is pushed from the pharynx to the stomach. The esophagus passes through an opening in the diaphragm (the large breathing muscle that separates the abdomen from the thorax) before reaching the stomach. In adults, the esophagus averages about 25 cm (10 in.) in length, depending on a person’s height. The inner lining of the esophagus consists of a mucous membrane, which provides a smooth, slippery surface for the passage of food. The cells of this membrane are constantly being replaced as they are worn away from the frequent passage of food over them.
When food is not being swallowed, the esophagus is closed at both ends by upper and lower esophageal sphincters. Sphincters are rings of muscle that can contract to close off openings between structures. The upper esophageal sphincter is triggered to relax and open by the act of swallowing, allowing a bolus of food to enter the esophagus from the pharynx. Then the esophageal sphincter closes again to prevent food from moving back into the pharynx.
Once in the esophagus, the food bolus travels down to the stomach, pushed along by the rhythmic contraction and relaxation of muscles (peristalsis). The lower esophageal sphincter is located at the junction between the esophagus and the stomach. This sphincter opens when the bolus reaches it, allowing the food to enter the stomach. The sphincter normally remains closed at other times to prevent the contents of the stomach from entering the esophagus. Failure of this sphincter to remain completely closed can lead to heartburn. If it happens chronically, it can lead to gastroesophageal reflux disease (GERD), in which the mucous membrane of the esophagus may become damaged by the highly acidic contents of the stomach.
Stomach
The stomach (Figure \(4\) is a J-shaped organ that is joined to the esophagus at its upper end and to the first part of the small intestine (duodenum) at its lower end. When the stomach is empty of food, it normally has a volume of about 75 mL. However, it can expand to hold up to about a liter of food. Waves of muscle contractions (peristalsis) passing through the muscular walls of the stomach cause the food inside to be mixed and churned. The wall of the stomach has an extra layer of muscle tissue not found in other organs of the GI tract that helps it squeeze and mix the food. These movements of the stomach wall contribute greatly to mechanical digestion by breaking the food into much smaller pieces. The churning also helps to mix the food with stomach secretions that aid in its chemical digestion.
Secretions of the stomach include gastric acid, which consists mainly of hydrochloric acid. This makes the stomach contents highly acidic, which is necessary so that the enzyme pepsin — also secreted by the stomach — can begin the digestion of protein. Mucus is secreted by the lining of the stomach to provide a slimy protective coating against the otherwise damaging effects of gastric acid. The fat-digesting enzyme lipase is secreted in small amounts in the stomach, but very little fat digestion occurs there.
By the time food has been in the stomach for about an hour, it has become the thick, semi-liquid chyme. When the small intestine is ready to receive chyme, a sphincter between the stomach and duodenum, called the pyloric sphincter, opens to allow the chyme to enter the small intestine for further digestion and absorption.
Feature: Reliable Sources
The ongoing epidemic of obesity has led to the development of several different bariatric surgeries that modify the stomach to help obese patients reduce their food intake and lose weight. Go online to learn more about bariatric surgery. Find sources you judge to be reliable that answer the following questions:
1. Who qualifies for bariatric surgery?
2. Describe the bariatric surgeries commonly called stomach stapling, lap band, and gastric sleeve. How does each type of surgery modify the stomach? How effective is each type in terms of weight loss?
3. What are the major potential risks of bariatric surgery?
4. Besides weight loss, what other benefits have been shown to result from bariatric surgery?
Review
1. List organs of the upper gastrointestinal tract.
2. Identify structures in the mouth that are specialized for digestion.
3. Describe digestion in the mouth.
4. What general role do the pharynx and esophagus play in the digestion of food?
5. How does food travel through the esophagus?
6. Describe digestion in the stomach.
7. In which of the following structures does chemical digestion occur?
A. Mouth
B. Esophagus
C. Stomach
D. A and C
E. A, B, and C
8. In which of the following structures does mechanical digestion occur?
A. Mouth
B. Pharynx
C. Stomach
D. A and C
E. A, B, and C
9. Describe the differences between how air and food normally move past the pharynx.
10. Name two structures in the mouth that contribute to mechanical digestion.
11. What structure normally keeps stomach contents from backing up into the esophagus?
12. True or False. Peristalsis occurs in the esophagus, but not in the stomach.
13. True or False. The sense of taste is due to the detection of chemicals by the tongue.
14. Where is most of your food located thirty minutes after you eat a meal? Explain your answer.
15. What are the two roles of mucus in the upper GI tract?
Explore More
In June of 2016, the U.S. Food and Drug Administration passed final approval on a new surgically implanted medical device that obese patients can use to pump out the contents of their stomach in order to lose weight. Although critics of the device refer to it as “assisted bulimia,” implanting the device is reversible and less invasive than traditional bariatric surgery. Clinical trials have also established its efficacy for significant weight loss. Watch this short video to see how the device works:
Check out this video to learn about how things like taste and smell are perceived by the brain:
Attributions
1. Yoga by Anant_762 via Pixabay license
2. Salivary glands by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
3. Head and neck by Prof. Squirrel, Public Domain via Wikimedia Commons
4. Esophagus and stomach by NIAID, CC BY 2.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.4%3A_Upper_Gastrointestinal_Tract.txt |
What Is It?
Figure \(1\) shows some of the cells of what has been called “the last human organ to be discovered.” This “organ” weighs about 200 grams (0.44 lb.) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It’s the mass of bacteria that live in our lower gastrointestinal tract.
Organs of the Lower Gastrointestinal Tract
Most of the bacteria that normally live in the lower gastrointestinal (GI) tract live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine. The latter is arguably the most important organ of the digestive system. It is where most chemical digestion and virtually all absorption of nutrients take place.
Small Intestine
The small intestine (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m (15 ft) in females and 6.9 m (22 ft 8 in.) in males. It is approximately 2.5 to 3.0 cm (1.0 to 1.2 in.) in diameter (it is called “small” because it is much smaller in diameter than the large intestine). The internal surface area of the small intestine totals an average of about 30 m2 (323 ft2). Structurally and functionally, the small intestine can be divided into three parts, called the duodenum, jejunum, and ileum, as shown in Figure \(2\) and described below.
The mucosa lining the small intestine is very wrinkled and covered with finger-like projections called villi. In fact, each square inch of mucosa contains around 20,000 villi. The individual cells on the surface of the villi also have many finger-like projections, the microvilli shown in Figure \(3\). There are thought to be well over 100 billion microvilli per square inch of intestinal mucosa! All of these wrinkles, villi, and microvilli greatly increase the surface area for chyme to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous surface area for the absorption of nutrients. Inside each of the villi is a network of tiny blood and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.
Duodenum
The duodenum is the first part of the small intestine, directly connected to the stomach. It is also the shortest part of the small intestine, averaging only about 25 cm (10 in.) in length in adults. Its main function is chemical digestion, and it is where most of the chemical digestion in the entire GI tract takes place.
The duodenum receives partially digested, semi-liquid chyme from the stomach. It receives digestive enzymes and alkaline bicarbonate from the pancreas through the pancreatic duct, and it receives bile from the liver via the gallbladder through the common bile duct (Figure \(4\)). In addition, the lining of the duodenum secretes digestive enzymes and contains glands — called Brunner’s glands — that secrete mucus and bicarbonate. The bicarbonate from the pancreas and Brunner’s glands as well as bile from the liver neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.
Digestion of Carbohydrates in the Duodenum
Complex carbohydrates such as starches are broken down by the digestive enzyme amylase from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple sugars). Disaccharides, including sucrose and lactose, are broken down into simple sugars by duodenal enzymes: sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum and ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.
Digestion of Proteins in the Duodenum
In the duodenum, the pancreatic enzymes trypsin and chymotrypsin cleave proteins into peptides. Then, these smaller molecules are broken down into amino acids. Their digestion is catalyzed by pancreatic enzymes called peptidases.
Digestion of Lipids in the Duodenum
Pancreatic lipase breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of bile secreted by the liver and stored in the gall bladder. Bile salts attach to triglycerides to help them emulsify or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases the access to the molecules by pancreatic lipase.
Jejunum
The jejunum is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (8.2 ft) long. Its main function is the absorption of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple diffusion (water and fatty acids), facilitated diffusion (the simple sugar fructose), or active transport (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the blood except for fatty acids and fat-soluble vitamins, which are absorbed into the lymph. Although most nutrients are absorbed in the jejunum, there are a few exceptions:
• Iron is absorbed in the duodenum.
• Vitamin B12 and bile salts are absorbed in the ileum.
• Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.
Ileum
The ileum is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (9.8 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in chyme that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.
Large Intestine
The large intestine, also called the large bowel, is the last organ of the GI tract. In adults, it averages about 1.5 m (5 ft) in length. It is shorter than the small intestine but at least twice as wide, averaging about 6.5 cm (2.5 in.) in diameter. Water is absorbed from the chyme as it passes through the large intestine, turning the chyme into solid feces. Feces is stored in the large intestine until it leaves the body during defecation.
Parts of the Large Intestine
Like the small intestine, the large intestine can be divided into several parts, as shown in Figure \(5\). The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. The first part of the large intestine, where chyme enters from the small intestine, is called the cecum. From the cecum, the large intestine continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The rectum stores feces until elimination occurs. It transitions to the final part of the large intestine, called the anus, which has an opening to the outside of the body for feces to pass through.
A projection from the cecum of the colon is known as the appendix. The function of the appendix is uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon’s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered to be dispensable. As such, it is often referred to as a vestigial organ, which is a previously useful organ that has been retained over evolutionary time as part of the anatomy, even though it no longer has a function in the body.
Functions of the Large Intestine
The removal of water from chyme to form feces starts in the ascending colon and continues throughout much of the length of the organ. Salts such as sodium are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts as well as water to be recycled in the body.
The large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases that are responsible for intestinal gas, or flatulence. These bacteria are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still, other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.
Feature: My Human Body
Colorectal cancer, or cancer of the colon or rectum, is the fourth most common cancer in the United States. It is also the second most common cause of cancer deaths in this country. Widespread screening of patients for signs of colorectal cancer has significantly lowered the death rate in recent years. Because early-stage colorectal cancer is usually asymptomatic, routine screening is important for identifying cancer early when chances of a cure are still high.
Screening for colorectal cancer has also become easier and less invasive in recent years. One way to test for colorectal cancer is to examine a sample of stool and look for occult (hidden from the unaided eye) blood in the stool. This test is based on the assumption that blood vessels in cancer are fragile and may be easily damaged by the passage of feces through the colon or rectum. The damaged vessels may bleed into the feces but rarely bleed enough for blood to be visible in the stool. Stool for the occult blood test can be collected by the patient at home with a test kit provided by a doctor. If occult blood is detected in the stool, a different type of follow-up test is generally needed to determine whether cancer is the cause of the bleeding.
A similarly simple and noninvasive but more definitive test for colorectal cancer looks for DNA from cancer cells in the stool. Again, the patient can collect the stool sample at home using a simple test kit and mail the specimen to a lab, which does the analysis. If the test comes back positive, then a direct visual examination of the colon and rectum by colonoscopy is required.
Colonoscopy is the gold standard for the diagnosis of colorectal cancer. Using a tiny camera at the end of a long tube inserted up into the colon, a doctor can directly visualize the lining of the large intestine and spot any suspicious areas that may be cancerous. While a colonoscopy is invasive and requires the patient to prepare for the test for a couple of days by changing his or her diet and drinking special fluids, it reveals more than just cancer. A colonoscopy also reveals any growths called polyps in the colon. Colon polyps are not cancer but often develop into cancerous lesions, so if they are found during a colonoscopy, a surgical instrument inserted with the scope is usually used to remove them. Therefore, a colonoscopy can not only detect cancer in its early stages, but it can even help prevent cancer by enabling the removal of potentially pre-cancerous polyps.
A test similar to colonoscopy may be done in some patients. Called a flexible sigmoidoscopy, it allows a doctor to use a small camera to inspect the rectum and lower third of the colon, where the majority of cases of colorectal cancer occur. However, the rest of the colon cannot be examined with a sigmoidoscopy. Another alternative to a full-blown colonoscopy is a virtual colonoscopy, in which a CT scan of the rectum and colon is used to make detailed cross-sectional images of the organs. The images can then be studied by a specialist to detect cancers or polyps. For both of these colonoscopy alternatives, a follow-up colonoscopy is required if polyps or potentially cancerous lesions are detected.
Unless you have a family history of colorectal cancer or certain other risk factors, you probably do not need to start routine screening for the disease until middle age. Your doctor can tell you the most appropriate starting age for your particular case, given your risk factors and current cancer guidelines. You may be able to be screened with one of the less invasive methods rather than colonoscopy until you are somewhat older. Again, check with your doctor for specific recommendations. All of the testing methods have pros and cons that should be taken into consideration by a given patient and medical provider.
Review
1. Which organs are included in the lower GI tract?
2. Name the parts of the small intestine.
3. How is the mucosa of the small intestine specialized for digestion and absorption?
4. What digestive substances are secreted into the duodenum, and what compounds in food do they help digest?
5. What is the main function of the jejunum?
6. What roles are played by the ileum?
7. Name the parts of the large intestine.
8. Identify the main functions of the large intestine.
9. How do beneficial bacteria in the large intestine help the human organism?
10. True or False. The first part of the large intestine is where most chemical digestion takes place.
11. True or False. The small intestine is actually longer than the large intestine.
12. When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer.
13. Arrange the following parts of the lower GI tract in order of how food passes through them, from earliest to latest. Note that not all parts are listed.
cecum; duodenum; sigmoid colon; jejunum; rectum; ileum
14. Which enzyme digests proteins?
A. trypsin
B. amylase
C. lipase
D. lactase
15. What is intestinal gas, or flatulence, due to?
Explore More
Did you know you have about 100 million neurons in your intestines? In this fascinating TED talk, food scientist Heribert Watzke talks about the "hidden brain" in our gut and the surprising things it makes us feel.
Attributions
1. Bacteroides biacutis by US government, Public Domain via Wikimedia Commons
2. Small intestine anatomy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
3. Human jejunum microvilli by Louisa Howard, Katherine Connollly, Public Domain via Wikimedia Commons
4. biliary tract medium by www.6xc.com.au6XC, CC BY-NC-ND 4.0
5. Large intestine by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.5%3A_Lower_Gastrointestinal_Tract.txt |
Jaundiced Eyes
Did you ever hear of a person looking at something or someone with a “jaundiced eye”? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice or yellowing of the eyes and skin. Jaundice is likely to be a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile also has an important role in digestion, making the liver an accessory organ of digestion.
What Are Accessory Organs of Digestion?
Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food but through which food does not actually pass as it is digested. Besides the liver, the major accessory organs of digestion are the gallbladder and pancreas. These organs secrete or store substances that are needed for digestion in the first part of the small intestine, the duodenum, where most chemical digestion takes place. You can see the three organs and their locations in Figure \(2\).
Liver
The liver is a vital organ located in the upper right part of the abdomen. It lies just below the diaphragm to the right of the stomach. The liver plays an important role in digestion by secreting bile. However, the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.
Structure of the Liver
The liver is a reddish-brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.
As shown in Figure \(3\), the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead to the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.
Functions of the Liver
The main digestive function of the liver is the production of bile. Bile is a yellowish alkaline liquid that consists of water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by hepatocytes; the rest are extracted from the blood.
As shown in the figure below, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct shown in Figure \(4\)
• The liver synthesizes glycogen from glucose and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.
• The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the minerals iron and copper.
• The liver synthesizes numerous proteins and many of the amino acids needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.
• The liver synthesizes many important lipids, including cholesterol, triglycerides, and lipoproteins.
• The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the kidneys, which excrete them in the urine.
The liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic location and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.
Gallbladder
The gallbladder is a small, hollow, pouch-like organ that lies just under the right side of the liver (Figure \(2\) and Figure \(3\)). It is about 8 cm (3.1 in.) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 and 60 mL (1-2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.
Pancreas
The pancreas is a glandular organ that is part of both the digestive system and the endocrine system. As shown in Figure \(4\), it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (6 in.) long, and it has two major ducts, the main pancreatic duct, and the accessory pancreatic duct. Both of these ducts drain into the duodenum.
As an endocrine gland, the pancreas secretes several hormones, including insulin and glucagon, which circulate in the blood. The endocrine hormones are secreted by clusters of cells called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive enzymes and also bicarbonate, which helps to neutralize acidic chyme after it enters the duodenum. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down carbohydrates, proteins, lipids, and nucleic acids in chyme. The pancreatic digestive enzymes include:
• amylase, which helps to digest starch and other carbohydrates.
• trypsin and chymotrypsin, which help to digest proteins.
• lipase, which helps to digest lipids.
• deoxyribonucleases and ribonucleases, which help to digest nucleic acids.
Review
1. Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?
2. Describe the liver and its blood supply.
3. Explain the main digestive function of the liver.
4. Besides its role as a digestive organ, what other vital functions does the liver have?
5. What is the gallbladder? How does it aid in digestion in the duodenum?
6. Which two body systems include the pancreas? What type of secretions does the pancreas release as part of each body system?
7. List pancreatic enzymes that work in the duodenum and the substances they help digest.
8. What are two substances produced by accessory organs of digestion that help neutralize chyme in the small intestine, and where are they produced?
9. People who have their gallbladder removed sometimes have digestive problems after eating high-fat meals. Why do you think this happens?
10. True or False. The liver is a gland.
11. True or False. Substances secreted by the pancreas enter into the duodenum from the common bile duct.
12. True or False. Bile contains wastes.
13. Which accessory organ of digestion synthesizes cholesterol?
Attributions
1. Jaundice eye by CDC/Dr. Thomas F. Sellers/Emory University, Public Domain via Wikimedia Common
2. Gallbladder, liver, pancreases location by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
3. Liver and nearby organs by Don Bliss (Illustrator) National Cancer Institute, Public Domain via Wikimedia Common
4. Pancreas anatomy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.6%3A_Accessory_Organs_of_Digestion.txt |
Crohn’s Rash
If you had a skin rash like the one in Figure \(1\), you probably wouldn’t assume that it was caused by a digestive system disease. However, that’s exactly why the individual in the picture has a rash. He has a gastrointestinal (GI) tract disorder called Crohn’s disease. This disease is one of a group of GI tract disorders that are known collectively as inflammatory bowel disease. Unlike other inflammatory bowel diseases, signs and symptoms of Crohn’s disease may not be confined to the GI tract.
Inflammatory Bowel Disease
Inflammatory bowel disease is a collection of inflammatory conditions primarily affecting the intestines. The two principal inflammatory bowel diseases are Crohn’s disease and ulcerative colitis. Unlike Crohn’s disease, which may affect any part of the GI tract and the joints as well as the skin, ulcerative colitis mainly affects just the colon and rectum. Both diseases occur when the body’s own immune system attacks the digestive system. Both diseases also typically first appear in the late teens or early twenties and occur equally in all sexes and genders.
Crohn’s Disease
Crohn’s disease is a type of inflammatory bowel disease that may affect any part of the GI tract from the mouth to the anus, among other body tissues. The most commonly affected region is the ileum, which is the final part of the small intestine. Signs and symptoms of Crohn’s disease typically include abdominal pain, diarrhea (with or without blood), fever, and weight loss. Malnutrition because of faulty absorption of nutrients may also occur. Potential complications of Crohn’s disease include obstructions and abscesses of the bowel. People with Crohn’s disease are also at a slightly greater risk than the general population of developing bowel cancer. Although there is a slight reduction in life expectancy in people with Crohn’s disease, if the disease is well managed, affected people can live full and productive lives.
Crohn’s disease is caused by a combination of genetic and environmental factors that lead to impairment of the generalized immune response (called innate immunity). The chronic inflammation of Crohn’s disease is thought to be the result of the immune system “trying” to compensate for the impairment. Dozens of genes are likely to be involved, only a few of which have been identified. Because of the genetic component, close relatives such as siblings of people with Crohn’s disease are many times more likely to develop the disease than people in the general population. Environmental factors that appear to increase the risk of the disease include smoking tobacco and eating a diet high in animal proteins. Crohn’s disease is typically diagnosed on the basis of a colonoscopy, which provides a direct visual examination of the inside of the colon and the ileum of the small intestine.
People with Crohn’s disease typically experience recurring periods of flare-ups followed by remission. There are no medications or surgical procedures that can cure Crohn’s disease, although medications such as anti-inflammatory or immune-suppressing drugs may alleviate symptoms during flare-ups and help maintain remission. Lifestyle changes, such as dietary modifications and smoking cessation, may also help control symptoms and reduce the likelihood of flare-ups. Surgery may be needed to resolve bowel obstructions, abscesses, or other complications of the disease.
Ulcerative Colitis
Ulcerative colitis is an inflammatory bowel disease that causes inflammation and ulcers (sores) in the colon and rectum. Unlike Crohn’s disease, other parts of the GI tract are rarely affected in ulcerative colitis. The primary symptoms of the disease are lower abdominal pain and bloody diarrhea. Weight loss, fever, and anemia may also be present. Symptoms typically occur intermittently with periods of no symptoms between flare-ups. People with ulcerative colitis have a considerably increased risk of colon cancer and should be screened for colon cancer more frequently than the general population. However, ulcerative colitis seems to reduce primarily the quality of life and not the lifespan.
The exact cause of ulcerative colitis is not known. Theories about its cause involve immune system dysfunction, genetics, changes in normal gut bacteria, and lifestyle factors such as a diet high in animal protein and the consumption of alcoholic beverages. Genetic involvement is suspected in part because ulcerative colitis tends to “run” in families. It is likely that multiple genes are involved. Diagnosis is typically made on the basis of colonoscopy and tissue biopsies.
Lifestyle changes, such as reducing the consumption of animal protein and alcohol, may improve symptoms of ulcerative colitis. A number of medications are also available to treat symptoms and help prolong remission. These include anti-inflammatory drugs and drugs that suppress the immune system. In cases of severe disease, removal of the colon and rectum may be required and can cure the disease.
Diverticulitis
Diverticulitis is a digestive disease in which tiny pouches in the wall of the large intestine become infected and inflamed. Symptoms typically include lower abdominal pain of sudden onset. There may also be fever, nausea, diarrhea or constipation, and blood in the stool. Having large intestine pouches called diverticula (Figure \(2\)) that are not inflamed is called diverticulosis. Diverticulosis is thought to be due to a combination of genetic and environmental factors and is more common in people who are obese. Infection and inflammation of the pouches (diverticulitis) occur in about 10 to 25 percent of people with diverticulosis and is more common at older ages. The infection is generally caused by bacteria.
Diverticulitis can usually be diagnosed with a CT scan. Mild diverticulitis may be treated with oral antibiotics and a short-term liquid diet. For severe cases, intravenous antibiotics, hospitalization, and complete bowel rest (no nourishment via the mouth) may be recommended. Complications such as abscess formation or perforation of the colon require surgery.
Peptic Ulcer
A peptic ulcer is a sore in the lining of the stomach or the duodenum (the first part of the small intestine). If the ulcer occurs in the stomach, it is called a gastric ulcer; if it occurs in the duodenum, it is called a duodenal ulcer. The most common symptoms of peptic ulcers are upper abdominal pain that often occurs at night and improves with eating. Other symptoms may include belching, vomiting, weight loss, and poor appetite. However, many people with peptic ulcers, particularly older people, have no symptoms. Peptic ulcers are relatively common, with about 10 percent of people developing a peptic ulcer at some point in their life.
The most common cause of peptic ulcers is infection with the bacterium Helicobacter pylori, which may be transmitted by food, contaminated water, or human saliva (for example, by kissing or sharing eating utensils). Surprisingly, the bacterial cause of peptic ulcers was not discovered until the 1980s. The scientists who made the discovery are Australians Robin Warren and Barry J. Marshall. Although the two scientists eventually won a Nobel Prize for their discovery, their hypothesis was poorly received at first. To demonstrate the validity of their discovery, Marshall used himself in an experiment. He drank a culture of bacteria from a peptic ulcer patient and developed symptoms of peptic ulcer in a matter of days. His symptoms resolved on their own within a couple of weeks, but he took antibiotics to kill any remaining bacteria at his wife’s urging (apparently because bad breath is also one of the symptoms of H. pylori infection). Marshall’s self-experiment was published in the Australian Medical Journal and is among the most cited articles ever published in the journal.
Another relatively common cause of peptic ulcers is the chronic use of non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen. Additional contributing factors may include tobacco smoking and stress, although these factors have not been demonstrated conclusively to cause peptic ulcers independent of H. pylori infection. Contrary to popular belief, diet does not appear to play a role in either causing or preventing peptic ulcers. Eating spicy foods and drinking coffee and alcohol were once thought to cause peptic ulcers. These lifestyle choices are no longer thought to have much if any effect on the development of peptic ulcers.
Peptic ulcers are typically diagnosed on the basis of symptoms or the presence of H. pylori in the GI tract. However, endoscopy (Figure \(3\)), which allows direct visualization of the stomach and duodenum with a camera, may be required for a definitive diagnosis. Peptic ulcers are usually treated with antibiotics to kill H. pylori, along with medications to temporarily decrease stomach acid and aid in healing. Unfortunately, H. pylori have developed resistance to commonly used antibiotics, so treatment is not always effective. If a peptic ulcer has penetrated so deep into the tissues that it causes perforation of the wall of the stomach or duodenum, then emergency surgery is needed to repair the damage.
Gastroenteritis
Gastroenteritis, also known as infectious diarrhea, is an acute and usually self-limiting infection of the GI tract by pathogens. Symptoms typically include some combination of diarrhea, vomiting, and abdominal pain. Fever, lack of energy, and dehydration may also occur. The illness generally lasts less than two weeks, even without treatment, but in young children, it is potentially deadly. Gastroenteritis is very common, especially in poorer nations. Worldwide, up to five billion cases occur each year, resulting in about 1.4 million deaths. In the United States, infectious diarrhea is the second most common type of infection after the common cold.
Commonly called “stomach flu,” gastroenteritis is unrelated to the influenza virus, although viruses are the most common cause of the disease (Figure \(4\)). In children, rotavirus is most often the cause, whereas norovirus is more likely to be the cause in adults. Besides viruses, other potential causes of gastroenteritis include fungi, protozoa (including Giardia lamblia, described below), and bacteria (most often Escherichia coli or Campylobacter jejuni). Transmission of pathogens may occur due to eating improperly prepared foods or foods left to stand at room temperature, drinking contaminated water, or having close contact with an infected individual.
Gastroenteritis is less common in adults than children, partly because adults have acquired immunity after repeated exposure to the most common infectious agents. Adults also tend to have better hygiene than children. If children have frequently repeated incidents of gastroenteritis, they may suffer from malnutrition, stunted growth, and developmental delays. Many cases of gastroenteritis in children can be avoided by giving them a rotavirus vaccine. Frequent and thorough hand washing can cut down on infections caused by other pathogens.
Treatment of gastroenteritis generally involves increasing fluid intake to replace fluids lost in vomitus or diarrhea. Oral rehydration solution, which is a combination of water, salts, and sugar, is often recommended. In severe cases, intravenous fluids may be needed. Antibiotics are not usually prescribed because they are ineffective against viruses that cause most cases of gastroenteritis.
Giardiasis
Giardiasis, popularly known as beaver fever, is a type of gastroenteritis caused by a GI tract parasite, the single-celled protozoan Giardia lamblia (Figure \(5\)). The parasite inhabits the digestive tract of a wide variety of domestic and wild animal species in addition to human beings, including cows, rodents, and sheep as well as beavers (hence its popular name). Giardiasis is one of the most common parasitic infections in people the world over, with hundreds of millions of people infected worldwide each year.
Transmission of G. lamblia is via a fecal-oral route. Those at greatest risk include travelers to countries where giardiasis is common, people who work in child-care settings, backpackers and campers who drink untreated water from lakes or rivers, and people who have close contact with infected people or animals in other settings. In the United States, giardiasis occurs more often during the summer than in other seasons, probably because people spend more time outdoors and in wild settings at that time of year.
Symptoms of giardiasis can vary widely. About a third of people with the infection have no symptoms, whereas others have severe diarrhea with poor absorption of nutrients. Problems with absorption occur because the parasites inhibit intestinal digestive enzyme production, cause detrimental changes in microvilli lining the small intestine, and kill off small intestinal epithelial cells. The illness can result in weakness, loss of appetite, stomach cramps, vomiting, and excessive gas. Without treatment, symptoms may continue for several weeks. Treatment with an antibiotic may be needed if symptoms persist longer or are particularly severe.
Review
1. What is inflammatory bowel disease?
2. Describe typical symptoms of inflammatory bowel disease.
3. Compare and contrast Crohn’s disease and ulcerative colitis.
4. What is diverticulosis? How is it related to diverticulitis?
5. Identify the locations and causes of peptic ulcers.
6. Define and describe gastroenteritis.
7. Identify the cause of giardiasis. Why may it cause malabsorption?
8. Which of the following does not normally affect the small intestine?
A. Peptic ulcers
B. Crohn's disease
C. Giardiasis
D. Ulcerative colitis
9. Name three disorders of the GI tract that can be due to bacteria.
10. True or False. A colonoscopy can be used to examine the small intestine.
11. True or False. Peptic ulcers are mainly due to diet.
12. Name one disorder of the GI tract that can be helped by anti-inflammatory medications and one that can be caused by chronic use of anti-inflammatory medications.
13. People with ulcerative colitis should be frequently screened for _________ cancer.
14. Describe one reason why it can be dangerous to drink untreated water.
15. Do you think the “stomach flu” can be prevented by an influenza vaccine? Why or why not?
Attributions
1. BADAS Crohn by Dayavathi Ashok and Patrick Kiely, CC BY 2.0 via Wikimedia Commons
2. Diverticula by Haymanj, a retired pathologist from Melbourne, Australia. public domain via Wikimedia Commons
3. Endoscopy training by Yuya Tamai, CC BY 2.0 via Wikimedia Commons
4. Gastroenteritis viruses by Graham Beards, CC BY 3.0 via Wikimedia Commons
5. Giardia lamblia by CDC / Janice Haney Carr, public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.7%3A_Disorders_of_the_Gastrointestinal_Tract.txt |
Case Study Conclusion: Please Don’t Pass the Bread
The garlic bread stuffed with spaghetti shown in Figure \(1\) may or may not look appetizing to you, but for people with celiac disease, it is certainly off-limits. Bread and pasta are traditionally made with wheat, which contains proteins called gluten. As you learned in the beginning of the chapter, even trace amounts of gluten can damage the digestive system of people with celiac disease. When Rania and Tui met for lunch, Rania chose a restaurant that she knew could provide her with gluten-free food because she has this disease.
When people with celiac disease eat gluten, it causes an autoimmune reaction that results in inflammation and flattening of the villi of the small intestine. What do you think happens if the villi are inflamed and flattened? Think about what you have learned about the functions of the villi and small intestine. The small intestine is where most chemical digestion and absorption of nutrients occurs in the body. The villi increase the surface area in the small intestine to maximize the digestion of food and absorption of nutrients into the blood and lymph. If the villi are inflamed and flattened, there is less surface area where digestion and absorption can occur. Therefore, damage from celiac disease can result in inadequate absorption of nutrients, called malabsorption.
Malabsorption explains why there can be so many different types of symptoms of celiac disease, ranging from diarrhea and other forms of digestive distress to anemia, nutritional deficiencies, skin rashes, osteoporosis, bone pain, depression, anxiety, and rarer but potentially serious complications such as cancer. Our bodies need to digest and absorb adequate amounts of nutrients in order to function properly and stay healthy. Lack of nutrients can affect and damage cells, tissues, and organs throughout the body, sometimes seriously and irreversibly. A person with celiac disease can limit and often heal intestinal damage just by not eating gluten. In fact, eliminating all gluten from the diet is the main treatment for celiac disease. In some people with celiac disease, a gluten-free diet may not be enough, and steroids and other medications may be used to reduce the inflammation in the small intestine.
Celiac disease is an autoimmune disorder in which the body’s immune system attacks its own tissues. It is thought to be caused by the presence of particular genes in combination with exposure to gluten. What are some other autoimmune disorders that you read about in this chapter that affect the digestive system? The two main inflammatory bowel diseases, Crohn’s disease and ulcerative colitis are both due to the body’s immune system attacking the digestive system, resulting in inflammation. Crohn’s disease can affect any part of the GI tract, most commonly the ileum of the small intestine, while ulcerative colitis mainly affects the colon and rectum. Similar to celiac disease, treatments for these diseases also focus on reducing GI tract damage through lifestyle changes and medications.
Gluten is clearly dangerous for people with celiac disease, but should people who do not have celiac disease or other diagnosed medical problems with gluten also eliminate gluten from their diet? Many medical experts say no; because gluten-free diets are so restrictive, they may cause nutritional deficiencies without providing any proven health benefits. They can also be expensive and, as Tui’s cousin found out, difficult to maintain given that gluten is present in so many foods. It is estimated that only 1% of the population has celiac disease. Most people should enjoy a varied diet and consult with their doctor if they are concerned about celiac disease, other types of gluten intolerance, or food allergies.
Chapter Summary
In this chapter, you learned about the digestive system, which allows the body to obtain needed nutrients from food. Specifically, you learned that:
• The digestive system consists of organs that break down food, absorb its nutrients, and expel any remaining food waste.
• Most digestive organs form a long, continuous tube through which food passes, called the gastrointestinal (GI) tract. It starts at the mouth, which is followed by the pharynx, esophagus, stomach, small intestine, and large intestine.
• Organs of the GI tract have walls that consist of several tissue layers that enable them to carry out digestion and/or absorption. For example, the inner mucosa has cells that secrete digestive enzymes and other digestive substances and also cells that absorb nutrients. The muscle layer of the organs enables them to contract and relax in waves of peristalsis to move food through the GI tract.
• Digestion is a form of catabolism, in which food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs when food moves through the GI tract. The digestive process is controlled by both hormones and nerves.
• Mechanical digestion is a physical process in which food is broken into smaller pieces without becoming chemically changed. It occurs mainly in the mouth and stomach.
• Chemical digestion is a chemical process in which macromolecules including carbohydrates, proteins, lipids, and nucleic acids in food are changed into simple nutrient molecules that can be absorbed into body fluids. Carbohydrates are chemically digested to sugars, proteins to amino acids, lipids to fatty acids, and nucleic acids to individual nucleotides. Chemical digestion requires digestive enzymes. Gut flora carries out additional chemical digestion.
• Absorption occurs when the simple nutrient molecules that result from digestion are absorbed into blood or lymph. They are then circulated through the body.
• Organs of the upper gastrointestinal (GI) tract include the mouth, pharynx, esophagus, and stomach.
• The mouth is the first organ of the GI tract. It has several structures that are specialized for digestion, including salivary glands, tongue, and teeth. Both mechanical digestion and chemical digestion of carbohydrates and fats begin in the mouth.
• The pharynx and esophagus move food from the mouth to the stomach but are not directly involved in the process of digestion or absorption. Food moves through the esophagus by peristalsis.
• Mechanical and chemical digestion continue in the stomach. Acid and digestive enzymes secreted by the stomach start the chemical digestion of proteins. The stomach turns masticated food into a semi-fluid mixture called chyme.
• The lower GI tract includes the small intestine and large intestine. The small intestine is where most chemical digestion and virtually all absorption of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.
• The small intestine consists of three parts: the duodenum, jejunum, and ileum. All three parts of the small intestine are lined with mucosa that is very wrinkled and covered with villi and microvilli, giving the small intestine a huge surface area for digestion and absorption.
• The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.
• The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.
• The duodenum secretes digestive enzymes and also receives bile from the liver or gallbladder and digestive enzymes and bicarbonate from the pancreas. These digestive substances neutralize acidic chyme and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.
• The large intestine consists of the colon (which in turn includes the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the appendix is attached to the cecum of the colon.
• The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid feces from the body through the anus. The large intestine is also the site where trillions of bacteria help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.
• Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the small intestine as needed for digestion.
• The liver is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic cells called hepatic cells, or hepatocytes. The liver receives oxygen in the blood from the aorta through the hepatic artery. It receives nutrients in blood from the GI tract and wastes in blood from the spleen through the portal vein.
• The main digestive function of the liver is the production of the alkaline liquid called bile. Bile is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic chyme that enters the duodenum from the stomach and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.
• Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.
• The gallbladder is a small, pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.
• The pancreas is a glandular organ that secretes both endocrine hormones and digestive enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches); trypsin and chymotrypsin (proteins); lipase (lipids); and ribonucleases and deoxyribonucleases (RNA and DNA).
• Inflammatory bowel disease is a collection of inflammatory conditions primarily affecting the intestines. The diseases involve the immune system attacking the GI tract, and they have multiple genetic and environmental causes. Typical symptoms include abdominal pain and diarrhea, which show a pattern of repeated flare-ups interrupted by periods of remission. Lifestyle changes and medications may control flare-ups and extend remission. Surgery is sometimes required.
• The two principal inflammatory bowel diseases are Crohn’s disease and ulcerative colitis. Crohn’s disease may affect any part of the GI tract from the mouth to the anus, among other body tissues. Ulcerative colitis affects the colon and/or rectum.
• Some people have little pouches, called diverticula, in the lining of their large intestine, a condition called diverticulosis. People with diverticulosis may develop diverticulitis, in which one or more of the diverticula become infected and inflamed. Diverticulitis is generally treated with antibiotics and bowel rest; sometimes surgery is required.
• A peptic ulcer is a sore in the lining of the stomach (gastric ulcer) or duodenum (duodenal ulcer). The most common cause is infection with the bacterium Helicobacter pylori. NSAIDs such as aspirin can also cause peptic ulcers, and some lifestyle factors may play contributing roles. Antibiotics and acid reducers are typically prescribed; surgery is not often needed.
• Gastroenteritis, or infectious diarrhea, is an acute and usually self-limiting infection of the GI tract by pathogens, most often viruses. Symptoms typically include diarrhea, vomiting, and/or abdominal pain. Treatment includes replacing lost fluids; antibiotics are not usually effective.
• Giardiasis is a type of gastroenteritis caused by infection of the GI tract with the protozoa parasite Giardia lamblia. It may cause malnutrition. It is generally self-limiting, but severe or long-lasting cases may require antibiotics.
Chapter Summary Review
1. Explain how the accessory organs of digestion interact with the GI tract.
2. In which of the following organs is food actually digested?
1. Pancreas
2. Small intestine
3. Gallbladder
4. A and B
3. True or False. Bile is one of the digestive fluids secreted in the stomach.
4. True or False. The smell of food can stimulate the release of digestive enzymes.
5. If the pH in the duodenum was too low (acidic), what effect do you think this would have on the processes of the digestive system?
6. Is the stomach involved in chemical digestion, mechanical digestion, or both? Explain your answer.
7. Absorption of most nutrients occurs in the:
1. Duodenum
2. Ileum
3. Cecum
4. Jejunum
8. Is food passing through the GI tract generally more solid in the small intestine or the large intestine? Explain your answer.
9. What is another name for the colon?
10. Discuss whether digestion occurs in the large intestine.
11. The appendix is attached to:
1. The large intestine
2. The small intestine
3. The pancreas
4. The liver
12. What is lacteal? In your answer, be sure to describe both its location and function in the digestive system.
13. Lipids are digested at different points in the digestive system. Describe how lipids are digested at two of these points.
14. Describe two different functions of stomach acid.
15. True or False. Proteins are only digested in the stomach.
16. True or False. A peptic ulcer can occur in the small intestine.
17. Match each of the following organs of the digestive system with the description that best fits it. Each organ is used only once: Organs: pancreas; liver; gallbladder
1. A small, pouch-like organ that stores and concentrates bile produced by a different organ.
2. Produces insulin as well as digestive enzymes and other needed substances.
3. Processes wastes in addition to aiding in digestive functions.
18. What is the name of the rhythmic muscle contractions that move food through the GI tract?
19. What are the major roles of the upper GI tract?
20. Diverticulitis causes infected and inflamed pouches in the:
1. stomach
2. small intestine
3. jejunum
4. large intestine
21. What is the physiological cause of heartburn?
22. Which disease most commonly affects the ileum of the small intestine?
1. Crohn’s disease
2. Peptic ulcer
3. Ulcerative colitis
4. Gastroenteritis
23. True or False. Smoking does not contribute to digestive system disorders.
24. True or False. In addition to obtaining nutrients, the digestive system plays a role in protecting the body from pathogens.
25. What are two ways in which the tongue participates in digestion?
26. Where is the epiglottis located?
27. If the epiglottis were to not close properly, what might happen?
28. The GI tract goes from the mouth to which structure?
Attributions
1. Spaghetti Stuffed Garlic Bread by Adam S licensed CC BY 2.0 via Flickr.com
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3. | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/18%3A_Digestive_System/18.8%3A_Case_Study_Conclusion%3A__Celiac_and_Chapter_Summary.txt |
This chapter discusses the concept of excretion and explains the excretory functions of the skin, liver, large intestine, lungs, and kidneys. It also describes the other organs of the urinary system and several urinary system disorders.
• 19.1: Case Study: Waste Management
“Wow, this line for the restroom is long!” Bintou says to Maeva, anxiously bobbing from side to side to ease the pressure in her bladder. Maeva nods and says, “It’s always like this at parties. It’s the alcohol.” As you will learn in this chapter, the liver and kidneys are important organs of the excretory system, and impairment of the functioning of these organs can cause serious health consequences.
• 19.2: Introduction to the Urinary System
The actual human urinary system, also known as the renal system, is shown in the drawing below. The system consists of the kidneys, ureters, bladder, and urethra, which is the only structure not visible in the sculpture above. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine. Between 1 and 2 liters of urine are normally produced every day in a healthy individual.
• 19.3: Kidneys
The two bean-shaped kidneys are located high in the back of the abdominal cavity on either side of the spine. A renal artery connects each kidney with the aorta and transports unfiltered blood to the kidney. The kidney has two main layers involved in the filtration of blood and formation of urine: the outer cortex and inner medulla. At least a million nephrons, which are the tiny functional units of the kidney, span the cortex and medulla.
• 19.4: Ureters, Urinary Bladder, and Urethra
Ureters are tube-like structures that connect the kidneys with the urinary bladder. They are paired structures, with one ureter for each kidney. The urinary bladder is a hollow, muscular organ that rests on the pelvic floor. It is also lined with transitional epithelium. The urethra is a tube that connects the urinary bladder to the external urethral orifice, which is the opening of the urethra on the surface of the body.
• 19.5: Disorders of the Urinary System
The kidneys play such vital roles in eliminating wastes and toxins and maintaining body-wide homeostasis that disorders of the kidneys may be life threatening. Gradual loss of normal kidney function commonly occurs with a number of disorders, including diabetes mellitus and high blood pressure. Other disorders of the kidneys are caused by faulty genes that are inherited. Loss of kidney function may eventually progress to kidney failure.
• 19.6: Case Study Conclusion: Alcohol and Chapter Summary
As you learned in the beginning of the chapter, consumption of alcohol inhibits a hormone that causes our bodies to retain water. The result is that more water is released in urine, increasing the frequency of restroom trips as well as the risk of dehydration. The excretory system is essential to remove toxic wastes from the body and regulate homeostasis. Limiting alcohol consumption can help preserve the normal functioning of the excretory system so that it can protect your health.
19: Urinary System
Case Study: Drink and Flush
“Wow, this line for the restroom is long!” Bintou says to Maeva, anxiously bobbing from side to side to ease the pressure in her bladder. Maeva nods and says, “It’s always like this at parties. It’s the alcohol.”
Bintou and Maeva are 21-year-old college students at a party. They and many other people have been drinking alcoholic beverages over the course of the evening. As the night has gotten later, the line for the restroom has gotten longer and longer. You may have noticed this phenomenon if you have been to places where large numbers of people are drinking alcohol, like at the ballpark below.
Bintou says, “I wonder why alcohol makes you have to pee?” Maeva learned about this in her Human Biology class and tells Bintou that alcohol inhibits a hormone that helps you retain water. So instead of your body retaining water, you urinate more out. This could lead to dehydration, so she suggests that after their trip to the restroom, they start drinking water instead of alcohol.
For people who drink occasionally or moderately, this effect of alcohol on the excretory system—the system that removes wastes such as urine—is usually temporary. However, in people who drink excessively, alcohol can have serious, long-term effects on the excretory system. For example, heavy drinking on a regular basis can cause liver and kidney disease.
As you will learn in this chapter, the liver and kidneys are important organs of the excretory system, and impairment of the functioning of these organs can cause serious health consequences. At the end of the chapter, you will learn which hormone Maeva was referring to, and some of the ways alcohol can affect the excretory system, both after the occasional drink and in cases of excessive alcohol use and abuse.
Chapter Overview: Excretory System
In this chapter, you will learn about the excretory system, which rids the body of toxic waste products and helps maintain homeostasis. Specifically, you will learn about:
• The organs of the excretory system, which include the skin, liver, large intestine, lungs, and kidneys, eliminate waste and excess water from the body.
• How wastes are eliminated through sweat, feces, urine, and exhaled gases; and how toxic substances in the blood are broken down by the liver.
• The urinary system, which includes the kidneys, ureters, bladder, and urethra.
• The main function of the urinary system, which is to filter the blood and eliminate wastes, mineral ions, and excess water from the body in the form of urine.
• How the kidneys filter the blood, retain needed substances, produce urine, and help maintain homeostasis, such as proper ion and water balance.
• How urine is stored, transported, and released from the body.
• Disorders of the urinary system, including bladder infections, kidney stones, polycystic kidney disease, urinary incontinence, and kidney damage due to factors such as uncontrolled diabetes and high blood pressure.
As you read the chapter, think about the following questions:
1. Which hormone do you think Maeva was referring to? Remember that this hormone causes the urinary system to retain water and excrete less water out in the urine.
2. How and where does this hormone work?
3. Long-term, excessive use of alcohol can affect the liver and kidneys. How do these two organs of excretion interact and work together?
Attributions
1. Gotta Pee (from QUACK design, Oslo based design agency that make interiors from recycled materials) by Jon-Eric Melsæter, CC BY 2.0 via Flickr.com
2. Line at ballpark by Dorothy, CC BY 2.0 via Flickr.com | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/19%3A_Urinary_System/19.1%3A_Case_Study%3A__Waste_Management.txt |
Sculpture Garden
In Figure \(1\), this interesting outdoor art installation can be viewed at the Hague in the Netherlands. It’s a colorful piece with an unusual subject. The wrinkled structures on each side of the sculpture represent the kidneys, and the striped structure in the distance represents the urinary bladder. The red and blue tubes are blood vessels, and the tan tubes are ureters. In short, the installation is a three-dimensional depiction of the human urinary system. Only one urinary system organ is not visible in the photo. Do you know what it is?
The actual human urinary system, also known as the renal system, is shown in Figure \(2\). The system consists of the kidneys, ureters, bladder, and urethra, which is the only structure not visible in the sculpture above. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine. Between 1 and 2 liters of urine are normally produced every day in a healthy individual.
Organs of the Urinary System
The urinary system is all about urine. It includes organs that form urine and also those that transport, store, or excrete urine.
Kidneys
Urine is formed by the kidneys, which filter many substances out of blood, allow the blood to reabsorb needed materials, and use the remaining materials to form urine. The human body normally has two paired kidneys, although it is possible to get by quite well with just one kidney. The anatomy and function of the kidney are discussed in detail in the next section.
Ureters, Bladder, Urethra
After urine forms in the kidneys, it is transported through the ureters (one per kidney) to the sac-like bladder, which stores the urine until urination. During urination, the urine is released from the bladder and transported by the urethra to be excreted outside the body through the external urethral opening.
Functions of the Urinary System
Waste products removed from the body with the formation and elimination of urine include many water-soluble metabolic products. The main waste products are urea, a by-product of protein catabolism, and uric acid, a by-product of nucleic acid catabolism. Excess water and mineral ions are also eliminated in urine.
Besides the elimination of waste products such as these, the urinary system has several other vital functions. These include:
• maintaining homeostasis of mineral ions in extracellular fluid. These ions are either excreted in urine or returned to the blood as needed to maintain the proper balance.
• regulating the acid-base balance in the body. For example, when pH is too low (blood is too acidic), the kidneys excrete less bicarbonate (which is basic) in the urine. When pH is too high (blood is too basic), the opposite occurs and more bicarbonate is excreted in the urine.
• controlling the volume of extracellular fluids, including the blood, which helps maintain blood pressure. The kidneys control fluid volume and blood pressure by excreting more or less salt and water in urine.
Control of the Urinary System
The formation of urine must be closely regulated to maintain body-wide homeostasis. Several endocrine hormones help control this function of the urinary system, including antidiuretic hormone, parathyroid hormone, and aldosterone.
• Antidiuretic hormone, also called vasopressin, is secreted by the hypothalamus. One of its main roles is conserving body water. It is released when the body is dehydrated and causes the kidneys to excrete less water in urine.
• Parathyroid hormone is secreted by the parathyroid glands. It works to regulate the balance of mineral ions in the body through its effects on several organs, including the kidneys. Parathyroid hormone stimulates the kidneys to excrete less calcium and more phosphorus in the urine.
• Aldosterone is secreted by the cortex of the adrenal glands, which rest atop the kidneys, as shown in Figure \(3\). It plays a central role in regulating blood pressure through its effects on the kidneys. It causes the kidneys to excrete less sodium and water in urine.
Once urine forms, it is excreted from the body in the process of urination. This process is controlled by both the autonomic and the somatic nervous systems. As the bladder fills with urine, it causes the autonomic nervous system to signal a muscle in the bladder wall to contract and the sphincter between the bladder and urethra to relax and open. This forces urine out of the bladder and through the urethra. Another sphincter at the distal end of the urethra is under voluntary control. When it relaxes under the influence of the somatic nervous system, it allows urine to leave the body through the external urethral opening.
Review
1. What organs make up the urinary system?
2. State the main function of the urinary system.
3. What is the primary function of the kidneys?
4. Describe how blood enters and leaves the kidneys.
5. What are nephrons?
6. What happens to urine after it forms in the kidneys?
7. Identify the functions of the urinary system besides the elimination of waste products.
8. How is the formation of urine regulated?
9. Explain how the process of urination is controlled.
10. What function do the adrenal glands carry out that is related to urine?
A. They form urine and transport it to the kidneys
B. They excrete concentrated urine into the ureters
C. They secrete a hormone that affects the composition of urine
D. They store urine when water and salts need to be retained by the body
11. Explain why it is important to have voluntary control over the sphincter at the end of the urethra.
12. Compare the aldosterone to antidiuretic hormones in terms of how they affect the kidneys.
13. If your body needed to retain more calcium, which of the hormones described in this concept is the most likely to rise in the level? Explain your reasoning.
14. True or False. Urine is composed of water, urea, and minerals only.
15. True or False. The renal artery contains blood that was filtered by the kidney.
Explore More
Many employers require a urine-based drug test upon hire, but are they really that accurate? Check this out:
Attributions
1. Nier met blaas / Kidney with bladder by FaceMePLS from The Hague, The Netherlands, CC BY 2.0, via Wikimedia Commons
2. Urinary system by Arcadian (National Cancer Institute), Public Domain via Wikimedia Commons
3. Adrenal Glands by Pearson Scott Foresman, Public Domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/19%3A_Urinary_System/19.2%3A_Introduction_to_the_Urinary_System.txt |
Kidneys on the Menu
Pictured here amidst a bed of mixed veggies are a steak and kidney pudding. More often made into a pie, this savory dish is a British favorite. Kidneys on the menu typically come from sheep, pigs, or cows. In these animals as in the human animal, kidneys are the main organs of excretion.
Location of the Kidneys
The two bean-shaped kidneys are located high in the back of the abdominal cavity, one on each side of the spine. Both kidneys sit just below the diaphragm, the large breathing muscle that separates the abdominal and thoracic cavities. As you can see in Figure \(2\), the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver, and the left kidney is behind the spleen. The location of the liver explains why the right kidney is smaller and lower than the left.
Kidney Anatomy
The shape of each kidney gives it a convex side and a concave side. You can see this clearly in the detailed diagram of kidney anatomy shown in Figure \(3\). The concave side is where the renal artery enters the kidney and the renal vein and ureter leave the kidney. This area of the kidney is called the hilum. The entire kidney is surrounded by tough fibrous tissue, called the renal capsule, which in turn is surrounded by two layers of protective, cushioning fat.
Internally, each kidney is divided into two major layers: the outer renal cortex and the inner renal medulla (see Figure \(3\)). These layers take the shape of many cone-shaped renal lobules, each containing a renal cortex surrounding a portion of the medulla called a renal pyramid. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons. Between the renal pyramids are projections of cortex called renal columns. The tip or papilla of each pyramid empties urine into a minor calyx (chamber). Several minor calyces empty into a major calyx, and the latter empties into the funnel-shaped cavity called the renal pelvis, which becomes the ureter as it leaves the kidney.
Renal Circulation
The renal circulation is an important part of the kidney’s main function of filtering waste products from the blood. Blood is supplied to the kidneys via the renal arteries. The right renal artery supplies the right kidney, and the left renal artery supplies the left kidney. These two arteries branch directly from the aorta, which is the largest artery in the body. Each kidney is only about 11 cm (4.4 in.) long and has a mass of just 150 grams (5.3 oz), yet it receives about 10 percent of the total output of blood from the heart. Blood is filtered through the kidneys about 20 times each hour, 24 hours a day, day after day.
As indicated in Figure \(4\), each renal artery carries blood with waste products into the kidney. Within the kidney, the renal artery branches into increasingly smaller arteries that extend through the renal columns between the renal pyramids. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons, the structures that actually filter the blood. After blood passes through the nephrons and is filtered, the clean blood moves through a network of venules that converge into small veins. Small veins merge into increasingly larger ones and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava.
Nephron Structure and Function
The illustration above gives an indication of the complex structure of a nephron. The nephron is the basic structural and functional unit of the kidney, and each kidney typically contains at least a million of them. As blood flows through a nephron, many materials are filtered out of the blood, needed materials are returned to the blood, and the remaining materials form urine. Most of the waste products removed from the blood and excreted in the urine are byproducts of metabolism. At last half of the waste is urea, a waste product produced by protein catabolism. Another important waste is uric acid, produced in nucleic acid catabolism.
Components of a Nephron
The diagram in Figure \(5\) shows in greater detail the components of a nephron. The renal corpuscle is a filtering structure that is consists of a network of capillaries called the glomerulus (plural, glomeruli) and the Bowman's capsule, a space surrounding the glomerulus. Bowman's capsule is the initial structure of a nephron. Extending from Bowman’s capsule is the renal tubule. The proximal end (nearest Bowman’s capsule) of the renal tubule is called the proximal convoluted (coiled) tubule. From here, the renal tubule continues as a loop (known as the loop of Henle), which in turn becomes the distal convoluted tubule. The latter finally joins with a collecting duct. As you can see in the diagram, peritubular capillaries surround the total length of the renal tubule.
The Function of a Nephron
The simplified diagram of a nephron in Figure \(6\) shows how the nephron functions. Blood enters the nephron through an arteriole called the afferent arteriole. Some of the blood next passes through the capillaries of the glomerulus. Any blood that doesn’t pass through the glomerulus, as well as blood after it passes through the glomerular capillaries, continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues to play roles in nephron functioning.
Filtration
As blood from the afferent arteriole flows through the glomerular capillaries, it is under pressure. Because of the pressure, water and solutes are filtered out of the blood and into the space made by Bowman’s capsule. This is the filtration stage of nephron function. The filtered substances, called filtrate, pass into Bowman’s capsule and from there into the proximal end of the renal tubule. At this stage, filtrate includes water, salts, organic solids such as nutrients, and waste products of metabolism such as urea.
Reabsorption and Secretion
As filtrate moves through the renal tubule, some of the substances it contains are reabsorbed from the filtrate back into the blood in the efferent arteriole (via peritubular capillaries). This is the reabsorption stage of nephron function. About two-thirds of the filtered salts and water and all of the filtered organic solutes (mainly glucose and amino acids) are reabsorbed from the filtrate by the blood in the peritubular capillaries. Reabsorption occurs mainly in the proximal convoluted tubule and the loop of Henle.
At the distal end of the renal tubule, some additional reabsorption generally occurs. This is also the region of the tubule where other substances from the blood are added to the filtrate in the tubule. The addition of other substances to the filtrate from the blood is called secretion. Both reabsorption and secretion in the distal convoluted tubule are largely under the control of endocrine hormones that maintain homeostasis of water and mineral salts in the blood. These hormones work by controlling what is reabsorbed into the blood from the filtrate and what is secreted from the blood into the filtrate to become urine. For example, the parathyroid hormone causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.
Collection of Urine and Excretion
By the time the filtrate has passed through the entire renal tubule, it has become the liquid waste known as urine. Urine empties from the distal end of the renal tubule into a collecting duct. From there, the urine flows into increasingly larger collecting ducts. As urine flows through the system of collecting ducts, more water may be reabsorbed from it. This will occur in the presence of antidiuretic hormone from the hypothalamus. This hormone makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by osmosis while preventing the passage of ions or other solutes. As much as three-fourths of the water may be reabsorbed from urine in the collecting ducts, making the urine more concentrated.
Urine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces and finally into the renal pelvis (see Figure \(3\). From there, it travels through the ureter to the urinary bladder for eventual excretion from the body. An average of about 1.5 liters of urine is excreted each day. Normally, urine is yellow or amber in color (Figure \(7\)). The darker the color, generally the more concentrated the urine is.
Other Functions of the Kidneys
Besides filtering blood and forming urine for the excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide homeostasis. Most of these functions are related to the composition or volume of urine formed by the kidneys. These functions include maintaining the proper balance of water and salts in the body, normal blood pressure, and the correct range of blood pH. Through the processes of absorption and secretion by nephrons, more or less water, salt ions, acids, or bases are returned to the blood or excreted in urine as needed to maintain homeostasis.
Kidney Hormones
• Aldosterone is secreted by the adrenal cortex. Aldosterone causes the kidneys to increase the reabsorption of sodium ions and water from the filtrate into the blood. This returns the concentration of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.
• Calcitriol is secreted by the kidneys in response to low levels of calcium in the blood. This hormone stimulates the uptake of calcium by the intestine, thus raising blood levels of calcium.
• Erythropoietin is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of red blood cells in the bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.
Feature: Human Biology in the News
Kidney failure is a complication of common disorders including diabetes mellitus and hypertension. Almost half a million Americans have end-stage kidney disease and need to either receive a donated kidney or have frequent hemodialysis, a medical procedure in which the blood is artificially filtered through a machine. Transplant generally has better outcomes than hemodialysis but demand for organs far outstrips the supply. At any given time, more than 100,000 people in the U.S. are on a waiting list for a kidney transplant, but each year fewer than 20,000 receive them. Every day, 13 Americans die while waiting for a donor's kidney.
For the past decade, Dr. William Fissell, a kidney specialist at Vanderbilt University, has been working to create an implantable part-biological and part-artificial kidney. Using microchips like those used in computers, he has produced an artificial kidney small enough to implant in the patient’s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is “... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.”
The filtration system in the artificial kidney consists of a stack of 15 microchips. Tiny pores in the microchips act as a scaffold for the growth of living kidney cells that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient’s blood as a biological kidney would, but with less risk of rejection by the patient’s immune system because they are embedded within the device. The new kidney doesn’t need a power source because it uses the natural pressure of blood flowing through arteries to push the blood through the filtration system. A major part of the design of the artificial organ was devoted to fine-tuning the fluid dynamics so blood flows through the device without clotting.
The implantable kidney was given fast-track approval for testing in people by the U.S. Food and Drug Administration because of the potentially life-saving benefits of the device. The artificial kidney is expected to be tested in pilot trials by 2018. Dr. Fissell says he has a long list of patients eager to volunteer for the trials.
Review
1. Where are the kidneys located?
2. Contrast the renal artery and renal vein.
3. Describe the structure of the kidney.
4. Identify the functions of a nephron.
5. Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.
6. Use the example of the renin-angiotensin-aldosterone system to illustrate how the kidneys control homeostasis with the help of endocrine hormones.
7. Identify two endocrine hormones secreted by the kidneys and the functions they control.
8. Put the following structures in order of how urine flows out of the kidney, from the earliest to the latest:
collecting ducts; renal tubule; renal pelvis; renal calyces
9. Name two regions in the kidney where water is reabsorbed.
10. True or False. Once the filtrate enters the renal tubule, no substances are added to it.
11. True or False. Some substances are reabsorbed in the distal end of the renal tubule.
12. Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.
13. How many nephrons are there per kidney?
A. One
B. Thirteen
C. At least one thousand
D. At least one million
14. If blood flow to the kidneys is blocked, what do you think would happen?
15. The loop of Henle is part of the:
A. glomerulus
B. renal tubule
C. collecting duct
D. ureter
Explore More
Another researcher working on an implantable kidney is surgeon Anthony Atala. In this fascinating TED talk, he shows how a 3D printer that uses living cells can potentially print out a transplantable kidney. Dr. Atala has already used similar technology to engineer a replacement bladder for a young patient, who is introduced during the talk.
Attributions
1. Steak and Kidney Pudding by Annie Mole from London, UK; CC BY 2.0, via Wikimedia Commons
2. Vessels of the abdomen by Henry Gray () Anatomy of the Human Body, Public Domain, via Wikimedia Commons
3. Kidney Anatomy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
4. Kidney by CNX OpenStax, CC BY 4.0, via Wikimedia Commons
5. Blood Flow in the Nephron by OpenStax College, CC BY 3.0, via Wikimedia Commons
6. Physiology of Nephron by Madhero88, CC BY 3.0, via Wikimedia Commons
7. Urine by Sustainable Sanitation Alliance, CC BY 2.0 via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/19%3A_Urinary_System/19.3%3A_Kidneys.txt |
Communicating with Urine
Why do dogs urinate on fire hydrants? Besides “having to go,” they are marking their territory with chemicals called pheromones in their urine. It’s a form of communication, in which they are “saying” with odors that the yard is theirs and other dogs should stay away. Dogs may urinate on fence posts, trees, car tires, and many other objects in addition to fire hydrants. Urination in dogs, as in people, is usually a voluntary process controlled by the brain. The process of forming urine, which occurs in the kidneys, occurs constantly and is not under voluntary control. What happens to all the urine that forms in the kidneys? It passes from the kidneys through the other organs of the urinary system, starting with the ureters.
Ureters
As shown in Figure \(2\), ureters are tube-like structures that connect the kidneys with the urinary bladder. They are paired structures, with one ureter for each kidney. In adults, ureters are between 25 and 30 cm (10 to 12 in.) long and about 3 to 4 mm (about 1/8 in.) in diameter.
Each ureter arises in the pelvis of a kidney (Figure \(3\)). It then passes down the side of the kidney and finally enters the back of the bladder.
The walls of the ureters are composed of multiple layers of different types of tissues. You can see the layers in Figure \(4\). The innermost layer is a special type of epithelium, called the transitional epithelium. Unlike the epithelium lining most organs, the transitional epithelium is capable of flattening and distending and does not produce mucus. It lines much of the urinary system, including the renal pelvis, bladder, and much of the urethra in addition to the ureters. Transitional epithelium allows these organs to stretch and expand as they fill with urine or allow urine to pass through. The next layer of the ureter walls is made up of loose connective tissue containing elastic fibers, nerves, and blood and lymphatic vessels. After this layer are two layers of smooth muscles, an inner circular layer, and an outer longitudinal layer. The smooth muscle layers can contract in waves of peristalsis to propel urine down the ureters from the kidneys to the urinary bladder. The outermost layer of the ureter walls consists of fibrous tissue.
Urinary Bladder
The urinary bladder is a hollow, muscular, and stretchy organ that rests on the pelvic floor. It collects and stores urine from the kidneys before the urine is eliminated through urination. As shown in Figure \(5\), urine enters the urinary bladder from the ureters through two ureteral openings on either side of the back wall of the bladder. Urine leaves the bladder through a sphincter called the internal urethral sphincter. When the sphincter relaxes and opens, it allows urine to flow out of the bladder and into the urethra.
Like the ureters, the bladder is lined with transitional epithelium, which can flatten out and stretch as needed as the bladder fills with urine. The next layer (lamina propria) is a layer of loose connective tissue, nerves, and blood and lymphatic vessels. This is followed by a submucosa layer, which connects the lining of the bladder with the detrusor muscle in the walls of the bladder. The outer covering of the bladder is the peritoneum, which is a smooth layer of epithelial cells that lines the abdominal cavity and covers most abdominal organs.
The detrusor muscle in the wall of the bladder is made of smooth muscle fibers that are controlled by both the autonomic and somatic nervous systems. As the bladder fills, the detrusor muscle automatically relaxes to allow it to hold more urine. When the bladder is about half full, the stretching of the walls triggers the sensation of needing to urinate. When the individual is ready to void, conscious nervous signals cause the detrusor muscle to contract and the internal urethral sphincter to relax and open. As a result, urine is forcefully expelled out of the bladder and into the urethra.
Urethra
The urethra is a tube that connects the urinary bladder to the external urethral orifice, which is the opening of the urethra on the surface of the body. As shown in Figure \(6\), the urethra in a person with XY chromosomes (anatomically male) travels through the penis, so it is much longer than the urethra in a person with XX chromosomes (anatomically female). In a genetically male person, the urethra averages about 20 cm (8 in.) long, whereas, in a genetically female individual, it averages only about 4.8 cm (1.9 in.) long. In an XY individual, the urethra carries semen as well as urine, but in the XX individual, it carries only urine.
Like the ureters and bladder, the proximal (closer to the bladder) two-thirds of the urethra are lined with transitional epithelium. The distal (farther from the bladder) third of the urethra is lined with mucus-secreting epithelium. The mucus helps protect the epithelium from urine, which is corrosive. Below the epithelium is loose connective tissue, and below that are layers of smooth muscle that are continuous with the muscle layers of the urinary bladder. When the bladder contracts to forcefully expel urine, the smooth muscle of the urethra relaxes to allow the urine to pass through.
In order for urine to leave the body through the external urethral orifice, the external urethral sphincter must relax and open. This sphincter is a striated muscle that is controlled by the somatic nervous system, so it is under conscious, voluntary control in most people (exceptions are infants, some elderly people, and patients with certain injuries or disorders). The muscle can be held in a contracted state and hold in the urine until the person is ready to urinate. Following urination, the smooth muscle lining the urethra automatically contracts to re-establish muscle tone, and the individual consciously contracts the external urethral sphincter to close the external urethral opening.
Review
1. What are ureters?
2. Describe the location of the ureters relative to other urinary tract organs.
3. Identify layers in the walls of a ureter and how they contribute to the ureter’s function.
4. Describe the urinary bladder.
5. What is the function of the urinary bladder?
6. How does the nervous system control the urinary bladder?
7. What is the urethra?
8. How does the nervous system control urination?
9. Identify the sphincters that are located along the pathway from the ureters to the external urethral orifice.
10. What are two differences between the male and female urethra?
11. True or False. Urine travels through the urinary system due solely to the force of gravity.
12. True or False. Urination refers to the process that occurs from the formation of urine in the kidneys to the elimination of urine from the body.
13. When the bladder muscle contracts, the smooth muscle in the walls of the urethra _________ .
14. Transitional epithelium lines the:
A. bladder
B. ureters
C. renal pelvis
D. All of the above
Explore More
You deposit it in the toilets and then you flush and never see it again. Could we be making use of all the pee (and poop) we usually flush away? Watch this fun and interesting TED talk to learn more about the potential use of pee and other human excrements to grow healthier plants and people.
Attributions
1. Dog urinating on hydrant by Jackie, CC BY 2.0 via Wikimedia Commons
2. Urinary System by Arcadian, Public Domain, via Wikimedia Commons
3. The kidney by OpenStax, CC BY 3.0 via Wikimedia Commons
4. Transverseureter, Public Domain via Wikimedia Commons
5. The bladder by OpenStax CC BY 3.0 via Wikimedia Commons
6. Female and Male Urethra by OpenStax CC BY 3.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/19%3A_Urinary_System/19.4%3A_Ureters_Urinary_Bladder_and_Urethra.txt |
Awareness Ribbon
Awareness ribbons are symbols meant to show support or raise consciousness for a cause. Different colors are associated with different issues, often relating to health problems. The first ribbon to gain familiarity for a health issue was the red ribbon for HIV/AIDS, created in 1991. The pink ribbon for breast cancer awareness is probably the best known today. Do you know what a green ribbon like the one pictured in Figure \(1\) represents? Among several other health problems, a green ribbon is meant to show support or raise awareness for kidney disorders.
The kidneys play such vital roles in eliminating wastes and toxins and maintaining body-wide homeostasis that disorders of the kidneys may be life-threatening. The gradual loss of normal kidney function commonly occurs with a number of disorders, including diabetes mellitus and high blood pressure. Other disorders of the kidneys are caused by faulty genes that are inherited. Loss of kidney function may eventually progress to kidney failure.
Diabetic Nephropathy
Diabetic nephropathy is a progressive kidney disease caused by damage to the capillaries in the glomeruli of the kidneys due to long-standing diabetes mellitus (see Figure \(2\)). It is not fully understood how diabetes leads to damage of glomerular capillaries, but it is thought that high levels of glucose in the blood are involved. In people with diabetes, nephropathy is more likely if their blood glucose is poorly controlled. Having high blood pressure, a history of cigarette smoking and a family history of kidney problems are additional risk factors. Diabetic nephropathy often has no symptoms at first. In fact, it may take up to a decade after kidney damage begins for symptoms to appear. When they do appear, they typically include severe tiredness, headaches, nausea, frequent urination, and itchy skin.
Proteins are large molecules that usually are not filtered out of blood in the glomeruli. When the glomerular capillaries are damaged, it allows proteins such as albumin to leak into the filtrate from the blood. As a result, albumin ends up being excreted in the urine. Finding a high level of albumin in the urine is one indicator of diabetic nephropathy and helps to diagnose the disorder. Drugs may be prescribed to reduce protein levels in the urine. Controlling high blood sugar levels and hypertension (high blood pressure) is also important to help slow kidney damage, as is a reduction of sodium intake.
Polycystic Kidney Disease
Polycystic kidney disease (PKD) is a genetic disorder in which multiple abnormal cysts develop and grow in the kidneys. Figure \(3\) shows a pair of kidneys that are riddled with cysts from PKD. In people who inherit PKD, the cysts may start to form at any point in life from infancy through adulthood. Typically, both kidneys are affected. Symptoms of the disorder may include high blood pressure, headaches, abdominal pain, blood in the urine, and excessive urination.
There are two types of PKD. The more common type is caused by an autosomal dominant allele, and the less common type is caused by an autosomal recessive allele. Both types collectively make PKD one of the most common hereditary diseases in the United States, affecting more than half a million people. There is little or no difference in the rate of occurrence of PKD between genders or ethnic groups. There is no known cure for this disease other than a kidney transplant.
Kidney Failure
Both diabetic nephropathy and PKD may lead to kidney (or renal) failure (classified as end-stage kidney disease), in which the kidneys are no longer able to adequately filter metabolic wastes from the blood. Long-term, uncontrolled high blood pressure is another common cause of kidney failure. Symptoms of kidney failure may include nausea, more or less frequent urination, blood in the urine, muscle cramps, anemia, swelling of the extremities, and shortness of breath due to the accumulation of fluid in the lungs. If kidney function drops below the level needed to sustain life, then the only treatment option is kidney transplantation or some means of artificial filtration of the blood, such as by hemodialysis.
Hemodialysis is a medical procedure in which blood is filtered externally through a machine. You can see how it works in the simplified diagram in Figure \(4\). During dialysis, waste products such as urea as well as excess water are removed from the patient’s blood before the blood is returned to the patient. Hemodialysis is typically done on an outpatient basis in a hospital or special dialysis clinic. Less frequently, it is done in the patient’s home. Depending on the patient’s size, among other factors, the blood is filtered for 3 to 4 hours about 3 times a week. Because the treatment is needed so frequently, hemodialysis is one of the most common procedures performed in U.S. hospitals.
Kidney Stones
A kidney stone, also known as a renal calculus, is a solid crystal that forms in a kidney from minerals in urine (see Figure \(5\)). The majority of kidney stones consist of crystals of calcium salts. Kidney stones typically leave the body in the urine stream. A small stone may pass through the ureters and other urinary tract organs without causing symptoms and go undetected. A larger stone may cause pain when it passes through the urinary tract. If a kidney stone grows large enough, it may block the ureter. Blockage of a ureter may cause a decrease in kidney function and damage to the kidney.
A kidney stone that causes pain is generally treated with pain medication until it passes through the urinary tract. A stone that causes a blockage may be treated with lithotripsy. This is a medical procedure in which high-intensity ultrasound pulses are applied externally to cause fragmentation of the stone into pieces small enough to pass easily through the urinary tract. Although lithotripsy is noninvasive, it can cause damage to the kidneys. An alternative treatment for a stone that blocks urine flow is to insert a stent into the ureter to expand it and allow both urine and the stone to pass. In some cases, surgery may be required to physically remove a large stone from the ureter.
A combination of lifestyle and genetic factors seem to predispose certain people to develop kidney stones. Risk factors include high consumption of cola soft drinks, eating a diet high in animal protein, being overweight, and not drinking enough fluids. Preventive measures are obvious. They include limiting cola consumption, eating less animal protein, losing weight, and increasing fluid intake.
Other Urinary System Disorders
Although disorders of the kidneys are generally the most serious urinary system disorders, problems that affect other organs of the urinary tract are generally more common. They include bladder infections and urinary incontinence.
Bladder Infection
A bladder infection, also called cystitis, is a very common type of urinary tract infection in which the urinary bladder becomes infected by bacteria (typically Escherichia coli), rarely by fungi. Symptoms of bladder infections may include pain with urination, frequent urination, and feeling the need to urinate despite having an empty bladder. In some cases, there may be blood in the urine. A much less common type of urinary tract infection is pyelonephritis, in which the kidney becomes infected. If a kidney infection occurs, it is generally because of an untreated bladder infection. Bladder infections are treated mainly with antibiotics.
Risk factors for urinary bladder infections include sexual intercourse (especially when spermicide or a diaphragm, as shown in Figure \(6\), is used for contraception), diabetes, obesity, and most importantly, female sex. Bladder infections are four times more common in women than in men. In fact, in women, they are the most common type of bacterial infection, and as many as 1 in 10 women has a bladder infection in any given year. Female anatomy explains the sex difference in the incidence of bladder infections. The urethra is much shorter and closer to the anus in females than in males, so contamination of the urethra and then the bladder with GI tract bacteria is more likely in females than in males. Once the bacteria reach the bladder, they can attach to the bladder wall and form a biofilm that resists the body’s immune response.
Urinary Incontinence
Urinary incontinence is a chronic problem of uncontrolled leakage of urine. It is very common, especially at older ages and especially in women. Sometimes urinary incontinence is a sign of another health problem, such as diabetes or obesity. Regardless of the underlying cause, the symptoms of urinary incontinence alone may have a large impact on the quality of life, frequently causing inconvenience, embarrassment, and distress.
In a person with male anatomy, urinary incontinence is most commonly caused by an enlarged prostate gland or treatment for prostate cancer. In genetically female individuals, there are two common types of urinary incontinence with different causes: stress incontinence and urge incontinence.
• Stress urinary incontinence is caused by loss of support of the urethra, usually due to stretching of pelvic floor muscles during childbirth. It is characterized by leakage of small amounts of urine with activities that increase abdominal pressure, such as coughing, sneezing, or lifting. Treatment of stress urinary incontinence may include Kegel exercises to strengthen the pelvic muscles (see Explore More below). More serious cases may call for surgery to improve support for the bladder.
• Urge urinary incontinence (commonly called “overactive bladder”) is caused by uncontrolled contractions of the detrusor muscle in the wall of the bladder. This causes the bladder to empty unexpectedly. Urge incontinence is characterized by leakage of large amounts of urine in association with an insufficient warning to get to the bathroom in time. Treatment of urge incontinence may include taking medication to relax the detrusor muscle.
Feature: My Human Body
You probably have had to “donate” a urine specimen for analysis in conjunction with a medical visit. A thorough medical exam often includes clinical tests for urine. Understanding what your urine may reveal about your health may help you appreciate the need for such tests.
The most common urine test is called a urinalysis. In a routine urinalysis, a urine sample may be analyzed by sight and smell and with simple urine test strips. If a particular disorder is suspected, urinalysis may be more extensive. For example, the urine may be analyzed with specific tests or viewed under a microscope to identify abnormal substances in the urine. If a bacterial infection is suspected, a sample of urine may be cultured in the lab to see if it grows bacteria and which type of bacteria grow. Knowing the type of bacteria is important for deciding which class of antibiotics is likely to be most effective in treating the infection.
The color and clarity of urine may be obvious first indicators of disorders or other abnormalities. Normal urine is yellow to amber in color and looks clear. If urine is nearly colorless, it could be a sign of excessive fluid intake, or it might be a sign of diabetes. Very dark urine may indicate dehydration, but it could also be caused by taking certain medications or ingesting some other substances. If the urine has a reddish tinge, it is often a sign of blood in the urine, which could be due to a urinary tract infection, kidney stone, or even cancer. If the urine appears cloudy instead of clear, it could be due to white blood cells in the urine, which may be another sign of a urinary tract infection.
Normal urine may have virtually no odor if it is very dilute. It will have a stronger odor if it is concentrated. Brief changes in the normal odor of urine often occur due to the ingestion of certain foods or medications. For example, after eating asparagus, urine may have a peculiar and distinctive odor for several hours. More significant is urine that has a sweet smell, because this may indicate sugar in the urine, which is a sign of diabetes.
Urine test strips, much like the familiar litmus test strips used to detect acids and bases in a chemistry lab, are used to identify abnormal levels of certain components in the urine. For example, urine test strips can detect and quantify the presence of nitrites in urine, which is usually a sign of infection with certain types of bacteria. Urine test strips can also be used to identify proteins such as albumin in urine, which may be a sign of a kidney infection or of kidney failure. Levels of sodium in urine can also be measured with test strips, and higher-than-normal levels may be another indication of kidney failure. In addition, test strips can identify and quantify the presence of white blood cells and blood in a urine specimen, both of which are likely to be a sign of a urinary tract infection or some other urinary system disorder.
Besides the use of urine test strips, other simple urine tests that are often performed include Benedict’s test, which is a test for the presence and quantity of glucose in urine. If the level is high, it is likely to indicate diabetes. The test is so simple that it may even be done in the home by the patient to monitor how well sugar levels are being controlled. Testing for some other substances in urine requires the patient to collect urine over a 24-hour period. This is the case, for example, when testing for the adrenal hormone cortisol in urine. When urine cortisol levels are higher than normal it may indicate Cushing’s syndrome, and when the levels are lower than normal it may indicate Addison’s disease.
Review
1. What is diabetic nephropathy and what causes it?
2. Describe polycystic kidney disease (PKD).
3. Define kidney failure.
4. What are potential treatments for kidney failure when kidney function drops below the level needed to sustain life?
5. Describe hemodialysis.
6. What are kidney stones?
7. How may a large kidney stone be removed from the body?
8. How are bladder infections usually treated?
9. Why are bladder infections much more common in females than in males?
10. Define urinary incontinence.
11. Compare and contrast stress incontinence and urge incontinence.
12. Why is the presence of a protein such as albumin in the urine a cause for concern?
13. Patients undergoing hemodialysis usually have to do this procedure a few times a week. Why does it have to be done so frequently?
14. Which of the following is considered a genetic disorder?
A. Polycystic kidney disease
B. Diabetic nephropathy
C. Kidney failure
D. Urinary incontinence
15. What is the most common cause of a kidney infection?
Explore More
Kegel exercises can strengthen the muscles of the pelvic floor and help many cases of urinary incontinence. You can learn more by watching this video:
Urinary tract infections can be very painful. Learn more about their symptoms, causes, complications and treatments here:
Attributions
1. Ribbon by rosanegra_1, Pixabay license
2. Diabetic Nephropathy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
3. Polycystic kidneys by CDC/ Dr. Edwin P. Ewing, Jr., Public Domain, via Wikimedia Commons
4. Hemodialysis by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
5. Kidney Stones by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
6. 'Durex' diaphragm by Wellcome Collection gallery (2018-03-21) CC-BY-4.0 ia Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/19%3A_Urinary_System/19.5%3A_Disorders_of_the_Urinary_System.txt |
Case Study Conclusion: Drink and Flush
You are probably aware that drinking alcohol can cause visual disturbances, slurred speech, drowsiness, impaired judgment, and loss of coordination because of its effects on the brain. Although it may be less obvious, alcohol also can have serious effects on the functioning of the excretory system.
As you learned in the beginning of the chapter from the conversation between Maeva and Bintou, who were in line for the restroom, consumption of alcohol inhibits a hormone that causes our bodies to retain water. The result is that more water is released in urine, increasing the frequency of restroom trips as well as the risk of dehydration.
Which hormone discussed in this chapter does this? If you answered antidiuretic hormone (ADH; also called vasopressin)—you are correct! ADH is secreted by the hypothalamus of the brain and acts on the kidneys. As you have learned, the kidneys filter the blood, reabsorb needed substances, and produce urine. ADH helps the body conserve water by influencing this process. ADH makes the collecting ducts in the kidneys permeable to water, allowing water molecules to be reabsorbed from the urine back into the blood through osmosis into capillaries.
Alcohol is thought to produce more dilute urine by inhibiting the release of ADH. This causes the collecting ducts to be more impermeable to water, so less water can be reabsorbed and more is excreted in the urine. Because the volume of urine is increased, the bladder fills up more quickly, and the urge to urinate occurs more frequently. This is part of the reason why you often see a long line for the restroom in situations where many people are drinking alcohol. In addition to producing more dilute urine, simply consuming many beverages can also increase urine output.
In most cases, moderate drinking causes only a minor and temporary effect on kidney function. However, when people consume a large quantity of alcohol in a short period of time or abuse alcohol over long time periods, there can be serious effects on the kidney. For example, binge drinking (i.e. consuming about four to five drinks in two hours), can cause a condition called “acute kidney injury,” serious and sudden impairment of kidney function that requires immediate medical attention. As with other cases of kidney failure that you learned about in this chapter, the treatment is to artificially filter the blood using hemodialysis. While normal kidney function may eventually return, acute kidney injury can sometimes cause long-term damage to the kidneys.
In cases where people abuse alcohol, particularly for an extended period of time, there can be many serious effects on the kidneys and other parts of the excretory system. The dehydrating effect of alcohol on the body can impair the function of many organs, including the kidneys themselves. Additionally, because of alcohol’s effect on kidney function and water and ion balance, chronic alcohol consumption can cause abnormalities in blood ion concentration and acid-base balance, which can be very dangerous.
Additionally, drinking more than two alcoholic beverages a day can increase your risk of high blood pressure. As you have learned, high blood pressure is a risk factor for some kidney disorders and a common cause of kidney failure. Therefore, drinking too much alcohol can damage the kidneys by raising blood pressure.
Finally, chronic excessive consumption of alcohol can cause liver disease. As you know, the liver is an important organ of the excretory system that breaks down toxic substances in the blood. The liver and kidneys work together to remove wastes from the bloodstream. For example, as you have learned, the liver transforms ammonia into urea, which is then filtered and excreted by the kidneys. When the liver is not functioning normally, it puts added strain on the kidneys, which can result in kidney dysfunction. This association between alcohol, liver disease, and kidney dysfunction is so strong that most of the patients in the United States with both liver disease and related kidney dysfunction are alcoholics.
As you have learned, the excretory system is essential to remove toxic wastes from the body and regulate homeostasis. Having an occasional drink can temporarily alter these functions, but excessive alcohol exposure can seriously and permanently damage this system in many ways. Limiting alcohol consumption can help preserve the normal functioning of the excretory system so that it can protect your health.
Chapter Summary
In this chapter, you learned about the excretory system. Specifically, you learned that:
• Excretion is the process of removing wastes and excess water from the body. It is an essential process in all living things and a major way the human body maintains homeostasis.
• Organs of the excretory system include the skin, liver, large intestine, lungs, and kidneys.
• The skin plays a role in excretion through the production of sweat by sweat glands. Sweating eliminates excess water and salts and also a small amount of urea, a byproduct of protein catabolism.
• The liver is a very important organ of excretion. The liver breaks down many substances in the blood, including toxins. The liver also excretes bilirubin, a waste product of hemoglobin catabolism, in bile. Bile then travels to the small intestine and is eventually excreted in feces by the large intestine.
• The main excretory function of the large intestine is to eliminate solid waste that remains after food is digested and water is extracted from the indigestible matter. The large intestine also collects and excretes wastes from throughout the body.
• The lungs are responsible for the excretion of gaseous wastes, primarily carbon dioxide from cellular respiration in cells throughout the body. Exhaled air also contains water vapor and trace levels of some other waste gases.
• The paired kidneys are often considered to be the main organs of excretion. Their primary function is the elimination of excess water and wastes from the bloodstream by the production of urine. The kidneys filter many substances out of blood, allow the blood to reabsorb needed materials, and use the remaining materials to form urine.
• The two bean-shaped kidneys are located high in the back of the abdominal cavity on either side of the spine. A renal artery connects each kidney with the aorta and transports unfiltered blood to the kidney. A renal vein connects each kidney with the inferior vena cava and transports filtered blood back to the circulation.
• The kidney has two main layers involved in the filtration of blood and formation of urine: the outer cortex and inner medulla. At least a million nephrons, which are the tiny functional units of the kidney, span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.
• As blood flows through a nephron, many materials are filtered out of the blood, needed materials are returned to the blood, and the remaining materials are used to form urine.
• In each nephron, the glomerulus and the surrounding Bowman’s capsule form the unit that filters blood. From Bowman’s capsule, the material filtered from the blood, called filtrate, passes through the long renal tubule. As it does, some substances are reabsorbed into the blood and other substances are secreted from the blood into the filtrate, finally forming urine. The urine empties into collecting ducts, where more water may be reabsorbed.
• The kidneys are part of the urinary system, which also includes the ureters, urinary bladder, and urethra. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine. After urine forms in the kidneys, it is transported through the ureters to the bladder. The bladder stores the urine until urination, when urine is transported by the urethra to be excreted outside the body.
• Besides the elimination of waste products such as urea, uric acid, excess water, and mineral ions, the urinary system has other vital functions. These include maintaining homeostasis of mineral ions in extracellular fluid, regulating acid-base balance in the blood, regulating the volume of extracellular fluids, and controlling blood pressure.
• The formation of urine must be closely regulated to maintain body-wide homeostasis. Several endocrine hormones help control this function of the urinary system, including antidiuretic hormone from the hypothalamus, parathyroid hormone from the parathyroid glands, and aldosterone from the adrenal glands. For example, the kidneys are part of the renin-angiotensin-aldosterone system that regulates the concentration of sodium in the blood to control blood pressure. In this system, the enzyme renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.
• The kidneys also secrete endocrine hormones, including calcitriol, which helps control the level of calcium in the blood; and erythropoietin, which stimulates the bone marrow to produce red blood cells.
• The process of urination is controlled by both the autonomic and the somatic nervous systems. The autonomic system causes the detrusor muscle in the bladder wall to relax as the bladder fills with urine, but the conscious contraction of the detrusor muscle expels urine from the bladder during urination.
• Ureters are tube-like structures that connect the kidneys with the urinary bladder. Each ureter arises at the renal pelvis of a kidney and travels down through the abdomen to the urinary bladder. The walls of the ureter contain smooth muscle that can contract to push urine through the ureter by peristalsis. The walls are lined with transitional epithelium that can expand and stretch.
• The urinary bladder is a hollow, muscular organ that rests on the pelvic floor. It is also lined with transitional epithelium. The function of the bladder is to collect and store urine from the kidneys before the urine is eliminated through urination. Filling of the bladder triggers the autonomic nervous system to stimulate the detrusor muscle in the bladder wall to contract. This forces urine out of the bladder and into the urethra.
• The urethra is a tube that connects the urinary bladder to the external urethral orifice. Somatic nerves control the sphincter at the distal end of the urethra. This allows the opening of the sphincter for urination to be under voluntary control.
• Diabetic nephropathy is a progressive kidney disease caused by damage to the capillaries in the glomeruli of the kidneys due to long-standing diabetes mellitus. Years of capillary damage may occur before symptoms first appear.
• Polycystic kidney disease (PKD) is a genetic disorder (autosomal dominant or recessive) in which multiple abnormal cysts grow in the kidneys.
• Diabetic nephropathy, PKD, or chronic hypertension may lead to kidney failure, in which the kidneys are no longer able to adequately filter metabolic wastes from the blood. Kidneys may fail to such a degree that kidney transplantation or repeated, frequent hemodialysis is needed to support life. In hemodialysis, the patient’s blood is filtered artificially through a machine and then returned to the patient’s circulation.
• A kidney stone is a solid crystal that forms in a kidney from minerals in the urine. A small stone may pass undetected through the ureters and the rest of the urinary tract. A larger stone may cause pain when it passes or be too large to pass so it blocks a ureter. Large kidney stones may be shattered with high-intensity ultrasound into pieces small enough to pass through the urinary tract, or they may be removed surgically.
• A bladder infection is generally caused by bacteria that reach the bladder from the GI tract and multiply. Bladder infections are much more common in females than males because the female urethra is much shorter and closer to the anus. Treatment generally includes antibiotic drugs.
• Urinary incontinence is a chronic problem of uncontrolled leakage of urine. It is very common, especially at older ages and in women. In men, urinary incontinence is usually caused by an enlarged prostate gland. In women, it is usually caused by stretching of pelvic floor muscles during childbirth (stress incontinence) or by an “overactive bladder” that empties without warning (urge incontinence).
You have learned that the excretory system protects your body through the removal of toxic wastes and the maintenance of homeostasis. But how does your body protect itself against pathogens and other threats? Read the next chapter on the immune system to find out.
Chapter Summary Review
1. Match each organ of the excretory system with the description that best fits it. Each organ is used only once. Organs: lungs, skin, kidneys, liver, large intestine
1. Excretes solid wastes from food
2. Cools the body while it eliminates water and other wastes
3. Part of the urinary system
4. Produces bile and breaks down toxins in the blood
5. Eliminates waste gases
2. In what ways can the alveoli of the lungs be considered analogous to the nephrons of the kidney?
3. What is urea? Where is urea produced and what is it produced from? How is urea excreted from the body?
4. True or False. The lungs help excrete excess water, in addition to carbon dioxide.
5. True or False. There are two kidneys in the human body, and both are the same size and shape.
6. True or False. The renal pelvis is a bone that surrounds the kidneys.
7. If a person has a large kidney stone that prevents urine that has left the kidney from reaching the bladder, where do you think this kidney stone is located? Explain your answer.
8. Match each of the following structures with the description that best fits it. Each structure is used only once. Structures: urethra; nephron; ureters; urinary bladder
1. Produces urine
2. Stores relatively large quantities of urine
3. Releases urine to the outside of the body
4. Transports urine between where it is produced and where it is stored
9. Explain what is meant by “Excretion = Filtration – Reabsorption + Secretion” regarding the production of urine.
10. Which disease discussed in the chapter specifically affects the glomerular capillaries of the kidneys? Where are the glomerular capillaries located within the kidneys, and what is their function?
11. Describe one way in which the excretory system helps maintain homeostasis in the body.
12. Secretion in a nephron occurs:
1. before filtration, and goes from the filtrate into the blood
2. after filtration, and goes from the filtrate into the blood
3. before filtration, and goes from the blood into the filtrate
4. after filtration, and goes from the blood into the filtrate
13. High blood pressure can both contribute to the development of kidney disorders and be a symptom of kidney disorders. What is a kidney disorder that can be caused by high blood pressure? What is a kidney disorder that has high blood pressure as a symptom? How does blood pressure generally relate to the function of the kidney?
14. If the body is dehydrated, what do the kidneys do? What does this do to the appearance of the urine produced?
15. Which of the following contain filtered blood in the kidney?
1. Arteries
2. Venules
3. Veins
4. B and C
16. True or False. Hormones influence the functions of the kidney, but the kidney itself does not produce hormones.
17. True or False. Both the lungs and the kidneys help maintain the pH of the blood.
18. Identify three risk factors for the development of kidney stones.
Attributions
1. Alcohol by geralt; Pixabay license
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/19%3A_Urinary_System/19.6%3A_Case_Study_Conclusion%3A__Alcohol_and_Chapter_Summary.txt |
This chapter outlines the layered responses of the human immune system, including both innate and adaptive immune responses. It also describes the structures and functions of the lymphatic system, with a focus on its roles in host defense. In addition, the chapter examines three different types of immune system disorders.
• 20.1: Case Study: Your Defense System
As you read this chapter, you will learn about the functions of the immune system, and the specific roles that its cells and organs - such as B and T cells and lymph nodes - play in defending the body. At the end of this chapter, you will learn what type of lymphoma Wei has and what some of his treatment options are, including treatments that make use of the biochemistry of the immune system to fight cancer with the immune system itself.
• 20.2: Introduction to the Immune System
The immune system is a host defense system. It comprises many biological structures - ranging from individual white blood cells to entire organs - as well as many complex biological processes. The function of the immune system is to protect the host from pathogens and other causes of disease such as tumor cells. To function properly, the immune system must be able to detect a wide variety of pathogens.
• 20.3: Lymphatic System
The lymphatic system is a collection of organs involved in the production, maturation, and harboring of white blood cells called lymphocytes. It also includes a network of vessels that transport or filter the fluid known as lymph in which lymphocytes circulate. The figure below shows major lymphatic vessels and other structures that make up the lymphatic system.
• 20.4: Innate Immune System
The innate immune system is a subset of the human immune system that produces rapid but non-specific responses to pathogens. Innate responses are generic rather than tailored to a particular pathogen. Every pathogen that is encountered is responded to in the same general ways by the innate system. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting immunity to them.
• 20.5: Adaptive Immune System
The adaptive immune system is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens and tumor cells. An adaptive immune response is set in motion by antigens that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen).
• 20.6: Disorders of the Immune System
An allergy is a disorder in which the immune system makes an inflammatory response to a harmless antigen. Any antigen that causes allergies is called an allergen. Common allergens include pollen, dust mites, mold, specific foods such as peanuts, insect stings, and certain medications such as aspirin. Autoimmune diseases occur when the immune system fails to recognize the body's own molecules as self and attacks them, causing damage to tissues and organs.
• 20.7: Human Microbiome
The scientific evidence supporting the gut microbiome in relation to health maintenance and links with various disease states afflicting humans, from metabolic to mental health, has grown dramatically in the last few years. Strategies addressing the positive modulation of microbiome functionality associated with these disorders offer huge potential to the food and pharmaceutical industries to innovate and provide therapeutic solutions to many of the health issues affecting modern society.
• 20.8: Case Study Conclusion: Lymphoma and Chapter Summary
About every three minutes, one person in the U.S. is diagnosed with blood cancer, the most common type of which is lymphoma. Wei, who was diagnosed with lymphoma in the beginning of this chapter, has a difficult road ahead, but he and his medical team are optimistic that he may be able to be cured. More research into how the immune system functions may lead to even better treatments for lymphoma, and other types of cancers, in the future.
Thumbnail: From left to right: erythrocyte, platelet, and lymphocyte. (Public Domain; The National Cancer Institute at Frederick ).
20: Immune System
Case Study: Defending Your Defenses
Twenty-six-year-old Wei isn’t feeling well. Wei uses he/him/his pronouns. He is more tired than usual, dragging through his workdays despite going to bed earlier and napping on the weekends. He doesn't have much of an appetite and has started losing weight. When he presses on the side of his neck, like the doctor is doing in Figure \(1\), he notices an unusual lump.
Wei goes to his doctor, who performs a physical exam and determines that the lump is a swollen lymph node. Lymph nodes are part of the immune system, and they will often become enlarged when the body is fighting off an infection. Dr. Bouazizi thinks that the swollen lymph node and fatigue could be signs of a viral or bacterial infection, or indicate a type of cancer called lymphoma. However, an infection is a more likely cause, particularly in a young person like Wei. Dr. Bouazizi prescribes an antibiotic in case Wei has a bacterial infection and advises him to return in a few weeks if his lymph node does not shrink or if he is not feeling better.
Wei returns a few weeks later. He is not feeling better and his lymph node is still enlarged. Dr. Bouazizi is concerned and orders a biopsy of the enlarged lymph node. A lymph node biopsy for suspected lymphoma often involves the surgical removal of all or part of a lymph node, to determine whether the tissue contains cancerous cells.
The initial results of the biopsy indicate that Wei does have lymphoma. Although lymphoma is more common in older people, young adults and even children can get this disease. There are many types of lymphoma, with the two main types being Hodgkin and non-Hodgkin lymphoma. Non-Hodgkin lymphoma (NHL), in turn, has many subtypes depending on factors such as which cell types are affected. For instance, some subtypes of NHL affect immune system cells called B cells, while others affect different immune system cells called T cells.
Dr. Bouazizi explains to Wei that it is important to determine which type of lymphoma he has, in order to choose the best course of treatment. Wei’s biopsied tissue will be further examined and tested to see which cell types are affected and which specific cell-surface proteins, called antigens, are present. This should help in identifying his specific type of lymphoma.
As you read this chapter, you will learn about the functions of the immune system, and the specific roles that its cells and organs—such as B and T cells and lymph nodes— play in defending the body. At the end of this chapter, you will learn what type of lymphoma Wei has and what some of his treatment options are, including treatments that make use of the biochemistry of the immune system to fight cancer with the immune system itself.
Chapter Overview: Immune System
In this chapter, you will learn about the immune system—the system that defends the body against infections and other causes of disease such as cancerous cells. Specifically, you will learn about:
• How the immune system identifies normal cells of the body as “self” and pathogens and damaged cells as “non-self.”
• The two major subsystems of the general immune system: the innate immune system, which provides a quick but non-specific response; and the adaptive immune system, which is slower but provides a specific response that often results in long-lasting immunity.
• The specialized immune system that protects the brain and spinal cord called the neuroimmune system.
• The organs, cells, and responses of the innate immune system, which include physical barriers such as skin and mucus, chemical and biological barriers, inflammation, activation of the complement system of molecules, and non-specific cellular responses such as phagocytosis.
• The lymphatic system—which includes white blood cells called lymphocytes; lymphatic vessels that transport a fluid called lymph; and organs such as the spleen, tonsils, and lymph nodes—and its important role in the adaptive immune system.
• Specific cells of the immune system and their functions, including B cells, T cells, plasma cells, and natural killer cells.
• How the adaptive immune system can generate specific and often long-lasting immunity against pathogens through the production of antibodies.
• How vaccines work to generate immunity.
• How cells in the immune system detect and kill cancerous cells.
• Some strategies that pathogens employ to evade the immune system.
• Disorders of the immune system, including allergies, autoimmune diseases (such as diabetes and multiple sclerosis), and immunodeficiency resulting from conditions such as HIV infection.
As you read the chapter, think about the following questions:
1. What are the functions of lymph nodes?
2. What are B and T cells and how do they relate to lymph nodes?
3. What are cell-surface antigens? How do they relate to the immune system and to cancer?
Attributions
1. Palpating lymph nodes by BodyParts3D/Anatomography (NIH), public domain via Wikimedia Commons
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3. | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/20%3A_Immune_System/20.1%3A_Case_Study%3A_Your_Defense_System.txt |
Worm Attack!
Does this organism look like a space alien? A scary creature from a nightmare? In fact, it’s a 1-cm long worm in the genus Schistosoma. It may invade and take up residence in the human body, causing a very serious illness known as schistosomiasis. The worm gains access to the human body while it is in a microscopic life stage. It enters through a hair follicle when the skin comes into contact with contaminated water. The worm then grows and matures inside the human organism, causing disease.
Host vs. Pathogen
The Schistosoma worm has a parasitic relationship with humans. In this type of relationship, one organism, called the parasite, lives on or in another organism, called the host. The parasite always benefits from the relationship and the host is always harmed. The human host of the Schistosoma worm is clearly harmed by the parasite when it invades the host’s tissues. The urinary tract or intestines may be infected, and signs and symptoms may include abdominal pain, diarrhea, bloody stool, or blood in the urine. Those who have been infected a long time may experience liver damage, kidney failure, infertility, or bladder cancer. In children, Schistosoma infection may cause poor growth and difficulty learning. Table \(1\) lists some of the microscopic pathogens, their images, description, and the diseases that they cause.
Like the Schistosoma worm, many other organisms can make us sick if they manage to enter our body. Any such agent that can cause disease is called a pathogen. Most pathogens are microorganisms, although some, such as the Schistosoma worm, are much larger. In addition to worms, common types of pathogens of human hosts include bacteria, viruses, fungi, and single-celled organisms called protists. You can see examples of each of these types of pathogens in Table \(1\). Fortunately for us, our immune system is able to keep most potential pathogens out of the body or to quickly destroy them if they do manage to get in. When you read this chapter, you’ll learn how your immune system usually keeps you safe from harm — including from scary creatures like the Schistosoma worm!
Table \(1\): Types of Pathogens
Type of Pathogen Example and their Image Description Human Disease caused by pathogens of that type
Bacteria
such as Escherichia coli
Single-celled organisms without a nucleus Strep throat, staph infections, tuberculosis, food poisoning, tetanus, pneumonia, syphilis
Viruses
such as Herpes simplex
Particles that reproduce by taking over living cells. Common cold, flu, genital herpes, cold sores, measles, AIDS, genital warts, chickenpox, smallpox
Fungi
such as Trichophyton rubrum
Organisms with a nucleus that grow as single cells or tread-like filaments Ringworm, athlete's foot, tineas, candidiasis, histoplasmosis
Protozoa
Such as Giarida lamblia
A single-celled organism with a nucleus Malaria, Traveler's diarrhea, giardiasis, trypanosomiasis (sleeping sickness)
What is the Immune System?
The immune system is a host defense system. It comprises many biological structures —ranging from individual white blood cells to entire organs — as well as many complex biological processes. The function of the immune system is to protect the host from pathogens and other causes of disease such as tumor cells. To function properly, the immune system must be able to detect a wide variety of pathogens. It also must be able to distinguish the cells of pathogens from the host’s own cells and also to distinguish cancerous or damaged host cells from healthy cells. In humans and most other vertebrates, the immune system consists of layered defenses that have increased specificity for particular pathogens or tumor cells. The layered defenses of the human immune system are usually classified into two subsystems called the innate immune system and the adaptive immune system.
Innate Immune System
Any discussion of the innate immune response usually begins with the physical barriers that prevent pathogens from entering the body, destroy them after they enter, or flush them out before they can establish themselves in the hospitable environment of the body’s soft tissues. Barrier defenses are part of the body’s most basic defense mechanisms. The barrier defenses are not a response to infections, but they are continuously working to protect against a broad range of pathogens.
The phagocytes are the body’s fast acting first line of immunological defense against organisms that have breached barrier defenses and have entered the vulnerable tissues of the body. For example, certain leukocytes (white blood cells) engulf and destroy pathogens they encounter in the process called phagocytosis. The body's response again a pathogen's breach is also called Inflammation. Phagocytosis and Inflammation will be discussed in detail in concept Innate Immune System.
Adaptive Immune System
The adaptive immune system is activated if pathogens successfully enter the body and manage to evade the general defenses of the innate immune system. An adaptive response is specific to the particular type of pathogen that has invaded the body or to cancerous cells. It takes longer to launch a specific attack, but once it is underway, its specificity makes it very effective. An adaptive response also usually leads to immunity. This is a state of resistance to a specific pathogen due to the ability of the adaptive immune system to “remember” the pathogen and immediately mount a strong attack tailored to that particular pathogen if it invades again in the future.
Self vs. Non-Self
Both innate and adaptive immune responses depend on the ability of the immune system to distinguish between self and non-self molecules. Self molecules are those components of an organism’s body that can be distinguished from foreign substances by the immune system. Virtually all body cells have surface proteins that are part of a complex called the major histocompatibility complex (MHC). These proteins are one way the immune system recognizes body cells as self. Non-self proteins, in contrast, are recognized as foreign because they are different from self-proteins.
Antigens and Antibodies
Many non-self molecules comprise a class of compounds called antigens. Antigens, which are usually proteins, bind to specific receptors on immune system cells and elicit an adaptive immune response. Some adaptive immune system cells (B cells) respond to foreign antigens by producing antibodies. An antibody is a molecule that precisely matches and binds to a specific antigen. This may target the antigen (and the pathogen displaying it) for destruction by other immune cells.
Antigens on the surface of pathogens are how the adaptive immune system recognizes specific pathogens. Antigen specificity allows for the generation of responses tailored to the specific pathogen. It is also how the adaptive immune system ”remembers” the same pathogen in the future.
Immune Surveillance
Another important role of the immune system is to identify and eliminate tumor cells. This is called immune surveillance. The transformed cells of tumors express antigens that are not found on normal body cells. The main response of the immune system to tumor cells is to destroy them. This is carried out primarily by aptly named killer T cells of the adaptive immune system.
Lymphatic System
The lymphatic system is a human organ system that is a vital part of the adaptive immune system. It is also part of the cardiovascular system and plays a major role in the digestive system (see the concept Lymphatic System).
Feature: Human Biology in the News
“They’ll have to rewrite the textbooks!”
That sort of response to scientific discovery is sure to attract media attention, and it did. It’s what Kevin Lee, a neuroscientist at the University of Virginia, said in 2016 when his colleagues told him they had discovered human anatomical structures that had never before been detected. The structures were tiny lymphatic vessels in the meningeal layers surrounding the brain.
How these lymphatic vessels could have gone unnoticed when all human body systems have been studied so completely is amazing in its own right. The suggested implications of the discovery are equally amazing:
• The presence of these lymphatic vessels means that the brain is directly connected to the peripheral immune system, presumably allowing a close association between the human brain and human pathogens. This suggests an entirely new avenue by which humans and their pathogens may have influenced each other’s evolution. The researchers speculate that our pathogens may have even influenced the evolution of our social behaviors.
• The researchers think there will also be many medical applications of their discovery. For example, the newly discovered lymphatic vessels may play a major role in neurological diseases that have an immune component, such as multiple sclerosis. The discovery might also affect how conditions such as autism spectrum disorders and schizophrenia are treated.
Review
1. What is a pathogen?
2. State the purpose of the immune system.
3. Compare and contrast the innate and adaptive immune systems.
4. Explain how the immune system distinguishes self molecules from non-self molecules.
5. What are antigens?
6. Define tumor surveillance.
7. Briefly describe the lymphatic system and its role in immune function.
8. Identify the neuroimmune system.
9. Which of the following is NOT a function of the immune system?
1. Protecting the body against fungi
2. Protecting the body against bacteria
3. Protecting the body against cancerous cells
4. None of the above
10. What does it mean that the immune system is not just composed of organs?
11. What are the general relationships between the terms lymphocytes, leukocytes, and white blood cells?
12. True or False. Phagocytosis occurs in the innate immune system.
13. True or False. Major histocompatibility complex proteins are antibodies.
14. True or False. Only the adaptive immune response requires the ability to distinguish between self and non-self.
15. Why is the immune system considered to be “layered?”
Explore More
Scientists predict we may be facing an antibiotic apocalypse, learn more here:
Attributions
1. Schistosome Parasite by Bruce Wetzel and Harry Schaefer, public domain via NCI NIH
2. Scanning electron micrograph of Escherichia coli by NIAID, public domain via Wikimedia Commons
3. Electron micrograph of Herpes virus by George W. Beran, public domain via Wikimedia Commons
4. Trichophyton rubrum by CDC/Dr. Libero Ajello, public domain via Wikimedia Commons
5. Giardia by schmidty4112, CC BY 2.0 via Flickr
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
7. Some text is adapted from 21.2 Barrier Defenses and the Innate Immune Response by OpenStax licensed CC BY 4.0. | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/20%3A_Immune_System/20.2%3A_Introduction_to_the_Immune_System.txt |
Tonsillitis
The white patches on either side of the throat in this picture are signs of tonsillitis. The tonsils are small structures in the throat that are very common sites of infection. The white spots on the tonsils pictured here are evidence of infection. The patches consist of large amounts of dead bacteria, cellular debris, and white blood cells; in a word, pus. Children with recurrent tonsillitis may have their tonsils removed surgically to eliminate this type of infection. The tonsils are organs of the lymphatic system.
What Is the Lymphatic System?
The lymphatic system is a collection of organs involved in the production, maturation, and harboring of white blood cells called lymphocytes. It also includes a network of vessels that transport or filter the fluid known as lymph in which lymphocytes circulate. Figure \(2\)shows major lymphatic vessels and other structures that make up the lymphatic system. Besides the tonsils, organs of the lymphatic system include the thymus, the spleen, and hundreds of lymph nodes that are distributed along the lymphatic vessels.
The lymphatic vessels form a transportation network similar in many respects to the blood vessels of the cardiovascular system. However, unlike the cardiovascular system, the lymphatic system is not a closed system. Instead, lymphatic vessels carry lymph in a single direction, always toward the upper chest, where the lymph empties from lymphatic vessels into blood vessels.
Cardiovascular Function of the Lymphatic System
The return of lymph to the bloodstream is one of the major functions of the lymphatic system. When blood travels through capillaries of the cardiovascular system, it is under pressure, which forces some of the components of blood (such as water, oxygen, and nutrients) through the walls of the capillaries and into the tissue spaces between cells, forming tissue fluid, also called interstitial fluid (Figure \(3\)). Interstitial fluid bathes and nourishes cells and also absorbs their waste products. Much of the water from the interstitial fluid is reabsorbed into the capillary blood by osmosis. Most of the remaining fluid is absorbed by tiny lymphatic vessels called lymph capillaries. Once interstitial fluid enters the lymphatic vessels, it is called lymph. Lymph is very similar in composition to blood plasma. Besides water, lymph may contain proteins, waste products, cellular debris, and pathogens. It also contains numerous white blood cells, especially the subset of white blood cells known as lymphocytes. In fact, lymphocytes are the main cellular components of lymph.
The lymph that enters lymph capillaries in tissues is transported through the lymphatic vessel network to two large lymphatic ducts in the upper chest. From there, the lymph flows into two major veins (called subclavian veins) of the cardiovascular system. Unlike blood, lymph is not pumped through its network of vessels. Instead, lymph moves through lymphatic vessels via a combination of contractions of the vessels themselves and forces applied to the vessels externally by skeletal muscles. Lymphatic vessels also contain numerous valves that keep lymph flowing in just one direction, thereby preventing backflow.
Digestive Function of the Lymphatic System
Lymphatic vessels called lacteals (Figure \(4\)) are present in the lining of the gastrointestinal tract, mainly in the small intestine. Each tiny villus in the lining of the small intestine has an internal bed of capillaries and lacteals. The capillaries absorb most nutrients from the digestion of food into the blood. The lacteals absorb mainly fatty acids from lipid digestion into the lymph, forming a fatty-acid-enriched fluid called chyle. Vessels of the lymphatic network then transport chyle from the small intestine to the main lymphatic ducts in the chest from which it drains into the blood circulation. The nutrients in chyle then circulate in the blood to the liver, where they are processed along with the other nutrients that reach the liver directly via the bloodstream.
Immune Function of the Lymphatic System
The primary function of the lymphatic system is host defense as part of the immune system. This function of the lymphatic system is centered on the production, maturation, and circulation of lymphocytes. Lymphocytes are leukocytes that are involved in the adaptive immune system. They are responsible for the recognition of, and tailored defense against, specific pathogens or tumor cells. Lymphocytes may also create a lasting memory of pathogens so they can be attacked quickly and strongly if they ever invade the body again. In this way, lymphocytes bring about long-lasting immunity to specific pathogens.
There are two major types of lymphocytes, called B cells and T cells, which are illustrated in Figure \(5\). Both B cells and T cells are involved in the adaptive immune response, but they play different roles. You can learn more about their immune functions by reading the concept Adaptive Immune System.
Production and Maturation of Lymphocytes
Like all other types of blood cells, including red blood cells as well as leukocytes, both B cells and T cells are produced from stem cells in the red marrow inside bones. After lymphocytes first form, they must go through a complicated maturation process before they are ready to search for pathogens. In this maturation process, they “learn” to distinguish self from non-self. Only those lymphocytes that successfully complete this maturation process go on to actually fight infections by pathogens.
B cells mature in the bone marrow, which is why they are called B cells. After they mature and leave the bone marrow, they travel first to the circulatory system and then enter the lymphatic system to search for pathogens. T cells, on the other hand, mature in the thymus, which is why they are called T cells. The thymus is illustrated in Figure \(6\). It is a small lymphatic organ in the chest that consists of an outer cortex and inner medulla, all surrounded by a fibrous capsule. After maturing in the thymus, T cells enter the rest of the lymphatic system to join B cells in the hunt for pathogens. The bone marrow and thymus are called primary lymphoid organs because of their role in the production and/or maturation of lymphocytes.
Lymphocytes in Secondary Lymphoid Organs
The tonsils, spleen, and lymph nodes are referred to as secondary lymphoid organs. These organs do not produce or mature lymphocytes. Instead, they filter lymph and store lymphocytes. It is in these secondary lymphoid organs that pathogens (or their antigens) activate lymphocytes and initiate adaptive immune responses. Activation leads to the cloning of pathogen-specific lymphocytes, which then circulate between the lymphatic system and the blood, searching for and destroying their specific pathogens by producing antibodies against them.
Tonsils
There are actually four pairs of human tonsils. Three of the four are shown in Figure \(7\). The fourth pair, called tubal tonsils, is located at the back of the nasopharynx. The palatine tonsils are the tonsils that are visible on either side of the throat. All four pairs of tonsils encircle a part of the anatomy where the respiratory and gastrointestinal tracts intersect and where pathogens have ready access to the body. This ring of tonsils is called Waldeyer's ring.
Spleen
The spleen (Figure \(8\)) is the largest of the secondary lymphoid organs and is centrally located in the body. Besides harboring lymphocytes and filtering lymph, the spleen also filters blood. Most dead or aged red blood cells are removed from the blood in the red pulp of the spleen. Lymph is filtered in the white pulp of the spleen. In the fetus, the spleen has the additional function of producing red blood cells. This function is taken over by bone marrow after birth.
Lymph Nodes
Each lymph node is a small but organized collection of lymphoid tissue (see green circular structures in Figure \(1\)) that contains many lymphocytes. Lymph nodes are located at intervals along the lymphatic vessels, and lymph passes through them on their way back to the blood. There are at least 500 lymph nodes in the human body. Many of them are clustered at the base of the limbs and in the neck. Figure \(9\) shows the major lymph node concentrations. The figure includes the spleen and the region named Waldeyer’s ring, consisting of the tonsils.
Feature: Myth vs. Reality
Lymph nodes near the surface of the body are obvious signs of immune system activity when they become enlarged and sometimes tender to the touch. Because it is easy to see and feel swollen lymph nodes, an individual can monitor his or her own health. It is important to be able to know the myths and realities of swollen lymph nodes.
Myth: You should see a doctor immediately whenever you have swollen lymph nodes.
Reality: Lymph nodes are constantly filtering lymph so it is expected that they will change in size with varying amounts of debris or pathogens that may be present. A minor, unnoticed infection may cause swollen lymph nodes that may last for a few weeks. Generally, lymph nodes that return to their normal size within three weeks are not a cause for concern.
Myth: Swollen lymph nodes mean you have a bacterial infection.
Reality: Although infection is the most common cause of swollen lymph nodes, not all infections are caused by bacteria. For example, mononucleosis commonly causes swollen lymph nodes, and it is caused by viruses. There are also other causes of swollen lymph nodes besides infections, such as cancer and certain medications.
Myth: A swollen lymph node means you have cancer.
Reality: Cancer is far less likely to be the cause of a swollen lymph node than is an infection.
Myth: Cancer in a lymph node always originates somewhere else. There is no cancer of the lymph nodes.
Reality: Cancers do commonly spread from their site of origin to nearby lymph nodes and then to other organs, but cancer may also originate in the lymph nodes. This type of cancer is called lymphoma.
Review
1. What is the lymphatic system?
2. Describe the composition of lymph.
3. Outline the cardiovascular function of the lymphatic system.
4. Describe the role of the lymphatic system in the absorption of nutrients from the digestive system.
5. Summarize the function of the lymphatic system in host defense.
6. Name primary lymphatic organs and their functions.
7. What are the secondary lymphatic organs? State their functions in the adaptive immune system.
8. How is interstitial fluid related to lymph?
9. B and T cells are types of:
1. Leukocytes
2. Lymphocytes
3. White blood cells
4. All of the above
10. For each of the following statements, indicate whether it applies to B cells, T cells, or both.
1. These cells are born in the red bone marrow.
2. These cells are part of the adaptive immune system.
3. These cells mature in the bone marrow.
11. Explain the difference between lymphocyte maturation and lymphocyte activation.
12. True or False. The spleen produces lymphocytes.
13. True or False. Tonsils are glands that produce lymph.
Attributions
1. Tonsillitis by Michaelbladon, Public Domain; via Wikimedia Commons
2. Lymphatic System By Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. via Wikimedia Commons, licensed CC BY 3.0, via Wikimedia Commons
3. Lymph Capillaries by US Government, Public Domain via Wikimedia Commons
4. Intestinal Villus by Snow93, public domain via Wikimedia Commons
5. White Blood Cells, CC BY 3.0, Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436, licensed CC BY 3.0, via Wikimedia Commons
6. Thymus by US Government, Public Domain via Wikimedia Commons
7. Tonsils CC BY by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0, via Wikimedia Commons
8. Spleen adapted from Illu Spleen by US government, public domain, via Wikimedia Commons
9. Lymph Nodes by Fred the Oyster, public domain via Wikimedia Commons
10. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/20%3A_Immune_System/20.3%3A_Lymphatic_System.txt |
Paper Cut
It’s just a paper cut, but the break in your skin could provide an easy way for pathogens to enter your body. If bacteria were to enter through the cut and infect the wound, your innate immune system would quickly respond with a dizzying array of general defenses.
The innate immune system is a subset of the human immune system that produces rapid but non-specific responses to pathogens. Innate responses are generic rather than tailored to a particular pathogen. Every pathogen that is encountered is responded to in the same general ways by the innate system. Although the innate immune system provides immediate and rapid defenses against pathogens, it does not confer long-lasting immunity to them. In most organisms, the innate immune system is the dominant system of host defense. Other than most vertebrates including humans, the innate immune system is the only system of host defense.
In humans, the innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses. Surface barriers of various types generally keep most pathogens out of the body. If these barriers fail, then other innate defenses are triggered. The triggering event is usually the identification of pathogens by pattern-recognition receptors on cells of the innate immune system. These receptors recognize molecules that are broadly shared by pathogens but distinguishable from host molecules. Alternatively, the other innate defenses may be triggered when damaged, injured, or stressed cells send out alarm signals, many of which are recognized by the same receptors as those that recognize pathogens.
Barriers to Pathogens
The body’s first line of defense consists of three different types of barriers that keep most pathogens out of body tissues. The types of barriers are mechanical, chemical, and biological barriers.
Mechanical Barriers
Mechanical barriers are the first line of defense against pathogens, and they physically block pathogens from entering the body. The skin is the most important mechanical barrier. In fact, it is the single most important defense the body has. The outer layer of skin, the epidermis, is tough and very difficult for pathogens to penetrate. It consists of dead cells that are constantly being shed from the body surface. This helps remove bacteria and other infectious agents that have adhered to the skin. The epidermis also lacks blood vessels and is usually lacking moisture, so it does not provide a suitable environment for most pathogens. Hair, which is an accessory organ of the skin, also helps to keep out pathogens. Hairs inside the nose may trap larger pathogens and other particles in the air before they can enter the airways of the respiratory system.
Mucous membranes provide a mechanical barrier to pathogens and other particles at body openings. These membranes also line the respiratory, gastrointestinal, urinary, and reproductive tracts. Mucous membranes secrete mucus, which is a slimy and somewhat sticky substance that traps pathogens. Many mucous membranes also have hair-like cilia that sweep mucus and trapped pathogens toward body openings where they can be removed from the body. When you sneeze or cough, mucus, and pathogens are mechanically ejected from the nose and throat, as you can see in the photo below. Other mechanical defenses include tears, which wash pathogens from the eyes, and urine, which flushes pathogens out of the urinary tract.
Chemical Barriers
Chemical barriers also protect against infection by pathogens. They destroy pathogens on the outer body surface, at body openings, and on inner body linings. Sweat, mucus, tears, saliva, and breastmilk all contain antimicrobial substances, such as the enzyme lysozyme, that kill pathogens, especially bacteria. Sebaceous glands in the dermis of the skin secrete acids that form a very fine, slightly acidic film on the surface of the skin that acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin. Urine and vaginal secretions are also too acidic for many pathogens to endure. Semen contains zinc, which most pathogens cannot tolerate, as well as defensins, which are antimicrobial proteins that act mainly by disrupting bacterial cell membranes. In the stomach, stomach acid and digestive enzymes called proteases, which break down proteins, kill most pathogens that enter the gastrointestinal tract in food or water.
Biological Barriers
Biological barriers are living organisms that help protect the body from pathogens. Trillions of harmless bacteria normally live on the human skin and in the urinary, reproductive, and gastrointestinal tracts. These bacteria use up food and surface space that help prevent pathogenic bacteria from colonizing the body. Some of these harmless bacteria also secrete substances that change the conditions of their environment, making it less hospitable to potentially harmful bacteria. For example, they may release toxins or change the pH. All of these effects of harmless bacteria reduce the chances that pathogenic microorganisms will be able to reach sufficient numbers to cause illness.
Inflammation
If pathogens manage to breach the barriers protecting the body, then one of the first active responses of the innate immune system kicks in. This response is inflammation. The main function of inflammation is to establish a physical barrier against the spread of infection. It also eliminates the initial cause of cell injury, clears out dead cells and tissues damaged from the original insult and the inflammatory process, and initiates tissue repair. Inflammation is often a response to infection by pathogens, but there are other possible causes, including burns, frostbite, and exposure to toxins.
The signs and symptoms of inflammation include redness, swelling, warmth, pain, and frequently some loss of function. These symptoms are caused by increased blood flow into infected tissue and a number of other processes, illustrated in Figure \(3\) and described below in the text.
Inflammation is triggered by chemicals such as cytokines and histamines, which are released by injured or infected cells or by immune system cells such as macrophages (described in Figure \(5\)) that are already present in tissues. These chemicals cause capillaries to dilate and become leaky, increasing blood flow to the infected area and allowing blood to enter the tissues. Pathogen-destroying leukocytes, complement proteins, and tissue-repairing proteins migrate into tissue spaces from the bloodstream to attack pathogens and repair their damage. Cytokines also promote chemotaxis, which is migration to the site of infection by leukocytes that destroy pathogens. Some cytokines have anti-viral, antifungal, and antibacterial effects, such as shutting down protein synthesis in host cells, which viruses need in order to survive and replicate.
Complement System
The complement system is a complex biochemical mechanism named for its ability to “complement” the killing of pathogens directly by creating holes in the body of the pathogen and by assisting antibodies. Antibodies are produced as part of an adaptive immune response. The complement system consists of more than two dozen proteins that are normally found in the blood and synthesized in the liver. The proteins usually circulate as non-functional precursor molecules until activated.
Cellular Responses
Cellular responses of the innate immune system involve a variety of different types of leukocytes. Many of these leukocytes circulate in the blood and act like independent, single-celled organisms, searching out and destroying pathogens in the human host. These and other immune cells of the innate system identify pathogens or debris and then help to eliminate them in some way. One way is phagocytosis.
Phagocytosis
Phagocytosis is an important feature of innate immunity that is performed by cells classified as phagocytes. In the process of phagocytosis, phagocytes engulf and digest pathogens or other harmful particles. Phagocytes generally patrol the body searching for pathogens, but they can also be called to specific locations by the release of cytokines when inflammation occurs. Some phagocytes reside permanently in certain tissues.
As shown in Figure \(4\), when a pathogen such as a bacterium is encountered by a phagocyte, the phagocyte extends a portion of its plasma membrane, wrapping the membrane around the pathogen until it is enveloped. Once inside the phagocyte, the pathogen becomes enclosed within an intracellular vesicle called a phagosome. The phagosome then fuses with another vesicle called a lysosome, forming a phagolysosome. Digestive enzymes and acids from the lysosome kill and digest the pathogen in the phagolysosome. The final step of phagocytosis is the excretion of soluble debris from the destroyed pathogen through exocytosis.
Leukocytes
Types of leukocytes that kill pathogens by phagocytosis include neutrophils, macrophages, and dendritic cells. Macrophages and dendritic cells are the derivatives of monocytes. Figure \(5\) shows five major types of leukocytes, lymphocytes, basophils, eosinophils, neutrophils, and monocytes. Because lymphocytes are mainly involved in the adaptive immune system, they are not discussed in this concept.
Neutrophils
Neutrophils are leukocytes that travel throughout the body in the blood and are usually the first immune cells to arrive at the site of an infection. As shown in Figure \(5\), these cells contain granules and carry a multilobed nucleus. They are the most numerous types of phagocytes and normally make up at least half of the total circulating leukocytes. The bone marrow of a normal healthy adult produces more than 100 billion neutrophils per day. During acute inflammation, more than 10 times that many neutrophils may be produced each day. Many neutrophils are needed to fight infections because after a neutrophil phagocytizes just a few pathogens, it generally dies.
Macrophages
Macrophages are large phagocytic leukocytes that develop from monocytes. Macrophages spend much of their time within the interstitial fluid in tissues of the body. As shown in Figure \(5\), monocytes do not contain granules and carry a big kidney-shaped nucleus. They are the most efficient phagocytes and can phagocytize a substantial number of pathogens or other cells. Macrophages are also versatile cells that produce a wide array of chemicals — including enzymes, complement proteins, and cytokines — in addition to their phagocytic action. As phagocytes, macrophages act as scavengers that rid tissues of worn-out cells and other debris as well as pathogens. In addition, macrophages act as antigen-presenting cells that activate the adaptive immune system. (To learn more about antigen-presenting cells, see the concept Adaptive Immune System.)
Eosinophils
Eosinophils are non-phagocytic leukocytes that are related to neutrophils. They specialize in defending against parasites. As shown in Figure \(5\), these cells contain granules and carry a bilobed earmuff-shaped nucleus. These leukocytes are very effective in killing large parasites such as worms by secreting a range of highly toxic substances when activated. Eosinophils may become overactive and cause allergies or asthma.
Basophils
Basophils are non-phagocytic leukocytes that are also related to neutrophils. They are the least numerous of all white blood cells. As shown in Figure \(5\), these cells contain granules and carry a bilobed nucleus. Basophils secrete two types of chemicals that aid in body defenses: histamines and heparin. Histamines are responsible for dilating blood vessels and increasing their permeability in inflammation. Heparin inhibits blood clotting and also promotes the movement of leukocytes into an area of infection.
Dendritic Cells
Like macrophages, dendritic cells develop from monocytes (see Figure \(6\). They reside in tissues that have contact with the external environment, so they are located mainly in the skin, nose, lungs, stomach, and intestines. Their plasma membrane has extensions. Besides engulfing and digesting pathogens, dendritic cells also act as antigen-presenting cells that trigger adaptive immune responses.
Mast Cells
Mast cells are non-phagocytic leukocytes that help to initiate inflammation by secreting histamines. In some people, histamines trigger allergic reactions as well as inflammation. Mast cells may also secrete chemicals that help defend against parasites.
Natural Killer Cells
Natural killer cells are in the subset of leukocytes called lymphocytes, which are produced by the lymphatic system. Natural killer cells destroy cancerous or virus-infected host cells, although they do not directly attack invading pathogens. Natural killer cells recognize these host cells by a condition they exhibit called “missing self.” Cells with missing self have abnormally low levels of cell-surface proteins of the major histocompatibility complex (MHC), which normally identify body cells as self.
Review
1. What is the innate immune system?
2. Identify the body’s first line of defense.
3. Define and give examples of mechanical and chemical barriers of the innate immune system.
4. What are biological barriers, and how do they protect the body?
5. State the purposes of inflammation.
6. What triggers inflammation, and what signs and symptoms does it cause?
7. Define the complement system. How does it help destroy pathogens?
8. List six different types of leukocytes and state their roles in innate immune responses.
9. Describe two ways that pathogens may evade the innate immune system.
10. Explain how mucus can contribute to the immune system as both a mechanical barrier and a chemical barrier.
11. Which type of immune system cell can both phagocytize pathogens and produce chemicals that promote inflammation?
A. Macrophages
B. Natural killer cells
C. Basophils
D. Mast cells
12. What are the ways in which phagocytes can encounter pathogens in the body?
13. Describe different two ways in which enzymes play a role in the innate immune response.
14. True or False. Complement proteins can be produced by macrophages.
15. True or False. The main function of inflammation is to secrete repair proteins at the site of damage.
Explore More
Watch this video to learn about the simple power of hand washing:
Attributions
1. Oww Papercut by Laurence Facun (Flickr), CC BY 2.0, via Wikimedia Commons
2. Sneeze by James Gathany; CDC Public Health Image library ID 11162, Public domain via Wikimedia Commons
3. Inflammatory Response by OpenStax, CC BY 3.0
4. Phagocytosis by OpenStax, CC BY 3.0
5. Leukocytes by Suzanne Wakim licensed CC BY 4.0 adapted from:
6. S8-Dendritic Cells Dragging Conidia in Collagen by Judith Behnsen, Priyanka Narang, Mike Hasenberg, Frank Gunzer, Ursula Bilitewski, Nina Klippel, Manfred Rohde, Matthias Brock, Axel A. Brakhage, Matthias Gunzer, CC BY 2.5 via wikimedia.org
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/20%3A_Immune_System/20.4%3A_Innate_Immune_System.txt |
The Kiss of Death
The photomicrograph in Figure \(1\) shows a group of killer T cells (green and red) surrounding a cancer cell (blue, center). When a killer T cell makes contact with the cancer cell, it attaches to and spreads over the dangerous target. The killer T cell then uses special chemicals stored in vesicles (red) to deliver the killing blow. This event has thus been nicknamed “the kiss of death.” After the target cell is killed, the killer T cells move on to find the next victim. Killer T cells like these are important players in the adaptive immune system.
The adaptive immune system (also called specific immunity) is a subsystem of the overall immune system. It is composed of highly specialized cells and processes that eliminate specific pathogens and tumor cells. An adaptive immune response is set in motion by antigens that the immune system recognizes as foreign. Unlike an innate immune response, an adaptive immune response is highly specific to a particular pathogen (or its antigen). An important function of the adaptive immune system that is not shared by the innate immune system is the creation of immunological memory or immunity. This occurs after the initial response to a specific pathogen. It allows a faster, stronger response on subsequent encounters with the same pathogen, usually before the pathogen can cause symptoms of illness.
Lymphocytes are the main cells of the adaptive immune system. They are leukocytes that arise and mature in organs of the lymphatic system, including the bone marrow and thymus. The human body normally has about 2 trillion lymphocytes, which constitute about a third of all leukocytes. Most of the lymphocytes are normally sequestered within tissue fluid or organs of the lymphatic system, including the tonsils, spleen, and lymph nodes. Only about 2 percent of the lymphocytes are normally circulating in the blood. There are two main types of lymphocytes involved in adaptive immune responses, called T cells and B cells. T cells destroy infected cells or release chemicals that regulate immune responses. B cells secrete antibodies that bind with antigens of pathogens so they can be removed by other immune cells or processes.
T Cells
There are multiple types of T cells or T lymphocytes. Major types are killer (or cytotoxic) T cells and helper T cells. Both types develop from immature T cells that become activated by exposure to an antigen.
T Cell Activation
T cells must be activated. After the pathogen is phagocytized and digested by macrophages, a part of the pathogen is displayed on the surface of the macrophage. The proteins that display the antigen (part of the pathogen) are called Major Histocompatibility Complex (MHC). There are two types of MHC, MHCI and MHCII. Therefore macrophages are called antigen-presenting cells as shown in Figure \(2\) and Figure \(3\). B lymphocytes can also act as antigen-presenting cells. Helper T cells are more easily activated than killer T cells. Activation of killer T cells is strongly regulated and may require additional stimulation from helper T cells.
Helper T Cells
Activated helper T cells do not kill infected or cancerous cells. Instead, their role is to “manage” both innate and adaptive immune responses by directing other cells to perform these tasks. They control other cells by releasing cytokines. These are proteins that can influence the activity of many cell types, including cytotoxic killer T cells (sometimes referred to as only killer T cells), B cells, and macrophages. For example, some cytokines released by helper T cells help activate killer T cells.
Killer T Cells (Cytotoxic T Cells)
When infected body cells present pathogen antigen to a killer T cell, it gets activated (see lower panel of Figure \(3\)). Activated killer T cells induce the death of cells that bear a specific non-self antigen because they are infected with pathogens or are cancerous. The antigen targets the cell for destruction by killer T cells, which travel through the bloodstream searching for target cells to kill. Killer T cells may use various mechanisms to kill target cells. One way is by releasing toxins in granules that enter and kill infected or cancerous cells (Figure \(3\)).
B Cells and B Cell Activation
B cells, or B lymphocytes, are the major cells involved in the creation of antibodies that circulate in blood plasma and lymph. Antibodies are large, Y-shaped proteins used by the immune system to identify and neutralize foreign invaders. Besides producing antibodies, B cells may also function as antigen-presenting cells or secrete cytokines that help control other immune cells and responses.
Before B cells can actively function to defend the host, they must be activated. As shown in Figure \(4\), B cell activation begins when a B cell engulfs and digests an antigen. The antigen may be either free-floating or on top of the pathogen. B cell internalize antigen and present it on its MHC to a helper T cell. The T cell activates and secretes cytokines that help the B cell to multiply and the daughter cells to mature into plasma cells and memory B cells. Plasma B cells produce antibodies.
Plasma Cells
Plasma cells are antibody-secreting cells that form from activated B cells. Each plasma cell is like a tiny antibody factory. It may secrete millions of copies of an antibody, each of which can bind to the specific antigen that activated the original B cell. The specificity of an antibody to a specific antigen is illustrated in Figure \(5\). When antibodies bind with antigens, it makes the cells bearing them easier targets for phagocytes to find and destroy. Antibody-antigen complexes may also trigger the complement system of the innate immune system, which destroys the cells in a cascade of protein enzymes. In addition, the complexes are likely to clump together (agglutinate). If this occurs, they are filtered out of the blood in the spleen or liver.
Immunity
Most activated T cells and B cells die within a few days once a pathogen has been cleared from the body. However, a few of the cells survive and remain in the body as memory T cells or memory B cells. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future. This is the basis of immunity.
The earliest known reference to the concept of immunity relates to the bubonic plague (see Figure \(6\)). In 430 B.C., a Greek historian and general named Thucydides noted that people who had recovered from a previous bout of the plague could nurse people sick with the plague without contracting the illness a second time. We now know that this is true of many diseases and it occurs because of active immunity.
Active Immunity
Active immunity is the ability of the adaptive immune system to resist a specific pathogen because it has formed an immunological memory of the pathogen. Active immunity is adaptive because it occurs during the lifetime of an individual as an adaptation to infection with a specific pathogen and prepares the immune system for future challenges from that pathogen. Active immunity can come about naturally or artificially.
Naturally Acquired Active Immunity
Active immunity is acquired naturally when a pathogen invades the body and activates the adaptive immune system. When the initial infection is over, memory B cells and memory T cells remain that provide immunological memory of the pathogen. As long as the memory cells are alive, the immune system is ready to mount an immediate response if the same pathogen tries to infect the body again.
Artificially Acquired Active Immunity
Active immunity can also be acquired artificially through immunization. Immunization is the deliberate exposure of a person to a pathogen in order to provoke an adaptive immune response and the formation of memory cells specific to that pathogen. The pathogen is introduced in a vaccine — usually by injection, sometimes by nose or mouth (Figure \(7\)) — so immunization is also called vaccination.
In a vaccine, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is typically used. This causes an adaptive immune response without making the immunized person sick. This is how you most likely became immune to diseases such as measles, mumps, and chickenpox. Immunizations may last for a lifetime or require periodic booster shots to maintain immunity. While immunization generally has long-lasting effects, it usually takes several weeks to develop full immunity.
Immunization is the most effective method ever discovered in preventing infectious diseases. As many as 3 million deaths are prevented each year because of vaccinations. Widespread immunity due to vaccinations is largely responsible for the worldwide eradication of smallpox and the near elimination of several other infectious diseases from many populations, including such diseases as polio and measles. Immunization is so successful because it exploits the natural specificity and inducibility of the adaptive immune system.
Passive Immunity
Passive immunity results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood. This is usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.
Naturally Acquired Passive Immunity
Passive immunity is acquired naturally by a fetus through its mother’s blood. Antibodies are transported from mother to fetus across the placenta, so babies have high levels of antibodies at birth. Their antibodies have the same range of antigen specificity as their mother’s. Passive immunity may also be acquired by an infant through the mother’s breast milk. This gives young infants protection from common pathogens in their environment while their own immune system matures.
Artificially Acquired Passive Immunity
Older children and adults can acquire passive immunity artificially through the injection of antibodies or activated T cells. This may be done when there is a high risk of infection and insufficient time for the body to develop active immunity through vaccination. It may also be done to reduce symptoms of ongoing disease or to compensate for immunodeficiency diseases (for the latter, see the concept Disorders of the Immune System).
Adaptive Immune Evasion
Many pathogens have been around for a long time, living with human populations for generations. To persist, some have evolved mechanisms to evade the adaptive immune system of human hosts. One way they have done this is by rapidly changing their non-essential antigens. This is called antigenic variation. An example of a pathogen that takes this approach is the human immunodeficiency virus (HIV). It mutates rapidly so the proteins on its viral envelope are constantly changing. By the time the adaptive immune system responds, the virus’s antigens have changed. Antigenic variation is the main reason that efforts to develop a vaccine against HIV have not yet been successful.
Another evasion approach some pathogens may take is to mask pathogen antigens with host molecules so the host’s immune system cannot detect the antigens. HIV takes this approach as well. The envelope that covers the virus is formed from the outermost membrane of the host cell.
Feature: My Human Body
If you think that immunizations are just for kids, think again. There are several vaccines recommended by the CDC for people over the age of 18. This link shows the vaccine schedule recommended for all adults aged 19 years and older. Additional vaccines may be recommended for certain adults based on specific medical conditions or other indications. Are you up to date with your vaccines? You can check with your doctor to be sure.
Review
1. What is the adaptive immune system?
2. Describe the main cells of the adaptive immune system.
3. How are lymphocytes activated?
4. Identify two common types of T cells and their functions.
5. How do activated B cells help defend against pathogens?
6. Define immunity.
7. What are two ways active immunity may come about?
8. How does passive immunity differ from active immunity?
9. How may passive immunity occur?
10. What ways of evading the human adaptive immune system evolved in the human immunodeficiency virus (HIV)?
11. Describe two ways in which B cells and T cells work together to generate adaptive immune responses.
12. Which cells directly kill pathogen-infected or cancerous cells?
1. A. Plasma cells
2. B. Killer T cells
3. C. Helper T cells
4. D. All of the above
13. Why do vaccinations involve the exposure of a person to a version of a pathogen?
14. True or False. Immunization is a form of passive immunity.
15. True or False. Antibodies transmitted from mother to child via breast milk cause the formation of memory B cells and long-term immunity.
Explore More
Watch this video to learn about the recent status of HIV vaccine development:
Attributions
1. Killer T cells surrounded by Cancer Cells by NIH, Public Domain via Wikimedia Commons
2. T cell receptors by Charles Molnar, CC BY 4.0 via BC Campus
3. Lymphocyte activation by Mikael Häggström, public domain via Wikimedia Commons
4. B cell activation by Fred the Oyster, public domain via Wikimedia Commons
5. Antibody by Fvasconcellos, Public domain via Wikimedia Commons
6. Acral gangrene of digits by CDC, Public Domain via Wikimedia Commons
7. Polio drops by USAID, Public Domain, via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/20%3A_Immune_System/20.5%3A_Adaptive_Immune_System.txt |
Allergy Eyes
Eyes that are red, watery, and itchy are typical of an allergic reaction known as allergic rhinitis. Commonly called hay fever, allergic rhinitis is an immune system reaction typically to the pollen of certain plants. Your immune system usually protects you from pathogens and keeps you well. However, like any other body system, the immune system itself can develop problems. Sometimes it responds to harmless foreign substances as though they were pathogens. That’s the basis of allergies such as hay fever.
Allergies
An allergy is a disorder in which the immune system makes an inflammatory response to a harmless antigen. It occurs when the immune system is hypersensitive to an antigen in the environment that causes little or no response in most people. Allergies are strongly familial: allergic parents are more likely to have allergic children and those children’s allergies are likely to be more severe. This is evidence that there is a heritable tendency to develop allergies. Allergies are more common in children than adults because many children outgrow their allergies by adulthood.
Allergens
Any antigen that causes an allergy is called an allergen. Common allergens are plant pollens, dust mites, mold, specific foods (such as peanuts or shellfish), insect stings, and certain common medications (such as aspirin and penicillin). Allergens may be inhaled or ingested, or they may come into contact with the skin or eyes. Symptoms vary depending on the type of exposure and the severity of the immune system response. Two common causes of allergies are ragweed and poison ivy. Inhaling ragweed pollen may cause symptoms of allergic rhinitis, such as sneezing and red itchy eyes. Skin contact with oils in poison ivy may cause an itchy rash. This type of allergy is called contact dermatitis.
Prevalence of Allergies
There has been a significant increase in the prevalence of allergies over the past several decades, especially in the rich nations of the world, where allergies are now very common disorders. In the developed countries, about 20 percent of people have or have had hay fever, another 20 percent have had contact dermatitis, and about 6 percent have food allergies. In the poorer nations of the world, on the other hand, allergies of all types are much less common.
One explanation for the rise in allergies in the developed world is called the hygiene hypothesis. According to this hypothesis, people in developed countries live in relatively sterile environments because of hygienic practices and sanitation systems. As a result, people in these countries are exposed to fewer pathogens than their immune system evolved to cope with. To compensate, their immune system “keeps busy” by attacking harmless antigens in allergic responses.
How Allergies Occur
The diagram in Figure \(3\) shows how an allergic reaction occurs. At the first exposure to an allergen, B cells are activated to form plasma cells that produce large amounts of antibodies to the allergen. These antibodies attach to leukocytes called mast cells. Subsequently, every time the person encounters the allergen again, the mast cells are already primed and ready to deal with it. The primed mast cells immediately release cytokines and histamines, which in turn cause inflammation and recruitment of leukocytes, among other responses. These responses are responsible for the signs and symptoms of allergies.
Treating Allergies
The symptoms of allergies can range from mild to life-threatening. Mild allergy symptoms are often treated with antihistamines. These are drugs that reduce or eliminate the effects of the histamines that produce allergy symptoms.
Treating Anaphylaxis
The most severe allergic reaction is a systemic reaction called anaphylaxis. This is a life-threatening response caused by a massive release of histamines. Many of the signs and symptoms of anaphylaxis are shown in Figure \(4\). Some of them include a drop in blood pressure, changes in heart rate, shortness of breath, and swelling of the tongue and throat, which may threaten the patient with suffocation unless emergency treatment is given. People who have had anaphylactic reactions may carry an epinephrine autoinjector (widely known by its brand name EpiPen®) so they can inject themselves with epinephrine if they start to experience an anaphylactic response. The epinephrine helps to control the immune reaction until medical care can be provided. Epinephrine constricts blood vessels to increase blood pressure, relaxes smooth muscles in the lungs to reduce wheezing and improve breathing, modulates heart rate, and works to reduce swelling that may otherwise block the airways.
Immunotherapy for Allergies
Another way to treat allergies is called immunotherapy, commonly called “allergy shots.” This approach may actually cure specific allergies, at least for several years if not lifelong. It may be particularly beneficial for allergens such as pollen that are difficult or impossible to avoid. First, however, patients must be tested to identify the specific allergens that are causing their allergies. As shown in Figure \(5\), this may involve scratching tiny amounts of common allergens into the skin and then observing whether there is a localized reaction to any of them. Each allergen is applied in a different numbered location on the skin so if there is a reaction, such as redness or swelling, the responsible allergens can be identified. Then, through periodic injections (usually weekly or monthly), patients are gradually exposed to larger and larger amounts of the allergens. Over time, generally from months to years, the immune system becomes desensitized to the allergens. This method of treating allergies is often effective for allergies to pollen or insect stings, but its usefulness for allergies to food is unclear.
Autoimmune Diseases
Autoimmune diseases occur when the immune system fails to recognize the body’s own molecules as self. As a result, instead of ignoring the body’s healthy cells, it attacks them, causing damage to tissues and altered organ growth and function. Most often, it is B cells that are at fault in autoimmune responses. They are generally the cells that lose tolerance for self. Why does this occur? Some autoimmune diseases are thought to be caused by exposure to pathogens that have antigens similar to the body’s own molecules. After this exposure, the immune system responds to body cells as though they were pathogens as well.
Certain individuals are genetically susceptible to developing autoimmune diseases. These individuals are also more likely to develop more than one such disease. Gender is also a risk factor for autoimmunity. Females are much more likely than males to develop autoimmune diseases, probably in part because of gender differences in sex hormones.
At a population level, autoimmune diseases are less common where infectious diseases are more common. The hygiene hypothesis has been proposed to explain the inverse relationship between infectious and autoimmune diseases as well as the prevalence of allergies. According to the hypothesis, without infectious diseases to “keep it busy,” the immune system may attack the body’s own cells instead.
Common Autoimmune Diseases
An estimated 15 million or more people worldwide have one or more autoimmune diseases. Two of the most common autoimmune diseases are type I diabetes and multiple sclerosis. Both are localized diseases in terms of the specific body cells that are attacked by the immune system. In the case of type I diabetes, the immune system attacks and destroys insulin-secreting islet cells in the pancreas. In the case of multiple sclerosis, the immune system attacks and destroys the myelin sheaths that normally insulate the axons of neurons and allow rapid transmission of nerve impulses.
Some relatively common autoimmune diseases are systemic, or body-wide, diseases. They include rheumatoid arthritis and systemic lupus erythematosus (SLE). In these diseases, many tissues and organs may be attacked and injured by the immune system. For example, as you can see in Figure \(6\), symptoms of SLE may involve the muscular, skeletal, integumentary, respiratory, and cardiovascular systems.
Treatment for Autoimmune Diseases
None of these common autoimmune diseases can be cured, although all of them have treatments that may help relieve symptoms and prevent some of the long-term damage they may cause. Traditional treatments for autoimmune diseases include immunosuppressive drugs to block the immune response and anti-inflammatory drugs to quell inflammation. Hormone replacement may be another option. For example, type I diabetes is treated with injections of the hormone insulin because islet cells in the pancreas can no longer secrete it.
Immunodeficiency
Immunodeficiency occurs when the immune system is not working properly, generally because one or more components of the immune system are inactive. As a result, the immune system may be unable to fight off pathogens or cancers that a normal immune system would be able to resist. Immunodeficiency may occur for a variety of reasons.
Causes of Immunodeficiency
Dozens of rare genetic diseases can result in a defective immune system. This type of immunodeficiency is called primary immunodeficiency. One is born with one of these diseases rather than acquiring it after birth. Probably the best known of these primary immunodeficiency diseases is severe combined immunodeficiency (SCID). It is also known as “bubble boy disease” because people with this disorder are extremely vulnerable to infectious diseases and some of them have become well known for living inside a bubble that provides a sterile environment. SCID is most often caused by an X-linked recessive mutation that interferes with normal B cell and T cell production.
Other types of immunodeficiency are not present at birth but acquired due to experiences or exposures that occur after birth. Acquired immunodeficiency is called secondary immunodeficiency because it is secondary to some other event or exposure. Secondary immunodeficiency may occur for a number of different reasons:
• The immune system naturally becomes less effective as people get older. This age-related decline, called immunosenescence, generally begins at about age 50 and worsens with increasing age. Immunosenescence is why older people are generally more susceptible to disease than younger people.
• The immune system may be damaged by another disorder, such as obesity, alcoholism, or the abuse of other drugs.
• In developing countries, malnutrition is the most common cause of immune system damage and immunodeficiency. Inadequate protein intake is especially damaging to the immune system. It can lead to impaired complement system activity, phagocyte malfunction, and lower-than-normal production of antibodies and cytokines.
• Surgical removal or disease of the thymus, where T lymphocytes normally mature, results in severe immunodeficiency. People without a functioning thymus are extremely susceptible to infections.
• Certain medications can suppress the immune system. This is the intended effect of immunosuppressant drugs given to people with transplanted organs so they do not reject them. In many cases, however, immunosuppression is an unwanted side effect of drugs used to treat other disorders.
• Some pathogens attack and destroy cells of the immune system. An example is a virus known as HIV, which attacks and destroys T cells.
Focus on HIV
The human immunodeficiency virus (HIV) is the most common cause of immunodeficiency in the world today, so it is the focus of the rest of this concept. It is also covered in the concept HIV and AIDS. HIV infections of human hosts are a relatively recent phenomenon. Scientists think that the virus originally infected monkeys but then jumped to human populations, probably sometime during the early to mid-1900s. This most likely occurred in West Africa, but the virus soon spread around the world. HIV was first identified by medical researchers in 1981. Since then, HIV has killed almost 40 million people worldwide, and its economic toll has also been enormous. The hardest hit countries are in Africa, where the virus has infected human populations the longest, and medications to control the virus are least available.
HIV Transmission
HIV is transmitted through direct contact of mucous membranes or body fluids such as blood, semen, or breast milk. As shown in Figure \(7\), the transmission of the virus can occur through sexual contact or the use of contaminated hypodermic needles. It can also be transmitted from an infected mother’s blood during late pregnancy or childbirth or through breast milk after birth. In the past, HIV was also transmitted occasionally through blood transfusions. Because donated blood is now screened for HIV, the virus is no longer transmitted this way.
HIV and the Immune System
HIV infects and destroys helper T cells, the type of lymphocytes that regulate the immune response. How this occurs is shown in Figure \(8\). The virus injects its own nucleic acid into a helper T cell and uses the T cell’s “machinery” to make copies of itself. In the process, the helper T cell is destroyed, and the virus copies go on to infect other helper T cells. HIV is able to evade the immune system and keep destroying helper T cells by mutating frequently so its surface antigens keep changing and by using the host cell’s membrane to hide its own antigens.
Acquired immunodeficiency syndrome (AIDS) may result from years of damage to the immune system by HIV. It occurs when helper T cells fall to a very low level and opportunistic diseases occur. Opportunistic diseases are infections and tumors that are rare except in people with a damaged immune system. The diseases take advantage of the “opportunity” presented by people whose immune systems cannot fight back. Opportunistic diseases are usually the direct cause of death of people with AIDS.
Treating HIV/AIDS
For patients who have access to HIV medications, infection with the virus has ceased to be the death sentence that it once was. By 1995, combinations of drugs called “highly active antiretroviral therapy” were developed. For some patients, these drugs can reduce the amount of virus they are carrying to undetectable levels. However, some amount of virus always hides in the body’s immune cells and will multiply again if a patient stops taking the medications. Researchers are trying to develop drugs to kill these hidden viruses as well. If their efforts are successful, it could bring an end to AIDS.
Feature: Human Biology in the News
EpiPens® and their sole manufacturer, pharmaceutical company Mylan, were featured in the news headlines in 2016 but not for a good reason. A drastic price hike in EpiPens® and Mylan’s apparent greed triggered the media outburst.
EpiPens® are auto-injectable syringes preloaded with a measured dose of epinephrine, a drug that can rapidly stop a life-threatening anaphylactic response to an allergen. Using the device is easy and does not require any special training. The injector just needs to be jammed against the thigh, which can be done through clothing or on bare skin. Each year, doctors write millions of prescriptions for EpiPens®. Many people with severe allergies always carry two of the devices with them just in case they experience anaphylaxis, although most of them never need to use them. Other people with severe allergies have literally had their lives saved multiple times by EpiPens® when they had anaphylactic reactions. Even when the devices haven’t been used, they must be replaced each year due to the expiration of the epinephrine.
You might think that EpiPens® would be relatively inexpensive, given their life-saving potential. As recently as 2009, a two-pack of EpiPens® cost about \$100. However, in just 7 years, the cost of the same two-pack of EpiPens® skyrocketed by an incredible 400 percent! By 2016, the cost was \$600 or more. Mylan apparently raised the price for the sole purpose of increasing profits. The company also raised prices significantly on many other drugs. The price hike in EpiPens® alone was certainly profitable. In 2015, the sale of EpiPens® earned Mylan \$1 billion. Mylan’s CEO took home almost \$19 million the same year, which was an increase of more than 600 percent over her prior salary.
News coverage of the price hike in EpiPens® began in the summer of 2016 after a price increase in May of that year. Both private citizens and elected officials expressed outrage over the price increase, especially when coupled with the gluttonous profits of the company and its CEO. By late August, Mylan responded to the backlash by offering discount coupons for EpiPens®. A few days later, the company promised to introduce a cheaper, generic version of the device. Analysts quickly determined that selling a generic version would allow Mylan to make more money on the product than reducing the price of the name-brand device, which they still declined to do. By September of 2016, Mylan was being investigated for antitrust violations relating to sales of EpiPens® to public schools in New York City.
The Mylan/EpiPen® story may still be making the news. But whatever its outcome, the story has already added fuel to public and private debates about important ethical issues — issues such as the excessive costs of life-saving drugs and the huge profits of big pharma. What is the most recent news on EpiPens® and Mylan? If you are interested, you can check the headlines online to find out. What are your views on the ethical issues they raise?
Review
1. What are allergies? What causes them?
2. Compare the prevalence of allergies in developed and developing countries. How does the hygiene hypothesis explain the differences in prevalence?
3. How do allergies occur?
4. How are mild allergy symptoms treated?
5. What is anaphylaxis, and how is it treated?
6. How does immunotherapy for allergies work?
7. What are autoimmune diseases?
8. Identify two risk factors for autoimmune diseases.
9. Autoimmune diseases may be specific to particular tissues, or they may be systemic. Give an example of each type of autoimmune disease.
10. What is immunodeficiency?
11. Compare and contrast primary and secondary immunodeficiency, and give an example of each.
12. What is the most common cause of immunodeficiency in the world today? How does this cause affect the immune system?
13. Distinguish between HIV and AIDS.
14. True or False. Allergies and autoimmune diseases both result from an over-reactive immune system.
15. True or False. An anaphylactic reaction can be stopped by administering a shot of histamine.
Attributions
1. Oedema By Championswimmer, Public domain via Wikimedia Commons
2. Pollen by Hans via Pixabay license
3. Poison Ivy by Sam Fraser-Smith, CC BY 2.0 via Wikimedia Commons
4. Mast Cells by NIH, Public domain, via Wikimedia Commons
5. Signs and symptoms of anaphylaxis by Mikael Häggström, CC0 via Wikimedia Commons
6. Skin prick test for allergies by NIH, public domain via Wikimedia Commons
7. Symptoms of SLE by Mikael Häggström, Public domain via Wikimedia Commons
8. HIV Infection by Hana Zavadska and Laura Guerin, CC BY-NC 3.0 via CK-12 foundation
9. AIDS life cycle illustration by NIH, Public domain, via Wikimedia Commons
10. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/20%3A_Immune_System/20.6%3A_Disorders_of_the_Immune_System.txt |
Beneficial Microbes: The pharmacy in the gut
While some bacteria can cause disease, others play beneficial roles in human health. We have co-evolved with microbes in and on our body, with everyone having a unique set of microorganisms. The most abundant and well-studied microbiome is found in the gut. It has been estimated that the number of bacteria in the human gut may outnumber the cells in the body by an order of magnitude. Thus, one may consider the gut microbiome as a multicellular organ similar in size to the liver. Indeed, it is sometimes referred to as our “forgotten organ”.
Generally, the microbiome within a given body habitat can be defined as the diversity and abundance distribution of distinct types of microorganisms. This microbial composition is highly influenced by individual factors such as diet, age, lifestyle, ethnicity, and host health, among others. Although no taxa are observed to be universally present among all individuals, some microbial patterns demonstrate broad prevalence. Most bacteria belong to the genera Bacteroides, Clostridium, Fusobacterium, Eubacterium, Ruminococcus, Peptococcus, Peptostreptococcus, and Bifidobacterium.
The evolution of microbiome during life
Recent research suggests early in-utero microbial exposure during pregnancy. Following birth, the newborn’s digestive tract is quickly colonized by microorganisms from the mother (vaginal, fecal, skin, breast milk, etc.) and the environment in which the delivery takes place. Following birth, the microbiome that enters and evolves in the infant's gut is dependent upon a number of factors, with delivery mode and feeding regime (breastfeeding vs infant formula feeding) of prime importance in the early days and weeks of life. By the age of 2 to 3 years, the microbiome becomes essentially established, having reached a steady state, and remains relatively stable throughout life. However, the gut microbiome continuously changes in response to daily variations in diet, lifestyle, age, and host physiological and immunological health.
Health benefits of the microbiome
On the basis of the currently available literature, the gut microbiome is known to contribute to a number of important functions in the host, from protective, immunomodulatory, metabolic to trophic roles. These are promoted via a number of mechanisms. For example, members of the gut microbiome can produce anti-inflammatory factors, pain relieving compounds, antioxidants, and vitamins to protect and nurture the body. Additionally, they may prevent attachment and action of harmful bacteria that can produce toxins causing chronic disease. This close and specific contact with human cells, exchanging nutrients and metabolic wastes, makes symbiotic bacteria essentially a human organ.
Gastrointestinal infection prevention
The indigenous intestinal microbiome serves as a line of resistance to colonization by exogenous microbes such as Clostridium difficile and Helicobacter pylori, and thus assists in competitive exclusion of pathogens preventing the potential invasion, termed colonization resistance. Indeed, antibiotic-associated diarrhea occurs when antibiotic treatment disturbs the natural balance of the gut microbiome causing harmful bacteria (i.e., Clostridium difficile) to proliferate and multiply. Oral probiotics may reduce antibiotic-associated diarrhea significantly.
Immunomodulatory effects
Commensal bacteria can interact with the host immune system in ways that control the host's immune response and counteracts the development of the disease. The complex interactions that may occur between ingested probiotic bacteria, commensals, and the mucosal surface are possible because of the mucosa-associated immune system, typically organized into MALT (Mucosal Associated Lymphoid Tissue, such as Peyer’s patches). This cross-talk interaction enhances cellular immune response characterized by activation of macrophages, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines. Furthermore, some probiotics may be effective in the prevention and/or alleviation of allergies and auto-immune diseases like irritable bowel syndrome and inflammatory bowel diseases (Crohn’s disease and ulcerative colitis).
Nutritional benefits
The metabolic activity of the gut microbiome makes an important contribution to the nutritional status of the host, via its ability to synthesize certain vitamins and various bioactive metabolites, such as short-chain fatty acids (SCFA) that then become bioavailable to the host. It has been reported that consumption of yogurt containing Lactobacillus bulgaricus or acidophilus could alleviate lactose intolerance during gastric passage through their enzyme lactase. However, the major metabolic function of the colonic microflora is the fermentation of nondigestible carbohydrates, which are key sources of energy in the colon. These carbohydrates also include large polysaccharides (i.e., resistant starches, pectins, and cellulose) and some oligosaccharides that escape digestion, as well as unabsorbed sugars and alcohols. Other benefits of the gut microbiome on human health, such as a role in supporting the health of the reproductive tract, oral cavity, lungs, skin, and gut-brain axis is currently under investigation.
Probiotic imbalance
When the normal composition of the microbiome is thrown off balance there is a potential risk of disease. A decrease in microbiome diversity has been linked to cancer, asthma, Parkinson's, obesity, Alzheimer's, type-2 diabetes, cardiovascular disease, and possibly even autism in comparison to healthy subjects. Over the counter probiotics can help. In order to arrive alive at their workplace (i.e. the gastrointestinal tract), orally administered probiotics must be able to resist stomach acid, bile, and the effects of digestive enzymes. Certain mechanisms of action (such as the delivery of certain enzymes to the intestine) may not require live cells to play a physiologic benefit. Hence, a probiotic must contain as many live bacteria as claimed on the label. In addition, to survive the stomach and arrive at the intestine in optimal numbers, probiotic strains must be able to adhere to the intestinal epithelium and/or mucus, persist and multiply in the gut to maintain its metabolic activity and confer their probiotic properties in the human body.
Resources
• BIOENGINEERED, 2016, VOL. 7, NO. 1, 11–20: http://dx.doi.org/10.1080/21655979.2015.1126015
• REVIEW: Beneficial Microbes: The pharmacy in the gut, Daniel M. Linares, Paul Ross, and Catherine Stanton, Food Biosciences Department, Teagasc Food Research Center, Moorepark, Fermoy, Cork, Ireland; APC, Microbiome Institute, University College Cork, Cork, Ireland; Biosciences Institute, University College Cork, Cork, Ireland
Explore More
You can learn more about the sea of microbes that live in and on the human body by watching this informative and entertaining TED talk.
The bacteria in our guts can break down food the body can’t digest, produce important nutrients, regulate the immune system, and protect against harmful germs. And while we can’t control all the factors that go into maintaining a healthy gut microbiome, we can manipulate the balance of our microbes by paying attention to what we eat. Shilpa Ravella shares the best foods for a healthy gut.
Attribution
1. Beneficial Microbes: The pharmacy in the gut by Daniel M. Linares, Paul Ross, and Catherine Stanton Beneficial Microbes, licensed CC-BY-NC via NCBI | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/20%3A_Immune_System/20.7%3A_Human_Microbiome.txt |
Case Study Conclusion: Defending Your Defenses
The person in Figure \(1\) is participating in a bike ride to raise funds for leukemia and lymphoma research. Leukemia and lymphoma are blood cancers. About every three minutes, one person in the U.S. is diagnosed with blood cancer. Lymphoma is the most common type of blood cancer. As a lymphoma patient, Wei, whom you learned about in the beginning of this chapter, may eventually benefit from research funded by a bike ride like this one.
What type of blood cell is affected in lymphoma? As the name implies, lymphoma is cancer that affects lymphocytes, which are a type of white blood cell. As you have learned in this chapter, there are different types of lymphocytes, including the B and T cells of the adaptive immune system. Different types of lymphoma affect different types of lymphocytes in different ways. It is important to correctly identify the type of lymphoma so that patients can be treated appropriately.
You may recall that one of Wei’s symptoms was a swollen lymph node, and he was diagnosed with lymphoma after a biopsy of that lymph node. Swollen lymph nodes are a common symptom of lymphoma. As you have learned, lymph nodes are distributed throughout the body along lymphatic vessels as part of the lymphatic system. The lymph nodes filter lymph and store lymphocytes and therefore play an important role in fighting infections. Because of this, they will often swell in response to an infection. In Wei’s case, the swelling and other symptoms did not improve after several weeks and a course of antibiotics, which caused Dr. Bouazizi to suspect lymphoma instead. The biopsy showed that Wei did indeed have cancerous lymphocytes in his lymph nodes.
But which type of lymphocytes were affected? Lymphoma most commonly affects B or T lymphocytes. The two major types of lymphoma are called Hodgkin (HL) or non-Hodgkin lymphoma (NHL). NHL is more common than HL. There are more than 70,000 cases of NHL diagnosed in the U.S. each year, compared to about 8,000 for HL. While HL is one distinct type of lymphoma, NHL has about 60 different subtypes, depending on which specific cells are affected and how.
Wei was diagnosed with a type of NHL called diffuse large B-cell lymphoma (DLBCL)—the most common type of NHL. This type of lymphoma affects B cells and causes them to appear large under the microscope. In addition to Wei’s symptoms of fatigue, swollen lymph nodes, loss of appetite, and weight loss, common symptoms of this type of lymphoma include fever and night sweats. It is an aggressive and fast-growing type of lymphoma that is fatal if not treated. But the good news is that with early detection and proper treatment, about 70% of patients with DLBCL can be cured.
How do physicians determine the specific type of lymphoma? Tissue obtained from a biopsy can be examined under a microscope to observe physical changes such as abnormal cell size or shape that are characteristic of a particular subtype of lymphoma. Additionally, tests can be performed on the tissue to determine which cell-surface antigens are present. Recall that antigens are molecules that bind to specific antibodies. Antibodies can be produced in the laboratory and labeled with compounds that can be identified by their color under a microscope. When these antibodies are applied to a tissue sample, this color will appear wherever the antigen is present, because it binds to the antibody. For example, this technique was used in the photomicrograph in Figure \(2\) to identify the presence of a cell-surface antigen (shown as reddish-brown) in a sample of skin cells. This technique, called immunohistochemistry, is also commonly used to identify antigens in tissue samples from lymphoma patients.
Why would identifying cell-surface antigens be important in diagnosing and treating lymphoma? As you have learned, the immune system uses antigens present on the surface of cells or pathogens to distinguish between self and non-self and to launch adaptive immune responses. Cells that become cancerous often change their cell-surface antigens, and this is one way that the immune system can identify and destroy them. Also, different cell types in the body can sometimes be identified by the presence of specific cell-surface antigens. Knowing the types of cell-surface antigens present in a tissue sample can help physicians identify which cells are cancerous, and possibly the specific subtype of cancer. Knowing this information can be helpful in choosing more tailored and effective treatments.
In fact, one treatment for NHL is the use of medications made from antibodies that bind to cell-surface antigens present on cells affected by the specific subtype of NHL. This is called immunotherapy. These drugs can directly bind to and kill the cancerous cells. For patients with DLBCL such as Wei, immunotherapy is often used in conjunction with chemotherapy and radiation as a course of treatment. Although Wei has a difficult road ahead, he and his medical team are optimistic that he may be able to be cured, given the high success rate when DLBCL is caught and treated early. More research into how the immune system functions may lead to even better treatments for lymphoma, and other types of cancers, in the future.
Chapter Summary
In this chapter, you learned about the immune system. Specifically, you learned that:
• Any agent that can cause disease is called a pathogen. Most human pathogens are microorganisms such as bacteria and viruses. The immune system is the body system that defends the human host from pathogens and cancerous cells.
• The innate immune system is a subset of the immune system that provides very quick but non-specific responses to pathogens. It includes multiple types of barriers to pathogens, leukocytes that phagocytize pathogens, and several other general responses.
• The adaptive immune system is a subset of the immune system that provides specific responses tailored to particular pathogens. It takes longer to put into effect, but it may lead to immunity to the pathogens.
• Both innate and adaptive immune responses depend on the ability of the immune system to distinguish between self and non-self molecules. Most body cells have major histocompatibility complex (MHC) proteins that identify them as self. Pathogens, infected cells, and tumor cells have non-self proteins called antigens that the immune system recognizes as foreign.
• Antigens are proteins that bind to specific receptors on immune system cells and elicit an adaptive immune response. Some immune cells (B cells) respond to foreign antigens by producing antibodies that bind with antigens and target pathogens for destruction.
• An important role of the immune system is tumor surveillance. Killer T cells of the adaptive immune system find and destroy tumor cells, which they can identify from their abnormal antigens.
• The neuroimmune system that protects the central nervous system is thought to be distinct from the peripheral immune system that protects the rest of the human body. The blood-brain and blood-spinal cord barriers are one type of protection of the neuroimmune system. Glial cells also play role in this system—for example, by carrying out phagocytosis.
• The lymphatic system is a human organ system that is a vital part of the adaptive immune system. It consists of several organs and a system of vessels that transport or filter the fluid called lymph. The main immune function of the lymphatic system is to produce, mature, harbor, and circulate white blood cells called lymphocytes, which are the main cells in the adaptive immune system and are circulated in the lymph.
• The return of lymph to the bloodstream is one of the functions of the lymphatic system. Lymph flows from tissue spaces, where it leaks out of blood vessels, to major veins in the upper chest, where it is returned to the cardiovascular system. Lymph is similar in composition to blood plasma. Its main cellular components are lymphocytes.
• Lymphatic vessels called lacteals are found in villi that line the small intestine. Lacteals absorb fatty acids from the digestion of lipids in the digestive system. The fatty acids are then transported through the network of lymphatic vessels to the bloodstream. Although it plays a role in digestion, the primary function of the lymphatic system is host defense.
• Lymphocytes, which include B cells and T cells, are the subset of white blood cells that are involved in adaptive immune responses. They may create a lasting memory of and immunity to specific pathogens.
• All lymphocytes are produced in the bone marrow and then go through a process of maturation in which they “learn” to distinguish self from non-self. B cells mature in the bone marrow, and T cells mature in the thymus. Both the bone marrow and thymus are considered primary lymphatic organs.
• Secondary lymphatic organs include the tonsils, spleen, and lymph nodes. There are four pairs of tonsils that encircle the throat. The spleen filters blood as well as lymph. There are hundreds of lymph nodes located in clusters along the lymphatic vessels. All of these secondary organs filter lymph and store lymphocytes, so they are sites where pathogens encounter and activate lymphocytes and initiate adaptive immune responses.
• Unlike the adaptive immune system, the innate immune system does not confer immunity. The innate immune system includes surface barriers, inflammation, the complement system, and a variety of cellular responses.
• The body’s first line of defense consists of three different types of barriers that keep most pathogens out of body tissues. The types of barriers are mechanical, chemical, and biological barriers.
• Mechanical barriers—which include the skin, mucous membranes, and fluids such as tears and urine—physically block pathogens from entering the body.
• Chemical barriers—such as enzymes in sweat, saliva, and semen—kill pathogens on body surfaces.
• Biological barriers are harmless bacteria that use up food and space so pathogenic bacteria cannot colonize the body.
• If pathogens breach the protective barriers, inflammation occurs. This creates a physical barrier against the spread of infection and repairs tissue damage. Inflammation is triggered by chemicals such as cytokines and histamines, and it causes swelling, redness, and warmth.
• The complement system is a complex biochemical mechanism that helps antibodies kill pathogens. Once activated, the complement system consists of more than two dozen proteins that lead to disruption of the cell membrane of pathogens and bursting of the cells.
• Cellular responses of the innate immune system involve various types of leukocytes (white blood cells). For example, neutrophils, macrophages, and dendritic cells phagocytize pathogens. Basophils and mast cells release chemicals that trigger inflammation. Natural killer cells destroy cancerous or virus-infected cells, and eosinophils kill parasites.
• Many pathogens have evolved mechanisms that help them evade the innate immune system. For example, some pathogens form a protective capsule around themselves, and some mimic host cells so the immune system does not recognize them as foreign.
• The main cells of the adaptive immune system are lymphocytes. There are two major types of lymphocytes: T cells and B cells. Both types must be activated by foreign antigens to become functioning immune cells.
• Most activated T cells become either killer T cells or helper T cells. Killer T cells destroy cells that are infected with pathogens or are cancerous. Helper T cells manage immune responses by releasing cytokines that control other types of leukocytes.
• Activated B cells form plasma cells that secrete antibodies, which bind to specific antigens on pathogens or infected cells. The antibody-antigen complexes generally lead to the destruction of the cells, for example, by attracting phagocytes or triggering the complement system.
• After an adaptive immune response occurs, long-lasting memory B cells and memory T cells may remain to confer immunity to the specific pathogen that caused the adaptive immune response. These memory cells are ready to activate an immediate response if they are exposed to the same antigen again in the future.
• Immunity may be active or passive. Active immunity occurs when the immune system has been presented with antigens that elicit an adaptive immune response. This may occur naturally as the result of an infection or artificially as the result of immunization. Active immunity may last for years or even for life.
• Passive immunity occurs without an adaptive immune response by the transfer of antibodies or activated T cells. This may occur naturally between a mother and her fetus or her nursing infant, or it may occur artificially by injection. Passive immunity lasts only as long as the antibodies or activated T cells remain alive in the body, generally just weeks or months.
• Many pathogens have evolved mechanisms to evade the adaptive immune system. For example, human immunodeficiency virus (HIV) evades the adaptive immune system by frequently changing its antigens and by forming its outer envelope from the host’s cell membrane.
• An allergy is a disorder in which the immune system makes an inflammatory response to a harmless antigen. Any antigen that causes allergies is called an allergen. Common allergens include pollen, dust mites, mold, specific foods such as peanuts, insect stings, and certain medications such as aspirin.
• The prevalence of allergies has been increasing for decades, especially in developed countries where they are much more common than in developing countries. The hygiene hypothesis posits that this has occurred because humans evolved to cope with more pathogens than we now typically face in our relatively sterile environments in developed countries. As a result, the immune system “keeps busy” by attacking harmless antigens.
• Allergies occur when B cells are first activated to produce large amounts of antibodies to an otherwise harmless allergen and the antibodies attach to mast cells. On subsequent exposures to the allergen, the mast cells immediately release cytokines and histamines that cause inflammation.
• Mild allergy symptoms are frequently treated with antihistamines that counter histamines and reduce allergy symptoms. A severe systemic allergic reaction, called anaphylaxis, is a medical emergency that is usually treated with injections of epinephrine. Immunotherapy for allergies involves injecting increasing amounts of allergens to desensitize the immune system to them.
• Autoimmune diseases occur when the immune system fails to recognize the body’s own molecules as self and attacks them, causing damage to tissues and organs. A family history of autoimmunity and female sex are risk factors for autoimmune diseases.
• In some autoimmune diseases, such as type I diabetes, the immune system attacks, and damages, specific body cells. In other autoimmune diseases, such as systemic lupus erythematosus, many different tissues and organs may be attacked and injured. Autoimmune diseases generally cannot be cured, but their symptoms can often be managed with drugs or other treatments.
• Immunodeficiency occurs when the immune system is not working properly, generally because one or more of its components are inactive. As a result, the immune system is unable to fight off pathogens or cancers that a normal immune system would be able to resist.
• Primarily immunodeficiency is present at birth and caused by rare genetic diseases. An example is severe combined immunodeficiency. Secondary immunodeficiency occurs because of some event or exposure experienced after birth. Possible causes include substance abuse, obesity, and malnutrition, among others.
• The most common cause of immunodeficiency in the world today is the human immunodeficiency virus (HIV), which infects and destroys helper T cells. HIV is transmitted through mucous membranes or body fluids. The virus may eventually lead to such low levels of helper T cells that opportunistic infections occur. When this happens, the patient is diagnosed with acquired immunodeficiency syndrome (AIDS). Medications can control the multiplication of HIV in the human body but not eliminate the virus completely.
Up to this point, this book has covered body systems that carry out processes within individuals, such as the digestive, muscular, and immune systems. Read the next chapter to learn about the body system that allows humans to produce new individuals—the reproductive system.
Chapter Summary Review
1. The skin plays a role in establishing mechanical, chemical, and biological barriers that protect the body against pathogens. Give one example of how the skin contributes to each type of barrier.
2. Compare and contrast a pathogen and an allergen.
3. Describe three ways in which pathogens can enter the body.
4. For each of the following immune responses, state whether it is an innate or adaptive immune response.
1. Inflammation
2. Lymphocyte activation
3. Phagocytosis by leukocytes
4. Plasma cell maturation
5. The complement system involves the activation of several proteins to kill pathogens. Why do you think this is considered part of the innate immune system instead of the adaptive immune system?
6. Why are innate immune responses generally faster than adaptive immune responses?
7. Rrue or False. There is more than one type of immune system cell that can carry out phagocytosis.
8. True or False. Semen can act as a chemical barrier against pathogens.
9. True or False. Leukocytes in the innate immune system are all phagocytic.
10. Which type of immunity is triggered by vaccination?
1. Artificially acquired active immunity
2. Artificially acquired natural immunity
3. Artificially acquired passive immunity
4. Innate immunity
11. Explain how an autoimmune disease could be triggered by a pathogen.
12. What is an opportunistic infection? Name two diseases or conditions that could result in opportunistic infections. Explain your answer.
13. Match each description below with the cell type that best fits it from the list provided. Each cell type is used only once. Cell types: mast cells; B cells; killer T cells
1. Directly destroys body cells that are cancerous or infected with a pathogen
2. Secretes histamines and are involved in allergies
3. Once activated, these cells multiply and their daughter cells mature into plasma cells
14. Which cell type in the immune system can be considered an “antibody factory”?
15. Besides foreign pathogens, what is one other thing that the immune system protects the body against?
16. What cell type in the immune system is infected and killed by HIV?
17. What is the difference between primary lymphoid organs and secondary lymphoid organs? Give one example of each in your answer.
18. Name two types of cells that produce cytokines in the immune system. What are two functions of cytokines in the immune system?
19. Many pathogens evade the immune system by altering their outer surface in some way. Based on what you know about the functioning of the immune system, why is this often a successful approach?
20. Major histocompatibility complex proteins:
1. are involved in antigen-presentation by B cells
2. help distinguish cells as self
3. produce histamines
4. A and B
21. What is “missing self” and how does this condition arise?
22. True or False. Sometimes only part of a pathogen is used to create a vaccine.
23. True or False. An antigen is the same thing as an allergen.
Attributions
1. Scenic Shore 150 by Joe Grant, CC BY 2.0 via Flickr.com
2. Langerhans Cells in Normal Epidermis, CD1a Immunostain by Ed Uthman, CC BY 2.0 via Flickr.com
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/20%3A_Immune_System/20.8%3A_Case_Study_Conclusion%3A__Lymphoma_and_Chapter_Summary.txt |
This chapter discusses disease as homeostatic dysfunction and explores causes and types of human diseases, including both infectious diseases and noninfectious diseases. Special emphasis is given to sexually transmitted infections, HIV/AIDS, and cancer.
• 21.1: Case Study: Threats to Our Health
Nineteen-year-old Ximena spent a relaxing week of summer vacation visiting her grandparents in New Jersey. She particularly enjoyed taking their dog on long walks in the woods near their home, occasionally spotting deer on the overgrown paths. The discovery of a distinctive rash upon her return, however, leads to a very serious diagnosis.
• 21.2: Homeostasis and Disease
When the human body is maintained in a steady state, the condition is called homeostasis. The body consists of trillions of cells that perform many different functions, but all of them require a similar internal environment with important variables kept within narrow ranges. For example, cells require a certain range of body temperature, pH of extracellular fluids, and concentrations of mineral ions and glucose in the blood. Each of these variables must be maintained within a narrow range of val
• 21.3: Infectious Diseases
Her real name was Mary Mallon (1869-1938), but she was nicknamed "Typhoid Mary." She gained notoriety (as evidenced by this newspaper article) by being the first person in the United States to be identified as an asymptomatic carrier of the pathogen that causes typhoid fever.
• 21.4: Sexually Transmitted Infections
Syphilis is one of many sexually transmitted infections. A sexually transmitted infection (STI)is an infection caused by a pathogen that spreads mainly through sexual contact. This generally involves direct contact between mucous membranes or their secretions. To be considered an STI, an infection must have only a small chance of spreading naturally in other ways.
• 21.5: HIV and AIDS
AIDS stands for acquired immunodeficiency syndrome and is a disease caused by infection with the human immunodeficiency virus, or HIV. HIV is a sexually transmitted virus that infects and destroys helper T cells of the human immune system. AIDS eventually develops in most people with untreated HIV infections, usually several years after the initial infection with the virus. AIDS is diagnosed when the immune system has been weakened to the point that it can no longer fight off diseases.
• 21.6: Noninfectious Diseases
Noninfectious diseases include all diseases that are not caused by pathogens. Instead, noninfectious diseases are generally caused by genetic or environmental factors other than pathogens, such as toxic environmental exposures or unhealthy lifestyle choices. Most noninfectious diseases have a complex, multifactorial set of causes, often including a mix of genetic and environmental variables.
• 21.7: Cancer
Cancer is actually a group of more than 100 diseases, all of which involve abnormal cell growth with the potential to invade or spread to other parts of the body. In general terms, cancer occurs when the cell cycle is no longer regulated due to DNA damage. The number of potential underlying causes of this DNA damage is great, so there are many different risk factors for cancer. Any cells that become cancerous divide more quickly than normal cells. They may form a mass of abnormal cells called a
• 21.8: Case Study Conclusion: Lyme and Chapter Summary
As you learned in the beginning of the chapter, Ximena came down with symptoms of Lyme disease, including the distinctive bulls-eye rash that allowed for a relatively quick diagnosis and the beginning of treatment. Lyme disease is a common infectious disease transmitted by ticks—and the most common vector-borne disease in the U.S. Thankfully, there are steps you can take to protect yourself!
Thumbnail: This colorized transmission electron microscopic (TEM) image revealed some of the ultrastructural morphology displayed by an Ebola virus virion. (Public Domain; Frederick A. Murphy via CDC).
21: Disease
Case Study: What's Lurking in the Woods
Nineteen-year-old Ximena spent a relaxing week of summer vacation visiting her grandparents in New Jersey. She particularly enjoyed taking their dog on long walks in the woods near their home, occasionally spotting deer on the overgrown paths. About a week after she returned home to California, Ximena came down with what she thought was the flu. She had a fever, chills, fatigue, headache, and body aches. But in the shower one morning, she noticed an unusual rash on her calf. It looked like a bulls-eye on a target, with a central circle surrounded by a ring, similar to the rash in Figure \(1\).
The rash caused Ximena to become concerned that she might have something other than the flu. She went to her doctor, who examined her and asked if she had taken any trips lately. Surprised, Ximena said yes, and told him about her trip to New Jersey. He told her that a bulls-eye rash combined with flu-like symptoms are often indications of Lyme disease. Lyme disease can occur in California, but it is much more prevalent in the northeastern United States, including New York and New Jersey (Figure \(2\)).
Lyme disease is caused by bacteria that are spread to people through tick bites. In the northeastern U.S., it is spread by the black-legged tick or deer tick. These ticks are commonly found in wooded areas, such as the paths Ximena walked on during her trip. So those relaxing walks in the woods might have caused Ximena to pick up an unwanted souvenir—a disease that was making her feel awful.
Although flu-like symptoms can indicate any number of diseases, a bulls-eye rash is a distinctive characteristic of Lyme disease. Therefore, based on Ximena’s symptoms and the fact that she was in an area likely to harbor Lyme disease, her doctor immediately starts her on medication to treat Lyme disease. To confirm the diagnosis, he also takes a blood sample to test for the disease. He tells Ximena that she may not test positive yet, even if she does have Lyme disease, because it can take a few weeks after infection for evidence to show up in the blood. In the meantime, the medication he prescribed should start helping her feel better soon if she does have Lyme disease.
In this chapter, you will learn about some of the major types of human diseases. These include infectious diseases, such as HIV, as well as noninfectious diseases, such as most cancers. You will learn about the causes of these diseases, the effects they have on the body, and the types of treatment. You will also learn about the ways in which infectious diseases are transmitted, and steps you can take to prevent infection. At the end of the chapter, you will learn more about how Lyme disease is transmitted, its effects on the body, how it is treated, Ximena’s path to recovery, and how you can protect yourself from this relatively common infectious disease.
Chapter Overview: Disease
In this chapter, you will learn about human diseases. Specifically, you will learn about:
• How problems in regulating homeostasis can result in disease.
• The differences between infectious and noninfectious diseases, and acute and chronic diseases.
• Epidemics, pandemics, endemic diseases, and emerging diseases.
• The science of epidemiology and how it is used to improve public health.
• The different types of pathogens that cause infectious diseases, how they are transmitted, and how they can be prevented and treated.
• Sexually transmitted diseases, such as genital herpes, human papillomavirus (HPV), and human immunodeficiency virus (HIV), and their effects on the body; how they can be prevented and treated; and their impact on public health.
• Noninfectious diseases such as diabetes, cardiovascular disease, cystic fibrosis, and cancer, and their mechanisms, risk factors, diagnosis, and treatments.
• Ways to prevent noninfectious disease through healthy lifestyle choices.
As you read the chapter, think about the following questions:
1. What kind of medication do you think Ximena’s doctor gave her to treat Lyme disease?
2. What type of pathogen transmission is involved in Lyme disease? What is another disease that is transmitted in a similar way?
3. Why, specifically, might Ximena’s blood test not be positive for Lyme disease for a few weeks, even if she does have the disease?
4. How do you think the term “endemic” relates to Lyme disease?
Attribution
1. Erythematous rash by CDC/ James Gathany, public domain
2. Reported Cases of Lyme Disease – the United States, 2018; Centers for Disease Control (CDC), public domain
3. Text is adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/21%3A_Disease/21.1%3A__Case_Study%3A__Threats_to_Our_Health.txt |
Cruise control
Imagine driving on this seemingly endless road. Hopefully, your imaginary car is equipped with cruise control. Cruise control can help keep you safe as well as help you avoid a speeding ticket by keeping the speed of the car at the speed limit. Cruise control works by monitoring the car’s speed and adjusting the throttle as needed to keep the speed within a narrow range around the set speed limit. If the car starts to go faster than the set limit, it causes the throttle to release less gas until the speed drops back down to the setpoint. The opposite happens if the car’s speed starts to fall below the set speed limit. Cruise control on a car is a good analogy for physiological mechanisms that maintain the human body in a steady state.
Homeostasis
When the human body is maintained in a steady state, the condition is called homeostasis. The body consists of trillions of cells that perform many different functions, but all of them require a similar internal environment with important variables kept within narrow ranges. For example, cells require a certain range of body temperature, pH of extracellular fluids, and concentrations of mineral ions and glucose in the blood. Each of these variables must be maintained within a narrow range of values regardless of changes in the external environment, food that has been consumed, the body’s activity level, or other changes in the human organism.
Homeostats
Keeping all of the body’s internal variables within normal ranges is the function of physiological mechanisms called homeostats. Different variables are controlled by different homeostats, but all homeostats work in the same general way. A stimulus from the variable in question is sensed and compared with the normal range of values for the variable. If the actual value of the variable is outside the normal range, it elicits a response that works to move the variable back within the normal range. All homeostats use negative feedback loops to bring excessively high or low values of a variable back within the normal range.
Controlling Blood Glucose
Consider the control of the concentration of glucose in the blood as an example. The homeostat that controls this variable is illustrated and described in Figure \(1\). The primary sensors that monitor blood glucose concentration are beta cells in the pancreas. If beta cells detect a rise in the blood glucose concentration above the normal range, they secrete the hormone insulin into the blood. Insulin, in turn, acts on cells throughout the body, stimulating them to take up glucose from the blood and use it for cellular respiration. Insulin also stimulates cells in the liver to take up glucose from the blood and turn it into the complex carbohydrate glycogen for storage. At the same time, insulin inhibits the liver from breaking down stored glycogen and releasing it as glucose. Insulin also inhibits the endoplasmic reticula (ER) of cells from converting amino acids and glycerol into glucose.
If the beta cells detect a drop in the blood glucose concentration below the normal range, they stop secreting insulin into the blood, and the alpha cells of the pancreas are stimulated to secrete the hormone glucagon into the blood. Glucagon, in turn, inhibits the uptake of glucose from the blood by the liver and by fat and muscle cells throughout the body. Instead, the liver is stimulated to make glucose by breaking down stored glycogen, and the ER in cells is stimulated to make glucose from amino acids and glycerol. Glucose from all these sources is released into the blood to bring the blood glucose concentration back to the normal range.
Homeostatic Imbalance and Disease
Sometimes homeostats fail to perform properly. This can cause homeostatic imbalance, a condition in which variables in the internal environment are no longer maintained within normal ranges. As a result, cells may not get everything they need, or toxic wastes may accumulate in cells. Eventually, homeostatic imbalance may lead to disease. The term disease can be broadly defined as a condition that is associated with the impairment of normal body functioning. Disease states can cause — as well as be caused by — the failure of homeostats to maintain homeostasis.
Blood Glucose Imbalance: Type 1 Diabetes
One of the best-known examples of a disease caused by homeostatic imbalance is type 1 diabetes. In type 1 diabetes, the blood glucose homeostat ceases to function because the beta cells of the pancreas are destroyed, most often by the body’s own immune system. Without beta cells, the body’s blood glucose sensors are absent and insulin is not produced in response to high blood concentrations of glucose. Without insulin, cells are not stimulated to take up glucose from the blood, the liver does not convert glucose to glycogen for storage, and the conversion of amino acids and glycerol into glucose is not inhibited. As a result, the concentration of glucose in the blood may rise to dangerously high levels (Figure \(2\)).
If the high blood glucose concentrations of type 1 diabetes are not controlled, they may lead to further homeostatic imbalances by damaging tissues and organs throughout the body. For example, high blood glucose concentrations may damage blood vessels in the kidneys and cause kidney disease and even kidney failure. The inability of the kidneys to function normally causes further homeostatic imbalances because the kidneys are unable to adequately filter the blood. This may lead to high acid levels in the blood and imbalances in blood concentrations of mineral ions. Imbalances of mineral ions, in turn, may adversely affect bone health. Kidney failure can also have deleterious effects on cardiovascular function and cause cardiovascular disease. In fact, death from cardiovascular disease causes close to half of all deaths of people with advanced kidney failure.
Aging, Homeostatic Imbalance, and Disease
The normal aging process may bring about a reduction in the efficiency of the body’s homeostats. This makes elderly people more susceptible to disease. For example, older people may have a harder time regulating their body temperature. This is one reason they are more likely than younger people to develop heat stroke and other diseases caused by the body overheating. Older people also have a harder time fighting off many infectious diseases and cancer.
Types of Disease
Although virtually all diseases involve homeostatic imbalances in some way, there are many different underlying causes of disease. For example, some diseases are caused by pathogens, whereas others are not.
Infectious vs. Noninfectious Diseases
Pathogens are agents — usually microorganisms — that cause disease. Diseases caused by pathogens are called infectious, or communicable, diseases because pathogens can spread the diseases by moving from host to host. Types of pathogens that commonly cause human diseases include bacteria, viruses, fungi, and protozoa (Figure \(3\)). Examples of infectious diseases include the common cold, influenza, chickenpox, cholera, and malaria. Some infectious diseases are spread only or mainly through sexual contact. These diseases are called sexually transmitted infections (STIs) and include gonorrhea and syphilis (see the concept of Sexually Transmitted Infections to learn more).
Diseases that are not caused by pathogens are called noninfectious diseases. They are also called non-communicable diseases because they do not spread from person to person. Examples of noninfectious diseases include diabetes, most types of cancer, and cardiovascular diseases. There are numerous possible causes of noninfectious diseases. They include inherited mutations, exposure to environmental toxins such as air pollution, and unhealthy lifestyle choices such as overeating or smoking.
Acute vs. Chronic Diseases
Most infectious diseases are also acute diseases. An acute disease is a short-term disease. After a person gets sick, an acute disease either runs its course (with or without medical intervention) until the person gets better, or the disease leads to the death of the infected individual.
Many noninfectious diseases are chronic diseases. Chronic disease is a long-term or even lifelong disease. For example, people who develop type 1 diabetes have the disease for life as do most people who develop cardiovascular diseases. Some noninfectious diseases, such as cancer, may be cured or they may be kept under control as chronic diseases with medications. Certain infectious diseases are also chronic rather than acute diseases because they are caused by pathogens that the body cannot eliminate. Examples are the viruses that cause herpes and AIDS.
Epidemic, Pandemic, and Endemic Diseases
Some infectious diseases spread through a population from time to time as large-scale disease outbreaks called epidemics, but are not always present in the population, at least not at high levels. Such diseases are called epidemic diseases. An example is the flu (influenza). In the United States, flu spreads through the population at a certain time each year (generally, from November through April), but is not commonly found at other times of the year.
Some epidemic diseases lead to pandemics. A pandemic is an epidemic that spreads across multiple populations, often across continents or even worldwide. Throughout human history, there have been many pandemics of infectious diseases. One of the most devastating pandemics was the Black Death (bubonic plague) pandemic that spread throughout Europe and much of Asia in the mid-1300s. In this pandemic, an estimated 75 million people died. More recent pandemics include influenza pandemics that occurred in 1918 and 2009 (see Explore More below). In December 2019, a novel coronavirus disease (COVID 19) spread over all continents. By April 2021, it killed 3.1 million people worldwide.
Some diseases are always present in a population. Such diseases are referred to as endemic diseases. For example, in many tropical countries, malaria is an endemic disease. It is transmitted year-round by mosquitoes in the hot climate, rather than appearing in periodic outbreaks that occur only at certain times of the year.
Studying Disease in Human Populations
Epidemiology is the science that focuses on the patterns, causes, and effects of diseases in human populations. It is the cornerstone of public health. It shapes health policy decisions and medical practice by identifying risk factors for disease and targets for preventive healthcare. The term epidemiology was first applied to the study of epidemics, but it is now widely applied to the study of disease in general and even to the study of many non-disease conditions, such as high blood pressure and obesity.
The Father of Epidemiology
The science of epidemiology has deep roots. The ancient Greek physician Hippocrates (about 460 to 379 BCE) was the first person known to have examined relationships between the occurrence of disease in populations and environmental factors. He also coined the terms epidemic and endemic. A century later, a Greek physician named Galen developed the idea that miasma (“bad air”) was the cause of diseases such as bubonic plague and cholera, an idea that was accepted by most physicians until well into the 1800s. However, the origin of modern epidemiology is generally attributed to an English physician named John Snow (Figure \(4\)), who is often referred to as the father of epidemiology. Snow gained fame for his scientific investigations into the cause of the cholera epidemics that led to many deaths in 19th century London.
Dr. Snow was skeptical of the still-dominant miasma theory, but the germ theory of disease would not be fully formed for several decades. As a result, Snow did not understand the mechanism by which infectious diseases such as cholera are transmitted. While treating cholera cases during an outbreak in London in the mid-1800s, Snow observed a pattern in the distribution of homes where people got the disease. Homes of cases appeared to cluster around a particular public well. Snow wondered if cholera was spread through the drinking water in this well. He plotted homes with cholera cases on a map (reproduced below) and then showed that all of these homes received their drinking water from the well in question. Snow also did chemical and microscopic examinations of water from the well but could not prove conclusively that it was causing cholera. However, his demonstration of the pattern of the disease in the population was convincing enough to persuade officials to close down the well. They removed the handle from the pump the next day, an action that has been credited with stopping the cholera outbreak
It was later discovered that the well in question had been dug only a few feet from an old cesspit, which had begun to leak fecal matter into the well water. This suggested that cholera spreads through a fecal-oral route. However, this explanation was not immediately accepted because it was too unpleasant for most people to contemplate. It took several more years — and additional cholera outbreaks — before this explanation for the spread of the disease could no longer be denied. Eventually, Snow’s work inspired fundamental changes in the water and waste systems of London and other cities and eventually led to significant improvements in public health around the world. Snow’s contributions to epidemiology and public health are now recognized by numerous honors and monuments to his work.
More Recent Contributions of Epidemiology
More recent pioneers in epidemiology include the British doctors Richard Doll and Austin Bradford Hill, whose large-scale observational studies in the mid-1900s showed statistically significant links between tobacco smoking and lung cancer. Doll and Hill were among the first scientists to use epidemiology to study noninfectious diseases. Since then, epidemiology has been applied to the study of many different diseases, and epidemiological methods have become more sophisticated and rigorous over time. The results of epidemiological studies continue to make a significant contribution to population-based health management. The results are used to assess the health status and needs of the public and to implement and evaluate interventions that are designed to improve public health.
Review
1. Define homeostasis.
2. What are homeostats?
3. Describe the homeostat that controls blood glucose concentration.
4. What happens if homeostats fail to perform properly? Give an example.
5. Compare and contrast infectious and noninfectious diseases.
6. Compare and contrast acute and chronic diseases. What is an example of each type of disease?
7. Define and relate the terms epidemic and pandemic.
8. What is an endemic disease?
9. What is the focus of epidemiology? What are the applications of epidemiological research?
10. Who was John Snow? What is the significance of his work in epidemiology?
11. True or False. Acute diseases are always less dangerous than chronic diseases.
12. True or False. Influenza can cause an epidemic or a pandemic.
13. Briefly describe the relationship between homeostasis and disease.
14. Can an infectious disease be a chronic disease? Why or why not?
15. If you were an epidemiologist studying a new infectious disease that is making many people ill, what are some things you would want to know so that you could distinguish between whether it is endemic, a pandemic, or an epidemic?
Attributions
1. The Homeostatic Regulation of Blood Glucose Levels by OpenStax College, CC BY 3.0, via Wikimedia Commons
2. Hyperglycemia by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
3. Plasmodium slides by CDC/Steven Glenn, public domain via Wikimedia Commons
4. John Snow, public domain via Wikimedia Commons
5. Snow Cholera map by John Snow, public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/21%3A_Disease/21.2%3A_Homeostasis_and_Disease.txt |
Typhoid Mary
Her real name was Mary Mallon (1869-1938), but she was nicknamed “Typhoid Mary.” She gained notoriety (as evidenced by this newspaper article in Figure \(1\)) by being the first person in the United States to be identified as an asymptomatic carrier of the pathogen that causes typhoid fever. Over the course of her career as a cook, Mary Mallon was thought to have infected 51 people, three of whom died. She was twice forcibly quarantined by public health authorities and died after a total of nearly 30 years in isolation. Typhoid fever is caused by bacteria that are spread by eating or drinking food or water contaminated with the feces of an infected person. Risk factors include poor sanitation and poor hygiene. Typhoid fever is one of many infectious diseases that can spread in human populations.
All infectious diseases are caused by infections with pathogens or disease-causing agents, but not all infections cause infectious diseases. Infection is the invasion of an organism’s body tissues by pathogens, which multiply and damage or poison the host tissues. The reaction of the host’s immune system to the pathogens may contribute to the tissue damage. Infectious disease is an illness resulting from an infection. It occurs when an infection causes noticeable symptoms.
Types of Pathogens
Infectious diseases kill more people in low-income countries than any other cause, and they are important causes of death elsewhere. Many different types of pathogens can cause infectious diseases. Besides bacteria and viruses, human pathogens include fungi, protists, helminths, and prions.
• Bacteria: The vast majority of bacteria are at least harmless if not beneficial to human hosts. Relatively few bacteria cause human diseases, but of those that do, the disease burden they exert on human populations may be great. Disease burden is the impact of a disease on a population as measured by financial cost, mortality, morbidity, or other indicators. One of the bacterial diseases with the highest disease burden worldwide is tuberculosis. It is caused by the bacterium Mycobacterium tuberculosis, which kills about 2 million people a year, most of them in sub-Saharan Africa. Other bacterial diseases that burden human populations include strep throat, pneumonia, shigellosis, tetanus, typhoid fever, cholera, diphtheria, syphilis, and leprosy.
• Viruses: Viruses are little more than DNA or RNA in a protein coat. Viruses are not usually classified as living things because they cannot survive or reproduce on their own. They require the cells of a host to provide the machinery for protein synthesis and reproduction. Many types of viruses are pathogenic to humans. Common human diseases caused by viruses include influenza, mumps, measles, chickenpox, hepatitis, AIDS, yellow fever, coronavirus disease, herpes, polio, and the common cold.
• Fungi: Fungi (singular, fungus) are eukaryotic organisms in the Fungus Kingdom. Some fungi are unicellular organisms; others are multicellular. Many fungi consume dead organisms. Many others are parasites of plants or animals, including humans. Human diseases caused by fungi include candidiasis, histoplasmosis, ringworm, and athlete’s foot. People with a compromised immune system are particularly susceptible to certain fungal diseases, such as candidiasis, which is pictured below, and cryptococcosis, which is a defining opportunistic infection for AIDS patients.
• Protists: Protists are an informal grouping of simple eukaryotic organisms that are not plants, animals, or fungi. Some protists — particularly those called protozoa — are significant parasites of human organisms. They include five species of the parasitic genus Plasmodium that cause malaria. Malaria places a tremendous disease burden on human populations. In 2015, there were 214 million cases of malaria worldwide resulting in an estimated 438,000 deaths, 90 percent of which occurred in Africa. Other human diseases caused by protists include giardiasis, toxoplasmosis, trichomoniasis, Chagas disease, leishmaniasis, trypanosomiasis (sleeping sickness), and amoebic dysentery.
• Helminths: Helminths, also commonly known as parasitic worms, are multicellular organisms, which when mature can generally be seen with the naked eye. Helminths infect animals including humans. Most live in the host’s intestines, but some live in other organs, such as muscles or blood vessels. Helminths take nourishment and protection from the host and cause disease in return. Examples of helminthic infections in humans include infections by tapeworms, roundworms, pinworms, and hookworms (Figure \(3\)).
• Prions: Prions are infectious agents composed entirely of proteins. Prions are misfolded proteins that replicate by converting their properly folded counterparts, in their host, to the same misfolded structure they possess. Prions are transmissible and a few of them are known to cause human diseases, including Creutzfeldt–Jakob disease (CJD), an incurable and universally fatal neurodegenerative disease. CJD is similar to the better-known prion disease in cows, called mad-cow disease. In both the human and bovine (cow) diseases, brain tissue degenerates rapidly, and the brain develops holes like a kitchen sponge.
Identifying Pathogens: Koch’s Postulates
The human body has more microorganisms than it does human cells. The majority of microorganisms that live in or on the human organism are actually beneficial or at least harmless to the human host (except in the case of immune-compromised individuals). Given a huge load of microorganisms in and on the human organism, how can scientists determine which species of microorganism causes a particular disease? The 19th-century physician and microbiologist Robert Koch (Figure \(4\)) is best known for developing four basic criteria, or postulates, for deciding whether a disease is caused by a particular microorganism. Koch’s postulates are tabulated below:
Koch’s postulates
1. The microorganism must be found in abundance in all individuals suffering from the disease and should not be found in healthy individuals.
2. The microorganism must be isolated from a diseased individual and grown in pure culture.
3. The cultured microorganism should cause disease when introduced into a healthy individual.
4. The microorganism must be re-isolated from the inoculated individual and then identified as being identical to the original microorganism.
In the first and third postulates, Koch originally used the word “must” instead of “should.” He changed the wording when he learned that some carriers of cholera and typhoid were asymptomatic and remained healthy.
Since Koch presented his postulates, scientists have come to realize that all four postulates may not apply to every pathogenic microorganism. For example, while bacteria can be grown and identified in pure culture (Figure \(5\)), this is not the case with all pathogens, including viruses and prions. Therefore, these pathogens fail to meet the second postulate. Taking these and other exceptions into account, Koch’s postulates can be viewed as sufficient but not necessary criteria for establishing a specific agent as the cause of a disease. The postulates still inform the basic approach scientists take to this research. Koch’s postulates are also important for their historical significance. They led to the scientific identification of many human pathogens, which allowed the development of ways to prevent and cure diseases.
How Pathogens Cause Disease
Pathogens usually gain entrance to a human host through the mucosa in orifices like the oral cavity, nose, eyes, genitalia, or anus. Some pathogens may be swallowed and gain access through the mucosa lining the digestive tract. Other pathogens may enter a human host through breaks in the skin. Once pathogens gain entrance to a host, they multiply inside the host, either at the site of entry or at other sites after migrating from the entry site. Some pathogens live and multiply inside host cells; others live and multiply in host body fluids.
Within the host’s tissues, pathogens may cause damage by releasing toxins. For example, Clostridium tetani releases a toxin that paralyzes muscles, causing the disease known as tetanus. Typically, the more pathogens that are present, the greater the severity of illness, but there is considerable variation in the virulence of pathogens. The poliovirus is not very virulent. Fewer than 5 percent of people infected with poliovirus actually develop any noticeable symptoms of the disease. On the other hand, CJD prions are so virulent that they cause severe disease and death in every infected individual.
How Pathogens Are Transmitted
For pathogens to survive and repeat the cycle of infection in other hosts, they or their progeny must have a means of leaving one host and entering another. Transmission of pathogens from infected to noninfected human hosts occurs through many different routes.
• Airborne Transmission: One of the most common routes is airborne transmission. As illustrated in Figure \(6\), this occurs when pathogens in droplets are expelled from an infected host’s respiratory system during coughing or sneezing. The pathogens are then inhaled by nearby people, who become new hosts for the pathogens. Flu and the common cold can spread this way.
• Direct Contact: Many pathogens spread from person to person through direct contact between an infected person and a new host. This can happen when people have skin-to-skin contact or touch the same surfaces. Athlete’s foot and warts are transmitted this way. Another form of direct contact is the oral transmission. This occurs when pathogens spread through direct oral contact, for example, by kissing, or by sharing items that go in the mouth, such as drinking glasses or eating utensils. Mononucleosis and oral herpes spread through oral contact.
• Fecal-Oral Transmission: Fecal-oral transmission is also very common. It occurs when pathogens in feces from an infected host enter the mouth of a new human host in fecally contaminated food or water or on contaminated fingers. Cholera and many types of gastroenteritis are transmitted this way. Helminth infections are also usually spread via a fecal-oral route. Adult worms may live and produce eggs in the human host for several years. Generally, thousands or even hundreds of thousands of eggs are released each day. The eggs are then shed from the human host in feces. The eggs that hatch develop into larvae that may be consumed by a new human host in contaminated food or water. After being ingested, the larvae develop into adult worms that parasitize the new human host.
• Vector Transmission: Vector transmission occurs when pathogens are carried by a vector organism from infected hosts to new hosts, usually through biting them. Mosquitoes, fleas, and other insects are common vectors. Figure \(7\) describes four diseases that are spread by mosquito vectors.
• Vertical Transmission: Vertical transmission occurs when pathogens travel from an infected woman to her embryo or fetus during pregnancy or to her infant during or soon after birth. Examples of diseases that can be transmitted this way include HIV infection and rubella. You can learn more about this type of transmission in the concept Embryonic Stage.
• Sexual Transmission: Sexual transmission occurs when pathogens spread through sexual activity between an infected host and a new host. Sexual transmission generally requires direct contact between mucous membranes or their secretions. This can occur during any type of sexual contact, including vaginal, anal, or oral contact. Sexually transmitted infections include chlamydia and gonorrhea. To learn more about sexual transmission, read the concept of Sexually Transmitted Infections.
• Transmission of Prions: Prions have unusual means of transmission. Some people have become infected with prions by eating meat from cows infected with mad cow disease. Prions that cause the human disease called kuru have been transmitted through cannibalism. Kuru is a deadly disease that was once commonly found in women and children of the Fore tribe in Papua New Guinea. Women and children were most often infected because they were more likely than males to eat highly infective brain material from the cannibalized bodies. Solving the mystery of this disease and its mode of transmission resulted in two Nobel Prizes.
Managing Infectious Disease
Infectious diseases must be correctly diagnosed so the appropriate treatment can be prescribed. Most infectious diseases can be treated if not cured. Many infectious diseases can be prevented through commonsense behaviors or immunizations.
Diagnosing Infectious Disease
Most minor infectious diseases, such as upper respiratory infections and diarrheal diseases, are usually diagnosed on the basis of their signs and symptoms. However, determining the specific pathogen that is causing the disease may be necessary to choose the best treatment. Many pathogens can be identified by growing samples from a patient on a culture medium or by examining samples from the patient under a microscope. Most bacteria can be identified from a culture based on the size, color, and shape of the colony. Viruses can be identified by using cells grown in culture as the medium. If viruses are pathogenic, they infect and kill the cultured cells. Biochemical tests can also be used to identify specific pathogens in patient samples. Some pathogens can be detected by testing for the chemical products they produce, such as acids, alcohols, or gases. Another potential diagnostic tool is a serological test, which identifies pathogens by their antigens and whether they bind to specific antibodies.
Treating Infectious Disease
Not all infectious diseases require treatment. Many minor infectious diseases are usually self-limiting and people get better on their own. For more serious infectious diseases, pharmaceutical drugs may be needed. Drugs have been developed to treat most types of infectious diseases. Different types of drugs are generally required to treat different types of pathogens.
• Bacteria can often be killed with antibiotic drugs. Antibiotics typically work by destroying the cell wall of bacterial cells, causing the DNA inside to spill out. This makes the bacterial cells incapable of producing proteins, so they die. This generally cures the disease. Several different classes of antibiotics have been developed. Different types of bacteria are susceptible only to certain classes of antibiotics. Some bacteria have evolved the ability to resist some or all classes of antibiotics (see Explore More below).
• Unlike bacteria, viruses are not killed by antibiotic drugs. However, antiviral drugs have been developed to help the immune system fight off viral infections. Antiviral drugs are generally not as effective at curing viral infections as antibiotics are at curing bacterial infections.
• Most fungal infections can be treated or cured with antifungal medications. These may be taken orally or applied topically, depending on the disease. Most antifungal drugs are available only with a doctor’s prescription, but a few can be purchased over the counter (OTC). An example of an OTC antifungal product is pictured in Figure \(8\).
• Infections by protozoa are treated with antiprotozoal drugs. Because protozoans may vary greatly in their biology, drugs effective against one pathogen may not be effective against another. Several anti-malarial drugs have been developed, but the Plasmodium pathogens are evolving resistance to most of them.
• Several drugs are available to kill worms in human hosts. Different drugs must be used for different helminthic parasites. The drugs kill off the adult worms, which are then shed in the host’s feces.
Unfortunately, infectious diseases caused by prions are not treatable at present. They are considered incurable and inevitably fatal diseases. The good news is that scientists are actively working to find ways to treat or cure prion diseases.
Preventing Infectious Disease
You have probably heard the expression “an ounce of prevention is worth a pound of cure.” It certainly applies to infectious diseases. Some side effects of treating minor infectious diseases can be worse than the disease symptoms. For example, taking an antibiotic for a minor sinus infection might lead to diarrhea or a vaginal yeast infection as beneficial bacteria are killed off along with harmful bacteria and causing homeostatic imbalance. It’s almost always better to avoid getting sick in the first place than to treat a disease after it occurs. Hygienic habits and immunizations are the most effective ways to prevent the spread of infectious diseases.
Hand washing and Other Behaviors
Frequent hand washing is the single most important defense against the spread of many pathogens, especially those transmitted through direct skin contact or the fecal-oral route. Hand washing can also reduce the spread of respiratory illnesses such as flu, coronavirus disease, and the common cold because the viruses can be spread on people’s hands when they touch their nose, mouth, or eyes. For the most effective way to wash your hands in order to prevent infection, see the Feature: My Human Body below.
What else can you do to protect yourself? You can use condoms to avoid sexually transmitted infections where there is a risk of transmission, for example, with a new partner. Condoms are the only method of contraception that also helps prevent the spread of such infections. Preventing the spread of infectious diseases transmitted by vectors often involves controlling the vectors or at least exposure to the vectors. For example, the number of mosquitoes can be reduced by removing sources of standing water around homes. Insect repellents and mosquito nets (like the one in Figure \(9\)) can be used to reduce human contact with vectors.
Immunization
Diseases that can be prevented with vaccinations include many otherwise common and potentially serious early childhood infections such as measles, mumps, chickenpox, whooping cough (pertussis), and diphtheria. Vaccinations also are recommended for older children against the human papillomavirus (HPV) that causes genital warts and may lead to cervical cancer in females. Annual vaccines for influenza (Figure \(10\)) are highly recommended as well, especially for young children and older adults. Pneumonia vaccines are also advised in certain people, particularly the elderly. Coronavirus diseases can also be combated by vaccination.
Some people cannot safely receive vaccines. For example, children with a compromised immune system or cancer may not be able to safely receive routine childhood vaccinations. To help protect such vulnerable people from being exposed to infectious diseases, it is important for populations to maintain high levels of vaccination. When a critical portion of a population is immunized against an infectious disease, most members of the population are protected against that disease even if they have not been immunized. This is known as herd immunity. You can see how it works in Figure \(11\). The principle of herd immunity applies to many infectious diseases, including influenza, measles, and mumps, to name just a few.
Emerging Infectious Diseases
New infectious diseases are showing up in human populations. Called emerging infectious diseases, they can come about in a number of ways, most of which are influenced by human actions. New infectious diseases can emerge when previously harmless microorganisms evolve to become pathogenic to humans or when microorganisms that infect nonhuman animals jump to human hosts. Infectious diseases can also spread to faraway populations where people have no prior exposure and natural immunity to them. Human actions that influence the emergence of new infectious diseases include:
• human encroachment on wild habitats. This may happen with residential development, mining, farming, or logging activities. It may bring humans into contact with insects and other animals that harbor previously unknown microorganisms that are pathogenic to people.
• changes in agriculture. The introduction of new crops attracts new crop pests and the microbes they carry to farming communities. This exposes people to new pathogens.
• uncontrolled urbanization. The rapid growth of cities in many developing countries concentrates large numbers of people in crowded areas with poor sanitation. Such conditions foster the transmission of pathogens that may not have been able to spread in small, dispersed rural populations.
• modern transportation. Ships and other cargo carriers often inadvertently carry microscopic pathogens or their infected vectors to distant places where they can infect people who have never been exposed to them before. International jet travel allows infected people to carry pathogens to distant populations, even before their first symptoms appear.
Feature: My Human Body
Proper hand washing is the single most important behavior you can adapt to avoid infection by pathogens. The most effective hand washing method is to use soap and warm running water and the following procedure:
1. Wet hands with warm water, keeping hands below the forearms to prevent contaminated water from moving from the hands to the arms.
2. Apply about 5 mL (1 teaspoon) of liquid soap and rub it all over the hands for at least 20 seconds. Be sure to wash the most commonly missed areas, which are the thumb, wrist, areas between the fingers, and skin under the fingernails. Ideally, you should use a nail brush to remove any debris or microorganisms under the fingernails.
3. Rinse thoroughly. Make sure the water flows from the wrist to the fingertips to ensure that any microorganisms are washed off the skin rather than up onto the arms.
4. Dry hands thoroughly with a clean towel or hot air blower. Properly dispose of any used towels. If possible, use towels to turn faucets on and off and to open the bathroom door.
Review
1. What are infectious diseases?
2. Name types of pathogens and give an example of a human disease caused by each type of pathogen.
3. What are Koch’s postulates? What is their current significance?
4. How do pathogens cause disease? Identify two factors that influence the severity of an infectious disease.
5. List six common routes of transmission of pathogens.
6. Why is the correct diagnosis of a pathogen important for selecting the appropriate treatment of an infectious disease?
7. What are the most effective ways to prevent the spread of infectious diseases?
8. How does herd immunity come about, and why is it important?
9. Explain how and why emerging infectious diseases are appearing in human populations.
10. Explain why Koch’s first postulate alone would not provide sufficient evidence to prove that a specific microorganism causes a disease. In particular, discuss why postulates three and four are required.
11. What is a disease burden?
A. How many infectious pathogens are in an infected person.
B. How many people are killed by an infectious disease each year?
C. How rapidly an infectious disease spreads.
D. The impact of a disease on a population, including the number of deaths and the associated financial impact.
12. Give an example of a human disease that is caused by a protist and transmitted by a vector.
13. What kind of treatment do you think might be given for leprosy? Explain your reasoning.
14. True or False. The direct contact route of pathogen transmission requires skin-to-skin contact.
15. True or False. Some people who are infected with a pathogen never show symptoms.
Explore More
For a fascinating read about an equally fascinating human infectious disease, check out Kuru Sorcery: Disease and Danger in the New Guinea Highlands by anthropologist Shirley Lindenbaum (2nd edition 2013, Routledge).
Scientists at Harvard Medical School have developed an innovative and fascinating way to study the emergence of antibiotic resistance in bacterial pathogens. To learn more, watch this video:
Attributions
1. Typhoid Mary, public domain via Wikimedia Commons
2. Candidiasis by CDC/ Sol Silverman, Jr., DDS, public domain via Wikimedia Commons
3. Hookworms by CDC, public domain via Wikimedia Commons
4. Robert Herman Koch, CC BY 4.0 via Wellcome Library
5. Nutrient Agar growing common Gram negative Bacteria by Eukaryotica, public domain via Wikimedia Commons
6. Disease Transmission sneezing by Centers for Disease Control and Prevention, public domain via Wikimedia Commons
7. Mosquito carried diseases by NIAID, CC BY 2.0 via Wikimedia Commons
8. Canesten by Editor182, public domain via Wikimedia Commons
9. Javan bed canopy by Azt3cs, public domain via Wikimedia Commons
10. A nurse vaccinates Barack Obama against H1N1 by White House (Pete Souza) / Maison Blanche (Pete Souza), public domain via Wikimedia Commons
11. Community Immunity by National Institutes of Health (NIH), public domain via Wikimedia Commons
12. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/21%3A_Disease/21.3%3A_Infectious_Diseases.txt |
Columbian Exchange
This artwork is entitled “Columbus and the Indian Maiden.” It was painted around 1875 by Constantino Brumidi, an Italian-American historical painter. It is a good image to represent the concept of Columbian exchange. This concept refers to the exchange of pathogens during initial contact between Europeans and Native Americans, starting when Columbus arrived in the New World in 1492. One of those pathogens may have been the sexually transmitted bacterium that causes syphilis. Syphilis is thought to have originated in the New World, and there is some evidence that Columbus himself was infected with it. The first recorded European outbreak of syphilis occurred in 1494. The outbreak began in Italy and quickly swept across the entire European continent. When Europeans were first exposed to syphilis, it was a much more virulent disease than it would eventually become. Back then, pustules covered the body of people with syphilis, and they caused the flesh to fall away from the face. The disease was also usually quickly fatal. In that first European epidemic, syphilis killed an estimated 5 million people.
Introduction to Sexually Transmitted Infections (STIs)
Syphilis is one of many sexually transmitted infections. A sexually transmitted infection (STI)is an infection caused by a pathogen that spreads mainly through sexual contact. This generally involves direct contact between mucous membranes or their secretions. To be considered an STI, an infection must have only a small chance of spreading naturally in other ways. Some infections that can spread through sexual contact, such as the common cold, spread much more commonly by other means, such as airborne transmission. These infections are not considered STIs.
You may have heard sexually transmitted infections (STIs) referred to as sexually transmitted diseases (STDs). The disease terminology is no longer used to avoid the misconception that STIs cannot be transmitted unless one has symptoms of the disease. In fact, many STIs do not cause symptoms but can still be spread by infected people, most of whom are unlikely to even realize they are infected. An even older term for STIs is a venereal disease (VD). The term comes from Venus, the Roman goddess of love. The World War I-era anti-VD poster shown below appeals to patriotism to encourage soldiers to avoid becoming infected. During that war, STIs caused the U.S. Army to lose the services of 18,000 servicemen per day.
Transmission of STIs
Most of the pathogens that cause STIs are either bacteria or viruses that enter the body through mucous membranes of the reproductive organs and often through the oral and anal mucosa as well. Pathogens that can only infect the body via direct contact between mucous membranes generally cannot spread through non-sexual skin contact, such as touching, hugging, or shaking hands. All sexual behaviors that involve contact between mucous membranes put a person at risk for infection with STIs. This includes vaginal, anal, and oral sexual behaviors. Some of the pathogens that cause STIs can also be transmitted through body fluids such as blood and breast milk. Therefore, sharing drug injection needles as well as the processes of childbirth and breastfeeding are other ways these STIs can potentially be spread. Most of the information in this section is about cis-gendered individuals because of the lack of data and information about the spread of STIs in the LGBTQ community.
Symptoms of STIs
Common symptoms of STIs include sores or rashes on the genitals, a vaginal or penile discharge, and painful urination. Many STIs are asymptomatic or cause such mild symptoms that they go unnoticed. Such infections are called “silent” infections. However, even in asymptomatic people, the pathogens can usually be transmitted to other people. Asymptomatic infections may also eventually cause serious health problems if they go untreated.
Diagnosis and Treatment of STIs
Most STIs are treatable if not curable, but the correct treatment depends on diagnosing the pathogen that is causing the infection. STIs caused by bacteria can generally be cured with antibiotics, although some bacteria may be evolving antibiotic resistance. STIs caused by viruses cannot be successfully treated or cured with antibiotics. Instead, viral STIs are treated with antiviral drugs, which may help control but usually not eliminate the virus. If the immune system cannot eliminate the virus, it may remain in the body for life.
Prevention of STIs
Vaccinations are available to prevent just a few STIs (including human papillomavirus and hepatitis infections). The only completely effective way to prevent other STIs is to avoid all sexual contact and other risky behaviors. Safe sex practices — such as using condoms, having few sexual partners, and maintaining mutually monogamous relationships — can reduce the risk of STIs but not prevent them for certain. Condoms, for example, are not foolproof. Pathogens may be present on areas of the body not covered by condoms, and condoms can also break or be used incorrectly. (See the Feature: My Human Body for the correct way to use condoms.) Practices that cannot prevent the transmission of STIs include washing the genitals, urinating, and/or douching after sexual contact.
Pathogens that Cause STIs
STIs are caused by many different types of pathogens. More than 30 different pathogens have been identified. Most are bacteria or viruses. A few of the pathogens are sexually transmitted parasites.
Parasites that Cause STIs
A very common sexually transmitted parasite is the crab louse (Pthirus pubis), pictured in Figure \(3\). It lives in human pubic hair where it bites the skin and sucks on blood. This may cause itching and irritation.
Another common sexually transmitted parasite is the single-celled protozoan named Trichomonas vaginalis. It infects the vagina and urethra, causing the STI called trichomoniasis. It may cause burning and itching at the site of infection but is often asymptomatic. It is easily cured with prescription drugs.
Bacterial STIs
Many STIs are caused by bacteria. Some of the most common bacterial STIs are chlamydia, gonorrhea, and syphilis.
Chlamydia
Chlamydia is an STI caused by infection with the bacterium Chlamydia trachomatis. It is the most frequently reported bacterial STI in the United States. In 2015, an estimated 2.9 million chlamydia infections occurred in the United States. Chlamydia is most common among young people, with about two-thirds of new cases occurring in people between the ages of 15 and 24. The high rates in young people are apparent in Figure \(4\). More young females than young males are diagnosed with chlamydia each year, largely because of sex differences in testing for the infection.
Chlamydia is transmitted through sexual contact with the penis, vagina, mouth, or anus of an infected sexual partner. Ejaculation of semen does not have to occur for chlamydia to be transmitted or acquired. People who have been treated and cured of chlamydia do not become immune to the bacteria and may become infected again if they have sexual contact with an infected person. Chlamydia can also spread from an untreated mother to her baby during childbirth. In infants infected at birth, the bacteria may infect the eyes, lungs, anus, or genitals.
In women, Chlamydia bacteria usually infect the cervix and sometimes the urethra. If symptoms occur, they are likely to include vaginal or urethral discharge or bleeding. Urination may also be painful. The infection may spread from the cervix to the upper reproductive tract, including the uterus and Fallopian tubes, causing pelvic inflammatory disease (PID). PID may also be asymptomatic, but even without symptoms, it can lead to permanent damage and increase the risk of ectopic pregnancy (in which the embryo implants outside of the uterus) or infertility. In men, chlamydia bacteria usually infect the urethra and sometimes the epididymis. If symptoms occur, they typically include a urethral discharge and pain in urinating. Occasionally, there is pain, tenderness, or swelling in one of the testes. In both sexes, the rectum can also be infected. If there are rectal symptoms, they may include rectal discharge, bleeding, or pain.
Chlamydia is easily cured with antibiotics. Because chlamydia is usually asymptomatic; screening is necessary to identify most infections so they can be treated and cured. Screening programs routinely test as many people as possible in high-risk groups using lab tests of patient specimens, such as urine samples or swabs of vaginal, oral, or anal discharge. Screening programs have been shown to reduce the adverse sequela of chlamydia in women (PID, ectopic pregnancy, infertility), so annual chlamydia testing is recommended for women in high-risk groups. These include all sexually active women younger than 25 as well as older women with certain risk factors, such as a new sexual partner, multiple sexual partners, or a sexual partner who has an STI. Pregnant women are also tested during their first prenatal care visit and sometimes again during the third trimester. Routine chlamydia screening is not generally recommended for men because the costs are thought to outweigh the potential benefits.
Gonorrhea
Gonorrhea is a common STI caused by the bacterium Neisseria gonorrhoeae. An estimated 820,000 new cases of gonorrhea occur in the United States each year, but fewer than half of them are actually diagnosed and reported. Approximately 70 percent of cases occur in people aged 15 to 24 years, as indicated by the graph in Figure \(5\) for the year 2014. The blue bars on the left represent the number of infected males and on the right, the red bars represent the number of females infected per 100,000 individuals. Again, sex differences in testing for the disease are reflected in higher rates for females.
Gonorrhea is transmitted through sexual contact with the penis, vagina, mouth, or anus of an infected partner. Ejaculation does not have to occur for gonorrhea to spread. After being cured of gonorrhea, a person can get the disease again through sexual contact with an infected partner. Gonorrhea can also be transmitted from an untreated mother to her baby when the infant passes through the birth canal. The bacteria may infect the baby’s eyes (Figure \(6\)) and possibly cause blindness. The baby can also develop a joint infection or a life-threatening blood infection.
Gonorrhea is often asymptomatic, especially in females. If symptoms do occur, they typically include a discharge from the penis or vagina and painful urination. Even when women do not have symptoms of gonorrhea, they are at risk of developing complications from the infection, such as PID and infertility. If left untreated, gonorrhea can also spread to the blood and cause a life-threatening systemic disease.
Because gonorrhea is so often asymptomatic, the majority of cases are diagnosed during routine screening. The Centers for Disease Control and Prevention (CDC) recommend annual gonorrhea screening for all sexually active females younger than 25 years old, as well for older women with risk factors such as new or multiple sex partners. Gonorrhea bacteria are detected with lab analysis of urine or of genital, oral, or rectal specimens. Gonorrhea usually can be cured with proper treatment. Successful treatment has become more difficult as the bacteria have started to evolve resistance to the most commonly used antibiotics. The CDC advises taking two different antibiotics concurrently for the best chances of a cure
Syphilis
Syphilis is an STI caused by the bacterium Treponema pallidum. During 2015, there were almost 75,000 new cases of syphilis reported in the United States. In some of these cases, the disease was diagnosed for the first time in people with long-term infections.
Syphilis is transmitted from person to person by direct contact with a syphilitic sore, known as a chancre. Chancres occur mainly on the external genitals or in the vagina or anus, but they may also occur on the lips or in the mouth (as shown in Figure \(7\)). Transmission of syphilis can occur during vaginal, anal, or oral sex. After being cured of syphilis, a person can get the disease again through sexual contact with an infected partner. A pregnant woman with untreated syphilis can pass the disease to her unborn child at any time during pregnancy or childbirth. Depending on when the bacteria are transmitted to the fetus, the outcome may be stillbirth or early infant death. Untreated syphilis in pregnant women results in infant death in up to 40 percent of cases.
Without treatment, syphilis typically progresses through several stages. The progression of the disease is likely to be stopped only if a person receives appropriate antibiotic therapy. Barring such treatment, an infected person may eventually progress through all of the stages of the disease, a process that may take a decade or more.
1. Primary syphilis is the first stage of the disease. It begins with the appearance of at least one chancre. The chancre is usually firm, round, and painless. It appears at the location where the syphilis bacteria entered the body. The chancre lasts from 3 to 6 weeks and then heals, regardless of whether the person is treated.
2. Secondary syphilis is the second stage of the disease. It is the most contagious of all the stages and is characterized by the spread of the bacteria throughout the body, causing systemic symptoms such as skin rashes (Figure \(8\)) and/or sores on the mucous membranes. Other symptoms may include fever and swollen lymph glands. This stage typically begins about 4 to 10 weeks after the initial infection and generally lasts 3 to 6 weeks.
3. Latent syphilis is the third stage of the disease. This stage begins when the symptoms of secondary syphilis resolve, which generally occurs even without treatment. There are no symptoms during the latent stage, but the bacteria are still present in the body. The latent stage of syphilis can last for years.
4. Tertiary syphilis is the final stage of the disease. In this stage, the disease may infect and damage internal organs, such as the brain, heart, liver, or bones. A severe joint infection in a person with tertiary syphilis is pictured below. Symptoms of the tertiary stage may include paralysis, blindness, seizures, and dementia. The damage may be serious enough to cause death
Syphilis is usually diagnosed on the basis of a blood test that detects antibodies that are specific for the syphilis bacterium. Because of the severity of infection in fetuses and the high risk of fetal death, it is recommended that all pregnant women be tested for syphilis at the first prenatal visit. Women at high risk of syphilis should be tested again during the third trimester and at the time of delivery. If a pregnant woman tests positive for syphilis, she is prescribed the antibiotic penicillin, which has a 98 percent success rate at preventing mother-to-fetus transmission. If the pregnant woman is allergic to penicillin, desensitization is required before treatment is given.
Penicillin is also the drug of choice for treating syphilis in the general population. A single intramuscular injection of long-acting penicillin can cure primary, secondary, or early latent syphilis. For people with late latent or tertiary syphilis, three doses of penicillin administered at weekly intervals are generally required for a cure. If patients are allergic to penicillin, other antibiotics may be used, but they tend to less effective and require retesting for syphilis after treatment to ensure that a cure has occurred. Antibiotics kill the syphilis bacteria and prevent further damage, but they do not repair any damage that is already done
Viral STIs
Two of the very common viral STIs, genital herpes and HPV infection are described below. Another important viral STI is HIV infection, which causes the disease known as AIDS. HIV infection is covered in the concept HIV and AIDS.
Genital Herpes
Genital herpes is a viral STI caused by a herpes simplex virus. The cause of genital herpes is most often herpes simplex virus type 2 (HSV-2). Increasingly, however, genital herpes is caused by herpes simplex virus type 1 (HSV-1). HSV-1 more commonly causes herpes infections of the mouth, resulting in “cold sores,” and is typically acquired during childhood. Genital herpes infections are very common in the United States, with about three-quarters of a million new cases occurring each year. HSV-2 infection is more common in women than men due to sex differences in transmission of the virus: genital herpes is more easily transmitted from males to females than from females to males.
Generally, a person can get an HSV-2 infection only through sexual contact with someone who has an infection with the virus. Transmission occurs through contact with lesions, mucosal surfaces, or genital or oral secretions. Transmission can occur even when the infected person does not have visible sores because the virus can be shed from body surfaces that appear normal. Genital herpes can also be passed from mother to child during pregnancy, childbirth, or shortly after birth.
Most people who are infected with genital herpes are unaware of their infection. In fact, in the United States, almost 90 percent of 14-49-year-olds infected with HSV-2 have never received a clinical diagnosis. That’s because most genital herpes infections are asymptomatic or have very mild symptoms that go unnoticed. When noticeable symptoms do occur, they typically appear as one or more blisters on or around the genitals (Figure \(10\)), rectum, or mouth. When the blisters break, they leave painful ulcers that take up to a month to heal. Sometimes the initial outbreak is accompanied by systemic symptoms such as fever and swollen lymph nodes. Recurrent outbreaks of blisters are common, especially during the first year of infection. Although the virus is likely to stay in the body indefinitely, the number, duration, and severity of outbreaks tend to decrease over time. Rare but serious complications of HSV-1 or HSV-2 infections may include blindness, encephalitis, or meningitis.
Herpes infections can be diagnosed with blood tests that detect antibodies to the virus. There is no cure for herpes infections, but antiviral medications can prevent or shorten outbreaks and lessen the risk of transmission during the time the patient is taking the medicine. Several clinical trials have tested vaccines against genital herpes, but none has yet been found to be effective. Pregnant women with genital herpes are usually prescribed antiviral medication during the last month of pregnancy to reduce the risk of an outbreak around the time of birth when the transmission is most likely. If an outbreak does occur, a cesarean delivery is recommended to prevent HSV transmission to the infant.
HPV Infection
The most common sexually transmitted infection in the United States is infection with the human papillomavirus (HPV). Almost 80 million Americans are estimated to be infected with HPV, and about 14 million people are thought to become infected each year. HPV is so common that nearly all sexually active people are eventually infected by it. There are more than 40 different types of HPV, but only some of them are likely to cause health problems.
HPV is acquired through vaginal, anal, or oral sex with someone who is infected with the virus. The infected person can transmit the virus even without showing any signs or symptoms of infection. In most cases, the immune system naturally clears HPV from the body before it causes any symptoms — generally within a couple of years of the original infection. However, in some cases, HPV is not naturally cleared. These cases may develop health problems years after the infection was first acquired. Some types of HPV can cause genital warts, and certain other types can cause cancer.
• Genital warts appear as one or more bumps on the skin or mucosa in the genital area. Warts may be small, like the genital wart in the photo below, or they may be much larger. Warts may be raised or flat; sometimes they are shaped like a cauliflower.
• Primary cancer caused by HPV is cervical cancer. However, HPV can also cause other cancers, including cancer of the vulva, vagina, penis, anus, or throat and mouth. It generally takes years or even decades for cancer to develop after a person is infected with HPV.
There is no cure for HPV infection. Once a person is infected, if the immune system does not clear the virus, it may remain in the body for life. There is also no general screening test to determine whether someone is infected with HPV. However, there are specific HPV tests that can identify the most common types of HPV that cause cervical cancer. These tests are recommended for women aged 30 and older. Women aged 21 to 65 should also receive routine Pap tests every 3 years to screen for cervical cancer, which has a high cure rate if it is discovered early.
Genital warts can usually be diagnosed visually by a healthcare provider. There is typically no need to treat warts unless they are unsightly or bothersome. Treatment consists of topical medications that cause warts to slowly resorb and disappear. However, treatment of genital warts does not get rid of the patient’s HPV infection, and warts may return. If genital warts are not treated, they may or may not eventually go away on their own.
Unlike the other STIs described above, infection with the most common and dangerous types of HPV can be prevented with a vaccine. The HPV vaccine is recommended for all girls and boys between the ages of 11 and 12 years. Young people who do not get vaccinated at those ages can still get the vaccine through age 21 (for males) or 26 (for females). The main purpose of the vaccine is to protect people from developing HPV-related cancers later in life.
Feature: My Human Body
Proper use of condoms can significantly reduce the risk of transmission of STIs. For infection protection, the best condoms to use are latex condoms, because some pathogens are able to pass through the tiny pores in natural skin condoms. When using male condoms, follow these guidelines for the most effective prevention of STI transmission:
• Always use a new condom; never reuse a condom, even if it does not contain ejaculate.
• Avoid putting on a condom tightly at the end. Leave about 1.5 cm (3/4 in.) of empty space at the tip of the condom to allow room for ejaculate. Otherwise, the force of ejaculation may cause the condom to fail.
• Avoid inverting or spilling a condom once worn, whether or not it contains ejaculate.
• If the user attempts to unroll the condom over the penis but realizes it is on the wrong side, the condom should be discarded and a new one used.
• Be careful when handling a condom, especially with long fingernails.
• Avoid using oil-based lubricants with latex condoms because the oil can weaken the latex.
• Avoid using flavored condoms, especially for vaginal intercourse, because the sugar in the flavoring may encourage yeast infections.
Review
1. Define the sexually transmitted infection.
2. Describe how sexually transmitted pathogens are spread.
3. Give examples of different types of sexually transmitted pathogens.
4. Describe common symptoms of STIs.
5. Contrast treatments for bacterial and viral STIs.
6. Why is it important to treat STIs even if they do not cause symptoms?
7. Discuss the role of vaccines in preventing STIs.
8. What are ways to prevent the transmission of STIs for which there are no vaccines?
9. Name two STIs that often go unnoticed because they commonly do not cause symptoms, or cause only very mild symptoms.
10. True or False. Proper use of condoms can completely prevent the transmission of STIs.
11. True or False. Antibiotics can be used to treat genital herpes.
12. What is the most common STI in the United States?
13. Describe the relationship between cancer and an STI.
14. What are two reasons why the number of diagnosed cases of some STIs is higher in females than in males?
Attributions
1. Columbus and the Indian Maiden by Architect of the Capitol public domain via Wikimedia Commons
2. Poster from the Library of Congress by US Government, public domain via Wikimedia Commons
3. Pthirus pubis by CDC, public domain via Wikimedia Commons
4. Chlamydia in the US by Delphi234, licensed CC0 via Wikimedia Commons
5. Gonorrhea in the US by Delphi234, licensed CC0 via Wikimedia Commons
6. Gonococcal ophthalmia neonatorum by CDC, public domain via Wikimedia Commons
7. Primary stage syphilis sore by CDC, public domain via Wikimedia Commons
8. Manifestations of secondary syphilis by CDC, public domain via Wikimedia Commons
9. Charcot joint tertiary syphilis by CDC/ Robert Sumpter, public domain via Wikimedia Commons
10. Genital Herpes by Mikael Häggström, licensed CC0 via Wikimedia Commons
11. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/21%3A_Disease/21.4%3A_Sexually_Transmitted_Infections.txt |
Myths and Realities
There are many misconceptions about HIV and AIDS. Knowing the realities is important to help prevent the spread of HIV and reduce the stigma associated with it.
Myth: HIV can be transmitted through nonsexual contacts such as kissing, sharing a glass, spitting, sitting on a public toilet seat, and coughing or sneezing.
Reality: HIV has not been shown to be transmissible through such contacts. People can safely interact and have casual contact with HIV-infected individuals without fear of acquiring the virus.
Myth: HIV can be acquired through an act of anal intercourse between two men, regardless of the HIV status of the sexual partners. In other words, homosexual acts in and of themselves increase the risk of acquiring HIV.
Reality: HIV infections can be acquired through sexual contact only with people who are infected with the virus. Uninfected individuals cannot transmit the virus. Homosexual contact is not necessary for the virus to be spread. In fact, most transmissions of HIV occur through heterosexual contacts.
Myth: HIV/AIDS was created by scientists, either accidentally or deliberately.
Reality: This conspiracy theory was created and spread by Operation INFEKTION, a Soviet KGB disinformation campaign. It purposely spread the misinformation that the United States invented HIV/AIDS as part of a biological weapons research project. The Soviet Union spread the now-discredited theory in order to undermine the United States’ credibility, foster anti-American attitudes, isolate America abroad, and create tensions between host countries and the U.S. over the presence of American military bases (which were often portrayed as the cause of AIDS outbreaks in local populations).
Introduction to HIV and AIDS
AIDS stands for acquired immunodeficiency syndrome. It is a disease caused by infection with the human immunodeficiency virus or HIV. HIV is a sexually transmitted virus that infects and destroys helper T cells of the human immune system (see the concept Disorders of the Immune System to learn more about how HIV infects immune system cells). AIDS eventually develops in most people with untreated HIV infections, usually several years after the initial infection with the virus. AIDS is diagnosed when the immune system has been weakened to the point that it can no longer fight off diseases that do not occur in healthy individuals.
Figure \(1\) shows the global distribution of deaths due to HIV and AIDS in 2017. The rate ranges from 0 to 250 deaths per 100,000 individuals. Most of those with HIV infections live in sub-Saharan Africa. This is partly due to the fact that this is where HIV first originated as a human disease when the virus jumped from nonhuman primate populations to human populations. Almost one million (954,000) people died from HIV/AIDS in 2017. HIV infections and AIDS are considered to be pandemic — a disease outbreak that is present in multiple populations around the world.
Transmission of HIV
HIV is considered to be a sexually transmitted infection (STI) because that is its most common mode of transmission. However, unlike some other pathogens that cause STIs, HIV is also commonly transmitted through nonsexual contact with HIV-contaminated blood and from HIV-infected mothers to their children.
Sexual Transmission
The majority of all HIV transmissions worldwide occur through sexual contact. Of these cases, most are the result of heterosexual contact. However, the pattern of transmission varies geographically. In the United States, HIV is transmitted more often in men who have sex with men. The risk of transmission from anal intercourse is especially high, whereas the risk of transmission from oral sex is relatively low.
The risk of HIV transmission increases when people are already infected with other sexually transmitted infections, and especially when they have open sores on their genitals. In fact, the presence of genital sores increases the risk of transmission by about fivefold. The risk of transmission is also greater during the early months of infection when infected people usually have the greatest viral load. Viral load refers to the amount of virus in a sample of an infected individual’s blood.
Transmission Through Contaminated Blood
The second most frequent mode of HIV transmission is via contaminated blood and blood products. Blood-borne transmission can occur through needle sharing during intravenous drug use, needle-stick injury in health professionals, transfusion of contaminated blood or blood products, or medical injections with unsterilized equipment. Theoretically, giving or receiving tattoos or piercings can also transmit HIV, but no confirmed cases have been documented. It is not possible for mosquitoes or other blood-sucking insects to transmit HIV.
In 2009 in the United States, intravenous drug users made up 12 percent of all new cases of HIV, and in some areas, more than 80 percent of people who injected drugs were infected with HIV. In rich nations, the risk of acquiring HIV from a blood transfusion is now virtually nil because of careful screening of blood donors and blood products. In poor nations, on the other hand, screening is less rigorous; therefore, rates of transmission through contaminated blood are higher. Unsafe medical injections and invasive medical procedures are also a significant mode of transmission in poor nations, particularly in sub-Saharan Africa. While it is possible to acquire HIV from the infected organ or tissue transplantation, this is rare because of screening.
Mother-to-Child Transmission
The third most common way HIV is transmitted worldwide is from an untreated mother to her child during pregnancy, childbirth, or breastfeeding. If the mother is infected with HIV, there is about a 15% chance that the virus will be transmitted to her infant through her breast milk. The transmission of pathogens from one generation to the next in these ways is called vertical transmission. This mode of transmission accounts for most cases of HIV infection in children.
Stages of HIV Infection
HIV is a type of virus called a retrovirus. Retroviruses are single-stranded RNA viruses that live as parasites inside host cells. HIV primarily infects helper T cells (CD4+ T cells) as well as some other cells of the human immune system. It spreads from helper T cells to helper T cells and causes illness by killing off the helper T cells.
Acute HIV Infection
After HIV enters the human body, there is a period of rapid viral replication, causing a high viral load in the person’s blood and a drop in the number of circulating helper T cells. This stage of infection is called acute HIV infection. It produces an immune system response, in which the number of killer T cells increases. The killer T cells start killing HIV-infected cells, and antibodies to HIV are also produced. As a consequence, the viral load starts to decline, and the number of helper T cells recovers. However, the virus is not eliminated and remains in the body.
Acute HIV infection may cause no noticeable symptoms, or it may cause a brief period of flu-like illness. The main symptoms of acute HIV infection are illustrated in Figure \(3\). Even when symptoms are present, they are not often recognized as signs of HIV infection due to their nonspecific nature.
Chronic HIV Infection
After any acute HIV symptoms subside, the infected individual enters the stage of chronic HIV infection. Typically, this begins with a prolonged period without symptoms. Without treatment, this stage may last from 3 to 20 years. However, the infection keeps progressing, and HIV keeps infecting and destroying helper T cells. Toward the end of the chronic stage, the infected person starts to experience symptoms again, such as fever, weight loss, and swollen lymph nodes.
AIDS
Most infected individuals will eventually progress to AIDS if their HIV infection is not treated. AIDS is diagnosed when the helper T cell count falls below 200 helper T cells per microliter of blood or when the infected individual starts to develop opportunistic diseases. These are diseases that rarely occur except in people with a compromised immune system. Such diseases are typically the immediate cause of death in people with AIDS.
Common opportunistic diseases in people with AIDS include pneumocystis pneumonia (fungal pneumonia) and esophageal candidiasis (yeast infection). They also include viral-induced cancers such as Kaposi’s sarcoma (Figure \(4\)) and Burkitt’s lymphoma. In addition to opportunistic diseases, many AIDS patients develop HIV wasting syndrome, in which they lose weight and muscle mass and experience extreme fatigue and weakness. People with AIDS also frequently experience systemic symptoms such as prolonged fevers, night sweats, swollen lymph nodes, and diarrhea.
Diagnosis and Treatment of HIV/AIDS
HIV/AIDS is diagnosed on the basis of blood tests and then staged based on the presence of signs and symptoms. In the United States, HIV screening is recommended for people aged 15 to 65 years, and especially for pregnant women. Testing for HIV is also recommended for people at high risk of HIV infection, which includes anyone diagnosed with another STI. Blood tests diagnose HIV infection by identifying antibodies to the virus. However, it may take up to 3 months after the initial infection for antibodies to show up in the blood. Antibody tests are also not accurate in children younger than 18 months because of the presence of maternal antibodies in their blood. Tests that identify viral RNA can detect the virus before antibodies develop, but these tests are not available in much of the world.
There is currently no cure for HIV infection or AIDS. However, the development of new antiretroviral drugs to treat HIV infections has changed HIV infections from a fatal to chronic disease. At present, treatment consists of a “cocktail” of at least three antiretroviral drugs that slow the progression of the disease by keeping viral loads relatively low. The medications should be started as soon as the diagnosis is made and continued without breaks. The sooner treatment is begun, the more effective it is likely to be. The goals of treatment are to decrease the risk of progression to AIDS and the risk of death. Taking the drugs consistently allows HIV-infected people to live longer, healthier lives. Another benefit of treatment is a decreased risk of transmission of the virus. Any opportunistic infections that occur in people with AIDS are also treated, generally with appropriate medications. For example, pneumocystis pneumonia is treated with anti-fungal drugs.
Prevention of HIV Transmission
There is currently no approved vaccine for HIV or AIDS, although vaccine trials are ongoing. Instead, prevention of HIV transmission depends on adopting safe behaviors and/or the administration of antiretroviral drugs.
Preventing Sexual Transmission of HIV
HIV transmission through sexual contact can be greatly reduced by the consistent use of condoms. When condoms are always used by couples in which one person is infected, the rate of transmission is less than 1 percent per year. Increasing condom use is typically an important public health policy in countries with very high rates of HIV/AIDS. For example, in South Africa, condom use is encouraged by dispensing condoms free of charge in public restrooms (Figure \(5\)). It is important to note that programs advocating sexual abstinence do not appear to diminish HIV risk. On the other hand, comprehensive sex education programs decrease risk by reducing high-risk behaviors and increasing condom use.
Preventing Nonsexual Transmission of HIV
HIV transmission through intravenous drug use can be reduced through harm-reduction strategies such as needle exchange programs or the substitution of prescription drugs for illegal drugs. In cases of unanticipated exposure to infected blood, such as a needle-stick injury or sexual assault by an HIV-positive perpetrator, the risk of HIV infection can be substantially reduced by the administration of antiretroviral medications within two or three days of the incident.
Rates of mother-to-child transmission can be reduced to about 1 percent by giving antiretroviral medications to the mother during pregnancy and to the infant after birth. Delivering infants by cesarean instead of vaginally also reduces the risk of transmission during childbirth. Substituting bottle feeding for breastfeeding, if feasible, eliminates the risk of HIV transmission through breast milk.
Review
1. What is HIV?
2. How is HIV transmitted?
3. How is HIV infection diagnosed?
4. What is AIDS?
5. How is AIDS diagnosed?
6. Describe the distribution of HIV infections in human populations.
7. Explain how HIV infection changed from a fatal to a chronic disease.
8. Without a vaccine, how can the risk of HIV transmission be reduced?
9. Explain why it is important to treat people exposed to or infected with HIV as early as possible with antiretroviral medications.
10. If your new sexual partner shows you recent test results indicating they are HIV negative, does that mean you cannot get HIV from them if you have unprotected sex? Why or why not?
11. Explain how HIV can cause cancer.
12. The immediate cause of death in people with AIDS is/are typically:
A. opportunistic diseases
B. weight loss
C. autoimmune disorders
D. Kaposi’s sarcoma
13. What is one risk factor that increases the rate of transmission of HIV?
14. True or False. Treatment with antifungal medications reduces the viral load in patients with HIV.
15. True or False. Different types of sexual intercourse (i.e. oral, anal, vaginal) have different risks of transmission of HIV.
Explore More
Check out this video to learn more about whether or not researchers believe AIDs will ever truly be cured:
Attributions
1. HIV data by Our World in Data, CC BY 4.0
2. AIDS poster, public domain via Wikimedia Commons
3. Symptoms of acute HIV infection by Mikael Häggström public domain via Wikimedia Commons
4. Kaposi's Sarcoma Lesions by OpenStax College, CC BY 3.0 via Wikimedia Commons
5. AIDS Prevention - Condom dispensers by Jorge Láscar from Australia, CC BY 2.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/21%3A_Disease/21.5%3A_HIV_and_AIDS.txt |
Apple Shape: Good for Apples, Bad for People
The person pictured in Figure \(1\) is obese. He has a lot of extra fat in his abdomen, giving him an apple shape. This type of fat distribution is called abdominal, or central, obesity. It is one of several indicators that are used to diagnose a condition called metabolic syndrome. Other indicators of metabolic syndrome include high blood pressure, high blood levels of glucose and triglycerides, and low blood levels of HDL (“good cholesterol”). Metabolic syndrome, in turn, is a major risk factor for many noninfectious disease killers.
Introduction to Noninfectious Diseases
Noninfectious diseases include all diseases that are not caused by pathogens. Instead, noninfectious diseases are generally caused by genetic or environmental factors other than pathogens, such as toxic environmental exposures or unhealthy lifestyle choices. Most noninfectious diseases have a complex, multifactorial set of causes, often including a mix of genetic and environmental variables. Examples of noninfectious diseases include cystic fibrosis, most cancers, cardiovascular diseases such as coronary artery disease, and diabetes mellitus.
While many noninfectious diseases are long-lasting, or chronic, diseases, their chronicity is not a defining factor because some infectious diseases, such as AIDS, are also chronic diseases. In addition, some noninfectious diseases are short-term, or acute, diseases because they generally result in rapid death. Some types of cancer and heart disease are examples of noninfectious diseases that may be acute for this reason.
Noninfectious diseases have also been called diseases of affluence or lifestyle diseases because they are often caused by unhealthy lifestyle choices and first became prevalent in the richer nations of the world, while infectious diseases remained at relatively high levels in the poorer nations. Globally, noninfectious diseases are the leading causes of death. Although infectious diseases still cause more deaths in low-income countries, rates of death from noninfectious diseases are expected to rise in these countries as well. By 2030, noninfectious diseases are expected to kill more than 50 million people a year worldwide, and most of these deaths will occur not in high-income nations but in low- and middle-income countries. National economies around the world will also suffer significant losses because of the growing noninfectious disease burden, as noninfectious diseases cause a great deal of worker disability and premature death.
Risk Factors for Noninfectious Diseases
Many of the same risk factors increase a person’s chances of developing a diversity of noninfectious diseases. These common risk factors include age, gender, genes, and exposure to environmental dangers such as radon. Behaviors such as smoking, unhealthy diet, and physical inactivity are also common environmental risk factors for many noninfectious diseases. These behaviors all contribute to obesity, high blood pressure, unbalanced blood lipid levels, and high blood glucose levels — in other words, to metabolic syndrome. This syndrome, in turn, is a major risk factor for cardiovascular diseases and type 2 diabetes. One of the single most important behavioral factors contributing to metabolic syndrome is the consumption of large amounts of sweetened beverages such as soft drinks (Figure \(2\)).
Most behavioral risk factors for noninfectious diseases can be avoided. That’s why many noninfectious diseases are considered preventable. Their risk can be reduced by modifying behaviors and making healthier lifestyle choices. In fact, an estimated 80 percent of cases of cardiovascular diseases and type 2 diabetes and 40 percent of cancer cases could be avoided through lifestyle changes. Interventions that target common behavioral risk factors can make a big impact on a nation’s noninfectious disease burden. For example, laws taxing tobacco products and curbing smoking in public places have been shown to reduce rates of smoking, which is the main risk factor for lung cancer.
Other risk factors for noninfectious diseases — including age, gender, and genes — cannot be avoided or modified. In terms of age, most noninfectious diseases become more common as people get older. Some noninfectious diseases, such as certain types of cancer, are more common, or occur only, in one sex or the other. Genes are wholly responsible for some inherited noninfectious diseases, such as cystic fibrosis. Genes may also affect individual susceptibility to many other noninfectious diseases that are caused mainly by environmental factors. For example, genes may influence how likely a person is to develop metabolic syndrome for a given lifestyle, and ultimately how likely the person is to develop cardiovascular disease and type 2 diabetes. It is important to take these unavoidable risk factors into account in diagnosing and screening for noninfectious diseases and establishing individual treatment and prevention guidelines.
Examples of Noninfectious Diseases
Several examples of noninfectious diseases are described below. The diseases represent a diversity of types of diseases, ranging from purely genetic to primarily environmental diseases.
Cystic Fibrosis
Cystic fibrosis is an example of a genetic noninfectious disease. It is caused by an inherited mutation in a gene called CFTR. Mutant versions of the gene produce a faulty protein that normally helps to move sodium chloride into and out of cells. The impaired salt transfer causes mucus to be abnormally thick and sticky. Figure \(3\) helps explain the diversity of negative health impacts that may occur in people with cystic fibrosis. The thick mucus accumulates in the organs of the airways. This may lead to resurrect respiratory and sinus infections. This may also lead to malabsorption. The mucus blocks passages in mucus-secreting organs such as the lungs, pancreas, reproductive system, and intestine.
There is no known cure for cystic fibrosis, but recent advances in the treatment of cystic fibrosis allow people with the disease to live healthier and longer lives. A few generations ago, a newborn with cystic fibrosis was unlikely to live beyond the first year of life. Today, people with cystic fibrosis are likely to live to middle adulthood. Lung infections and other lung problems cause the greatest disability and premature death in people with cystic fibrosis. Therefore, treatment usually includes the proactive use of antibiotics and other drugs to fight off infections, along with pulmonary rehabilitation to maximize lung function. Even with treatment, however, lung damage may eventually progress to the point where a lung transplant is needed.
The mutant CFTR gene for cystic fibrosis is a recessive gene located on an autosome (chromosome 7). As with any autosomal recessive trait, an individual must have two copies of the mutant gene to develop the disease. An individual with just one copy of the normal CFTR gene can produce enough of the functioning protein to secrete normal mucus and avoid the signs and symptoms of cystic fibrosis. Such a person is called a carrier of cystic fibrosis. Carriers can pass the mutant gene to their offspring. The inheritance pattern of an autosomal recessive disease such as cystic fibrosis is shown in the pedigree diagram in Figure \(4\).
Without medical intervention, cystic fibrosis is fatal in infancy, yet the mutant gene that causes it has been maintained at relatively high levels in some human populations for tens of thousands of years. The mutant gene is most common in people of Northern European ancestry. In these populations, about 1 in 25 people is a carrier, and about 1 in 3,000 newborns have cystic fibrosis. The most common explanation for the persistence of the cystic fibrosis mutation is some type of heterozygote advantage in carriers of the mutant gene. For example, it has been hypothesized that carriers of the cystic fibrosis mutation may have greater-than-normal resistance to certain infectious diseases, such as cholera, typhoid fever, or tuberculosis.
Cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. Cancer is one of the top ten causes of death in high-income countries. Most cancers are diagnosed in people over the age of 65; only a few types of cancer occur in children. It is likely that if one were to live long enough and avoid other common causes of death, such as cardiovascular diseases and diabetes, sooner or later a person would succumb to cancer.
About 90 percent of cancers are noninfectious diseases. (About 10 percent of cancers are infectious diseases caused by pathogens, such as the human papillomavirus, which causes cervical cancer.) Rather than pathogens, noninfectious cancers are caused by some combination of genetic and environmental factors. About 10 percent of cancers are caused largely by genes or have a very strong genetic influence. For example, inheriting genes called BRCA1 and BRCA2 increase the risk of women developing breast or ovarian cancer by as much as 75 percent.
Most cancers are caused largely by environmental factors, including human behaviors. For example, tobacco smoke contains 50 known carcinogens or cancer-causing agents, and smoking causes 90 percent of lung cancers. You can see the connection between smoking and lung cancer in Figure \(5\). Like most such environmental factors and cancer, it generally takes many years of exposure to tobacco smoke before lung cancer develops. Lung cancer is not the only kind of cancer caused by tobacco use. Smoking also increases the risk of cancer of the larynx, head, neck, stomach, bladder, kidney, esophagus, and pancreas.
Other behaviors that play major roles in causing cancer include poor diet and physical inactivity, both of which contribute to high rates of obesity. These factors are responsible for at least a third of cancer deaths. Additional environmental causes of cancer include the radioactive gas radon from underground rocks and ultraviolet radiation from the sun. Radon increases lung cancer risk, and UV radiation is the primary cause of skin cancer.
Many treatment options exist for cancer. The primary treatments include surgery, chemotherapy, and radiation therapy. Which treatments are used depends on factors such as the type and location of cancer and whether cancer has spread. Treatments may or may not be curative. You can learn more about cancer by reading the concept of Cancer.
Cardiovascular Disease
Cardiovascular disease refers to a class of diseases that involve the heart or blood vessels. The diseases include coronary artery disease, stroke, and peripheral artery disease. (You can read more about specific types of cardiovascular disease in the concept of Cardiovascular Disease.) Cardiovascular disease is the leading cause of death worldwide, with about 30 percent of deaths attributable mainly to cardiovascular disease. By the year 2030, an estimated 23 million people a year will die from cardiovascular disease.
Two major precursors of cardiovascular disease are hypertension and atherosclerosis.
• Hypertension is defined as blood pressure that is persistently elevated. Controlling hypertension either through medications or lifestyle changes is important for reducing the risk of all types of cardiovascular diseases, but especially stroke.
• Atherosclerosis is a condition in which artery walls thicken and stiffen as a result of the buildup of fatty plaques inside the arteries (Figure \(6\)). The buildup of plaques in arteries actually starts in childhood and continues in most people throughout life. The progression of atherosclerosis can be controlled through lifestyle approaches, including eating a healthy diet, getting regular exercise, and avoiding tobacco smoke. Medications to lower blood triglycerides and raise HDL levels may also help.
Obesity and diabetes are additional major risk factors for cardiovascular disease. Obesity is associated with other risk factors for cardiovascular disease, including hypertension and high blood triglycerides, but it may also have an independent effect on cardiovascular disease risk. People with diabetes are two to four times more likely than nondiabetics to die of cardiovascular disease.
Most cases of cardiovascular disease could be prevented by modifying risk factors. Some risk factors, such as hypertension and high blood triglycerides, can be controlled with medications. Other risk factors, such as obesity and physical inactivity, can be controlled by adopting healthy behaviors (such behaviors may also help control hypertension and high blood lipids even without medications). Although modifiable environmental factors such as these are the main risk factors for cardiovascular disease, genes also play an important role. A person’s risk of developing cardiovascular disease is three times greater than the average if the person’s parents had cardiovascular disease. However, age is by far the most important risk factor for diseases of the heart or arteries. There is a tripling of cardiovascular disease risk with each passing decade of life.
Type 2 Diabetes
Diabetes is diagnosed in people who have abnormally high levels of blood glucose over prolonged periods of time. Symptoms of untreated high blood glucose include frequent urination, increased thirst, and increased hunger. As of 2016, an estimated 422 million people worldwide had diabetes, with the rates being somewhat higher in developed countries.
There are several types of diabetes, but type 2 diabetes is by far the most common. It accounts for about 90 percent of all cases of diabetes. Type 2 diabetes generally develops due to insulin resistance, rather than lack of insulin, which occurs in type 1 diabetes. As illustrated in Figure \(7\), insulin resistance occurs when cells of the body become increasingly unresponsive to insulin due to malfunctioning insulin-receptor sites. Cells can no longer take up enough glucose from the blood to maintain glucose homeostasis. In many cases of type 2 diabetes, the problem of insulin resistance is exacerbated by a secondary reduction in insulin secretion.
Type 2 diabetes typically starts after the age of 40. It is most likely to be diagnosed in people who are obese and have other indicators of metabolic syndrome, which is sometimes referred to as pre-diabetes for this reason. Because of the dramatic increase in recent decades in obesity in younger people, the age at which type 2 diabetes is diagnosed has fallen. Even children are now being diagnosed with type 2 diabetes. Today, about 30 million Americans have type 2 diabetes, and another 90 million Americans have pre-diabetes.
Unless diabetes is carefully monitored and controlled, high blood sugar levels can eventually lead to heart attacks, strokes, blindness, kidney failure, and many other serious health problems. These complications of diabetes are primarily due to damage to small blood vessels caused by inadequately controlled blood glucose levels. All else being equal, the risk of death in adults with diabetes is 50 percent greater than it is in adults without diabetes.
Controlling type 2 diabetes usually requires frequent blood glucose testing, watching what and when you eat and taking oral medications or even insulin injections. Changing your lifestyle may stop the progression of type 2 diabetes or even reverse it. By adopting healthier behaviors, you may be able to keep your blood glucose level within the normal range without medications or insulin.
Review
1. Define noninfectious disease.
2. In general, what causes noninfectious diseases?
3. Identify risk factors for noninfectious diseases.
4. Why are many noninfectious diseases considered preventable diseases?
5. What noninfectious disease risk factors cannot be avoided? Why are these risk factors still important to identify?
6. What is cystic fibrosis?
7. How does the cystic fibrosis mutation cause disease?
8. Define cancer.
9. What causes most cancers?
10. List three types of treatment for cancer.
11. What is a cardiovascular disease? What is its significance as a cause of death worldwide?
12. Identify major precursors and risk factors for cardiovascular disease.
13. What causes type 2 diabetes?
14. Identify risk factors for type 2 diabetes.
15. List possible health problems that can result from poorly controlled diabetes.
16. Could metabolic syndrome ultimately result in kidney failure? Explain your answer.
Attributions
1. Obesity by FatM1ke, Public Domain via Wikimedia Commons
2. Double big gulp by Russell Bernice from New York City, U.S., CC BY 2.0 via Wikimedia Commons
3. Cystic Fibrosis by National Heart Lung and Blood Institute (NIH), Public Domain via Wikimedia Commons
4. Autosomal Recessive Inheritance by OpenStax College CC BY 3.0
5. Smoking and Lung Cancer Correlation by Sakurambo, Public Domain via Wikimedia Commons
6. Coronary Artery Disease by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
7. Insulin Resistance by Manu5, CC BY 4.0 via Wikimedia.org
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3. | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/21%3A_Disease/21.6%3A_Noninfectious_Diseases.txt |
Feel the Burn
During his many months on the campaign trail in 2015 and 2016, U.S. Democratic presidential candidate Bernie Sanders was feeling the burn — sunburn, that is. There’s no doubt that candidate Sanders was exposed to a lot of UV light during all those outdoor rallies in states across the nation. Exposure to UV light is the single greatest risk factor for skin cancer, so it’s perhaps not surprising that Sanders was diagnosed with a small skin cancer on his cheek by December of 2016 (although most of his cumulative UV exposure probably occurred earlier in life). In Sanders's case, the cheek cancer was a basal cell carcinoma. This is the most common type of skin cancer. Fortunately, basal cell carcinoma is also a slowly growing and highly curable cancer. Sanders had his basal cell carcinoma completely removed in an hour-long outpatient procedure, after which he was able to return immediately to work. Unfortunately, most other types of cancer are faster growing and more difficult to cure than basal cell carcinoma, including a relatively rare but potentially deadly form of skin cancer called melanoma.
Defining Cancer
Cancer is actually a group of more than 100 diseases, all of which involve abnormal cell growth with the potential to invade or spread to other parts of the body. In general terms, cancer occurs when the cell cycle is no longer regulated due to DNA damage. The number of potential underlying causes of this DNA damage is great, so there are many different risk factors for cancer. Any cells that become cancerous divide more quickly than normal cells. They may form a mass of abnormal cells called a tumor. The rapidly dividing cells take up nutrients and space, damaging the normal cells around them. If the cancer cells spread to other parts of the body, they invade and damage other tissues and organs. They may eventually lead to death.
By far, the most common of the 100-plus types of human cancer is basal cell carcinoma, the type of skin cancer Bernie Sanders had removed in 2016. Basal cell carcinoma makes up 40 percent of all new cancers each year in the United States. Other common types of cancer include lung, colorectal, prostate (in males), and breast (in females) cancers. These cancers are not as common as skin cancer, but they cause the majority of cancer deaths.
How Cancer Occurs
Cancer occurs when a normal cell is transformed into a cancer cell. This happens when the genes that regulate cell growth and differentiation are altered through mutations. The genes involved include proto-oncogenes and tumor-suppressor genes. Proto-oncogenes are genes that normally promote the growth and division of normal cells. Tumor-suppressor genes are genes that normally inhibit the division and survival of abnormal cells. Genetic alterations may result in the formation of cancer-causing oncogenes, over-expression of normal proto-oncogenes, or under-expression or disabling of tumor-suppressor genes.
As shown in Figure \(2\), changes in multiple genes are typically required to transform a normal cell into a cancer cell. The entire process is similar to a chain reaction. Initial DNA errors are compounded by additional errors, and each error progressively allows the cell to escape more controls on cell growth and cell division.
How Cancer Spreads
Once a normal cell transforms into a cancer cell and starts dividing out of control, cancer cells can spread from the original site (called the primary tumor) to other tissues. This can occur in three different ways. One way is local spread, in which aggressively dividing cancer cells directly invade nearby tissues. Another way involves the lymphatic system. Cancer cells can spread to regional lymph nodes through lymph vessels that pass by the primary tumor.
The third way cancer cells can spread is through the blood to distant sites. This is called metastasis, and the new cancers that form are called metastases. Although the blood can carry cancer cells to tissues everywhere in the body, cancer cells generally grow only in certain sites (Figure \(3\)). Different types of cancers tend to metastasize to particular organs. The most common places for metastases to occur are the brain, lungs, bones, and liver. Almost all cancers can metastasize, especially during the late stages of the disease. Cancer that has metastasized generally has the worst prognosis and is associated with most cancer deaths.
Risk Factors for Cancer
The direct cause of any cancer is DNA damage in genes that control the cell cycle. In this sense, all cancers are genetic diseases. The DNA damage can be inherited from parents or result spontaneously from environmental exposures to cancer-causing agents, or carcinogens. Having risk factors for cancer increases your chances of getting cancer either way.
Inherited Risk Factors
Certain mutant genes greatly increase the risk of particular cancers, such as the BRCA1 and BRCA2 genes, which increase the risk for breast and ovarian cancer in women to 75 percent. However, environmental factors also play a role even in this example because the genes do not cause cancer in every woman who inherits them. Genes with such a great effect on cancer risk cause fewer than 10 percent of human cancers.
Environmental Risk Factors
Environmental risk factors are the major risk factors for at least 90 percent of human cancers. Environmental risk factors include certain pathogens, such as hepatitis viruses that increase the risk of liver cancer. Other, nonpathogenic environmental factors that increase cancer risk include radon, which increases the risk of lung cancer, especially in smokers; air pollution, which increases the risk of lung cancer and bladder cancer; and UV light, which increases the risk of both carcinoma and melanoma skin cancers.
Avoidable lifestyle choices — especially smoking tobacco, eating an unhealthy diet, and not exercising — are some of the most important but preventable environmental risk factors for cancer. In fact, the majority of cancer deaths could be prevented by making healthy lifestyle choices. Not using tobacco would prevent an estimated 25 percent of cancer deaths. Eating right, getting adequate exercise, and avoiding obesity would prevent another 35 percent of cancer deaths.
Diagnosing Cancer
Early diagnosis and treatment are the keys to curing cancer, although not all cancers can be cured. Many cancers are first detected through routine screening of asymptomatic people. Many other cancers are noticed by early warning signs of cancer. A definitive diagnosis of cancer requires a biopsy before treatment can begin.
Cancer Screening
Screening for cancer has the aim of detecting common cancers in people who do not yet have any noticeable symptoms. Examples of cancers for which screening is usually recommended for high-risk groups include colon cancer (older people), breast cancer (older females), prostate cancer (older men), and skin cancer (e.g., light-skinned people, people with excessive UV light exposure). Screening of asymptomatic people for cancer may involve physical examinations (e.g., visual inspection of the skin for skin cancer), medical imaging (mammogram for breast cancer, pictured in Figure \(4\)), blood test (PSA test for prostate cancer), other tissue tests (Pap test for cervical cancer), or endoscopy (colonoscopy for colon cancer).
Routine cancer screening is somewhat controversial. Screening has both risks and benefits, and not everyone agrees that the benefits always outweigh the risks. Ideally, potential benefits are early detection, treatment, and cure of the disease in more people. However, if no treatment is available or treatment is unlikely to result in a cure, there is obviously less benefit to screening. Potential risks of cancer screening include false positives, which may lead to unnecessary anxiety and more invasive testing. Other potential risks include harm from excessive radiation and excessive cost or pain from screening procedures. Given the controversial nature of screening, it is not surprising that sometimes-conflicting screening recommendations are made. As a general rule, people should follow the advice of a health care provider who is familiar with their status.
Warning Signs of Cancer
There is no routine screening for many types of cancers. Instead, it is up to patients and healthcare providers to notice signs that might indicate cancer. Most cancers do not initially cause pain, but there may be other early warning signs. A good way to remember the signs is using the mnemonic CAUTION:
• Change in the bowel of bladder habits
• A sore that does not heal
• Unusual bleeding or discharge
• Thickening or lump in the breast or elsewhere
• Indigestion or difficulty in swallowing
• Obvious change in wart or mole
• Nagging cough or hoarseness
Most of the early warning signs of cancer are likely to be caused by the mass of a primary tumor or its ulceration. For example, the mass of a tumor in the rectum might cause a change in bowel habits. If the tumor was ulcerated, it might cause bleeding in the rectum and blood in the stool. A tumor in a breast might produce a detectable lump; a tumor in a lung might interfere with normal breathing and cause a persistent cough.
The early warning signs of cancer are not diagnostic of cancer because they could have many other causes — and chances are they do. However, the presence of one or more of the signs should prompt a visit to the doctor to find out for sure.
Biopsy
A definitive diagnosis of cancer requires a biopsy. In a biopsy, a tissue sample from the patient is examined microscopically by a pathologist (doctor specializing in disease diagnosis based on tissue changes). Cancers are classified and named by the type of tissue where cancer began (Table \(1\)). For example, carcinoma — such as Bernie Sanders’ basal cell carcinoma — is cancer derived from epithelial cells. Besides the skin, carcinomas include cancers of the lung, breast, and colon. Often, different types of cancer can be further distinguished on the basis of the size or shape of the cancer cells. For example, carcinomas of the lung include small-cell carcinomas and non-small-cell carcinomas. This is an important distinction because the two types of lung carcinoma have different prognoses and treatments.
Table \(1\): Four Common Types of Cancer
Type of Cancer Type of Tissue or Cells Where It Originates Example of this Type of Cancer
Carcinoma epithelial system skin
Sarcoma connective tissue bone
Leukemia blood-forming tissue bone marrow
Lymphoma immune system cells lymph nodes
Cancer Staging
A cancer diagnosis generally includes cancer staging. Staging is the use of a classification system that reflects the seriousness of a cancer, such as how large a tumor is and the extent to which cancer has spread. These factors are important to consider in determining the prognosis and appropriate treatment for a given cancer. There are several different staging systems in use for different types of cancer. A general staging system commonly used by cancer registries includes the following stages:
1. In situ — Abnormal cells are present but have not spread to nearby tissue.
2. Localized — Cancer is limited to the place where it started, with no sign that it has spread beyond local tissue.
3. Regional — Cancer has spread to nearby lymph nodes, tissues, or organs.
4. Distant — Cancer has spread to distant parts of the body.
Treating Cancer
Many treatments for cancer exist. Some of the options for removing or killing cancer cells and potentially curing cancer are surgery, chemotherapy, radiation therapy, and immunotherapy. Which treatment is used generally depends on the type of cancer and its stage. In many cases, two or more types of curative treatments are used. Palliative treatments, such as drugs to relieve pain, are often provided in addition to curative treatments.
Surgery
Surgery is the primary method to treat most isolated, solid cancers. In an isolated tumor, surgery typically attempts to remove the entire mass, often along with local lymph nodes. If the cancer is still localized, surgery is likely to cure it. If not, surgery may at least lessen symptoms and prolong survival.
Chemotherapy
Chemotherapy is the treatment of cancer with one or more drugs that kill cancer cells. The drugs are often delivered directly into the bloodstream rather than orally (Figure \(5\)). Chemotherapy may be used alone or, more commonly, in conjunction with other treatments. Most chemotherapy drugs target rapidly dividing cells, not specifically cancerous cells. For example, chemotherapy drugs often target rapidly growing cells in hair roots, causing a temporary loss of hair. A more sophisticated form of chemotherapy, called targeted chemotherapy, targets specific molecules that distinguish cancerous from normal cells. Targeted therapies generally cause fewer adverse side effects. Targeted therapies exist for breast cancer, prostate cancer, melanoma, and several other cancers.
Radiation Therapy
About half of the cancers are treated with radiation therapy, usually in addition to surgery and/or chemotherapy. Radiation therapy is the use of ionizing radiation such as X-rays to kill cancerous tissues. To spare normal tissues through which the radiation must pass to reach cancer, multiple rays are directed toward cancer from different angles. The rays all intersect at the site of cancer, providing a much larger dose than in the surrounding healthy tissue.
Immunotherapy
A variety of newer therapies are directed at helping the immune system fight cancer. This type of therapy is called immunotherapy. Cancer immunotherapy attempts to stimulate the immune system to destroy cancer cells. A variety of such strategies are in use or are undergoing research and testing (see the Feature: Human Biology in the News below). If immunotherapy is used, it is usually in conjunction with other types of treatment, such as surgery or chemotherapy.
Stigma of Cancer
Many other diagnoses have a worse prognosis than most diagnoses of cancer, but a cancer diagnosis usually causes greater fear and dread. There are widespread misconceptions that cancer is always painful or always terminal, and there is a stigma associated with cancer. The stigma is reflected in how people refer to cancer deaths. An obituary is likely to say the deceased “died after a long illness” rather than “died of cancer.” Fear of cancer and cancer stigma may delay cancer diagnoses and treatment and increase the risk of cancer death.
Review
1. What is cancer?
2. What are some common types of cancer?
3. How does a normal cell transform into a cancer cell?
4. Describe three ways that cancer cells can spread from the original site of cancer.
5. Define metastases.
6. Explain why there are many different risk factors for cancer, and identify some common cancer risk factors.
7. Why is it important to diagnose cancer early? What are two ways in which cancer can be detected early?
8. What is a biopsy, what is its role in a cancer diagnosis, and what else can a biopsy reveal?
9. Explain cancer staging and why it is done.
10. Discuss types of treatments for cancer.
11. Many other diagnoses have a worse prognosis than most diagnoses of cancer, but cancer usually causes greater fear and dread. Why?
12. Which stage(s) of cancer, as defined here, are the most likely to be able to be fully cured with surgery? Explain your answer.
13. True or False. Activation of tumor-suppressor genes is a common cause of cancer.
14. True or False. Lung cancer can commonly spread to the brain.
15. Explain why some cancer treatments can damage normal tissue.
Explore More
About 1 in every 3 families is now affected by cancer. Learn more about 2 new treatment options that are giving patients hope again:
Attributions
1. Feel the Bern by Carl Glover from Long Beach, California, United States; CC BY 2.0 via Wikimedia Commons
2. Cancer requires multiple mutations by National Cancer Institute, NIH, Public Domain via Wikimedia Commons
3. Metastasis sites for common cancers by Mikael Häggström, CC0 1.0 via Wikimedia Commons
4. Woman receives mammogram by National Cancer Institute, NIH, Public Domain via Wikimedia Commons
5. Chemotherapy IV by National Cancer Institute, NIH, Public Domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/21%3A_Disease/21.7%3A_Cancer.txt |
Case Study Conclusion: What’s Lurking in the Woods
The bacteria in Figure \(1\), labeled with red, yellow, and green in the photomicrograph, are the tiny culprits responsible for Lyme disease. They are the bacteria Borrelia burgdorferi, which, when transmitted through the bite of an infected tick, can make humans very sick.
As you learned in the beginning of the chapter, Ximena came down with symptoms of Lyme disease after visiting her grandparents in New Jersey and spending time in the woods there. Ximena’s symptoms included a distinctive bulls-eye rash that is characteristic of Lyme disease (known formally as an erythema migrans rash—shown in Figure \(2\)) and flu-like symptoms, including fever, chills, fatigue, headache, and body aches. In addition to these symptoms, Lyme disease can cause facial palsy (loss of muscle tone in the face, which can occur on one or both sides), and joint pain and swelling. These symptoms are illustrated below. Lyme disease can also cause swollen lymph nodes; neck stiffness; pain, numbness, or tingling in various parts of the body; heart problems; dizziness; inflammation of the brain and spinal cord; and short-term memory problems. Clearly, Lyme disease can be quite serious, which is why early diagnosis and treatment are so important.
In a way, Ximena was lucky that her symptoms included the bulls-eye rash because it helped her get diagnosed early. In about 20-30% of cases of Lyme disease, there is no rash. Even if there is a rash, it may not have the “classic” bulls-eye appearance, which can make it hard to identify.
In addition to noting Ximena’s symptoms and time spent in the woods of the northeastern U.S. where Lyme disease is prevalent, Ximena’s doctor took a blood sample to confirm the diagnosis of the disease. The first blood test for Lyme disease is usually an enzyme immunoassay (EIA), which detects antibodies against the bacteria that cause Lyme disease. Similar to HIV testing, which you learned about in this chapter, it can take time after the initial infection for antibodies against the pathogen to be produced by the body and detectable in the blood. In the case of HIV, this can take up to three months, while in Lyme disease it can take up to two months.
Ximena’s EIA test results came back as “indeterminate” (neither conclusively positive nor negative), possibly because she was recently infected. The Centers for Disease Control and Prevention (CDC) recommends two-step testing for Lyme disease if the initial EIA test is indeterminate or positive, meaning that the second type of test should be performed to confirm the diagnosis. This second test is called an immunoblot, or Western blot test, which is another way of detecting antibodies in the blood.
Why would a second test be needed if the first test came back positive? This is because the EIA test can give false positives, often due to the presence of other diseases, such as tick-borne relapsing fever, syphilis, and some autoimmune disorders. The Western blot test gives more information and may be able to distinguish between Lyme and other diseases. Even the Western blot can give false positives, so it must be administered correctly (i.e. at the correct time after infection) and the results must be interpreted carefully by an experienced medical professional. The risk of false positives and the need for careful interpretation is similar to the reasons why widespread screening for some types of cancer can be controversial, as you learned earlier in the chapter.
When Ximena’s doctor did the Western blot test, it confirmed that she did have antibodies against the bacteria that cause Lyme disease. This result, combined with her symptoms and presence in areas where Lyme-disease infected ticks are common, caused her doctor to confirm a diagnosis of Lyme disease. Recall that he started Ximena on medication immediately, because of the high likelihood that she had Lyme disease and the importance of early treatment. Given what you have learned in this chapter, what type of medication do you think he prescribed? If you guessed an antibiotic, you are correct! Because the pathogen is bacterial, antibiotics are generally effective in treating Lyme disease. A two- to four-week course of oral antibiotics is usually sufficient, although extreme cases may require intravenous antibiotics.
Within a week of starting the antibiotics, Ximena was beginning to feel better, although she continued to have fatigue and body aches for several weeks afterward, which is common. By two months after treatment, Ximena was back to normal. A small percentage of people with Lyme disease are not so lucky. In those people, Lyme disease symptoms can continue for more than six months after treatment. This is called Post-treatment Lyme Disease Syndrome (PTLDS) and the cause is not yet known. Most medical experts think that PTLDS is due to damage to body tissues and the immune system that occurred during the original infection, and not a continued active infection with B. burgdorferi, although the causes are still under investigation. More research needs to be done to better understand this more chronic version of Lyme disease.
Ximena is relieved to have recovered, but she wonders how she got infected in the first place. She never saw a tick on her or felt a tick bite, which is not uncommon. The ticks that spread Lyme disease are very small (Figure \(3\)) and their saliva contains a substance that has anesthetic properties, so a person may not feel their bite. They often bite and attach themselves on areas of the body where they are hard to see, such as the scalp, armpit, and groin.
As you now know, this method of infectious disease transmission is called vector transmission. The disease-causing pathogen is the B. burgdorferi bacteria and the vector is the tick. This is similar to malaria, where the pathogen is transmitted through the bite of a mosquito vector. Like malaria, Lyme disease is endemic to particular geographic regions, based on the presence of the vector organism.
Lyme disease risk is high in certain areas of the U.S., because of where the tick species that transmit Lyme disease live. In the northeast, mid-Atlantic, and north-central U.S., the tick species that transmits Lyme disease is the black-legged tick, or deer tick, Ixodes scapularis, and on the Pacific coast, it is transmitted by the western black-legged tick, Ixodes pacificus. A research study published in 2016 showed that the range of these species is rapidly expanding, and that half of all counties in the U.S. are now home to these tick species. Cases of Lyme disease have tripled in the U.S. in the last 20 years, and it is estimated that 300,000 Americans are infected each year. Lyme disease is the most common vector-borne disease in the United States.
How can you prevent getting this common infectious disease? You don’t necessarily need to avoid spending time in nature, but you should take preventative measures if you are outside in an area with Lyme disease. These include avoiding walking through thick vegetation where ticks commonly live, using insect repellent, bathing after being outdoors, and checking yourself for ticks daily if you are likely to be exposed. You may want to enlist a friend or family member to check areas you can’t easily see, such as your scalp.
If you do see a tick attached to your body, it is important to remove it quickly and carefully. Removing a tick within 24 hours of attachment can greatly reduce your chance of getting Lyme disease since it can take 36-48 hours for a tick to transmit the disease-causing bacteria. Remove the tick with tweezers by steadily pulling straight up, as shown in Figure \(4\). Visit the CDC website and consult with your physician for more detailed instructions on proper tick removal.
If you are concerned about a tick bite or think you may have symptoms of Lyme disease, please consult a physician. Many websites or laboratories advertise types of tests for Lyme disease that have not been scientifically proven to be valid. The good news is that when properly diagnosed and treated early, as in Ximena's case, Lyme disease can usually be cured.
Chapter Summary
In this chapter, you learned about the general causes of disease, and details about several specific diseases. Specifically, you learned that:
• Homeostasis is needed for good health. Homeostasis refers to maintaining internal conditions in a steady state. Homeostats are physiological mechanisms that keep internal variables within normal ranges.
• The homeostat that controls blood glucose concentration involves pancreatic beta cells, which secrete insulin, and alpha cells, which secrete glucagon. These two hormones control blood glucose concentration in two negative feedback loops, with insulin-lowering values that are too high, and glucagon raising values that are too low.
• If homeostats fail to perform properly, homeostatic imbalance and disease may result. For example, failure of the homeostat that controls blood glucose concentration causes high blood glucose levels and diabetes. Homeostats also start to fail as people age.
• There are many underlying causes of homeostatic imbalances that lead to disease. Infectious diseases are caused by pathogens such as bacteria and viruses. Noninfectious diseases are caused by genes or environmental factors other than pathogens, such as toxic exposures or unhealthy habits.
• Some diseases, such as flu, are acute, or short-term, diseases. Other diseases, such as heart disease, are long-term or even lifelong diseases.
• At the population level, diseases may occur as sudden outbreaks, called epidemics. If epidemics spread through multiple populations or even worldwide, they are called pandemics. Endemic diseases, in contrast, occur at about the same rate year-round in populations.
• The science that studies diseases in human populations is epidemiology. The results of epidemiological research form the cornerstone of public health. The father of epidemiology is John Snow, a 19th-century English physician whose investigations pinpointed the cause of cholera outbreaks in London. His work eventually led to significant improvements in public health around the world.
• All infectious diseases are caused by infections with pathogens, or disease-causing agents, many of which are microorganisms. Types of pathogens and examples of the diseases each type causes include: bacteria (e.g., tuberculosis and strep throat), viruses (e.g., influenza and the common cold), fungi (e.g., ringworm and athlete’s foot), protists (e.g., malaria and giardiasis), helminths (e.g., tapeworm and hookworm), and prions (e.g., CJD and mad-cow disease).
• In the 19th century, Robert Koch developed four criteria, or postulates, for deciding whether a disease is caused by a particular microorganism. The postulates are now viewed as sufficient but not necessary criteria. They still inform the basic approach to identifying pathogens and historically led to the discovery of many human pathogens.
• Pathogens cause disease by invading and multiplying in host tissues, causing damage and releasing toxins. Typically, the more pathogens there are in the host, the greater is the severity of the disease. However, pathogens also vary greatly in their virulence.
• Transmission of pathogens from infected to noninfected human hosts can occur through a variety of different routes: airborne transmission, direct contact, fecal-oral transmission, vector transmission, vertical transmission, and sexual transmission. Prions can be transmitted via eating contaminated nervous tissue from an infected individual.
• Infectious diseases must be correctly diagnosed so the appropriate treatment can be prescribed. Most infectious diseases can be treated with drugs if not cured. Hygienic habits, especially frequent handwashing, and immunizations are the most effective ways to prevent the spread of infectious diseases. A high level of vaccination in a population provides herd immunity to population members who cannot be vaccinated for medical reasons.
• Emerging infectious diseases are new infectious diseases that are appearing for the first time in human populations, mainly because of human actions such as encroachment on wild lands. Emerging infectious diseases come about in a number of ways. For example, some pathogens of nonhuman hosts jump to human hosts and start causing disease in them.
• A sexually transmitted infection (STI) is an infection caused by a pathogen that spreads mainly through sexual contact. This may include vaginal, anal, and/or oral contact.
• Most STIs are caused by pathogens that can infect the body only via direct contact between mucous membranes. Such pathogens generally cannot spread through nonsexual skin contact, although some can also be transmitted through body fluids such as blood and breast milk.
• Types of pathogens that are sexually transmitted include parasites, such as crab lice and the protozoa that cause trichomoniasis; bacteria, including those that cause chlamydia, gonorrhea, and syphilis; and viruses, such as those that genital herpes, genital warts, and AIDS.
• Common symptoms of STIs include genital sores, genital discharge, and painful urination. However, many cases of STIs are asymptomatic.
• Bacterial STIs can generally be cured with antibiotics. Viral STIs can be treated with anti-viral drugs, but the viruses may not be completely eliminated.
• If STIs go untreated, some may eventually lead to more serious diseases, especially in females, who may develop the pelvic inflammatory disease (PID) and its sequelae of infertility or ectopic pregnancy. Untreated syphilis is dangerous in both sexes. It typically advances through several stages over the decades to invade internal organs and cause death.
• A few STIs can be prevented with vaccines. An example is a human papillomavirus (HPV) infection, which sometimes leads to genital warts or cervical cancer. The HPV vaccine is recommended for all girls and boys aged 11-12 years old.
• For STIs without vaccines, avoiding sexual contact is the only sure way to prevent transmission. Practicing safe sex behaviors — such as proper condom use — can greatly reduce but not totally eliminate the risk of transmission.
• Human immunodeficiency virus (HIV) is a sexually transmitted virus that infects and destroys helper T cells of the immune system. It is usually transmitted through sexual contact but can also be transmitted through contaminated blood or breast milk. HIV infection is diagnosed on the basis of a blood test for antibodies to the virus.
• AIDS stands for acquired immunodeficiency syndrome. AIDS is a disease that develops in people with untreated HIV infections, typically several years after their initial infection with the virus. AIDS is diagnosed when the immune system has been weakened to the point that it can no longer fight off opportunistic diseases that do not normally occur in healthy individuals.
• HIV infection and AIDS are a worldwide pandemic with the highest population rates in sub-Saharan Africa where the virus first emerged. Death and disability due to AIDS have been economically devastating to these populations.
• The development of new anti-retroviral drugs to treat HIV infection has changed HIV infection from a fatal to a chronic disease. The drugs keep the virus at low levels, reducing the risk of transmission as well as reducing the risk of the infection progressing to AIDS.
• Until an HIV vaccine is developed, reducing the risk of HIV transmission depends on factors ranging from individual behaviors such as effective condom use to public health policies such as needle-exchange programs.
• Noninfectious diseases include all diseases that are not caused by pathogens. Noninfectious diseases are generally caused by a combination of genetic and environmental factors other than pathogens. Noninfectious diseases are the leading causes of death globally.
• Risk factors for noninfectious diseases include age, gender, inherited genes, and environmental factors including exposures such as radon and behaviors such as smoking. Most behavioral risk factors for noninfectious diseases can be avoided, so many noninfectious diseases are considered preventable diseases. Risk factors such as age, gender, and genes cannot be avoided but should be considered in diagnosing, treating, and preventing noninfectious diseases in individuals.
• Cystic fibrosis is an example of a genetic noninfectious disease. It is inherited as an autosomal recessive trait, caused by a mutation in a gene called CFTR. It results in thick mucus that blocks mucus-secreting organs such as the lungs and intestines, causing recurrent respiratory infections and malabsorption of nutrients. Medical interventions can help people with cystic fibrosis live into middle adulthood.
• Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. It is a major cause of death in developed countries. Most cancers are noninfectious diseases caused by a combination of genetic and environmental factors. Few are caused mainly by inherited genes.
• Cardiovascular disease is a class of diseases that involve the heart or blood vessels, such as coronary artery disease and stroke. Cardiovascular disease is the leading cause of death globally. Major precursors of cardiovascular disease include hypertension and atherosclerosis. Obesity and diabetes are additional major risk factors. Most cases of cardiovascular disease could be prevented by modifying risk factors through medications and behavioral changes.
• Type 2 diabetes accounts for 90% of all diabetes cases. It generally develops due to insulin resistance, although the reduction in insulin secretion may exacerbate the problem. Risk factors for type 2 diabetes include obesity, with or without the other indicators of metabolic syndrome, which is called pre-diabetes. Poorly controlled diabetes can lead to heart attacks, blindness, kidney failure, and other serious health problems.
• Cancer is a group of more than 100 diseases, all of which involve abnormal cell growth with the potential to invade or spread to other parts of the body. The most common type of cancer is the type of skin cancer called basal cell carcinoma, which is usually easy to cure. Less common but more deadly cancers include lung, colorectal, prostate, and breast cancers.
• Cancer generally occurs when the cell cycle is no longer regulated due to DNA damage to two types of genes: proto-oncogenes, which normally promote division of normal cells; and tumor-suppressor genes, which normally inhibit the division of abnormal cells. Transformation of a normal cell into a cancer cell is a multi-stage process involving accumulated damage to these genes.
• Once a normal cell transforms into a cancer cell and starts dividing out of control, cancer cells can spread from the original site. Cancer cells can invade nearby tissues, spread through lymph vessels to regional lymph nodes, or spread through the bloodstream to distant sites in the body, which is called metastasis. New cancers that forms at a distant sites are called metastases.
• There are many possible underlying causes of the DNA damage that leads to cancer, so cancer has many risk factors. DNA damage can be inherited from parents or result spontaneously from environmental exposures to carcinogens. Environmental risk factors include radon, UV light, air pollution, and behaviors such as smoking, unhealthy diet, and lack of exercise.
• Early diagnosis and treatment are the keys to curing cancer, although not all cancers are curable. Cancers may be detected early through routine screening (e.g., by mammograms) or by patients or health care providers noticing early warning signs, such as unusual bleeding or a nagging cough. A definitive diagnosis of cancer requires a biopsy, in which a tissue sample from the patient is examined microscopically. A biopsy may also reveal the type of cancer (e.g., carcinoma or sarcoma) and its stage (degree of severity, such as whether it has spread).
• Many types of treatments for cancer exist, including surgery, chemotherapy, radiation therapy, and immunotherapy. The first three types of treatment directly target cancer cells, while the last type of treatment is directed at helping the immune system fight cancer.
Chapter Summary Review
1. What type of feedback loops help maintain homeostasis by keeping variables within a normal range?
2. Explain what generally happens to homeostatic mechanisms as people age, and how this relates to susceptibility to disease in elderly people.
3. Give an example of a noninfectious disease that can cause an infectious disease.
4. True or False. Epidemiologists only study diseases that can be transmitted between people.
5. True or False. Some cases of cancer are preventable.
6. Explain how type 1 diabetes can lead to cardiovascular disease.
7. For each of the following diseases, state whether the pathogen is a bacterium, virus, fungus, protist, helminth, or prion.
1. Creutzfeldt–Jakob disease
2. Strep throat
3. Ringworm
4. Giardiasis
5. Hepatitis
6. Pinworm
8. Which type(s) of pathogens (listed in question 7) are not considered to be living organisms? Explain your answer.
9. True or False. Transmission of viral pathogens can sometimes be prevented by immunization.
10. True or False. Human papillomavirus can cause cancer of the penis.
11. Proper washing can help prevent the spread of:
1. Sexually transmitted diseases
2. Infectious respiratory illnesses
3. Cystic fibrosis
4. A and B
12. What does “screening” for a disease mean? Compare and contrast screening for STIs to screening for cancer in terms of potential benefits and drawbacks.
13. Which is more likely to result in a chronic disease instead of an acute disease—a bacterial STI or a viral STI? Assume the proper treatment is given. Explain your answer and give an example of each type of STI.
14. Can a couple that does not engage in penile-vaginal intercourse still transmit STIs to each other if they engage in other types of unprotected sexual activity? Why or why not?
15. What are two ways that STIs can be transmitted that do not involve sexual activity?
16. What is metabolic syndrome and why is it such a cause for concern?
17. What is the relationship between HIV and AIDS?
1. HIV causes AIDS.
2. AIDS causes HIV.
3. They are different terms for the same thing.
4. AIDS can make a person more susceptible to HIV.
18. Viral load refers to:
1. The financial impact of a viral disease.
2. The amount of damage a virus does to an individual.
3. How widely a virus has spread across a population.
4. The amount of virus in a sample of an infected person’s blood.
19. Explain the roles of genetics and the environment in the development of cancer.
20. If breast cancer metastasized to a patient’s brain, what stage would this cancer most likely be classified as? Explain your answer.
21. What are two healthy lifestyle choices that you can make that can reduce your risk of disease? Explain your answer, and identify some of the diseases that may be able to be avoided.
22. What is the most frequently reported bacterial STI in the United States?
1. HPV
2. Genital herpes
3. Chlamydia
4. PID
23. What is the difference between a disease vector and a pathogen?
24. Antiretroviral drugs are used to treat:
1. HIV
2. Tetanus
3. Cholera
4. Malaria
25. What is one way that screening for skin cancer is performed?
Attributions
1. Lyme Disease Bacteria by NIAID/NIH, CC BY 3.0 via Flickr.com
2. Signs and Symptoms of Lyme Disease by Centers for Disease Control, Public Domain
3. Transmission of Lyme Disease by Centers for Disease Control, Public Domain
4. Tick removal by Centers for Disease Control, Public Domain
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/21%3A_Disease/21.8%3A_Case_Study_Conclusion%3A__Lyme_and_Chapter_Summary.txt |
Please note there are not only two genders in the human population. When male or female is mentioned in this chapter, it only refers to the biological male and female sexes. This chapter outlines the structures and functions of the male and female reproductive systems, explains how fertilization occurs, and discusses the menstrual cycle's role. The chapter describes the causes of and treatments for male and female reproductive system disorders, infertility, and contraception methods.
• 22.1: Case Study: Making Babies
Isabella, 28, and Omar, 30, have been together for three years. A year ago, they decided they wanted to have a baby, and they stopped using birth control. At first, they did not pay attention to the timing of their sexual activity in relation to Isabella's menstrual cycle, but after six months passed without Isabella becoming pregnant, they decided to try to maximize their efforts.
• 22.2: Introduction to the Reproductive System
The reproductive system is the human organ system responsible for the production and fertilization of gametes (sperm or eggs) and, in females, the carrying of a fetus. Both male and female reproductive systems have organs called gonads that produce gametes. Besides producing gametes, the gonads also produce sex hormones.
• 22.3: Structures of the Male Reproductive System
The two testes are sperm- and testosterone-producing male gonads. They are contained within the scrotum, a pouch that hangs down behind the penis. The testes are filled with hundreds of tiny, tightly coiled seminiferous tubules, where sperm are produced.
• 22.4: Functions of the Male Reproductive System
A mature sperm cell has several structures that help it reach and penetrate an egg.
• 22.5: Disorders of the Male Reproductive System
Erectile dysfunction (ED) is a disorder characterized by the regular and repeated inability of a sexually mature male to obtain and maintain an erection. ED occurs when normal blood flow to the penis is disturbed, or when there are problems with the nervous control of penile arousal.
• 22.6: Structures of the Female Reproductive System
The female reproductive system is made up of internal and external organs that function to produce haploid female gametes called eggs (or oocytes), secrete female sex hormones (such as estrogen), and carry and give birth to a fetus.
• 22.7: Menstrual Cycle
The menstrual cycle refers to natural changes that occur in the ovaries and uterus each month during the reproductive years of a female.
• 22.8: Functions of the Female Reproductive System
At birth, a female's ovaries contain all the eggs she will ever produce, which may include a million or more eggs. The eggs don't start to mature, however, until she enters puberty and attains sexual maturity. After that, one egg typically matures each month and is released from an ovary, until she reaches menopause.
• 22.9: Disorders of the Female Reproductive System
Cervical cancer is one of three disorders of the female reproductive system described in detail in this concept.
• 22.10: Infertility
Infertility is the inability of a sexually mature adult to reproduce by natural means and is generally defined as the failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse.
• 22.11: Contraception
Contraception, also known as birth control, is any method or device used to prevent pregnancy. Birth control methods have been used for centuries, but safe and effective methods only became available in the 20th century.
• 22.12: Case Study Conclusion: Trying to Conceive and Chapter Summary
In the beginning of the chapter, you learned that Isabella and Omar have been actively trying to get pregnant for a year, which, as you now know, is the time-frame necessary for infertility to be diagnosed.
22: Reproductive System
Case Study: Trying to Conceive
Isabella, 28, and Omar, 30, have been together for three years. A year ago, they decided they wanted to have a baby, and they stopped using birth control. At first, they did not pay attention to the timing of their sexual activity in relation to Isabella’s menstrual cycle, but after six months passed without Isabella becoming pregnant, they decided to try to maximize their efforts.
They knew that in order for a woman to become pregnant, the man’s sperm must encounter the woman’s egg, which is typically released once a month through a process called ovulation. They had also heard that for the average woman, ovulation occurs around day 14 of the menstrual cycle. To maximize their chances of conception, they tried to have sexual intercourse on day 14 of Isabella’s menstrual cycle each month.
After several months of trying this method, Isabella is still not pregnant. She is concerned that she may not be ovulating on a regular basis because her menstrual cycles are irregular and often longer than the average 28 days. Omar is also concerned about his own fertility. He had some injuries to his testicles (testes) when he was younger, and wonders if that may have caused a problem with his sperm.
Isabella calls her doctor for advice. Dr. Bashir recommends that she try taking her temperature each morning before she gets out of bed. This temperature is called basal body temperature (BBT), and recording BBT throughout a woman’s menstrual cycle can sometimes help identify if and when they ovulate. Additionally, Dr. Bashir recommends she try using a home ovulation predictor kit, which predicts ovulation by measuring the level of luteinizing hormone (LH) in the urine. In the meantime, Dr. Bashir sets up an appointment for Omar to give a semen sample, so that his sperm may be examined with a microscope.
As you read this chapter, you will learn about the male and female reproductive systems, how sperm and eggs are produced, and how they meet each other to ultimately produce a baby. You will learn how these complex processes are regulated, and how they can be susceptible to problems along the way. Problems in either the male or female reproductive systems can result in infertility, or difficulty in achieving a successful pregnancy. As you read the chapter, you will understand exactly how BBT and LH relate to ovulation, why Dr. Bashir recommended that Isabella monitor these variables, and the types of problems she will look for in Omar’s semen. At the end of the chapter, you will find out the results of Isabella and Omar’s fertility assessments, steps they can take to increase their chances of conception, and whether they are ultimately able to get pregnant.
LGBTQ +
Most of the information in this chapter is in terms of cis-gendered individuals because there is a lack of data on lesbian, gay, bisexual, transgender, and queer (LGBTQ) individuals. About 3.5% of Americans identify themselves as lesbian, gay, or bisexual, and 0.3% identify themselves as transgender. The acronym LGBTQIA+ is an umbrella term that includes a number of groups: lesbian (homosexual woman), gay (homosexual man or woman), bisexual (person who is attracted to both genders), transgender (person who identifies their gender as different from their biological one), queer (a synonym for gay; some people prefer to identify themselves as queer to empower themselves and take their identity “back from the bullies”), questioning (people who are unsure about their gender identity/sexuality), intersex (people with two sets of genitalia), asexual (people who are not sexually attracted to anyone and who don’t identify with any orientation), allies (the loving supporters of the community, though not necessarily part of it), two spirits (a tradition in many First Nations that considers sexual minorities to have both male and female spirits), and pansexual (person sexually attracted to others of any sex or gender).
Chapter Overview: Reproductive System
In this chapter, you will learn about the male and female reproductive systems. Specifically, you will learn about:
• The functions of the reproductive system, which include the production and fertilization of gametes (eggs and sperm), the production of sex hormones by the gonads (testes and ovaries), and, in females, the carrying of a fetus
• How the male and female reproductive systems differentiate in the embryo and fetus, and how they mature during puberty
• The structures of the male reproductive system, including the testes, epididymis, vas deferens, ejaculatory ducts, seminal vesicles, prostate gland, bulbourethral glands, and the penis
• How sperm are produced, matured, stored, and deposited into the female
• The fluids in semen that protect and nourish sperm, and where those fluids are produced
• Disorders of the male reproductive system, including erectile dysfunction, epididymitis, prostate cancer, and testicular cancer—some of which predominantly affect younger men
• The structures of the female reproductive system, including the ovaries, fallopian tubes, uterus, cervix, vagina, and external structures of the vulva
• How eggs are produced in the female fetus, and how they then mature after puberty through the process of ovulation
• The menstrual cycle, its purpose, and the hormones that control it
• How fertilization and implantation occur, the stages of pregnancy and childbirth, and how the mother’s body produces milk to feed the baby
• Disorders of the female reproductive system, including cervical cancer, endometriosis, and vaginitis (which includes yeast infections)
• Some causes and treatments of male and female infertility
• Forms of contraception (birth control), including barrier methods (such as condoms), hormonal methods (such as the birth control pill), behavioral methods, intrauterine devices, and sterilization
As you read the chapter, think about the following questions:
1. Why might sexual intercourse on day 14 of Isabella's menstrual cycle not necessarily be optimal timing to achieve a pregnancy?
2. Why is Isabella concerned about her irregular and long menstrual cycles? How could tracking her BBT and LH level help identify if she is ovulating and when?
3. Why do you think Omar is concerned about past injuries to his testes? How might an analysis of his semen help assess whether he has a fertility issue—and, if so, the type of issue?
Attributions
1. Couple holding hands by aprilsylvester; Pixabay license
2. Day 222 - temperature by Phil and Pam Gradwell licensed CC BY 2.0 via Flickr
3. Some text is adapted from Health Care Disparities Among Lesbian, Gay, Bisexual, and Transgender Youth: A Literature Review; Hudaisa Hafeez, Muhammad Zeshan, Muhammad A Tahir, Nusrat Jahan, and Sadiq Naveed; . 2017 Apr; 9(4): e1184. Published online 2017 Apr 20. doi: 10.7759/cureus.1184; CC BY 4.0.
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.01%3A__Case_Study%3A_Making_Babies.txt |
It’s All About Sex
A tiny sperm breaks through the surface of a huge egg. Voilà! In nine months, a new baby will be born. Like most other multicellular organisms, human beings reproduce sexually. In human sexual reproduction, individuals with testes produce sperm, and individuals with ovaries produce eggs, and a new offspring forms when a sperm unites with an egg. How do sperm and eggs form? And how do they arrive together at the right place and time so they can unite to form a new offspring? These are functions of the reproductive system.
What Is the Reproductive System?
The reproductive system is the human organ system responsible for the production and fertilization of gametes (sperm or eggs) and carrying of a fetus. Both both sexes gonads produce gametes. A gamete is a haploid cell that combines with another haploid gamete during fertilization, forming a single diploid cell called a zygote. Besides producing gametes, the gonads also produce sex hormones. Sex hormones are endocrine hormones that control the development of sex organs before birth, sexual maturation at puberty, and reproduction once sexual maturation has occurred. Other reproductive system organs have various functions, such as maturing gametes, delivering gametes to the site of fertilization, and providing an environment for the development and growth of offspring.
Sex Differences in the Reproductive System
The reproductive system is the only human organ system that is significantly different between males and females. Embryonic structures that will develop into the reproductive system start out the same in males and females, but by birth, the reproductive systems have differentiated. How does this happen?
Sex Differentiation
Starting around the seventh week after conception in genetically male (XY) embryos, a gene called SRY on the Y chromosome (Figure \(2\)) initiates the production of multiple proteins. These proteins cause undifferentiated gonadal tissue to develop into testes. Testes secrete hormones — including testosterone — that trigger other changes in the developing offspring (now called a fetus), causing it to develop a complete male reproductive system. Without a Y chromosome, an embryo will develop ovaries, that will produce estrogen. Estrogen, in turn, will lead to the formation of the other organs of a female reproductive system.
Homologous Structures
Undifferentiated embryonic tissues develop into different structures in male and female fetuses. Structures that arise from the same tissues in males and females are called homologous structures. The testes and ovaries, for example, are homologous structures that develop from the undifferentiated gonads of the embryo. Likewise, the penis and clitoris are homologous structures that develop from the same embryonic tissues.
Sex Hormones and Maturation
Male and female reproductive systems are different at birth, but they are immature and incapable of producing gametes or sex hormones. Maturation of the reproductive system occurs during puberty when hormones from the hypothalamus and pituitary gland stimulate the testes or ovaries to start producing sex hormones again. The main sex hormones are testosterone and estrogen. Sex hormones, in turn, lead to the growth and maturation of the reproductive organs, rapid body growth, and the development of secondary sex characteristics, such as body and facial hair and breasts.
Role of Sex Hormones in Transgender Treatment
Feminizing or masculinizing hormone therapy is the administration of exogenous endocrine agents to induce changes in physical appearance. Since hormone therapy is inexpensive relative to surgery and highly effective in the development of secondary sex characteristics (e.g., facial and body hair in female-to-male [FTM] individuals or breast tissue in male-to-females [MTFs]), hormone therapy is often the first, and sometimes only, medical gender affirmation intervention accessed by transgender individuals looking to develop masculine or feminine characteristics consistent with their gender identity. In some cases, hormone therapy may be required before surgical interventions can be conducted. Trans-females are prescribed estrogen and anti-testosterone medication, such as cyproterone acetate and spironolactone. Trans-men are prescribed testosterone.
Male Reproductive System
The main structures of the male reproductive system are external to the body and illustrated in Figure \(3\). The two testes (singular, testis) hang between the thighs in a sac of skin called the scrotum. The testes produce both sperm and testosterone. Resting atop each testis is a coiled structure called the epididymis (plural, epididymes). The function of the epididymes is to mature and store sperm. The penis is a tubular organ that contains the urethra and has the ability to stiffen during sexual arousal. Sperm passes out of the body through the urethra during a sexual climax (orgasm). This release of sperm is called ejaculation.
In addition to these organs, there are several ducts and glands that are internal to the body. The ducts, which include the vas deferens (also called the ductus deferens), transport sperm from the epididymis to the urethra. The glands, which include the prostate gland and seminal vesicles, produce fluids that become part of semen. Semen is the fluid that carries sperm through the urethra and out of the body. It contains substances that control pH and provide sperm with nutrients for energy.
Female Reproductive System
The main structures of the female reproductive system are internal to the body and shown in Figure \(4\). They include the paired ovaries, which are small, oval structures that produce eggs and secrete estrogen. The two Fallopian tubes (aka uterine tubes) start near the ovaries and end at the uterus. Their function is to transport eggs from the ovaries to the uterus. If an egg is fertilized, it usually occurs while it is traveling through a Fallopian tube. The uterus is a pear-shaped muscular organ that functions to carry a fetus until birth. It can expand greatly to accommodate a growing fetus, and its muscular walls can contract forcefully during labor to push the baby into the vagina. The vagina is a tubular tract connecting the uterus to the outside of the body. The vagina is where sperm are usually deposited during sexual intercourse and ejaculation. The vagina is also called the birth canal because a baby travels through the vagina to leave the body during birth.
The external structures of the female reproductive system are referred to collectively as the vulva. They include the clitoris, which is homologous to the male penis. They also include two pairs of labia (singular, labium), which surround and protect the openings of the urethra and vagina.
Review
1. What is the reproductive system?
2. Define gonad.
3. What are sex hormones? What are their general functions?
4. Distinguish between male and female sex hormones.
5. How does the differentiation of the reproductive system occur in males and females?
6. In the context of the human male and female reproductive systems, what are homologous structures?
7. When and how does the human reproductive system mature?
8. List the organs of the male reproductive system.
9. List the organs of the female reproductive system.
10. What are female gametes called? What are male gametes called?
11. True or False: The vagina is the homologous structure to the penis.
12. rue or False: In the absence of a Y chromosome in humans, ovaries will develop.
13. Which are secondary sex characteristics?
1. Fallopian tubes
2. ovaries
3. breasts
4. all of the above
14. Fertilization usually occurs in which of these structures?
1. ovary
2. Fallopian tube
3. uterus
4. vagina
15. Explain the difference between the vulva and the vagina.
Explore More
People's sense of gender identity does not always match their anatomy. Some people do not identify as either male or female, and instead, they identify as non-binary, or genderqueer. Others may identify as a gender that is the opposite of what is typically associated with their chromosomes or reproductive organs. These people are called transgender, and they may choose to transition to the opposite gender, a process that may or may not involve physical modifications. Watch the video below to learn about the use of hormones in gender transitioning.
Sex determination may be more complicated than originally thought. Check out this video to learn more:
Attributions
1. Sperm-Egg; public domain via Wikimedia Commons
2. Human Y chromosome by National Center for Biotechnology Information (NCBI); public domain via Wikimedia Commons
3. Male Reproductive System by Charles Molnar and Jane Gair; CC BY 4.0 from Concepts of Biology - 1st Canadian edition
4. Female Reproductive System by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
6. Some text is adapted from White Hughto JM, Reisner SL.. A systematic review of the effects of hormone therapy on psychological functioning and quality of life in transgender individuals. Transgend Health. 2016;1(1):21-31 CC BY 4.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.02%3A_Introduction_to_the_Reproductive_System.txt |
Rocky Mountain Oysters
First, they are peeled and pounded flat. Then, they are coated in flour, seasoned with salt and pepper, and deep-fried. What are they? They are often called Rocky Mountain oysters, but they don’t come from the sea. They may also be known as Montana tendergroin, cowboy caviar, or swinging beef — all names that hint at their origins. Here’s another hint: they are harvested only from male animals, such as bulls or sheep. What are they? In a word: testes.
Testes and Scrotum
The two testes (singular, testis) are sperm- and testosterone-producing gonads in male mammals, including male humans. These and other organs of the human male reproductive system are shown in Figure \(2\). The testes are contained within the scrotum, a pouch made of skin and smooth muscle that hangs down behind the penis.
Testes Structure
The testes are filled with hundreds of tiny tubes, called seminiferous tubules, which are the functional units of the testes. The seminiferous tubules are coiled and tightly packed within divisions of the testis called lobules. Lobules are separated from one another by internal walls (or septa).
One or more seminiferous tubules are tightly coiled within each of the hundreds of lobules in the testis. A single testis normally contains a total of about 30 m (90 ft) of these tightly packed tubules! As shown in the cross-sectional drawing of a seminiferous tubule in Figure \(2\), the tubule contains sperm in several different stages of development
Other Scrotal Structures
Besides the two testes, the scrotum also contains a pair of organs called epididymes (singular, epididymis) and part of each of the paired vas deferens (or ducti deferens). Both structures play important functions in the production or transport of sperm.
Epididymis
The seminiferous tubules within each testis join together to form ducts (called efferent ducts) that transport immature sperm to the epididymis associated with that testis. Each epididymis (plural, epididymes) consists of a tightly coiled tubule with a total length of about 6 m (20 ft). As shown in Figure \(2\) the epididymis is generally divided into three parts: the head (which rests on top of the testis), the body (which drapes down the side of the testis), and the tail (which joins with the vas deferens near the bottom of the testis). The functions of the two epididymes are to mature sperm, and then to store that mature sperm until they leave the body during an ejaculation when they pass the sperm on to the vas deferens.
Vas Deferens
The vas deferens, also known as sperm ducts, are a pair of thin tubes, each about 30 cm (1 ft) long, which begin at the epididymis in the scrotum and continue up into the pelvic cavity. They are composed of ciliated epithelium and smooth muscle. These structures help the vas deferens fulfill their function of transporting sperm from the epididymes to the ejaculatory ducts, which are accessory structures of the male reproductive system.
Accessory Structures
In addition to the structures within the scrotum, the male reproductive system includes several internal accessory structures. They include the ejaculatory ducts, seminal vesicles, and the prostate and bulbourethral (Cowper’s) glands. See Figure \(3\). The major reproductive structures represented in this figure are explained below.
Seminal Vesicles
The seminal vesicles are a pair of glands that each consist of a single tube, which is folded and coiled upon itself. Each vesicle is about 5 cm (2 in.) long and has an excretory duct that merges with the vas deferens to form one of the two ejaculatory ducts. Fluid secreted by the seminal vesicles into the ducts makes up about 70 percent of the total volume of semen, which is the sperm-containing fluid that leaves the penis during an ejaculation. The fluid from the seminal vesicles is alkaline, so it gives semen a basic pH that helps prolong the lifespan of sperm after it enters the acidic secretions inside the female vagina. Fluid from the seminal vesicles also contains proteins, fructose (a simple sugar), and other substances that help nourish sperm.
Ejaculatory Ducts
The ejaculatory ducts form where the vas deferens join with the ducts of the seminal vesicles in the prostate gland. They connect the vas deferens with the urethra. The ejaculatory ducts carry sperm from the vas deferens, as well as secretions from the seminal vesicles and the prostate gland that together form semen. The substances secreted into semen by the glands as it passes through the ejaculatory ducts control its pH and provide nutrients to sperm, among other functions. The fluid itself provides sperm with a medium in which to “swim.”
Prostate Gland
The prostate gland is located just below the seminal vesicles. It is a walnut-sized organ that surrounds the urethra and its junction with the two ejaculatory ducts. The function of the prostate gland is to secrete a slightly alkaline fluid that constitutes close to 30 percent of the total volume of semen. The prostate fluid contains small quantities of proteins, such as enzymes. In addition, it has a very high concentration of zinc, which is an important nutrient for maintaining sperm quality and motility.
Bulbourethral Glands
Also called Cowper’s glands, the two bulbourethral glands are each about the size of a pea and located just below the prostate gland. The bulbourethral glands secrete a clear, alkaline fluid that is rich in proteins. Each of the glands has a short duct that carries the secretions into the urethra, where they make up a tiny percentage of the total volume of semen. The function of the bulbourethral secretions is to help lubricate the urethra and neutralize any urine (which is acidic) that may remain in the urethra.
Penis
The penis is the external male organ that has the reproductive function of delivering sperm to the female reproductive tract. This function is called intromission. The penis also serves as the organ that excretes urine.
Structure of the Penis
The structure of the penis and its location relative to other reproductive organs are shown in Figure \(4\). The part of the penis that is located inside the body and out of sight is called the root of the penis. The shaft of the penis is the part of the penis that is outside the body. The enlarged, bulbous end of the shaft is called the glans penis.
Urethra
The urethra passes through the penis to carry urine from the bladder — or semen from the ejaculatory ducts — through the penis and out of the body. After leaving the urinary bladder, the urethra passes through the prostate gland, where the urethra is joined by the ejaculatory ducts. From there, the urethra passes through the penis to its external opening at the tip of the glans penis. Called the external urethral orifice, this opening provides a way for urine or semen to leave the body.
Tissues of the Penis
The penis is covered with skin (epithelium) that is unattached and free to move over the body of the penis. In an uncircumcised male, the glans penis is also mainly covered by epithelium, which (in this location) is called the foreskin, and below which is a layer of the mucous membrane. The foreskin is attached to the penis at an area on the underside of the penis called the frenulum.
As shown in Figure \(5\), the interior of the penis consists of three columns of spongy tissue that can fill with blood and swell in size, allowing the penis to become erect. This spongy tissue is called corpus cavernosum (plural, corpora cavernosa). Two columns of this tissue run side by side along the top of the shaft, and one column runs along the bottom of the shaft. The urethra runs through this bottom column of spongy tissue, which is sometimes called corpus spongiosum. The glans penis also consists mostly of spongy erectile tissue. Veins and arteries run along the top of the penis, allowing blood circulation through the spongy tissues.
Feature: Human Biology in the News
Lung, heart, kidney, and other organ transplants have become relatively commonplace, so when they occur, they are unlikely to make the news. However, when the nation’s first penis transplant took place, it was considered very newsworthy.
In 2016, Massachusetts General Hospital in Boston announced that a team of its surgeons had performed the first penis transplant in the United States. The patient who received the donated penis was a 64-year-old cancer patient. During the 15-hour procedure, the intricate network of nerves and blood vessels of the donor penis were connected with those of the penis recipient. The surgery went well, but doctors reported it would be a few weeks until they would know if normal urination would be possible, and even longer before they would know if sexual functioning would be possible. At the time that news of the surgery was reported in the media, the patient had not shown any signs of rejecting the donated organ. The surgeons also reported they were hopeful that such transplants would become relatively common, and that patient populations would expand to include wounded warriors and transgender males seeking to transition.
The 2016 Massachusetts operation was not the first penis transplant ever undertaken. The world’s first successful penis transplant was actually performed in 2014 in Cape Town, South Africa. A young man who had lost his penis from complications of a botched circumcision at age 18 was given a donor penis three years later. That surgery lasted nine hours and was highly successful. The young man made a full recovery and regained both urinary and sexual functions in the transplanted organ.
In 2005, a man in China also received a donated penis in a technically successful operation. However, the patient asked doctors to reverse the procedure just two weeks later, because of psychological problems associated with the transplanted organ for both himself and his wife.
Review
1. What are the testes? Where are they located?
2. Describe the structure of a testis.
3. Identify the epididymis and its functions.
4. What are the vas deferens? What do they do?
5. Where are the seminal vesicles located? What is their reproductive role?
6. Which parts of the male reproductive system are connected by the ejaculatory ducts? What fluids enter and leave the ejaculatory ducts?
7. Identify the location of the prostate gland relative to other male reproductive organs. What is the prostate’s function?
8. Where are the bulbourethral glands? What is their function?
9. Relate the structure of the penis to its two basic functions.
10. For each of the descriptions below, match the part of the male reproductive system from the list that best fits it. Each part is used only once. Parts of the male reproductive system: urethra, seminal vesicle, epididymis, testes
1. Sperm are produced here.
2. Sperm mature here.
3. Sperm are transported through the penis in this structure.
4. This is a gland that produces fluid that is a major component of semen.
11. A vasectomy is a form of birth control for men that is performed by surgically cutting or blocking the vas deferens so that sperm cannot be ejaculated out of the body. Do you think men who have a vasectomy emit semen when they ejaculate? Why or why not?
12. Which of the following structures are located internally within the body? Choose all that apply.
1. testes
2. seminal vesicles
3. epididymis
4. prostate
5. glans penis
Explore More
Morning erections are part of the normal sleep cycle in men. Learn more here:
Attributions
1. Lamb Fries by Paul Lowry, CC BY 2.0 via Wikimedia Commons
2. Testicle by (public domain; National Cancer Institute via Wikimedia.org)
3. Male Reproductive Anatomy by OpenStax College licensed CC BY 3.0
4. Anatomical Illustration by Grant, John Charles Boileau licensed public domain, via Wikimedia Commons
5. Cross-section of the penis, by Gray's Anatomy, licensed public domain, via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.03%3A_Structures_of_the_Male_Reproductive_System.txt |
Colorful Sperm
The false-color image in Figure \(1\) shows real human sperm. The tiny gametes are obviously greatly magnified in the picture because they are actually the smallest of all human cells. In fact, human sperm cells are small, even when compared with sperm cells of other animals. Mice sperm are about twice the length of human sperm! Human sperm may be small in size, but in a normal, healthy man, huge numbers of them are usually released during each ejaculation. There may be hundreds of millions of sperm cells in a single teaspoon of semen. Producing sperm is one of the major functions of the male reproductive system.
Sperm Anatomy
A mature sperm cell has several structures that help it reach and penetrate an egg. These are labeled in the drawing of a sperm shown in Figure \(2\).
• The head is the part of the sperm that contains the nucleus — and not much else. The nucleus, in turn, contains tightly coiled DNA that is the male parent’s contribution to the genetic makeup of a zygote (if one forms). Each sperm is a haploid cell, containing half the chromosomal complement of a normal, diploid body cell.
• The front of the head is an area called the acrosome. The acrosome contains enzymes that help the sperm penetrate an egg (if it reaches one).
• The midpiece is the part of the sperm between the head and the flagellum tail. The midpiece is packed with mitochondria that produce the energy needed to move the flagellum.
• The flagellum (also called the tail) can rotate like a propeller, allowing the sperm to “swim” through the female reproductive tract to reach an egg if one is present.
Spermatogenesis
The process of producing sperm is known as spermatogenesis. Spermatogenesis normally starts when a boy reaches puberty, and it usually continues uninterrupted until death, although a decrease in sperm production generally occurs at older ages. A young, healthy male may produce hundreds of millions of sperm a day! Only about half of these, however, are likely to become viable, mature sperm.
Where Sperm Are Produced
Spermatogenesis occurs in the seminiferous tubules in the testes. Spermatogenesis requires high concentrations of testosterone. Testosterone is secreted by Leydig cells, which are adjacent to the seminiferous tubules in the testes.
Sperm production in the seminiferous tubules is very sensitive to temperature. This may be the most important reason the testes are located outside the body in the scrotum. The temperature inside the scrotum is generally about 2 degrees Celsius (almost 4 degrees Fahrenheit) cooler than core body temperature. This lower temperature is optimal for spermatogenesis. The scrotum regulates its internal temperature as needed by contractions of the smooth muscles lining the scrotum. When the temperature inside the scrotum becomes too low, the scrotal muscles contract. The contraction of the muscles pulls the scrotum higher against the body, where the temperature is warmer. The opposite occurs when the temperature inside the scrotum becomes too high.
Events of Spermatogenesis
Figure \(3\) summarizes of the main cellular events that occur in the process of spermatogenesis. The process begins with a diploid stem cell called a spermatogonium (plural, spermatogonia), and involves several cell divisions. The entire process takes at least ten weeks to complete, including maturation in the epididymis.
1. A spermatogonium undergoes mitosis to produce two diploid cells called primary spermatocytes. One of the primary spermatocytes goes on to produce sperm. The other replenishes the reserve of spermatogonia.
2. The primary spermatocyte undergoes meiosis I to produce two haploid daughter cells called secondary spermatocytes.
3. The secondary spermatocytes rapidly undergo meiosis II to produce a total of four haploid daughter cells called spermatids.
4. The spermatids begin to form a tail, and their DNA becomes highly condensed. Unnecessary cytoplasm and organelles are removed from the cells, and they form a head, midpiece, and flagellum. The resulting cells are sperm (spermatozoa).
As shown in Figure \(3\), the events of spermatogenesis begin near the wall of the seminiferous tubule — where spermatogonia are located — and continue inward toward the lumen of the tubule. Sertoli cells extend from the wall of the seminiferous tubule inward toward the lumen, so they are in contact with developing sperm at all stages of spermatogenesis. Sertoli cells play several roles in spermatogenesis:
• They secrete endocrine hormones that help regulate spermatogenesis.
• They secrete substances that initiate meiosis.
• They concentrate testosterone (from Leydig cells), which is needed at high levels to maintain spermatogenesis.
• They phagocytize the extra cytoplasm that is shed from developing sperm cells.
• They secrete a testicular fluid that helps carry sperm into the epididymis.
• They maintain a blood-testis barrier, so immune system cells cannot reach and attack the sperm
Maturation in the Epididymis
Although the sperm produced in the testes have tails, they are not yet motile (able to “swim”). The non-motile sperms are transported to the epididymis in the testicular fluid that is secreted by Sertoli cells with the help of peristaltic contractions. In the epididymis, the sperms gain motility, so they are capable of swimming up the female genital tract and reaching an egg. The mature sperms are stored in the epididymis until ejaculation occurs.
Ejaculation
Sperms are released from the body during ejaculation, which typically occurs during orgasm. Hundreds of millions of mature sperm — contained within a small amount of thick, whitish fluid called semen — are propelled from the penis during a normal ejaculation.
How Ejaculation Occurs
Ejaculation occurs when peristalsis of the muscle layers of the vas deferens and other accessory structures propel sperm from the epididymes, where mature sperm are stored. The muscle contractions force the sperm through the vas deferens and the ejaculatory ducts, and then out of the penis through the urethra. Due to the peristaltic action of the muscles, the ejaculation occurs in a series of spurts.
The Role of Semen
As sperms travel through the ejaculatory ducts during ejaculation, they mix with secretions from the seminal vesicles, prostate gland, and bulbourethral glands to form semen (Figure \(4\)). The average amount of semen per ejaculate is about 3.7 mL, which is a little less than a teaspoonful. Most of this volume of semen consists of glandular secretions, with the hundreds of millions of sperm cells actually contributing relatively little to the total volume.
The secretions in semen are important for the survival and motility of sperm. They provide a medium through which sperm can swim. They also include sperm-sustaining substances, such as high concentrations of the sugar fructose, which is the main source of energy for sperm. In addition, semen contains many alkaline substances that help neutralize the acidic environment in the female vagina. This protects the DNA in sperm from being denatured by the acid and prolongs the life of sperm in the female reproductive tract.
Erection
Besides providing a way for sperm to leave the body, the main role of the penis in reproduction is intromission or depositing sperm in the vagina of the female reproductive tract. Intromission depends on the ability of the penis to become stiff and erect, a state referred to as an erection. The human penis, unlike that of most other mammals, contains no erectile bone. Instead, in order to reach its erect state, it relies entirely on engorgement with the blood of its columns of spongy tissue. During sexual arousal, the arteries that supply blood to the penis dilate, allowing more blood to fill the spongy tissue. The now-engorged spongy tissue presses against and constricts the veins that carry blood away from the penis. As a result, more blood enters than leaves the penis, until a constant erectile size is achieved.
In addition to sperm, the penis also transports urine out of the body. These two functions cannot occur simultaneously. During an erection, the sphincters that prevent urine from leaving the bladder are controlled by centers in the brain so they cannot relax and allow urine to enter the urethra.
Testosterone Production
The final major function of the male reproductive system is the production of the male sex hormone testosterone. In mature males, this occurs mainly in the testes. Testosterone production is under the control of luteinizing hormone (LH) from the pituitary gland. LH stimulates Leydig cells in the testes to secrete testosterone.
Testosterone is important for male sexual development at puberty. It stimulates maturation of the male reproductive organs, as well as the development of secondary male sex characteristics (such as facial hair). Testosterone is also needed in mature males for normal spermatogenesis to be maintained in the testes. Follicle stimulating hormone (FSH) from the pituitary gland is also needed for spermatogenesis to occur, in part because it helps Sertoli cells in the testes concentrate testosterone to high enough levels to maintain sperm production. Testosterone is also needed for the proper functioning of the prostate gland. In addition, testosterone plays a role in erection, allowing sperm to be deposited within the female reproductive tract.
Feature: My Human Body
If you’re a man and you use a laptop computer on your lap for long periods of time, you may be decreasing your fertility. The reason? A laptop computer generates considerable heat, and its proximity to the scrotum during typical use results in a significant rise in temperature inside the scrotum. Spermatogenesis is very sensitive to high temperatures, so it may be adversely affected by laptop computer use. If you want to avoid the potentially fertility-depressing effect of laptop computer use, you might want to consider using your laptop computer on a table or other surface rather than on your lap — at least when you log on for long computer sessions. Other activities that raise scrotal temperature and have the potential to reduce spermatogenesis including soaking in hot tubs, wearing tight clothing, and biking. Although the effects of short-term scrotal heating on fertility seem to be temporary, years of such heat exposure may cause irreversible effects on sperm production.
Review
1. List parts of mature sperm.
2. What is spermatogenesis? When does it occur?
3. Where does spermatogenesis take place? State one role of Sertoli cells in spermatogenesis.
4. Summarize the steps of sperm production, naming the cells and processes involved.
5. What must happen to sperm before they are able to “swim”?
6. What is ejaculation?
7. Describe semen and its components.
8. Define intromission. How is it related to erection?
9. Explain how an erection occurs.
10. What cells secrete testosterone? What controls this process?
11. Identify the functions of testosterone in males.
12. Which of the following cells are haploid? Choose all that apply.
1. spermatids
2. spermatogonia
3. primary spermatocytes
4. secondary spermatocytes
5. mature sperm
13. Describe one way in which Leydig and Sertoli cells work together to maintain spermatogenesis.
14. True or False: When it is cold outside the body, the scrotal muscles relax.
15. True or False: During an erection, the arteries and veins of the penis dilate.
Explore More
How do queer couples have babies? Learn more here:
Watch this video to learn more about spermatogenesis:
Attributions
1. Sperm by Gilberto Santa Rosa licensed CC BY 2.0 via Wikimedia Commons
2. sperm anatomy by Anatomy & Physiology OpenStax CNX. CC BY 4.0 via lumen learning
3. spermatogenesis and seminiferous tubule by OpenStax College licensed CC BY 3.0 via Wikimedia.org
4. Human semen in a Petri dish by Digitalkil, public domain via Wikimedia Commons
5. Homework by Tony Alter, CC BY 2.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.04%3A_Functions_of_the_Male_Reproductive_System.txt |
Offering to the Gods
The marble penis and scrotum depicted in Figure \(1\) comes from ancient Rome, during the period from about 200 BCE to 400 CE. During that time, offerings like this were commonly given to the gods by people with health problems, either in the hopes of a cure or as thanks for receiving one. The offerings were generally made in the shape of the afflicted body part. Scholars think this marble penis and scrotum may have been an offering given in hopes of — or thanks for — a cure for impotence, known medically today as erectile dysfunction.
Erectile Dysfunction
Erectile dysfunction (ED) is sexual dysfunction characterized by the regular and repeated inability of a sexually mature individual to obtain or maintain an erection. It is a common disorder that affects about 40 percent of people with penises.
Causes of Erectile Dysfunction
The penis normally stiffens and becomes erect when the columns of spongy tissue within the shaft of the penis (the corpora cavernosa and corpus spongiosum) become engorged with blood. Anything that hampers normal blood flow to the penis may, therefore, interfere with its potential to fill with blood and become erect. The normal nervous control of sexual arousal or penile engorgement may also fail and lead to problems obtaining or maintaining an erection
Specific causes of ED include both physiological and psychological causes. Physiological causes include the use of therapeutic drugs (such as antidepressants), aging, kidney failure, diseases (such as diabetes or multiple sclerosis), tobacco smoking, and treatments for other disorders (such as prostate cancer). Psychological causes are less common but may include stress, performance anxiety, or mental disorders. The risk of ED may also be greater in people with obesity, cardiovascular disease, poor dietary habits, and overall poor physical health. Having an untreated hernia in the groin may also lead to ED.
Treatments for Erectile Dysfunction
Treatment of ED depends on its cause or contributing factors. For example, for tobacco smokers, smoking cessation may bring significant improvement in ED. Improving overall physical health by losing weight and exercising regularly may also be beneficial. The most common first-line treatment for ED, however, is the use of oral prescription drugs, known by brand names such as Viagra® and Cialis®. These drugs help ED by increasing blood flow to the penis. Other potential treatments include topical creams applied to the penis, injection of drugs into the penis, or the use of a vacuum pump that helps draw blood into the penis by applying negative pressure. More invasive approaches may be used as a last resort if other treatments fail. These usually involve surgery to implant inflatable tubes or rigid rods into the penis.
Ironically, the world’s most venomous spider —the Brazilian wandering spider (Figure \(2\)) — may offer a new treatment for ED. The venom of this spider is known to cause priapism in humans. Priapism is a prolonged erection that may damage the reproductive organs and lead to infertility if it continues too long. Researchers are investigating one of the components of the spider’s venom as a possible treatment for ED if taken in minute quantities.
Epididymitis
Epididymitis is inflammation of the epididymis. The epididymis is one of the paired organs within the scrotum where sperms mature and are stored. Discomfort or pain and swelling in the scrotum are typical symptoms of epididymitis, which is a relatively common condition, especially in young individuals. In the U.S. alone, more than half a million cases of epididymitis are diagnosed annually between the ages of 18 to 35.
Acute vs. Chronic Epididymitis
Epididymitis may be acute or chronic. Acute diseases are generally short-term conditions, whereas chronic diseases may last years — or even lifelong.
Acute Epididymitis
Acute epididymitis generally has a fairly rapid onset and is most often caused by a bacterial infection. Bacteria in the urethra can back-flow through the urinary and reproductive structures to the epididymis. In sexually active individuals, many cases of acute epididymitis are caused by sexually transmitted bacteria. Besides pain and swelling, common symptoms of acute epididymitis include redness, warmth in the scrotum, and a fever. There may also be a urethral discharge.
Chronic Epididymitis
Chronic epididymitis is epididymitis that lasts for more than three months. In some, the condition may last for years. It may occur with or without a bacterial infection being diagnosed. Sometimes, it is associated with lower back pain that occurs after an activity that stresses the lower back, such as heavy lifting or a long period spent driving a vehicle.
Treatment of Epididymitis
If a bacterial infection is suspected, both acute and chronic epididymitis are generally treated with antibiotics. For chronic epididymitis, antibiotic treatment may be prescribed for as long as four to six weeks to ensure the complete eradication of any possible bacteria. Additional treatments often include anti-inflammatory drugs to reduce inflammation of the tissues and painkillers to control the pain, which may be severe. Physically supporting the scrotum and applying cold compresses may also be recommended to help relieve swelling and pain.
Regardless of symptoms, treatment is important for both acute and chronic epididymitis, because major complications may occur otherwise. Untreated acute epididymitis may lead to an abscess — which is a buildup of pus — or to the infection spreading to other organs. Untreated chronic epididymitis may lead to permanent damage to the epididymis and testis, and it may even cause infertility.
Male Reproductive Cancers
Why does the Brazilian hospital pictured in Figure \(3\) have a huge blue mustache on its “face”? The mustache is a symbol of “Movember.” This is an international campaign to raise awareness of prostate cancer, as well as money to fund prostate cancer research.
Prostate Cancer
The prostate gland is an organ located in the male pelvis. The urethra passes through the prostate gland after it leaves the bladder and before it reaches the penis. The function of the prostate is to secrete zinc and other substances into semen during ejaculation. In the United States, prostate cancer is the most common type of cancer and the second leading cause of cancer death in people carrying prostate gland. About 80 percent of American individuals with the prostate will have cancerous cells in their prostate gland by the age of 80.
How Prostate Cancer Occurs
Prostate cancer occurs when glandular cells of the prostate mutate into tumor cells. Eventually, the tumor, if undetected, may invade nearby structures, such as the seminal vesicles. Tumor cells may also metastasize and travel in the bloodstream or lymphatic system to organs elsewhere in the body. Prostate cancer most commonly metastasizes to the bones, lymph nodes, rectum, or lower urinary tract organs.
Symptoms of Prostate Cancer
Early in the course of prostate cancer, there may be no symptoms. When symptoms do occur, they mainly involve urination, because the urethra passes through the prostate gland. The symptoms typically include frequent urination, difficulty starting and maintaining a steady stream of urine, blood in the urine, and painful urination. Prostate cancer may also cause problems with sexual function, such as difficulty achieving an erection or painful ejaculation.
Risk Factors for Prostate Cancer
Some factors that increase the risk of prostate cancer can be changed, and others cannot.
• Risk factors that can be changed include a diet high in meat, a sedentary lifestyle, obesity, and high blood pressure.
• Risk factors that cannot be changed include older age, a family history of prostate cancer, and African ancestry. Family history is an important risk factor, so genes are clearly involved. Many different genes have been implicated.
Diagnosing Prostate Cancer
The only definitive test to confirm a diagnosis of prostate cancer is a biopsy. In this procedure, a small piece of the prostate gland is surgically removed and then examined microscopically. A biopsy is done only after less invasive tests have found evidence that a patient may have prostate cancer.
A routine exam by a doctor may find a lump on the prostate, which might be followed by a blood test that detects an elevated level of prostate-specific antigen (PSA). PSA is a protein secreted by the prostate that normally circulates in the blood. Higher-than-normal levels of PSA can be caused by prostate cancer, but they may also have other causes. Ultrasound or magnetic resonance imaging (MRI) might also be undertaken to provide images of the prostate gland and additional information about cancer.
Treatment of Prostate Cancer
The average age at which men are diagnosed with prostate cancer is 70. Prostate cancer typically is such a slow-growing cancer that elderly patients may not require treatment. Instead, the patients are watched carefully over the subsequent years to make sure the cancer isn’t growing and posing an immediate threat — an approach that is called active surveillance. It is used for at least 50 percent of patients who are expected to die from other causes before their prostate cancer causes symptoms.
Treatment of younger patients — or those with more aggressively growing tumors — may include surgery to remove the prostate, chemotherapy, and/or radiation therapy (such as brachytherapy, see Figure \(4\)). All of these treatment options can have significant side effects, such as erectile dysfunction or urinary incontinence. Patients should learn the risks and benefits of the different treatments, and discuss them with their healthcare provider to decide on the best treatment options for their particular case.
Testicular Cancer
Reproductive cancer that is rare and most commonly affects young individuals is testicular cancer. The testes are the paired reproductive organs in the scrotum that produce sperm and secrete testosterone. The risk of testicular cancer is about four to five times greater in individuals of European than African ancestry. The cause of this difference is unknown.
Signs and Symptoms of Testicular Cancer
One of the first signs of testicular cancer is often a lump or swelling in one of the two testes. The lump may or may not be painful. If pain is present, it may occur as a sharp pain or a dull ache in the lower abdomen or scrotum. Some people with testicular cancer report a feeling of heaviness in the scrotum. Testicular cancer does not commonly spread beyond the testis, but if it does, it most often spreads to the lungs, where it may cause shortness of breath or a cough.
Diagnosis of Testicular Cancer
The main way that testicular cancer is diagnosed is by detection of a lump in the testis. This is likely followed by further diagnostic tests. An ultrasound may be done to determine the exact location, size, and characteristics of the lump. Blood tests may be done to identify and measure tumor-marker proteins in the blood that are specific to testicular cancer. CT scans may also be done to determine whether the disease has spread beyond the testis. However, unlike the case with prostate cancer, a biopsy is not recommended, because it increases the risk of cancer cells spreading into the scrotum.
Treatment of Testicular Cancer
Testicular cancer has one of the highest cure rates of all cancers. Three basic types of treatment for testicular cancer are surgery, radiation therapy, and/or chemotherapy. Generally, the initial treatment is surgery to remove the affected testis. If the cancer is caught at an early stage, the surgery is likely to cure the cancer and has nearly a 100 percent five-year survival rate. When just one testis is removed, the remaining testis (if healthy) is adequate to maintain fertility, hormone production, and other normal functions. Radiation therapy and/or chemotherapy may follow surgery to kill any tumor cells that might exist outside the affected testis, even when there is no indication that cancer has spread. In many cases, however, surgery is followed by surveillance instead of additional treatments.
Review
1. What is erectile dysfunction? When does it occur?
2. Underlying causes of erectile dysfunction may include physiological and/or psychological factors. Identify some of these factors.
3. Discuss types of treatment for erectile dysfunction.
4. Define epididymitis. What is its most common cause?
5. Identify possible treatments for epididymitis. Why is treatment important, even when there are no symptoms?
6. Rank prostate cancer as a cause of cancer and cause of cancer death in men. What are some of the symptoms of prostate cancer?
7. List risk factors for prostate cancer.
8. How is prostate cancer detected?
9. In many cases, treatment for prostate cancer is unnecessary. Why? When is treatment necessary, and what are treatment options?
10. Testicular cancer is generally rare, but it is the most common cancer in one age group. What age group is it?
11. Identify possible signs and symptoms of testicular cancer.
12. How can testicular cancer be diagnosed?
13. Describe how testicular cancer is typically treated.
14. Which of the following is common in younger individuals (i.e. under age 39)?
1. prostate cancer
2. testicular cancer
3. epididymitis
4. B and C
15. A biopsy is important in cases of suspected:
1. epididymitis
2. testicular cancer
3. prostate cancer
4. B and C
Attributions
1. Votive male genitalia by Wellcome Images, CC BY 4.0 via Wikimedia Commons
2. Phoneutria nigriventer by João P. Burini, CC BY-SA 3.0 via Wikimedia Commons
3. Novembro Azul by Marcelo Camargo / Agência Brasil, CC BY 3.0 via Wikimedia Commons
4. Brachytherapy by James Heilman, MD, CC BY-SA 4.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.05%3A_Disorders_of_the_Male_Reproductive_System.txt |
Fertility Symbol
The geometric design on this ancient stone carving represents a powerful symbol of fertility: the vagina. The symbol is called yoni in Hindu, and it reflects the value placed by Hindu culture on the ability to give birth. The vagina is one of several organs in the female reproductive system.
Female Reproductive Organs
The female reproductive system is made up of internal and external organs that function to produce haploid gametes called eggs (or oocytes), secrete sex hormones (such as estrogen), and carry and give birth to a fetus. As shown in Figure \(2\), the internal reproductive organs include the vagina, uterus, Fallopian (uterine) tubes, and ovaries. The external organs — collectively called the vulva — include the clitoris and labia.
The vagina is an elastic, muscular canal leading from its opening in the vulva to the neck of the uterus, called the cervix. It is about 7.5 cm (3.0 in.) long at the front, and about 9 cm (3.5 in.) long at the back. The vagina accommodates the penis and is the site where sperm are usually ejaculated during sexual intercourse. In the context of pregnancy and natural (vaginal) childbirth, the vagina is also referred to as the birth canal. In addition, it channels the flow of menstrual blood from the uterus.
Structure of the Vagina
The vagina, shown at the bottom of Figure \(2\), is a muscular canal (approximately 10 cm long) that serves as the entrance to the reproductive tract. It also serves as the exit from the uterus during menses and childbirth. The outer walls of the anterior and posterior vagina are formed into longitudinal columns or ridges, and the superior portion of the vagina—called the fornix—meets the protruding uterine cervix. The walls of the vagina are lined with an outer, fibrous adventitia; a middle layer of smooth muscle; and an inner mucous membrane with transverse folds called rugae. Together, the middle and inner layers allow the expansion of the vagina to accommodate intercourse and childbirth. The thin, perforated hymen can partially surround the opening to the vaginal orifice. The hymen can be ruptured with strenuous physical exercise, penile-vaginal intercourse, and childbirth. The Bartholin’s glands and the lesser vestibular glands (located near the clitoris) secrete mucus, which keeps the vestibular area moist.
The vagina is home to a normal population of microorganisms that help to protect against infection by pathogenic bacteria, yeast, or other organisms that can enter the vagina. In a healthy individual, the most predominant type of vaginal bacteria is from the genus Lactobacillus. This family of beneficial bacterial flora secretes lactic acid, and thus protects the vagina by maintaining an acidic pH (below 4.5). Potential pathogens are less likely to survive in these acidic conditions. Lactic acid, in combination with other vaginal secretions, makes the vagina a self-cleaning organ. However, douching—or washing out the vagina with fluid—can disrupt the normal balance of healthy microorganisms, and actually increase the risk for infections and irritation. Indeed, the American College of Obstetricians and Gynecologists recommend that people do not douche and that they allow the vagina to maintain its normal healthy population of protective microbial flora.
Uterus
The uterus (commonly called the womb) is a pear-shaped, muscular organ that is about 7.6 cm (3 in.) long. It is located above the vagina and behind the bladder in the center of the pelvis. The position of the uterus in the pelvis is stabilized by several ligaments and bands of supportive tissue. The uterus is where a fetus develops during gestation, and the organ provides mechanical protection and support for the developing offspring. Contractions of the muscular wall of the uterus are responsible for pushing the fetus out of the uterus during childbirth.
Parts of the Uterus
As shown in Figure \(3\), the lower end of the uterus forms the cervix, which is also called the neck of the uterus. The cervix is about 2.5 cm (1 in.) long and protrudes downward into the vagina. A small canal runs the length of the cervix, connecting the uterine cavity with the lumen of the vagina. This allows semen deposited in the vagina to enter the uterus, and a baby to pass from the uterus into the vagina during birth. Glands in the cervix secrete mucus that varies in water content and thickness, so it can function either as a barrier to keep microorganisms out of the uterus during pregnancy or as a transport medium to help sperm enter the uterus around the time of ovulation. The rest of the uterus above the cervix is called the body of the uterus. The upper end of the uterus is connected with the two Fallopian tubes.
Tissues of the Uterus
As indicated in Figure \(3\), the uterus consists of three tissue layers, called the endometrium, myometrium, and perimetrium.
• The endometrium is the innermost tissue layer of the uterus. It consists of epithelial tissue, including mucous membranes. This layer thickens during each menstrual cycle and, unless an egg is fertilized, sloughs off during the following menstrual period. If an egg is fertilized, the thickened endometrium is maintained by hormones and provides nourishment to the embryo. As gestation progresses, the endometrium develops into the maternal portion of the placenta. The placenta is a temporary organ that consists of a mass of maternal and fetal blood vessels through which the mother’s and fetus’s blood exchange substances.
• The myometrium is the middle layer of the uterus. It consists mostly of a thick layer of smooth muscle tissue. Powerful contractions of the smooth muscle allow the uterus to contract and expel an infant during childbirth.
• The perimetrium is the outermost layer of the uterus. It covers the outer surface of the uterus. This layer actually consists of two layers of epithelium that secrete fluid into the space between them. The fluid allows for small movements of the uterus within the pelvis, without causing it to rub against other organs.
Fallopian Tubes
The Fallopian tubes (Figure \(2\)) are two thin tubes that lie between the ovaries and the uterus. The Fallopian tubes are not attached to the ovaries, but their broad upper ends — called infundibula — lie very close to the ovaries. The infundibula also have fringe-like extensions called fimbriae that move in a waving motion to help guide eggs from the ovaries into the Fallopian tubes. The lower ends of the Fallopian tubes are attached to the upper part of the body of the uterus on either side of the body. They open into the uterus. Egg is fertilized in the fallopian tubes.
Ovaries
The ovaries are the gonads. Paired ovals, they are each about 2 to 3 cm in length, about the size of an almond. The ovaries are located within the pelvic cavity and are supported by many ligaments. The ovaries are the ovum-producing organs of the internal reproductive system. The ovary is an ovum-producing reproductive organ, typically found in pairs. Ovaries are analogous to testes in that both are gonads and endocrine glands. Ovaries secrete both estrogen and progesterone. Estrogen is responsible for the appearance of secondary sex characteristics of females at puberty and for the maturation and maintenance of the reproductive organs in their mature functional state. Progesterone functions with estrogen by promoting menstrual cycle changes in the endometrium.
Vulva
The external reproductive structures are referred to collectively as the vulva (Figure \(4\)). The mons pubis is a pad of fat that is located at the anterior, over the pubic bone. After puberty, it becomes covered in pubic hair. The labia majora (labia = “lips”; majora = “larger”) are folds of hair-covered skin that begin just posterior to the mons pubis. The thinner and more pigmented labia minora (labia = “lips”; minora = “smaller”) extend medially to the labia majora. Although they naturally vary in shape and size, the labia minora serve to protect the urethra and the entrance to the reproductive tract.
The superior, anterior portions of the labia minora come together to encircle the clitoris (or glans clitoris), an organ that originates from the same cells as the glans penis and has abundant nerves that make it important in sexual sensation and orgasm. The hymen is a thin membrane that sometimes partially covers the entrance to the vagina. An intact hymen cannot be used as an indication of “virginity”; even at birth, this is only a partial membrane, as menstrual fluid and other secretions must be able to exit the body, regardless of penile-vaginal intercourse. The vaginal opening is located between the opening of the urethra and the anus. It is flanked by outlets to the Bartholin’s glands (or greater vestibular glands).
Breasts
The breasts are not directly involved in reproduction, but because they contain mammary glands, they can provide nourishment to an infant after birth. The breasts overlay major muscles in the chest from which they project outward in a conical shape. The two main types of tissues in the breast are adipose (fat) tissue and glandular tissue that produces milk. As shown in Figure \(6\), each mature breast contains many lobules, where milk is produced and stored during pregnancy.
During nursing (or lactation), the milk drains into ducts and sacs, which in turn converge at the nipple. Milk exits the breast through the nipple in response to the suckling action of an infant. The nipple is surrounded by a more darkly pigmented area called the areola. The areola contains glands that secrete an oily fluid, which lubricates and protects the nipple during breastfeeding.
In 2018, during a survey, the respondents mentioned about two protocols of lactation induction in a trans woman, Zil Goldstein and Newman-Goldfarb protocols, which was initially designed for a cis woman to nurse a baby born to a surrogate mother. The Zil Goldstein protocol starts with 10 mg domperidone (a drug) three times daily while the Newman-Goldfarb protocol recommends 10 mg domperidone four times daily. Both regimens subsequently increase the dose to 20 mg four times daily. Since domperidone is not approved by the Food and Drug Administration, the patients obtain the drug elsewhere. Both regimens also utilize estradiol and progesterone sex hormones. Subsequently, the physical stimulation of the nipples is recommended for milk production. Informal reports of trans women who induced lactation have emerged recently. However, there are no data to support any of the other reports. There is a critical gap in evidence-based medicine for this population.
Review
1. State the general functions of the female reproductive system.
2. Describe the vagina and its reproductive functions.
3. Outline the structure and basic functions of the uterus.
4. What is the endometrium? How does it change during the monthly cycle?
5. Relate features of the Fallopian tubes to the functions of these structures.
6. Define ovary.
7. Identify the functional units of the ovaries.
8. Discuss the timing of egg production and ovulation in the context of a female’s lifetime.
9. What is the vulva? What structures does it include?
10. Why are breasts included in discussions of reproduction, if they are not organs of the female reproductive system?
11. What is the function of the folds in the mucous membrane lining of the vagina?
12. What are two ways in which the female reproductive system protects itself from pathogens?
13. True or False: The fallopian tube runs through the cervix and allows sperm to enter the uterus.
14. True or False: The nipple and the areola are not the same things.
15. Immature eggs are located in __________, which are located within _________ .
Attributions
1. Cattien stone yoni by Binh Giang, public domain via Wikimedia Commons
2. Female reproductive organs by The Open University, licensed CC BY-NC-SA 4.0
3. Uterus regions by The Open University, licensed CC BY-NC-SA 4.0
4. Vulva by The Open University, licensed CC BY-NC-SA 4.0
5. Vulva by OpenStax, licensed CC BY 3.0
6. Breast diagram by NCI NIH, public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
8. Some text is adapted from Trautner, Emily et al. “Knowledge and practice of induction of lactation in trans women among professionals working in trans health.” International breastfeeding journal vol. 15,1 63. 16 Jul. 2020, doi:10.1186/s13006-020-00308-6; CC BY 4.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.06%3A_Structures_of_the_Female_Reproductive_System.txt |
Taboo Topic
The banner in Figure \(1\) was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and the lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year that symbolizes the five days of bleeding during menstruation each month. The 28th day was chosen because the menstrual cycle averages about 28 days.
What Is the Menstrual Cycle?
The menstrual cycle refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of eggs and the preparation of the uterus for pregnancy. It involves changes in both the ovaries and the uterus and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period, when bleeding from the uterus begins as the built-up endometrium lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if pregnancy does not occur. In the ovaries, the menstrual cycle includes the development of a follicle, ovulation of a secondary oocyte, and the degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.
Menarche and Menopause
The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as menarche. There is considerable variation among individuals in the age of menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.
The cessation of menstrual cycles at the end of a woman’s reproductive years is termed menopause. The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors. It may occur earlier as a result of certain illnesses or medical treatments.
Variation in the Menstrual Cycle
The length of the menstrual cycle — as well as its phases — may vary considerably, not only among different individuals but also from month to month for a given person. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman’s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.
Ovarian Cycle
The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle. It consists of changes that occur in the follicles of one of the ovaries. The ovarian cycle is divided into the following three phases: the follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure \(2\).
Follicular Phase
The follicular phase is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several ovarian follicles are stimulated to begin maturing, but usually only one — called the Graafian follicle — matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of follicle-stimulating hormone (FSH), which is secreted by the pituitary gland. The maturing follicle releases estrogen, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the chart in Figure \(3\).
Ovulation
Ovulation is the second phase of the ovarian cycle. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its egg. Ovulation is stimulated by a sudden rise in the blood level of luteinizing hormone (LH) from the pituitary gland. This is called the LH surge. You can see the LH surge in the top hormone graph above. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the follicular phase, the rising estrogen level actually suppresses LH secretion by the pituitary. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and estrogen stimulates the release of a large amount of LH. The surge in LH matures the egg and weakens the wall of the follicle, causing the fully developed follicle to release its secondary oocyte.
Luteal Phase
The luteal phase is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, FSH and LH cause the Graafian follicle that ovulated the egg to transform into a structure called a corpus luteum. The corpus luteum secretes progesterone, which in turn suppresses FSH and LH production by the pituitary and stimulates the continued buildup of the endometrium in the uterus. How this phase ends depends on whether or not the egg has been fertilized.
• If fertilization has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).
• If fertilization has occurred so a zygote forms and then divides to become a blastocyst, the outer layer of the blastocyst produces a hormone called human chorionic gonadotropin. This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.
Uterine Cycle
The events of the menstrual cycle that take place in the uterus make up the uterine cycle. This cycle consists of changes that occur mainly in the endometrium, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure \(4\).
Menstruation
Menstruation (also called the menstrual period or menses) is the first phase of the uterine cycle. It occurs if fertilization has not taken place during the preceding menstrual cycle. During menstruation, the endometrium of the uterus, which has built up during the preceding cycle, degenerates and is shed from the uterus. The average loss of blood during menstruation is about 35 mL. The flow of blood is often accompanied by uterine cramps, which may be severe in some people.
Proliferative Phase
The proliferative phase is the second phase of the uterine cycle. During this phase, estrogen secreted by cells of the maturing ovarian follicle causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the cervix of the uterus to secrete larger amounts of thinner mucus that can help sperm swim through the cervix and into the uterus, making fertilization more likely.
Secretory Phase
The secretory phase is the third and final phase of the uterine cycle. During this phase, progesterone produced by the corpus luteum in the ovary stimulates further changes in the endometrium so it is more receptive to implantation of a blastocyst. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of smooth muscle tissue in the uterine wall.
My body: Menstruators, Not Menstruating Women
Within the field of critical menstruation studies, we must pay attention to our depictions of menstruation and menstruators, and the knowledge we produce in the pursuit to de-stigmatize menstruation. Not all women menstruate, for example, trans women, postmenopausal women, pregnant women, and those experiencing amenorrhea, and not all who menstruate are women, for example, transmen. Experiences of menstruating later in life vary among menstruators as well. Some do not suffer from their periods in direct relation to their gender identity. Others do, as they disidentify with the body as a whole and/or with certain body parts such as the genitalia or the uterus, or with the bodily function of menstruation. This suffering is sometimes related to gender dysphoria. Testosterone treatments are a method adopted by some trans menstruators to get rid of unwanted bleeding. Preventing the menstrual period is not necessarily the main reason for using testosterone, but it can be one among several desired outcomes. Menstruators are of a variety of gender identities (far beyond those who identify as trans) and, hence, menstruation cannot be equated singularly with cis/womanhood.
Review
1. What is the menstrual cycle?
2. Why is the menstrual cycle necessary in order for pregnancy to occur?
3. What organs are involved in the menstrual cycle? What hormones control the cycle?
4. Identify the two major events that mark the beginning and end of the reproductive period in females. When do these events typically occur?
5. Discuss the average length of the menstrual cycle and menstruation, as well as variations that are considered normal.
6. Define the ovarian cycle.
7. Summarize the phases of the ovarian cycle.
8. Compare and contrast events that occur in the ovaries and uterus, depending on whether or not an egg is fertilized during the menstrual cycle.
9. Define the uterine cycle.
10. Give an overview of the phases of the uterine cycle.
11. If the LH surge did not occur in a menstrual cycle, what do you think would happen? Explain your answer.
12. Give one reason why FSH and LH levels drop in the luteal phase of the menstrual cycle.
13. What does the follicle that housed the ovulated egg become in the luteal phase of the menstrual cycle?
14. True or False: Day 1 of the menstrual cycle is when the secondary oocyte is released from its follicle.
15. True or False: The secretory phase of the uterine cycle generally aligns with the luteal phase of the ovarian cycle.
Explore More
Have you ever heard of premenstrual syndrome, also known as PMS? Learn more about what it is and why some women get it here:
Attributions
1. Menstrual Hygiene Day by Water for People, CC BY 2.0 via Wikimedia Commons
2. Ovarian cycle by CK-12 licensed CC BY-NC 3.0
3. Pituitary and ovarian hormone levels by OpenStax College CC BY 3.0 via Wikimedia Commons
4. Menstrual cycle by OpenStax College, CC BY 3.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
6. Some text is adapted from Rydström K. (2020) Degendering Menstruation: Making Trans Menstruators Matter. In: Bobel C., Winkler I.T., Fahs B., Hasson K.A., Kissling E.A., Roberts TA. (eds) The Palgrave Handbook of Critical Menstruation Studies. Palgrave Macmillan, Singapore. https://doi.org/10.1007/978-981-15-0614-7_68; CC BY 4.0. | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.07%3A_Menstrual_Cycle.txt |
Waiting Expectantly
A mother-to-be waits patiently for her fetus to grow as her belly swells. Reproduction is all about making babies, and the female reproductive system is specialized for this purpose. Its functions include producing gametes called eggs, secreting sex hormones (such as estrogen), providing a site for fertilization, gestating a fetus if fertilization occurs, giving birth to a baby, and breastfeeding a baby after birth. The only thing missing is sperm.
Egg Production
At birth, ovaries contain all the eggs that will ever be produced, which may include a million or more eggs. The eggs don't start to mature, however, until a female enters puberty and attains sexual maturity. After that, one egg typically matures each month and is released from an ovary. This continues until menopause (cessation of monthly periods), typically by age 52. By then, viable eggs may be almost depleted, and hormone levels can no longer support the monthly cycle. During the reproductive years, which of the two ovaries releases an egg in a given month seems to be a matter of chance. Occasionally, both ovaries will release an egg at the same time. If both eggs are fertilized, the offspring are fraternal twins (dizygotic, or "two-zygote," twins), and they are no more alike genetically than non-twin siblings.
Oogenesis
The process of producing eggs in the ovaries of a fetus carrying XX chromosomes is called oogenesis. Eggs are haploid gametes, and their production occurs in several steps that involve different types of cells, as summarized in Figure \(2\). Oogenesis is completed long before birth. It occurs when diploid germ cells called oogonia (singular, oogonium) undergo mitosis. Each such cell division produces two diploid cells. One is called the primary oocyte, and the other is retained to help maintain a reserve of oogonia. The primary oocyte, in turn, starts to go through the first cell division of meiosis (meiosis I). However, it does not complete meiosis I until puberty.
Maturation of a Follicle
Beginning in puberty, about once a month, one of the follicles in an ovary undergoes maturation, and an egg is released. As the follicle matures, it goes through changes in the numbers and types of its cells. The primary oocyte within the follicle also resumes meiosis. It completes meiosis I, which began long before birth, to form a secondary oocyte and a smaller cell, called the first polar body. Both the secondary oocyte and the first polar body are haploid cells. The secondary oocyte has most of the cytoplasm from the primary oocyte and is much larger than the first polar body, which soon disintegrates and disappears. The secondary oocyte begins meiosis II, but only completes it if the egg is fertilized.
Release of an Egg
It typically takes 12 to 14 days for a follicle to mature in an ovary, and for the secondary oocyte to form. Then, the follicle bursts open and the ovary ruptures, releasing the secondary oocyte from the ovary. This event is called ovulation. The now-empty follicle starts to change into a structure called a corpus luteum. The expelled secondary oocyte is usually swept into the nearby Fallopian tube by its waving, fingerlike fimbriae.
Uterine Changes
While the follicle is maturing in the ovary, the uterus is also undergoing changes to prepare it for an embryo if fertilization occurs. For example, the endometrium gets thicker and becomes more vascular. Around the time of ovulation, the cervix undergoes changes that help sperm reach the oocyte to fertilize it. The cervical canal widens, and the cervical mucus becomes thinner and more alkaline. These changes help promote the passage of sperm from the vagina into the uterus and make the environment more hospitable to sperm.
Fertilization — or Not
Fertilization of an egg by a sperm normally occurs in a Fallopian tube, most often in the part of the tube that passes above the ovary (Figure \(3\)). In order for fertilization to occur, sperm must “swim” from the vagina where they are deposited, through the cervical canal to the uterus and then through the body of the uterus to one of the Fallopian tubes. Once sperm enters a Fallopian tube, tubular fluids help carry them through the tube toward the secondary oocyte at the other end. The secondary oocyte also functions to promote fertilization. It releases molecules that guide the sperm and allow the surface of the egg to attach to the surface of the sperm. The egg can then absorb the sperm, allowing fertilization to occur.
If Fertilization Occurs
If the secondary oocyte is fertilized by a sperm as it passes through the Fallopian tube, the secondary oocyte quickly completes meiosis II, forming a diploid zygote and another polar body. (This second polar body, like the first, normally breaks down and disappears.) The zygote then continues the journey through the fallopian tube to the uterus, during which it undergoes several mitotic cell divisions. By the time it reaches the uterus up to five days after fertilization, it consists of a ball of cells called a blastocyst. Within another day or two, the blastocyst implants itself in the endometrium lining the uterus, and gestation begins.
If Fertilization Does Not Occur
What happens if the secondary oocyte is not fertilized by a sperm as it passes through the Fallopian tube? It continues on its way to the uterus without ever completing meiosis II. It is likely to disintegrate within a few days while still in the Fallopian tube. Any remaining material will be shed from the woman’s body during the next menstrual period.
Pregnancy
Pregnancy is the carrying of one or more offspring from fertilization until birth. This is one of the major functions of the female reproductive system. It involves virtually every other body system including the cardiovascular, urinary, and respiratory systems, to name just three. The maternal organism plays a critical role in the development of the offspring. They must provide all the nutrients and other substances needed for the normal growth and development of the offspring, and they must also remove the wastes excreted by the offspring. Most nutrients are needed in greater amounts by a pregnant individual to meet fetal needs, but some are especially important, including folic acid, calcium, iron, and omega-3 fatty acids. A healthy diet, along with prenatal vitamin supplements, is recommended for the best pregnancy outcome. A pregnant person should also avoid ingesting substances (such as alcohol) that can damage the developing offspring, especially early in the pregnancy when all of the major organs and organ systems are forming.
When counted from the first day of the last menstrual period, the average duration of pregnancy is about 40 weeks (38 weeks when counted from the time of fertilization), but a pregnancy that lasts between 37 and 42 weeks is still considered within the normal range. From the point of view of the maternal organism, the total duration of pregnancy is typically divided into three periods, called trimesters, each of which lasts about three months. This division of the total period of gestation is useful for summarizing the typical changes during pregnancy. From the point of view of the developing offspring, however, the major divisions are different. They are the embryonic and fetal stages. The offspring is called an embryo from the time it implants in the uterus through the first eight weeks of life. After that, it is called a fetus for the duration of the pregnancy.
First Trimester
The first trimester begins at the time of fertilization and lasts for the next 12 weeks. Even before a pregnant person knows they are pregnant, they may experience signs and symptoms of pregnancy. They may notice a missed menstrual period, and they may also experience a tender nipple area, increased appetite, and more frequent urination. Many individuals also experience nausea and vomiting in the first trimester. This is often called “morning sickness” because commonly occurs in the morning, but it may occur at any time of day. Some lose weight during the first trimester because of morning sickness.
Second Trimester
The second trimester occurs during weeks 13 to 28 of pregnancy. A pregnant person may feel more energized during this trimester. If she experienced nausea and vomiting during the first trimester, these symptoms often subside during the second trimester. Weight gain starts occurring during this trimester, as well. By about week 20, the fetus is getting large enough that the mother can feel its movements. The photo on the left in Figure \(4\) shows a pregnant woman at week 26, toward the end of the second trimester. (For comparison, the same woman is shown on the right of Figure \(4\) at the end of the third trimester.)
Third Trimester
The third trimester occurs during weeks 29 through birth (at about 40 weeks). During this trimester, the uterus expands rapidly, making up a larger and larger portion of the woman's abdomen. Weight gain is also more rapid. During the third trimester, the movements of the fetus become stronger and more frequent, and they may become disruptive to the mother. As the fetus grows larger, its weight and the space it takes up may lead to symptoms in the mother such as back pain, swelling of the lower extremities, more frequent urination, varicose veins, and heartburn. By the end of the third trimester, the woman's abdomen often will transform in shape as it drops, due to the fetus turning to a downward position before birth so its head rests on the cervix. This relieves pressure on the upper abdomen, but reduces bladder capacity and increases pressure on the pelvic floor and rectum.
Childbirth
Near the time of birth, the amniotic sac — a fluid-filled membrane that encloses the fetus within the uterus — breaks in a gush of fluid. This is commonly called “breaking water.” Labor usually begins within a day of this event, although it may begin prior to it. Labor is the general term for the process of childbirth in which regular uterine contractions push the fetus and placenta out of the body. Labor can be divided into three stages, which are illustrated in Figure \(5\): dilation, birth, and after birth.
1. During the dilation stage of labor, uterine contractions begin and become increasingly frequent and intense. The contractions push the baby’s head (most often) against the cervix, causing the cervical canal to dilate, or become wider. This lasts until the cervical canal has dilated to about 10 cm (3.9 in.) in width, which may take 12 to 20 hours — or even longer. The cervical canal must be dilated to this extent in order for the baby’s head to fit through it.
2. During birth, the baby descends (usually headfirst) through the cervical canal and vagina, and into the world outside. This is the stage when the mother generally starts bearing down during the contractions to help push out the fetus. The fetus exits the vagina with its face facing the mother's posterior. It takes a 90-degree turn as the shoulders are delivered. This stage may last from about 20 minutes to two hours or more. Usually, within a minute or less of birth, the umbilical cord is cut, so the baby is no longer connected to the placenta.
3. During the afterbirth stage, the placenta is delivered. This stage may last from a few minutes to a half hour.
Delivery is a matter of concern for all individuals, however, additional concerns arise for individuals with disabilities. According to a study done by Lipson and Rogers, delivery decisions for disabled individuals are made arbitrarily without their content. They are forced to deliver via cesarean section because they think that a disabled person wouldn't be able to handle the process.
Feature: Myth vs. Reality
There are many myths associated with pregnancy. Most are harmless, but some may put the pregnant woman or fetus at risk. As always, knowledge is power.
Myth: You should avoid petting your cat during pregnancy.
Reality: Cat feces may be contaminated with microscopic parasites that can cause a disease called toxoplasmosis. Pregnant women who contract this disease are at risk of stillbirth, miscarriage, or giving birth to an infant with serious health problems. Pregnant women should not have contact with a cat’s litter box or feces, but petting a cat poses no real risk of infection.
Myth: You should not dye your hair during pregnancy, because the chemicals can harm the fetus.
Reality: Whereas some chemicals (such as certain pesticides) have been shown to be associated with birth defects, there is no evidence that using hair dye during pregnancy increases this risk.
Myth: A pregnant woman needs to eat for two, so she should double her pre-pregnancy caloric intake.
Reality: Throughout a typical pregnancy, a person needs only about 300 extra calories per day, on average, to support her growing fetus. Most of the extra calories are needed during the last trimester when the fetus is growing most rapidly. Doubling her caloric intake during pregnancy is likely to cause too much weight gain, which can be detrimental to her baby. Babies that weigh much more than the average 7.5 pounds at birth are more likely to develop diabetes and obesity in later life.
Myth: Women who are pregnant have strange food cravings, such as ice cream with pickles.
Reality: Some women do have food cravings during pregnancy, but they are not necessarily cravings for strange foods or unusual food combinations. For example, a pregnant woman might crave starchy foods for a few weeks, or she may be put off by certain foods that she loved before pregnancy.
Myth: A pregnant woman has skin that glows.
Reality: Pregnancy can actually be hard on the skin and its appearance. Besides stretch marks on the abdomen and breasts, pregnancy may lead to spider veins, varicose veins, new freckles, darkening of moles, and acne flare-ups. In addition, as many as 75 percent of pregnant women experience chloasma, which is the emergence of blotchy brown patches of skin on the face due to high estrogen levels. Chloasma is often referred to as the “mask of pregnancy.”
Myth: Men cannot carry a baby.
Reality: Transgender men can get pregnant using alternative methods.
Review
1. What is oogenesis? How does it occur?
2. Describe the maturation of an ovarian follicle.
3. Define ovulation.
4. What is happening in the uterus while a follicle in the ovary is maturing?
5. After a secondary oocyte is ovulated from the ovary, it may or may not be fertilized. Contrast what happens next in each of these different outcomes.
6. What is pregnancy, and what is the role of the maternal organism in pregnancy?
7. What is the average duration of pregnancy? Identify the trimesters of pregnancy.
8. Define labor. What event is often a sign that labor will soon begin?
9. Identify the stages of labor.
10. Describe the physiological function of female breasts. How is this function controlled?
11. Identify the functions of the female sex hormones estrogen and progesterone.
12. True or False: All of the developing gametes in an ovary complete meiosis I at the time of puberty.
13. True or False: After fertilization, meiosis II is completed, and then mitosis occurs.
14. A fertilized egg that has not yet implanted in the uterus is called a(n) ________________.
A. embryo
B. zygote
C. fetus
D. secondary oocyte
15. Describe the roles of the cervix in fertilization and childbirth.
Attributions
1. Pregnant woman by Øyvind Holmstad, CC BY-SA 4.0 via Wikimedia Commons
2. Oogenesis by OpenStax College, CC BY 3.0 via Wikimedia Commons
3. Fertilization by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
4. Pregnancy comparison by Maustrauser, public domain via Wikimedia Commons
5. Stages of labor by OpenStax College, CC BY 3.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
7. Some text is adapted from Trautner, Emily et al. “Knowledge and practice of induction of lactation in trans women among professionals working in trans health.” International breastfeeding journal vol. 15,1 63. 16 Jul. 2020, doi:10.1186/s13006-020-00308-6; CC BY 4.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.08%3A_Functions_of_the_Female_Reproductive_System.txt |
Vaccinating Against Cancer
Can a vaccine prevent cancer? In the case of cervical cancer, it can. Cervical cancer is one of three disorders of the female reproductive system described in detail in this concept. Of the three, only cervical cancer can be prevented with a vaccine.
Cervical Cancer
Cervical cancer occurs when cells of the cervix (neck of the uterus) grow abnormally and develop the ability to invade nearby tissues or spread to other parts of the body, such as the abdomen or lungs. Figure \(2\) shows the location of the cervix and the appearance of normal and abnormal cervical cells when examined with a microscope.
Cervical Cancer Prevalence and Death Rates
Worldwide, cervical cancer is the second most common type of cancer (after breast cancer) and the fourth most common cause of cancer death. In the United States and other high-income nations, the widespread use of cervical cancer screening has detected many cases of precancerous cervical changes and has dramatically reduced rates of cervical cancer deaths. About three-quarters of cervical cancer cases occur in developing countries, where routine screening is less likely because of cost and other factors. Cervical cancer is also the most common cause of cancer death in low-income countries.
Symptoms of Cervical Cancer
Early in the development of cervical cancer, there are typically no symptoms. As the disease progresses, however, symptoms are likely to occur. The symptoms may include abnormal vaginal bleeding, pelvic pain, or pain during sexual intercourse. Unfortunately, by the time symptoms start to occur, cervical cancer has typically progressed to a stage at which treatment is less likely to be successful.
Cervical Cancer Causes and Risk Factors
More than 90 percent of cases of cervical cancer are caused at least in part by human papillomavirus (HPV), which is a sexually transmitted virus that also causes genital warts. HPV infection can cause cervical cancer by interfering with a normal cell division. When HPV is not present, cervical cells containing mutations are not allowed to divide, so the cervix remains healthy. When HPV is present, however, cervical cells with mutations may be allowed to divide, leading to uncontrolled growth of mutated cells and the formation of a tumor.
Other risk factors for cervical cancer include smoking, a weakened immune system (for example, due to HIV infection), use of birth control pills, becoming sexually active at a young age, and having many sexual partners. However, these risk factors are less important than HPV infection. Instead, the risk factors are more likely to increase the risk of cervical cancer in individuals who are already infected with HPV. For example, among HPV-infected, current and former smokers have roughly two to three times the incidence of cervical cancer as non-smokers. Passive smoking is also associated with an increased risk of cervical cancer but to a lesser extent.
Diagnosis of Cervical Cancer
Diagnosis of cervical cancer is typically made by looking for microscopic abnormal cervical cells in a smear of cells scraped off the cervix. This is called a Pap smear. If cancerous cells are detected or suspected in the smear, this test is usually followed up with a biopsy to confirm the Pap smear results. Medical imaging (by CT scan or MRI, for example) is also likely to be done to provide more information, such as whether the cancer has spread.
Prevention of Cervical Cancer
It is now possible to prevent HPV infection with a vaccine. The first HPV vaccine was approved by the U.S. Food and Drug Administration in 2006. The vaccine protects against the strains of HPV that have the greatest risk of causing cervical cancer. It is thought that widespread use of the vaccine will prevent up to 90 percent of cervical cancer cases. Current recommendations are to be given the vaccine between the ages of nine and 26. (All sexes should be vaccinated against HPV, because the virus may also cause cancer of the penis and certain other cancers.) The vaccine is effective only if it is given before HPV infection has occurred. Using condoms during sexual intercourse can also help prevent HPV infection and cervical cancer, in addition to preventing pregnancy and sexually transmitted infections (such as HIV).
Even for those who have received the HPV vaccine, there is still a small risk of developing cervical cancer. Therefore, it is recommended that individuals with cervix continue to be examined with regular Pap smears.
Treatment of Cervical Cancer
Treatment of cervical cancer generally depends on the stage at which the cancer is diagnosed, but it is likely to include some combination of surgery, radiation therapy, and/or chemotherapy. Outcomes of treatment depend largely on how early the cancer is diagnosed and treated. For surgery to cure cervical cancer, the entire tumor must be removed with no cancerous cells found at the margins of the removed tissue on microscopic examination. If cancer is found and treated very early when it is still in the microscopic stage, the five-year survival rate is virtually 100 percent.
Vaginitis
Vaginitis is inflammation of the vagina — and sometimes the vulva, as well. Symptoms may include a discharge that is yellow, gray, or green; itching; pain; and a burning sensation. There may also be a foul vaginal odor and pain or irritation with sexual intercourse.
Causes of Vaginitis
About 90 percent of cases of vaginitis are caused by infection with microorganisms. Most commonly, vaginal infections are caused by the yeast Candida albicans (Figure \(3\)). Such infections are referred to as vaginal candidiasis. Other possible causes of vaginal infections include bacteria, especially Gardnerella vaginalis, and some single-celled parasites, notably the protist parasite Trichomonas vaginalis, which is usually transmitted through vaginal intercourse. The risk of vaginal infections may be greater in those who wear tight clothing, are taking antibiotics for another condition, use birth control pills, or have improper hygiene. Poor hygiene allows organisms that are normally present in the stool (such as yeast) to contaminate the vagina. Most of the remaining cases of vaginitis are due to irritation by — or allergic reactions to — various products. These irritants may include condoms, spermicides, soaps, douches, lubricants, and even semen. Using tampons or soaking in hot tubs may be additional causes of this type of vaginitis.
Diagnosis of Vaginitis
Diagnosis of vaginitis typically begins with symptoms reported by the patient. This may be followed by a microscopic examination or culture of the vaginal discharge in order to identify the specific cause. The color, consistency, acidity, and other characteristics of the discharge may be predictive of the causative agent. For example, infection with Candida albicans may cause a cottage cheese-like discharge with a low pH, whereas infection with Gardnerella vaginalis may cause a discharge with a fish-like odor and a high pH.
Prevention of Vaginitis
Prevention of vaginitis includes wearing loose cotton underwear that helps keep the vulva dry. Yeasts and bacteria that may cause vaginitis tend to grow best in a moist environment. It is also important to avoid the use of perfumed soaps, personal hygiene sprays, and douches, all of which may upset the normal pH and bacterial balance in the vagina. To help avoid vaginitis caused by infection with Trichomonas vaginalis, the use of condoms during sexual intercourse is advised.
Treatment of Vaginitis
The appropriate treatment of vaginitis depends on the cause. In many cases of vaginitis, there is more than one cause, and all of the causes must be treated to ensure a cure.
• Yeast infections of the vagina are typically treated with topical anti-fungal medications, which are available over the counter. The medications may be in the form of tablets or creams that are inserted into the vagina. Depending on the particular medication used, treatment may involve one, three, or seven days of application.
• Bacterial infections of the vagina are usually treated with antibiotics. These may be taken orally as pills or applied topically to the vagina in creams.
• Trichomonas vaginalis infections of the vagina are generally treated with a single dose of an oral antibiotic. Sexual partners should be treated at the same time, and intercourse should be avoided for at least a week until both partners have completed treatment and been followed up by a physician.
Endometriosis
Endometriosis is a disease in which endometrial tissue, which normally grows inside the uterus, grows outside of the uterus (Figure \(4\)). Most often, the endometrial tissue grows around the ovaries, Fallopian tubes, and uterus. In rare instances, the tissue may grow elsewhere in the body. The areas of endometriosis typically bleed each month during the menstrual period, and this often results in inflammation, pain, and scarring. An estimated six to ten percent of individuals with a uterus are believed to have endometriosis. It is most common in their thirties and forties, and only rarely occurs before menarche or after menopause.
Signs and Symptoms of Endometriosis
The main symptom of endometriosis is pelvic pain, which may range from mild to severe. There appears to be little or no relationship between the amount of endometrial tissue growing outside the uterus and the severity of the pain. For many with the disease, the pain occurs mainly during menstruation. However, nearly half of those affected have chronic pelvic pain. The pain of endometriosis may be caused by bleeding in the pelvis, which triggers inflammation. Pain can also occur from internal scar tissue that binds internal organs to each other.
Another problem often associated with endometriosis is infertility, or the inability to conceive or bear children. Among patients with endometriosis, up to half may experience infertility. Infertility can be related to scar formation or to anatomical distortions due to abnormal endometrial tissue. Other possible symptoms of endometriosis may include diarrhea or constipation, chronic fatigue, nausea and vomiting, headaches, and heavy or irregular menstrual bleeding.
Causes of Endometriosis
The causes of endometriosis are not known for certain, but several risk factors have been identified, including a family history of endometriosis. People who have a genetic relationship with a person with endometriosis have about six times the normal risk of developing the disease themselves. It has been suggested that endometriosis results from mutations in several genes. It is likely that endometriosis is multifactorial, involving the interplay of several factors.
At the physiological level, the predominant idea for how endometriosis comes about is retrograde menstruation. This happens when some of the endometrial debris from a menstrual flow exits the uterus through the Fallopian tubes, rather than through the vagina. The debris then attaches itself to the outside of organs in the abdominal cavity, or to the lining of the abdominal cavity itself. Retrograde menstruation, however, does not explain all cases of endometriosis, so other factors are apparently involved. Suggestions include environmental toxins and autoimmune responses.
Diagnosis of Endometriosis
Diagnosis of endometriosis is usually based on self-reported symptoms and a physical examination by a doctor, often combined with medical imaging, such as ultrasonography. The only way to definitively diagnose endometriosis, however, is through visual inspection of the endometrial tissue. This can be done with a surgical procedure called laparoscopy, in which a tiny camera is inserted into the abdomen through a small incision (Figure \(5\)). The camera allows the physician to visually inspect the area where endometrial tissue is suspected.
Treatment of Endometriosis
The most common treatments for endometriosis are medications to control the pain, and surgery to remove the abnormal tissue. Frequently used pain medications are non-steroidal inflammatory drugs (NSAIDS), such as naproxen. Opiates may be used in cases of severe pain. Laparoscopy can be used to surgically treat endometriosis, as well as to diagnose the condition. In this type of surgery, an additional small incision is made to insert instruments that the surgeon can manipulate externally in order to burn (cauterize) or cut away the endometrial growths. In younger patients who want to have children, surgery is conservative to keep the reproductive organs intact and functional. However, with conservative surgery, endometriosis recurs in 20 to 40 percent of cases within five years of the surgery. In older patients who have completed childbearing, hysterectomy may be undertaken to remove all or part of the internal reproductive organs. This is the only procedure that is likely to cure endometriosis and prevent relapses.
Feature: My Human Body
A Pap smear is a method of cervical cancer screening used to detect potentially pre-cancerous and cancerous cells in the cervix. It is the most widely used screening test for this type of cancer, and it is very effective. The test may also detect vaginal infections and abnormal endometrial cells, but it is not designed for these purposes.
If you are sexually active, you should start receiving routine Pap smears by age 21. Because most cases of cervical cancer are caused by infection with human papillomavirus (HPV), which is a sexually transmitted infection, there is little or no benefit to screening people who have not had vaginal intercourse. Starting at age 21, general guidelines are for Pap smears to be repeated every three years until age 50, and then every five years until age 65. Screening may be discontinued after age 65 if the last three Pap smears were normal. If a person has a complete hysterectomy so they no longer have a cervix, there is also no need for further Pap smears. On the other hand, if a person has had a history of abnormal Pap smears or cancer, they will likely be screened more frequently. Pap smears can be done safely during the first several months of pregnancy and resumed about three months after childbirth. Generally, better results are obtained if Pap smears are not done during menstruation.
If you’ve never had a Pap smear, knowing what to expect may help prepare you for the procedure. The patient lies on the examining table with their feet in “stirrups” to hold the legs up and apart. An instrument called a speculum is inserted into the vagina to hold back the vaginal walls and give access to the cervix. A tiny amount of tissue is brushed off the cervix and smeared onto a microscope slide. The speculum is then removed, and the procedure is over. The slide is later examined under a microscope for abnormal cells. Some people experience light spotting or mild diarrhea after a Pap smear, but most have no lasting effects.
Pap smears are uncomfortable and may be somewhat painful for some people. there may also be a pelvic exam where doctors insert their fingers into the vagina during the Pap smear test. If you experience pain during a Pap smear, tell your health care provider. Many steps can be taken to minimize the pain, which might include using a smaller speculum, using warm instruments and a lubricant, and applying a topical anesthetic such as lidocaine to the cervix before obtaining the smear. Any pain is generally very brief, and the potential reward is worth it. Pap tests are estimated to reduce up to 80 percent of cervical cancer deaths. One of the lives saved could be your own.
Review
1. What is cervical cancer? Worldwide, how prevalent is it, and how does it rank as a cause of cancer deaths?
2. Identify the symptoms of cervical cancer.
3. What are the causes of — and risk factors for — cervical cancer?
4. What roles can Pap smears and HPV vaccines play in preventing cervical cancer cases and cervical cancer deaths?
5. How is cervical cancer treated?
6. Define vaginitis and identify its symptoms.
7. What are some of the causes of vaginitis? Which cause is responsible for most of the cases?
8. How is vaginitis diagnosed and treated?
9. What is endometriosis, and what are its symptoms?
10. Discuss possible causes of endometriosis.
11. How is endometriosis treated? Which treatment is most likely to prevent the recurrence of the disorder?
12. Which disorder below is the most likely to cause symptoms, specifically during menstruation?
1. endometriosis
2. cervical cancer
3. HPV infection
4. vaginitis
13. True or False: Yeast infections are normally treated with antibiotics.
14. True or False: In the absence of HPV, there are no mutated cells in the cervix.
15. In the case of infection with Trichomonas vaginalis, why is the woman’s sexual partner usually treated at the same time?
Attributions
1. Young girl getting a shot by Amanda Mills, USCDCP, CC0 via Pixnio
2. Cervical cancer by Lolaia, CC BY 4.0 via Wikimedia Commons
3. Candida albicans by CDC/Dr. William Kaplan, public domain via Wikimedia Commons
4. Endometriosis by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
5. Laparoscopy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.09%3A__Disorders_of_the_Female_Reproductive_System.txt |
Family Portrait
This family portrait in Figure \(1\) shows that human societies value having children. Indeed, for most people, parenthood is an important life goal. Unfortunately, some people are unable to achieve that goal because of infertility.
What Is Infertility?
Infertility is the inability of a sexually mature adult to reproduce by natural means. For scientific and medical purposes, infertility is generally defined as the failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse. Infertility may be primary or secondary. Primary infertility applies to cases in which an individual has never achieved a successful pregnancy. Secondary infertility applies to cases in which an individual has had at least one successful pregnancy, but fails to achieve another after trying for at least a year. Infertility is a common problem. About ten percent (6.1 million) of U.S. women aged 15 to 44 have difficulty getting or staying pregnant.
Causes of Infertility
Pregnancy is the result of a multi-step process. In order for a normal pregnancy to occur, an egg must be released from one of the ovaries, the egg must go through a Fallopian tube, a sperm must fertilize the egg as it passes through the Fallopian tube, and then the fertilized egg must implant in the uterus. If there is a problem with any of these steps, infertility can result.
Causes of Male Infertility
Male infertility occurs when there are no or too few sperms, or when the sperms are not healthy and motile and cannot travel through the female reproductive tract to fertilize an egg. A common cause of inadequate numbers or motility of sperm is varicocele, which is an enlargement of blood vessels in the scrotum. This may raise the temperature of the testes and adversely affect sperm production. In other cases, there is no problem with the sperm, but there is a blockage in the male reproductive tract that prevents the sperm from being ejaculated.
Factors that increase a man’s risk of infertility include heavy alcohol use, drug abuse, cigarette smoking, exposure to environmental toxins (such as pesticides or lead), certain medications, serious diseases (such as kidney disease), and radiation or chemotherapy for cancer. Another risk factor is advancing age. Male fertility normally peaks in the mid-twenties and gradually declines after about age 40, although it may never actually drop to zero.
Causes of Female Infertility
Female infertility generally occurs due to one of two problems: failure to produce viable eggs by the ovaries, or structural problems in the Fallopian tubes or uterus. The most common cause of female infertility is a problem with ovulation. Without ovulation, there are no eggs to be fertilized. Anovulatory cycles (menstrual cycles in which ovulation does not occur) may be associated with no or irregular menstrual periods, but even regular menstrual periods may be anovulatory for a variety of reasons. The most common cause of anovulatory cycles is polycystic ovary syndrome (PCOS), which causes hormone imbalances that can interfere with normal ovulation. Another relatively common cause of anovulation is primary ovarian insufficiency. In this condition, the ovaries stop working normally and producing viable eggs at a relatively early age, generally before the age of 40.
Structural problems with the Fallopian tubes or uterus are less common causes of infertility. The Fallopian tubes may be blocked as a result of endometriosis. Another possible cause is a pelvic inflammatory disease, which occurs when sexually transmitted infections spread to the Fallopian tubes or other female reproductive organs (Figure \(2\)). The infection may lead to scarring and blockage of the Fallopian tubes. If an egg is produced and the Fallopian tubes are functioning — and a woman has a condition such as uterine fibroids — implantation in the uterus may not be possible. Uterine fibroids are non-cancerous clumps of tissue and muscle that form on the walls of the uterus.
Factors that increase a woman’s risk of infertility include tobacco smoking, excessive use of alcohol, stress, poor diet, strenuous athletic training, and being overweight or underweight. Advanced age is even more problematic for females than males. Female fertility normally peaks in the mid-twenties, and continuously declines after age 30 and until menopause around the age of 52, after which the ovary no longer releases eggs. About one-third of couples in which the woman is over age 35 have fertility problems. In older women, more cycles are likely to be anovulatory, and the eggs may not be as healthy.
Diagnosing Causes of Infertility
Diagnosing the cause(s) of a couple’s infertility often requires testing both partners for potential problems. The semen is likely to be examined for the number, shape, and motility of sperm. If problems are found with sperm, further studies are likely to be done, such as medical imaging to look for structural problems with the testes or ducts.
In individuals with ovaries and uterus, the first step is most often determining whether ovulation is occurring. This can be done at home by carefully monitoring body temperature (it rises slightly around the time of ovulation) or using a home ovulation test kit, which is available over the counter at most drugstores. Whether or not ovulation is occurring can also be detected with blood tests or ultrasound imaging of the ovaries. If ovulation is occurring normally, then the next step may be an X-ray of the Fallopian tubes and uterus to see if there are any blockages or other structural problems. Another approach to examining the reproductive tract for potential problems is laparoscopy. In this surgical procedure, a tiny camera is inserted into the abdomen through a small incision. This allows the doctor to directly inspect the reproductive organs.
Treating Infertility
Infertility often can be treated successfully. The type of treatment depends on the cause of infertility.
Treating Male Infertility
Medical problems that interfere with sperm production may be treated with medications or other interventions that may lead to the resumption of normal sperm production. If, for example, if an infection is interfering with sperm production, antibiotics may resolve the problem. If there is a blockage in the ejaculation of sperm, surgery may be able to remove the blockage. Alternatively, the sperm may be removed from his body and then used for the artificial insemination of their partner. In this procedure, the sperms are injected into the uterus.
Treating Female Infertility
It is possible to correct blocked Fallopian tubes or uterine fibroids with surgery. Ovulation problems, on the other hand, are usually treated with hormones that act either on the pituitary gland or on the ovaries. Hormonal treatments that stimulate ovulation often result in more than one egg being ovulated at a time, thus increasing the chances of a person conceiving with twins, triplets, or even higher multiple births. Multiple fetuses are at greater risk of being born too early or having health and developmental problems. The mother is also at greater risk of complications arising during pregnancy. Therefore, the possibility of multiple fetuses should be weighed in making a decision about this type of infertility treatment.
Assisted Reproductive Technology
Some cases of infertility are treated with assisted reproductive technology (ART). This is a collection of medical procedures in which eggs and sperms are removed to be manipulated in ways that increase the chances of fertilization occurring. The eggs and sperm may be injected into one of the Fallopian tubes for fertilization to take place in vivo (in the body). More commonly, however, the eggs and sperm are mixed together outside the body so fertilization takes place in vitro (in a test tube or dish in a lab). The latter approach is illustrated in Figure \(3\). With in vitro fertilization, the fertilized eggs may be allowed to develop into embryos before being placed in the woman’s uterus.
ART has about a 40 percent chance of leading to a live birth in women under the age of 35, but only about a 20 percent chance of success after the age of 35. Some studies have found a higher-than-average risk of birth defects in children produced by ART procedures, but this may be due to the generally higher ages of the parent — not the technologies used. Same-sex couples take advantage of the ART process to expand their family.
Other Approaches
Other approaches for certain causes of infertility and same-sex couples include the use of a surrogate mother, a gestational carrier, or sperm donation.
• A surrogate mother is a woman who agrees to become pregnant using the man’s sperm and her own egg. The child, who will be the biological offspring of the surrogate and the male partner, is given up at birth for adoption by the couple. Surrogacy might be selected by women with no eggs or unhealthy eggs. A woman who carries a mutant gene for a serious genetic disorder might choose this option to ensure that the defective gene is not passed on to the offspring.
• A gestational carrier is a woman who agrees to receive a transplanted embryo from a couple and carry it to term. The child, who will be the biological offspring of the couple, is given to the parents at birth. A gestational carrier might be used by women who have normal ovulation but no uterus, or who cannot safely carry a fetus to term because of a serious health problem (such as kidney disease or cancer). This method is commonly used by gay men couples.
• Sperm donation is the use of sperm from a fertile man (generally through artificial insemination) for cases in which the male partner in a couple is infertile, or in which a woman seeks to become pregnant without a male partner. A lesbian couple may use donated sperm to enable one of them to become pregnant and have a child. Sperm can be obtained from a sperm bank, which buys and stores sperm for artificial insemination, or a male friend or another individual may donate sperm to a specific woman.
Feature: Human Biology in the News
More than 14 million people in the United States have polycystic ovary syndrome (PCOS), an endocrine disorder with a genetic basis that is the most common cause of infertility. Most patients with PCOS grow many small cysts on their ovaries. The cysts are not usually harmful, but they lead to hormone imbalances, such as higher-than-normal levels of testosterone in affected individuals. The hormonal imbalances are the primary cause of infertility associated with PCOS. The disorder also increases the risk of a whole slew of other serious health problems, including endometrial cancer, heart disease, high blood pressure, type 2 diabetes, asthma, obesity, depression, and anxiety.
Despite the prevalence of PCOS and its serious potential effects, until recently, its cause was poorly understood. There were also no effective early diagnostic or treatment strategies for it. All that appears to be changing now. A review of the research literature on PCOS published in 2016 provides new insights into causes, diagnosis, and treatment for the disorder.
Among the research cited in the review is promising new work on nonhuman animal models, including monkeys and mice. One line of research suggests that PCOS may be programmed into a fetus during the second trimester of pregnancy. Another line of research indicates that hair taken from an infant can be analyzed for early risk factors for PCOS, even though PCOS symptoms do not show up until puberty. In addition, the research is helping scientists identify a constellation of genes that are suspected to play a role in PCOS.
The new research on PCOS is important for those who suffer from the disorder and its consequences, including infertility and life-threatening chronic conditions (such as heart disease and diabetes). The hope is that such research will lead to new ways of diagnosing PCOS at an early age, when medical interventions and lifestyle choices may be used to head off the more serious complications. It is likely that the research will also eventually lead to new and more effective treatment options for the millions of people who battle PCOS.
Review
1. What is infertility? How is infertility defined scientifically and medically?
2. What percentage of infertility in couples is due to male infertility? What percentage is due to female infertility?
3. Identify causes of and risk factors for male infertility.
4. Identify causes of and risk factors for female infertility.
5. How are the causes of infertility in couples diagnosed?
6. How is infertility treated?
7. Discuss some of the social and ethical issues associated with infertility or its treatment.
8. Why is infertility an under-appreciated problem in developing countries?
9. Describe two similarities between the causes of male and female infertility.
10. True or False: A person who already has a biological child can suffer from infertility.
11. True or False: ART always involves fertilization outside of the body.
12. Explain the difference between males and females in terms of how age affects fertility.
13. If a woman has no viable eggs, which method below is the most likely to help her and her partner have a baby?
1. gestational carrier
2. surrogate mother
3. in vitro fertilization
4. in vivo fertilization
14. Do you think that taking medication to stimulate ovulation is likely to improve fertility in cases where infertility is due to endometriosis? Explain your answer.
15. If a semen sample shows no sperm, what is a possible cause?
1. blockage in the male reproductive tract
2. lack of spermatogenesis
3. PCOS
4. A and B
Attributions
1. Family Portrait by Taylor McKenzie via flickr.com CC BY-SA 2.0
2. Pelvic Inflammatory Disease by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
3. Assisted Reproductive Technology by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.10%3A_Infertility.txt |
Family Planning Pioneer
Her name was Marie Stopes, and she was a British author and paleobotanist who lived from 1880 to 1958. She is pictured here in her lab next to her microscope. Stopes made significant contributions to science and was the first woman on the faculty of the University of Manchester in England. Her primary claim to fame was her work as a family planning pioneer.
Along with her husband, Stopes founded the first birth control clinic in Britain. She also edited a newsletter called Birth Control News, which gave explicit practical advice on how to avoid unwanted pregnancies. In 1918, she published a sex manual titled Married Love. The book was controversial and influential, bringing the subject of contraception into wide public discourse for the first time.
What is Contraception?
About a century after Married Love, more than half of all fertile married couples worldwide use some form of contraception. Contraception, also known as birth control, is any method or device used to prevent pregnancy. Birth control methods have been used for centuries, but safe and effective methods only became available in the 20th century, in part because of the work of people like Marie Stopes.
Many different birth control methods are currently available, but they differ considerably in their effectiveness at preventing pregnancy. The effectiveness of contraception is generally expressed as the failure rate, which is the percentage of women who become pregnant using a given method during the first year of use. Virtually no one uses any method of birth control perfectly, so the failure rate with typical use is almost always higher — and often much higher — than the failure rate with perfect use. For example, with perfect use, a birth control method might have a failure rate of just one percent, whereas, with typical use, the failure rate might be 25 percent. For comparison, there is an average one-year pregnancy rate of 85 percent if no contraception is used.
All methods of birth control have potential adverse effects, but their health risks are less than the health risks associated with pregnancy. Using contraception to space the children in a family is also good for the children’s health and development, as well as for the health of the mother.
Types of Contraception and Their Effectiveness
Types of birth control methods include barrier methods, hormonal methods, intrauterine devices, behavioral methods, and sterilization. With the exception of sterilization, all of these methods are reversible. Examples of each type of birth control method and their failure rates with typical use are described below.
Barrier Methods
Barrier methods are devices that are used to physically block sperm from entering the uterus. They include condoms and diaphragms. Eighteen out of 100 individuals who use this method may get pregnant.
Condoms
Condoms are the most commonly used method of birth control globally. There are condoms for vaginas and penises, but penis condoms are more widely used, less expensive, and more readily available. Both types of condoms are pictured in Figure \(2\).
Whichever type of condom is used, it must be put in place before sexual intercourse occurs. Condoms work by physically blocking ejaculated sperm from entering the vagina of the sexual partner. With typical use, penis condoms have an 18 percent failure rate, and vagina condoms have a 21 percent failure rate. Unlike virtually all other birth control methods, condoms also help prevent the spread of sexually transmitted infections (STIs), in addition to helping to prevent pregnancy.
Diaphragms
Diaphragms, like the one in Figure \(3\), ideally prevent sperm from passing through the cervical canal and into the uterus. A diaphragm is inserted vaginally before sexual intercourse occurs and must be placed over the cervix to be effective. It is usually recommended that a diaphragm be covered with spermicide before insertion for extra protection. It is also recommended that the diaphragm be left in place for at least six hours after intercourse. The failure rate of diaphragms with typical use is about 12 percent, which is about half that of condoms. However, diaphragms do not help prevent the spread of STIs, and their use is also associated with an increased frequency of urinary tract infections.
Hormonal Methods
Hormonal contraception is the administration of hormones to prevent ovulation. Hormones can be taken orally in birth control pills, implanted under the skin, injected into a muscle, or received transdermally from a skin patch. Six to twelve pregnancies in hundred individuals who use this method may occur.
Hormonal methods are currently available only for individuals with uterus and ovaries. Birth control pills are the most common form of hormonal contraception. There are two types of pills: the combined pill (which contains both estrogen and progesterone) and the progesterone-only pill. Both types of pills inhibit ovulation and thicken cervical mucus. The failure rate of birth control pills is only about one percent or less if used perfectly. However, the failure rate rises to about ten percent with typical use, because individuals do not always remember to take the pill at the same time every day. The combined pill is associated with a slightly increased risk of blood clots, but a reduced risk of ovarian and endometrial cancers. The progesterone-only pill does not increase the risk of blood clots, but it may cause irregular menstrual periods. It may take a few weeks or even months for fertility to return to normal after the long-term use of birth control pills.
Intrauterine Devices
An intrauterine device (IUD) is a T-shaped or coiled plastic structure that is inserted into the uterus via the vagina and cervix that contains either copper or a hormone. You can see an IUD in the uterus in the drawing in Figure \(4\). An IUD is inserted by a physician and may be left in place for months or even years. A physician also must remove an IUD, using the strings attached to the device. The copper in copper IUDs prevents pregnancy by interfering with the movement of sperm so they cannot reach and fertilize an egg. The copper may also prevent implantation in the unlikely circumstance of a sperm managing to reach and fertilize an egg. Almost no one gets pregnant who uses this method.
The hormones in hormonal IUDs prevent pregnancy by thickening cervical mucus and trapping sperm. The hormones may also interfere with ovulation, so there is no egg to fertilize. For both types of IUDs, the failure rates are less than one percent, and failure rates with typical use are virtually the same as failure rates with perfect use. Their effectiveness is one reason that IUDs are among the most widely used forms of reversible contraception. Once removed, even after long-term use, fertility returns to normal immediately. On the other hand, IUDs do have a risk of complications, including increased menstrual bleeding and more painful menstrual cramps. IUDs are also occasionally expelled from the uterus, and there is a slight risk of perforation of the uterus by the IUD.
Behavioral Methods
The least effective methods of contraception are behavioral methods. They involve regulating the timing or method of intercourse to prevent the introduction of sperm into the uterus, either altogether or when an egg may be present. Behavioral methods include fertility awareness methods and withdrawal. Abstinence from sexual activity, or at least from vaginal intercourse, is sometimes considered a behavioral method, as well — but it is unlikely to be practiced consistently enough by most people to prevent pregnancy. Even teens who receive abstinence-only sex education do not have reduced rates of pregnancy. Abstinence is also ineffective in cases of non-consensual sex.
Fertility Awareness Methods
Fertility awareness methods involve estimating the most fertile days of the menstrual cycle and then avoiding unprotected vaginal intercourse on those days. The most fertile days are generally a few days before ovulation occurs, the day of ovulation, and another day or two after that. Unless unprotected sex occurs on those days, pregnancy is unlikely. Techniques for estimating the most fertile days include monitoring and detecting minor changes in basal body temperature or cervical secretions. This requires daily motivation and diligence, so it is not surprising that typical-use failure rates of these methods are at least 20 to 25 percent, and for some individuals may be as high as using no contraception at all (85 percent).
Basal body temperature is the lowest body temperature when the body is at rest (usually during sleep). It is most often estimated by a temperature measurement taken immediately upon awakening in the morning and before any physical activity has occurred. Basal body temperature normally rises after ovulation occurs, as shown in Figure \(5\). The increase in temperature is small but consistent and may be used to determine when ovulation occurs, around which time unprotected intercourse should be avoided to prevent pregnancy. However, basal body temperature only shows when ovulation has already occurred, and it cannot predict in advance when ovulation will occur. Sperm can live for up to a week in the female reproductive tract, so determining the occurrence of ovulation only after the fact is a major drawback of this method.
Monitoring cervical mucus has the potential for being more effective than monitoring basal body temperature because it can predict ovulation ahead of time. As ovulation approaches, cervical secretions usually increase in the amount and become thinner (which helps sperm swim through the cervical canal). By recognizing the changing characteristics of cervical mucus, ovulation timing can be predicted. From this information, it can be determined when to avoid unprotected sex to prevent pregnancy.
Withdrawal
Withdrawal (also called coitus interruptus) is the practice of withdrawing the penis from the vagina before ejaculation occurs. The main risk of the withdrawal method is that penis is not withdrawn in a timely manner. The fluid typically released from the penis before ejaculation occurs may also contain some sperm. In addition, if sperms are ejaculated just outside of the vagina, there is a chance they will be able to enter the vagina and travel up to fertilize an egg. For all these reasons, the withdrawal method has a relatively high failure rate of about 22 percent with typical use.
Sterilization
The most effective contraceptive method is sterilization. In both sexes, sterilization generally involves surgical procedures that are considered irreversible. Additional surgery may be able to reverse a sterilization procedure, but there are no guarantees. Male sterilization is generally less invasive and less risky than female sterilization.
Male Sterilization
Male sterilization is usually achieved with a vasectomy. In this surgery, the vas deferens from each testis is clamped, cut, or otherwise sealed (Figure \(6\)). This prevents sperm from traveling from the epididymis to the ejaculatory ducts and being ejaculated from the penis. The same amount of semen will still be ejaculated, but it will not contain any sperm, making fertilization impossible. After a vasectomy, the testes continue to produce sperm, but the sperms are reabsorbed. It usually takes several months after a vasectomy for all remaining sperm to be ejaculated or reabsorbed. In the meantime, another method of birth control should be used.
Female Sterilization
The procedure undertaken for female sterilization is usually tubal ligation. The Fallopian tubes may be tied or cut in a surgical procedure, which permanently blocks the tubes. Alternatively, tiny metal implants may be inserted into the Fallopian tubes in a nonsurgical procedure. Over time, scar tissue grows around the implants and permanently blocks the tubes. Either method stops eggs from traveling from the ovaries through the Fallopian tubes, where fertilization usually takes place.
Emergency Contraception
Emergency contraception is any form of contraception that is used after unprotected vaginal intercourse. One method is the so-called “morning-after” pill. This is essentially a high-dose birth control pill that helps prevent pregnancy by temporarily preventing ovulation. It works only if ovulation has not already occurred, and when taken within five days after unprotected sex. The sooner the pill is taken, the more likely it is to work. Another method of emergency contraception is the IUD. An IUD that is inserted up to five days after unprotected sex can prevent nearly 100 percent of pregnancies. It keeps sperm from reaching and fertilizing an egg or inhibits implantation if an egg has already been fertilized. The IUD can then be left in place to prevent future pregnancies.
Review
1. Define contraception. Globally, how prevalent is the use of contraception by fertile married couples?
2. How is the effectiveness of contraceptive methods typically measured?
3. List five different types of birth control methods. Which (if any) methods are reversible? Which (if any) methods can prevent the spread of sexually transmitted infections, as well as pregnancies?
4. What are barrier methods? Give two examples.
5. Describe hormonal contraception. What is the most commonly used form of hormonal contraception?
6. What is an IUD?
7. Generally, describe behavioral methods of contraception, and identify two specific behavioral methods.
8. Discuss sterilization as a birth control method. How is sterilization typically achieved in males? In females?
9. What is emergency contraception? When is it used? What are the two forms of emergency contraception?
10. How does the thickness of cervical mucus relate to fertility? How do two methods of contraception take advantage of this relationship?
11. Arrange the following methods of contraception in order of typical effectiveness, from the least effective to most effective: birth control pill; fertility awareness method; IUD; male condom
12. If a newly developed method of contraception had a 35 percent failure rate, would you consider this to be an effective method? Explain your answer.
13. Which method of contraception prevents ovulation?\
1. female condom
2. tubal ligation
3. birth control pill
4. fertility awareness method
14. True or False: A vasectomy prevents the production of sperm.
15. True or False: If a man and woman have unprotected intercourse four days before ovulation occurs, the woman cannot get pregnant.
Explore More
bio.libretexts.org/link?17799#Explore_More
Attributions
1. Marie Stopes, public domain via Wikimedia Commons
2. Condom rolled by Tom Hannen, public domain via NCBI
3. Female condom by Anka Grzywacz, public domain via Wikimedia Commons
4. contraceptive diaphragm by Axfean2, public domain via Wikimedia Commons
5. Intrauterine Device (IUD) by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
6. Menstrual cycle by Isometrik, CC BY-SA 3.0 via Wikimedia Commons
7. Vasectomy by Rhcastilhos, CC0 via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.11%3A_Contraception.txt |
Case Study Conclusion: Trying to Conceive
The person in Figure \(1\) is holding a home pregnancy test. The two pink lines in the middle are the type of result that Isabella and Omar are desperately hoping to see themselves one day—a positive pregnancy test. At the beginning of the chapter, you learned that Isabella and Omar have been actively trying to get pregnant for a year, which, as you now know, is the timeframe necessary for infertility to be diagnosed.
Isabella and Omar tried having sexual intercourse on day 14 of her menstrual cycle to optimize their chances of having his sperm meet her egg. Why might this not be successful, even if they do not have fertility problems? As you have learned, although the average menstrual cycle is 28 days, with ovulation occurring around day 14, many women vary widely from these averages (either consistently or variably) from month to month. Recall, for example, that menstrual cycles may vary from 21 to 45 days in length, and a woman’s cycle is considered to be regular if it varies within as many as eight days from shortest to longest cycle. This variability means that ovulation often does not occur on or around day 14, particularly if the woman has significantly shorter, longer, or irregular cycles—as Isabella does. Therefore, by aiming for day 14 without knowing when Isabella is actually ovulating, they may not be successful in helping Omar’s sperm encounter Isabella’s egg.
Lack of ovulation entirely can also cause variability in menstrual cycle length. As you have learned, the regulation of the menstrual cycle depends on an interplay of hormones from the pituitary and ovary, including FSH and LH from the pituitary and estrogen and progesterone from the ovary—specifically from the follicle which surrounds the maturing egg and becomes the corpus luteum after ovulation. Shifts in these hormones and processes can affect ovulation and menstrual cycle length. This is why Isabella was concerned about her long and irregular menstrual cycles. If there is a sign that she is not ovulating, that could be the reason why she is having trouble getting pregnant.
In order to get a better idea of whether Isabella is ovulating, Dr. Bashir recommended that she take her basal body temperature (BBT) each morning before getting out of bed, and track it throughout her menstrual cycle. As you have learned, BBT typically rises slightly and stays high after ovulation. While tracking BBT is not a particularly effective form of contraception, since the temperature rise occurs only after ovulation, it can be a good way to see whether a person is ovulating at all. Although not every individual will see a clear shift in BBT after ovulation, it is a relatively easy way to start assessing fertility and is used as part of a more comprehensive fertility assessment by some physicians.
Dr. Bashir also recommended that Isabella use a home ovulation predictor kit. This is another relatively cheap and easy way to assess ovulation. Most ovulation predictor kits work by detecting the hormone LH in urine using test strips, like the ones shown in Figure \(2\). Why can this predict ovulation? Think about what you have learned about how ovulation is triggered. Rising levels of estrogen from the maturing follicle in the ovary causes a surge in the level of LH secreted from the pituitary gland, which triggers ovulation. This surge in LH can be detected by the home kit, which compares the level of LH in urine to that of a control on the strip. After the LH surge is detected, ovulation will typically occur within one to two days.
By tracking her BBT and using the ovulation predictor kit, Isabella has learned that she is most likely ovulating—but not in every cycle, and sometimes she ovulates much later than day 14. Because frequent anovulatory cycles can be a sign of an underlying hormonal disorder, such as polycystic ovary syndrome (PCOS) or problems with the pituitary or other glands that regulate the reproductive system, Dr. Bashir orders blood tests for Isabella and sets up an appointment for a physical exam.
But because Isabella is sometimes ovulating, the problem may not lie solely with her. Recall that infertility occurs in equal proportions in all sexes, and can be due to problems in both partners. This is why it is generally recommended that both partners get assessed for fertility issues when they are having trouble getting pregnant after a year of trying.
Therefore, Omar proceeds with the semen analysis that Dr. Bashir recommended. In this process, the man provides a semen sample by ejaculating into a cup or special condom, and the semen is then examined under a microscope. As you have learned, the semen is then checked for sperm number, shape, and motility. Sperm with an abnormal shape or trouble moving will likely have trouble reaching and fertilizing an egg. A low amount of sperm will also reduce the chances of conception. In this way, a semen analysis can provide insight into the possible underlying causes of infertility. For instance, a low sperm count could indicate problems in sperm production or a blockage that is preventing sperm from being emitted from the penis. Further testing would have to be done to dissociate between these two possible causes.
Omar had been worried that past injuries to his testes may have affected his fertility. As you have learned, the testes are where sperm are produced, and because they are external to the body, they are vulnerable to injury. In addition to physical damage to the testes and other parts of the male reproductive tract, a testicular injury could potentially cause the creation of antibodies against a person's own sperm. As you have learned, Sertoli cells lining the seminiferous tubules are tightly packed so that the developing sperm are normally well-separated from the body’s immune system. However, in the case of an injury, this barrier can be breached, which can cause the creation of anti-sperm antibodies. These antibodies can hamper fertility by killing the sperm or otherwise interfering with their ability to move or fertilize an egg. When infertility is due to such antibodies, it is called “immune infertility.”
However, Omar’s semen analysis shows that he has normal numbers of healthy sperm. Dr. Bashir recommends that while they investigate whether Isabella has an underlying medical issue, she continues to track her BBT and use ovulation predictor kits to try to pinpoint when she is ovulating. She recommends that once Isabella sees an LH surge, the couple try to have intercourse within three days to maximize their chances of conception. If Isabella is found to have a medical problem that is inhibiting ovulation, depending on what it is, they may either address the problem directly, or she can take medication that stimulates ovulation, such as clomiphene citrate (often sold under the brand name Clomid). This medication works by increasing the amount of FSH secreted by the pituitary.
Fortunately, tracking ovulation at home and timing intercourse appropriately was all Isabella and Omar needed to do to finally get pregnant! After their experience, they, like you, now have a much deeper understanding of the intricacies of the reproductive system and the complex biology that is involved in the making of a new human organism.
Chapter Summary
In this chapter, you learned about the male and female reproductive systems. Specifically, you learned that:
• The reproductive system is the human organ system responsible for the production and fertilization of gametes and the carrying of a fetus.
• Both male and female reproductive systems have organs called gonads (testes in males, ovaries in females) that produce gametes (sperm or eggs) and sex hormones (such as testosterone and estrogen). Sex hormones are endocrine hormones that control the prenatal development of sex organs, sexual maturation at puberty, and reproduction after puberty.
• The reproductive system is the only organ system that is significantly different between male and female sexes. A Y-chromosome gene called SRY is responsible for undifferentiated embryonic tissues developing into a male reproductive system. Without a Y chromosome, the undifferentiated embryonic tissues develop into a female reproductive system.
• Male and female reproductive systems are different at birth but immature and nonfunctioning. Maturation of the reproductive system occurs during puberty when hormones from the hypothalamus and pituitary gland stimulate the gonads to produce sex hormones again. The sex hormones, in turn, cause the changes of puberty.
• Male reproductive system organs include the testes, epididymis, penis, vas deferens, prostate gland, and seminal vesicles.
• The two testes are sperm- and testosterone-producing male gonads. They are contained within the scrotum, a pouch that hangs down behind the penis. The testes are filled with hundreds of tiny, tightly coiled seminiferous tubules, where sperm are produced. The tubules contain sperm in different stages of development, as well as Sertoli cells, which secrete substances needed for sperm production. Between the tubules are Leydig cells, which secrete testosterone.
• The two epididymes are contained within the scrotum. Each epididymis is a tightly coiled tubule where sperms mature and are stored until they leave the body during an ejaculation.
• The two vas deferens are long, thin tubes that run from the scrotum up into the pelvis. During ejaculation, each vas deferens carries sperm from one of the epididymes to one of the pair of ejaculatory ducts.
• The two seminal vesicles are glands within the pelvis that secrete fluid through ducts into the junction of each vas deferens and ejaculatory duct. This alkaline fluid makes up about 70 percent of semen, the sperm-containing fluid that leaves the penis during ejaculation. Semen contains substances and nutrients that sperm need to survive and “swim” in the female reproductive tract.
• The prostate gland is located just below the seminal vesicles and surrounds the urethra and its junction with the ejaculatory ducts. The prostate secretes an alkaline fluid that makes up close to 30 percent of semen. The prostatic fluid contains a high concentration of zinc, which sperm need to be healthy and motile.
• The ejaculatory ducts form where the vas deferens joins with the ducts of the seminal vesicles in the prostate gland. They connect the vas deferens with the urethra. The ejaculatory ducts carry sperm from the vas deferens whereas the secretions from the seminal vesicles and prostate gland form semen.
• The paired bulbourethral glands are located just below the prostate gland. They secrete a tiny amount of fluid into semen. The secretions help lubricate the urethra and neutralize any acidic urine it may contain.
• The penis is the external male organ that has the reproductive function of intromission, which is delivering sperm to the female reproductive tract. The penis also serves as the organ that excretes urine. The urethra passes through the penis and carries urine or semen out of the body. Internally, the penis consists largely of columns of spongy tissue that can fill with blood and make the penis stiff and erect. This is necessary for sexual intercourse so intromission can occur.
• Parts of a mature sperm include the head, acrosome, midpiece, and flagellum. The process of producing sperm is called spermatogenesis. This normally starts during puberty and continues uninterrupted until death.
• Spermatogenesis occurs in the seminiferous tubules in the testes and requires high concentrations of testosterone. Sertoli cells in the testes play many roles in spermatogenesis, including concentrating testosterone under the influence of follicle-stimulating hormone (FSH) from the pituitary gland.
• Spermatogenesis begins with a diploid stem cell called a spermatogonium, which undergoes mitosis to produce a primary spermatocyte. The primary spermatocyte undergoes meiosis I to produce haploid secondary spermatocytes, and these cells in-turn, undergo meiosis II to produce spermatids. After the spermatids grow a tail and undergo other changes, they become sperm.
• Before sperms are able to “swim,” they must mature in the epididymis. The mature sperms are then stored in the epididymis until ejaculation occurs.
• Ejaculation is the process in which semen is propelled by peristalsis in the vas deferens and ejaculatory ducts from the urethra in the penis.
• Leydig cells in the testes secrete testosterone under the control of luteinizing hormone (LH) from the pituitary gland. Testosterone is needed for male sexual development and to maintain normal spermatogenesis after puberty. It also plays a role in the prostatatic function and the ability of the penis to become erect.
• Disorders of the male reproductive system include erectile dysfunction (ED), epididymitis, prostate cancer, and testicular cancer.
• ED is a disorder characterized by the regular and repeated inability of a sexually mature male to obtain and maintain an erection. ED is a common disorder that occurs when normal blood flow to the penis is disturbed or there are problems with the nervous control of penile engorgement or arousal.
• Possible physiological causes of ED include aging, illness, drug use, tobacco smoking, and obesity, among others. Possible psychological causes of ED include stress, performance anxiety, and mental disorders.
• Treatments for ED may include lifestyle changes, such as stopping smoking and adopting a healthier diet and regular exercise. However, the first-line treatment is prescription drugs such as Viagra® or Cialis® that increase blood flow to the penis. Vacuum pumps or penile implants may be used to treat ED if other types of treatment fail.
• Epididymitis is inflammation of the epididymis. It is a common disorder, especially in young men. It may be acute or chronic and is often caused by a bacterial infection. Treatments may include antibiotics, anti-inflammatory drugs, and painkillers. Treatment is important to prevent the possible spread of infection, permanent damage to the epididymis or testes, and even infertility.
• Prostate cancer is the most common type of cancer in men and the second leading cause of cancer death in men. If there are symptoms, they typically involve urination, such as frequent or painful urination. Risk factors for prostate cancer include older age, family history, high-meat diet, and sedentary lifestyle, among others.
• Prostate cancer may be detected by a physical exam or a high level of prostate-specific antigen (PSA) in the blood, but a biopsy is required for a definitive diagnosis. Prostate cancer is typically diagnosed relatively late in life and is usually slow growing, so no treatment may be necessary. In younger patients or those with faster-growing tumors, treatment is likely to include surgery to remove the prostate, followed by chemotherapy and/or radiation therapy.
• Testicular cancer, or cancer of the testes, is the most common cancer in males between the ages of 20 and 39 years. It is more common in males of European than African ancestry. A lump or swelling in one testis, fluid in the scrotum, and testicular pain or tenderness are possible signs and symptoms of testicular cancer.
• Testicular cancer can be diagnosed by a physical exam and diagnostic tests, such as ultrasound or blood tests. Testicular cancer has one of the highest cure rates of all cancers. It is typically treated with surgery to remove the affected testis, and this may be followed by radiation therapy, and/or chemotherapy. Normal male reproductive functions are still possible after one testis is removed if the remaining testis is healthy.
• The female reproductive system is made up of internal and external organs that function to produce haploid female gametes called eggs, secrete female sex hormones (such as estrogen), and carry and give birth to a fetus.
• Female reproductive system organs include the ovaries, Fallopian tubes, uterus, vagina, clitoris, and labia.
• The vagina is an elastic, muscular canal that can accommodate the penis. It is where sperm are usually ejaculated during sexual intercourse. The vagina is also the birth canal, and it channels the flow of menstrual blood from the uterus. A healthy vagina has a balance of symbiotic bacteria and an acidic pH.
• The uterus is a muscular organ above the vagina where a fetus develops. Its muscular walls contract to push out the fetus during childbirth. The cervix is the neck of the uterus that extends down into the vagina. It contains a canal connecting the vagina and uterus for sperm or an infant to pass through. The innermost layer of the uterus, the endometrium, thickens each month in preparation for an embryo but is shed in the following menstrual period if fertilization does not occur.
• The Fallopian tubes extend from the uterus to the ovaries. Waving fimbriae at the ovary ends of the Fallopian tubes guide ovulated eggs into the tubes where fertilization may occur as the eggs travel to the uterus. Cilia and peristalsis help eggs move through the tubes. Tubular fluid helps nourish sperm as they swim up the tubes toward eggs.
• The ovaries are gonads that produce eggs and secrete sex hormones including estrogen. Nests of cells called follicles in the ovarian cortex are the functional units of ovaries. Each follicle surrounds an immature egg. At birth, a baby girl’s ovaries contain at least a million eggs, and they will not produce any more during her lifetime. One egg matures and is typically ovulated each month during a woman’s reproductive years.
• The vulva is a general term for external female reproductive organs. The vulva includes the clitoris, two pairs of labia, and openings for the urethra and vagina. Secretions from Bartholin’s glands near the vaginal opening lubricate the vulva.
• The breasts are technically not reproductive organs, but their mammary glands produce milk to feed an infant after birth. Milk drains through ducts and sacs and out through the nipple when a baby sucks.
• Oogenesis is the process of producing eggs in the ovaries of a female fetus. Oogenesis begins when a diploid oogonium divides by mitosis to produce a diploid primary oocyte. The primary oocyte begins meiosis I and then remains at this stage in an immature ovarian follicle until after birth.
• After puberty, one follicle a month matures and its primary oocyte completes meiosis I to produce a secondary oocyte, which begins meiosis II. During ovulation, the mature follicle bursts open and the secondary oocyte leaves the ovary and enters a Fallopian tube.
• While a follicle is maturing in an ovary each month, the endometrium in the uterus is building up to prepare for an embryo. Around the time of ovulation, cervical mucus becomes thinner and more alkaline to help sperm reach the secondary oocyte.
• If the secondary oocyte is fertilized by a sperm, it quickly completes meiosis II and forms a diploid zygote, which will continue through the Fallopian tube. The zygote will go through multiple cell divisions before reaching and implanting in the uterus. If the secondary oocyte is not fertilized, it will not complete meiosis II, and will soon disintegrate.
• Pregnancy is the carrying of one or more offspring from fertilization until birth. The maternal organism must provide all the nutrients and other substances needed by the developing offspring, and also remove its wastes. She should also avoid exposures that could potentially damage the offspring, especially early in the pregnancy when organ systems are developing.
• The average duration of pregnancy is 40 weeks (from the first day of the last menstrual period) and is divided into three trimesters of about three months each. Each trimester is associated with certain events and conditions that a pregnant woman may expect, such as morning sickness during the first trimester, feeling fetal movements for the first time during the second trimester, and rapid weight gain in both fetus and mother during the third trimester.
• Labor, which is the general term for the birth process, usually begins around the time the amniotic sac breaks and its fluid leaks out. Labor occurs in three stages: dilation of the cervix, the birth of the baby, and delivery of the placenta (afterbirth).
• The physiological function of female breasts is lactation or the production of breastmilk to feed an infant. Sucking on the breast by the infant stimulates the release of the hypothalamic hormone oxytocin from the posterior pituitary, which causes the flow of milk. The release of milk stimulates the baby to continue sucking, which in turn keeps the milk flowing. This is one of the few examples of positive feedback in the human organism.
• The ovaries produce female sex hormones, including estrogen and progesterone. Estrogen is responsible for sexual maturation and secondary sex characteristics at puberty. It is also needed to help regulate the menstrual cycle and ovulation after puberty until menopause. Progesterone prepares the uterus for pregnancy each month during the menstrual cycle and helps maintain the pregnancy if fertilization occurs.
• The menstrual cycle refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of eggs and the preparation of the uterus for pregnancy. It involves changes in both the ovaries and uterus and is controlled by pituitary hormones (FSH and LH) and ovarian hormones (estrogen and progesterone).
• The female reproductive period is delineated by menarche, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.
• The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle. It includes the follicular phase, when a follicle and its egg mature due to rising levels of FSH; ovulation, when the egg is released from the ovary due to a rise in estrogen and a surge in LH; and the luteal phase, when the follicle is transformed into a structure called a corpus luteum that secretes progesterone. In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.
• The events of the menstrual cycle that take place in the uterus make up the uterine cycle. It includes menstruation, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the proliferative phase, during which the endometrium builds up again until ovulation occurs; and the secretory phase, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an embryo.
• Disorders of the female reproductive system include cervical cancer, vaginitis, and endometriosis.
• Cervical cancer occurs when cells of the cervix grow abnormally and develop the ability to invade nearby tissues, or spread to other parts of the body. Worldwide, cervical cancer is the second-most common type of cancer in females and the fourth-most common cause of cancer death in females. Early on, cervical cancer often has no symptoms; later, symptoms such as abnormal vaginal bleeding and pain are likely.
• Most cases of cervical cancer occur because of infection with human papillomavirus (HPV), so the HPV vaccine is expected to greatly reduce the incidence of the disease. Other risk factors include smoking and a weakened immune system. A Pap smear can diagnose cervical cancer at an early stage. Where Pap smears are done routinely, cervical cancer death rates have fallen dramatically. Treatment of cervical cancer generally includes surgery, which may be followed by radiation therapy or chemotherapy.
• Vaginitis is an inflammation of the vagina. A discharge is likely, and there may be itching and pain. About 90 percent of cases of vaginitis is caused by infection with microorganisms, typically by the yeast Candida albicans. A minority of cases are caused by irritants or allergens in products such as soaps, spermicides, or douches.
• Diagnosis of vaginitis may be based on characteristics of the discharge, which can be examined microscopically or cultured. Treatment of vaginitis depends on the cause and is usually an oral or topical anti-fungal or antibiotic medication.
• Endometriosis is a disease in which endometrial tissue grows outside the uterus. This tissue may bleed during the menstrual period and cause inflammation, pain, and scarring. The main symptom of endometriosis is pelvic pain, which may be severe. Endometriosis may also lead to infertility.
• Endometriosis is thought to have multiple causes, including genetic mutations. Retrograde menstruation may be the immediate cause of endometrial tissue escaping the uterus and entering the pelvic cavity. Endometriosis is usually treated with surgery to remove the abnormal tissue and medication for pain. If surgery is more conservative than hysterectomy, endometriosis may recur.
• Infertility is the inability of a sexually mature adult to reproduce by natural means. It is defined scientifically and medically as the failure to achieve a successful pregnancy after at least one year of regular, unprotected sexual intercourse.
• About 30 percent of infertility in couples is due to female infertility, and another 30 percent is due to male infertility. In the remaining cases, a couple’s infertility is due to problems in both partners or to unknown causes.
• Male infertility occurs when there are no or too few healthy, motile sperm. This may be caused by problems with spermatogenesis or by blockage of the male reproductive tract that prevents sperm from being ejaculated. Risk factors for male infertility include heavy alcohol use, smoking, certain medications, and advancing age, to name just a few.
• Female infertility occurs due to failure to produce viable eggs by the ovaries or structural problems in the Fallopian tubes or uterus. Polycystic ovary syndrome is the most common cause of failure to produce viable eggs. Endometriosis and uterine fibroids are possible causes of structural problems in the Fallopian tubes and uterus. Risk factors for female infertility include smoking, stress, poor diet, and older age, among others.
• Diagnosing the cause(s) of a couple’s infertility generally requires testing both the man and the woman for potential problems. For men, semen is likely to be examined for adequate numbers of healthy, motile sperm. For women, signs of ovulation are monitored, for example, with an ovulation test kit or ultrasound of the ovaries. For both partners, the reproductive tract may be medically imaged to look for blockages or other abnormalities.
• Treatments for infertility depend on the cause. For example, if a medical problem is interfering with sperm production, medication may resolve the underlying problem so sperm production is restored. Blockages in either the male or the female reproductive tract can often be treated surgically. If there are problems with ovulation, hormonal treatments may stimulate ovulation.
• Some cases of infertility are treated with assisted reproductive technology (ART). This is a collection of medical procedures in which eggs and sperm are taken from the couple and manipulated in a lab to increase the chances of fertilization occurring and an embryo forming. Other approaches for certain causes of infertility include the use of a surrogate mother, gestational carrier, or sperm donation.
• Infertility can negatively impact a couple socially and psychologically, and it may be a major cause of marital friction or even divorce. Infertility treatments may raise ethical issues relating to the costs of the procedures and the status of embryos that are created in vitro but not used for pregnancy. Infertility is an under-appreciated problem in developing countries where birth rates are high and children have high economic as well as a social value. In these countries, poor health care is likely to lead to more problems with infertility and fewer options for treatment.
• More than half of all fertile couples worldwide use contraception (birth control), which is any method or device used to prevent pregnancy. Different methods of contraception vary in their effectiveness, typically expressed as the failure rate, or the percentage of women who become pregnant using a given method during the first year of use. For most methods, the failure rate with typical use is much higher than the failure rate with perfect use.
• Types of birth control methods include barrier methods, hormonal methods, intrauterine devices, behavioral methods, and sterilization. Except for sterilization, all of the methods are reversible.
• Barrier methods are devices that block sperm from entering the uterus. They include condoms and diaphragms. Of all birth control methods, only condoms can also prevent the spread of sexually transmitted infections.
• Hormonal methods involve the administration of hormones to prevent ovulation. Hormones can be administered in various ways, such as in an injection, through a skin patch, or, most commonly, in birth control pills. There are two types of birth control pills: those that contain estrogen and progesterone, and those that contain only progesterone. Both types are equally effective, but they have different potential side effects.
• An intrauterine device (IUD) is a small T-shaped plastic structure containing copper or a hormone that is inserted into the uterus by a physician and left in place for months or even years. It is highly effective even with typical use, but it does have some risks, such as increased menstrual bleeding and, rarely, perforation of the uterus.
• Behavioral methods involve regulating the timing or method of intercourse to prevent the introduction of sperm into the female reproductive tract, either altogether or when an egg may be present. In the fertility awareness methods, unprotected intercourse is avoided during the most fertile days of the cycle as estimated by basal body temperature or the characteristics of cervical mucus. In withdrawal, the penis is withdrawn from the vagina before ejaculation occurs. Behavioral methods are the least effective methods of contraception.
• Sterilization is the most effective contraceptive method, but it requires a surgical procedure and may be irreversible. Male sterility is usually achieved with a vasectomy, in which the vas deferens are clamped or cut to prevent sperm from being ejaculated in semen. Female sterility is usually achieved with a tubal ligation, in which the Fallopian tubes are clamped or cut to prevent sperm from reaching and fertilizing eggs.
• Emergency contraception is any form of contraception that is used after unprotected vaginal intercourse. One method is the “morning after” pill, which is a high-dose birth control pill that can be taken up to five days after unprotected sex. Another method is an IUD, which can be inserted up to five days after unprotected sex.
In this chapter, you learned how the male and female reproductive systems work together to produce a zygote. In the next chapter, you will learn about how the human organism grows and develops throughout life—from a zygote all the way through old age.
Chapter Summary Review
1. Which glands produce semen? What is the rough percentage of each fluid in semen?
2. What is one reason why semen's alkalinity assists in reproduction?
3. True or False: The hormones FSH and LH are involved in regulating the female reproductive system, but not the male reproductive system.
4. True or False: A majority of American males have some cancerous cells in their prostate gland by age 80.
5. True or False: The secretory phase of the menstrual cycle is when menstruation occurs.
6. Menarche is:
1. the part of the menstrual cycle known as a period
2. the first menstrual period
3. the conclusion of menstrual periods when a woman is around 50
4. an anovulatory menstrual cycle
7. Where are sperm located when they develop tails?
1. the testes
2. the epididymis
3. the vas deferens
4. the seminal vesicles
8. For each of the descriptions below, choose whether it applies to the male or female reproductive systems.
1. The gametes are present at birth.
2. Gametogenesis begins at puberty.
3. The gametes complete meiosis II upon fertilization.
4. The mature gametes have a reduced amount of cytoplasm.
9. What are three things that pass through the cervical canal of females, going in either direction?
10. If a man and a woman have unprotected vaginal intercourse, what are the structures of the female reproductive tract, in order, that sperm would move through? (Assume that these sperm do not die prematurely before they reach their final destination.)
11. Other than where the cancer originates, what is one difference between the prostate and testicular cancer?
12. Progesterone is relatively high...
1. in the follicular phase of the menstrual cycle
2. in the luteal phase of the menstrual cycle
3. during pregnancy
4. B and C
13. If a woman is checking her basal body temperature each morning as a form of contraception, and today is day 12 of her menstrual cycle and her basal body temperature is still low, is it safe for her to have unprotected sexual intercourse today? Why or why not?
14. True or False: If a young woman does not get pregnant after nine months of regular, unprotected intercourse, the couple will be diagnosed with infertility.
15. True or False: In the developing male fetus, the gonads start out as ovaries, but then differentiate into testes because of the Y chromosome.
16. True or False: A cause of endometriosis can be retrograde menstruation, where some of the endometrial tissue flows backward from the uterus out through the Fallopian tubes.
17. Where are sperm produced?
1. the seminal vesicles
2. the spermatogonium
3. the epididymis
4. the seminiferous tubules
18. Which of the following methods of contraception is the most effective, while also being reversible? A. withdrawal B. tubal ligation C. intrauterine device D. condoms
19. Where is a diaphragm placed? How does it work to prevent pregnancy?
20. Why are the testes located outside of the body?
21. Why is it important to properly diagnose the causative agent when a woman has vaginitis?
22. True or False: When a baby nurses, the stimulation negatively feeds back to decrease milk flow to the nipple.
23. True or False: Part of the penis is located inside of the male body.
24. What is the best description of what occurs at menopause?
1. Eggs stop maturing in the ovaries on a monthly basis.
2. Oogenesis stops occurring.
3. Ovulated eggs cannot be fertilized, so they are automatically reabsorbed.
4. The endometrium can no longer support a fetus, so fertilized eggs cannot implant.
25. The transition from spermatid to sperm involves
1. mitosis
2. meiosis I
3. meiosis II
4. no cell division
26. Describe two ways in which sperm can move through the male and/or female reproductive tracts.
Attributions
1. Dos Rayitas by Esparta Palma, licensed CC BY 2.0 via Flickr
2. Urine-based ovulation test by Sapp, public domain via Wikimedia Commons
3. Abnormal sperm by Xenzo, CC BY-SA 3.0, Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/22%3A_Reproductive_System/22.12%3A_Case_Study_Conclusion%3A_Trying_to_Conceive_and_Chapter_Summary.txt |
This chapter describes how the human organism grows and develops from fertilization through death. The following stages of life are described in detail: germinal stage, embryonic stage, fetal stage, infancy, childhood, adolescence, and adulthood.
• 23.1: Case Study: How Our Bodies Change Throughout Life
Paul and Vanessa are shocked to discover that their toddler Lucas' blood lead level is 10 µg/dL, which is considered high. Since Vanessa is three months pregnant, they are worried about whether Vanessa was also exposed to lead. If so, what effects could it have on the developing baby?
• 23.2: Germinal Stage
The germinal stage of development is the first and shortest of the stages of the human lifespan. The main events in this stage of development are illustrated in the figure below and described in detail in the rest of this concept. The germinal stage lasts a total of eight to nine days. It begins in a Fallopian tube when an ovum is fertilized by a sperm to form a zygote (day 0). The germinal stage continues as the zygote undergoes several initial cell divisions to a morula.
• 23.3: Embryonic Stage
In many cultures, marriage - along with birth and death - is considered the most pivotal life event. For pioneering developmental biologist Lewis Wolpert, however, these life events are overrated. According to Wolpert, "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Gastrulation is a major biological event that occurs early in the embryonic stage of human development.
• 23.4: Fetal Stage
This mother-to-be is holding an ultrasound image of her fetus. She is nearly nine months pregnant, so the fetus is fully developed and almost ready to be born. The fetus has grown tremendously and changed in many other ways since it was a tiny embryo seven months previously.
• 23.5: Infancy
Infancy refers to the first year of life after birth, and an infant is defined as a human being between birth and the first birthday. The term baby is usually considered synonymous with infant, although it is commonly applied to the young of other animals, as well as humans. Human infants seem weak and helpless at birth, but they are actually born with a surprising range of abilities. Most of their senses are quite well developed, and they can also communicate their needs by crying.
• 23.6: Childhood
Legally, childhood is defined as the period of minority, which lasts from birth until adulthood (majority). The age of majority varies by place and purpose. For example, in the United States, at age 18, you are considered an adult for military service, but a minor for buying alcohol. Biologically, childhood is defined as the stage of a human organism between birth and adolescence.
• 23.7: Adolescence and Puberty
Adolescence is the period of transition between childhood and adulthood. It is generally considered to start with puberty, during which sexual maturation occurs and adolescents go through a spurt in growth. In many children, however, puberty actually begins during the stage called pre-adolescence, which covers the ages 11 to 12 years. Puberty may begin before adolescence, but it usually continues for several years, well into the adolescent stage, which ends during the late teens.
• 23.8: Adulthood
This family image includes an elderly woman and her young-adult daughters and granddaughters from the Hmong ethnic group in Laos. Grandmother and daughters are adults, but they are obviously far apart in age. What ages define the beginning and end of adulthood?
• 23.9: Case Study Conclusion: Lead Danger and Chapter Summary
Earlier in this chapter, you met Paul, Vanessa, and Lucas, who were concerned by elevated levels of lead in Lucas' blood. Many experts agree that preventing lead exposure and more widespread blood lead level screening is critical to prevent permanent damage to children’s health. Infancy and early childhood is a wonderful time of tremendous growth and change in a person’s lifespan, but it is also a time that is highly vulnerable to damage—with potential lifelong consequences.
Thumbnail: Father and son on a beach via Pixabay
23: Human Growth and Development
Case Study: Lead Danger
Instead of using a phone to make a call, the infant in Figure \(1\) is using it for a purpose more suited to their current stage of life—to relieve the pain of teething. Although this may look cute, the tendency that infants and young children have of putting objects in their mouths makes them particularly vulnerable to being exposed to toxic substances in their environment that can seriously—and sometimes permanently—damage their health.
One such toxic substance is lead. Lead is a metal that can be found throughout the environment—including inside homes—and is toxic to humans. According to the Centers for Disease Control and Prevention (CDC), there are about a half million children in the U.S. between the ages of one and five who have blood lead levels above 5 micrograms per deciliter (µg/dL), the level at which steps should be taken to reduce lead exposure. There is no known safe blood level of lead in children.
This is why Paul, the father of a toddler named Lucas, brings Lucas to his pediatrician, Dr. Morrison, to test his blood for lead. Eighteen-month-old Lucas seems to be healthy, but the detrimental effects of lead exposure are often not apparent until later in life, so many medical professionals routinely screen children for lead toxicity between the ages of one and two.
Paul and his wife Vanessa are shocked to find out that Lucas’ blood lead level is 10 µg/dL, which is considered high. Medical treatment for lead poisoning is not recommended in children who do not have symptoms unless their blood level is at or over 45 µg/dL. However, Dr. Morrison tells Paul and Vanessa that they must take action to limit any further exposure, such as finding and eliminating the source of lead and limiting Lucas’ contact with potential lead-containing substances. Sources of lead that children may be exposed to include deteriorating lead-based paint, dust from peeling and cracking paint, water from lead pipes, toys, and jewelry, among others. Figure \(2\) illustrates some possible sources and routes of lead exposure in the home. One reason that young children are particularly susceptible to lead exposure is that they tend to put objects and unwashed hands into their mouths, which can directly introduce lead objects or lead-containing dust into their bodies.
Lead exposure in infants and young children can cause a variety of adverse health effects, some of which may not be noticeable until later in childhood. These effects include developmental delays, lower IQ, hyperactivity, behavior and learning problems, slowed growth, hearing problems, and anemia. When there is a very high level of exposure, serious immediate consequences of lead poisoning can occur, such as seizures, coma, and even death.
Paul and Vanessa are very concerned, not only for Lucas but also because Vanessa is three months pregnant. They are worried about whether Vanessa was also exposed to lead. If so, what effects could it have on the developing baby? Dr. Morrison shares their concern and strongly recommends that Vanessa get her blood tested for lead. Paul wonders if he should get tested, as well. Dr. Morrison says that testing Paul is less urgent than testing Vanessa, especially since Lucas’ lead level is not extremely high and Paul is not having any symptoms of lead poisoning—but if there is a source of lead in the home, it would be good for him to be tested eventually.
Lead clearly can cause significant adverse health effects, but its impact varies depending on the stage of life of the person exposed. Although lead exposure can cause health problems in adults, exposure to low levels of lead usually has much more of an impact on humans in earlier developmental stages, such as the embryo, fetus, infants, and young children. As you read this chapter, you will learn about these early stages, as well as the later stages of adolescence, early and middle adulthood, and old age. Many changes occur across a human’s lifespan, including physical characteristics, motor and cognitive abilities, behavior, and susceptibility to damage and disease.
At the end of this chapter, you will learn how Lucas likely became exposed to lead, whether his parents and developing sibling have been exposed, the potential impact on the family members at their different life stages, and what they—and you—can do to protect against the dangerous effects of lead exposure.
Chapter Overview: Human Growth and Development
In this chapter, you will learn about the growth and development of humans from fertilization to old age. Specifically, you will learn about:
• The germinal stage of human development, which starts at fertilization; goes through the early cell divisions and developmental stages of the zygote, morula, and blastocyst; and ends when the blastocyst implants in the uterus to become an embryo
• The embryonic stage, which starts at implantation and lasts until the eighth week after fertilization. This period involves significant growth and changes in the developing embryo, which occur through processes such as gastrulation, neurulation, and organogenesis.
• The three germ layers (which ultimately develop into different tissues of the body), and the extraembryonic tissues which nourish and protect the developing embryo and fetus, including the yolk sac, amnion, and placenta
• The fetal stage, which starts at the ninth week after fertilization and lasts until birth. This stage includes the final stages of prenatal growth and development, including the functioning of most organs and sensory systems.
• The differences between fetal and postnatal blood circulation and hemoglobin, due to the lungs not being used until birth
• Factors that affect fetal growth, birth weight, and viability
• Characteristics of newborns, and how health is assessed at birth
• Infancy, which is the first year of life—and the physical, motor, sensory, and cognitive changes that occur during this time period
• Childhood, which is defined biologically as the period between birth and adolescence—and the physical, cognitive, behavioral, and social changes that occur at different sub-stages of childhood
• Adolescence, which is the period between childhood and adulthood. This stage includes puberty—the period when sexual and physical maturation occurs—as well as further maturation of the brain, a stronger sense of personal identity, and changes in relationships.
• The stages of adulthood—early, middle, and old age—and the physical, cognitive, and social changes that typically occur during these times
• Susceptibility to diseases and common causes of death at different stages of adulthood, along with possible causes of aging
As you read the chapter, think about the following questions:
1. Vanessa is three months pregnant. What are the major developmental events that have occurred in her pregnancy so far? If she has been exposed to lead, what effects might it have on her developing offspring?
2. Lead exposure in infants and toddlers can cause developmental delays and other effects that may only become obvious later in childhood. What do you think is meant by a developmental delay? Why do you think that some of the effects of lead are only noticeable at older ages?
3. Why is Dr. Morrison less concerned about Paul’s lead level than he is about Vanessa’s and Lucas’ levels?
Attributions
1. Yummies by cplbasilisk, licensed CC BY 2.0 via Flickr
2. Lead Infographic by Center for Disease Control, public domain
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.1%3A_Case_Study%3A__How_Our_Bodies_Change_Throughout_Life.txt |
Experienced Newborn
This newborn baby is just starting out in life. They have their whole life ahead of them!
Actually, that’s not really true. While most of their life is still ahead of them — including life stages of infancy, childhood, adolescence, and adulthood — this newborn baby is not just starting out in life. They are already nine months old, and what happened to them during those nine months will help shape the rest of their life. Some of the shortest — but most important — life stages occur before birth. These stages include the germinal, embryonic, and fetal stages. This concept focuses on the earliest of all human life stages: the germinal stage.
What Is the Germinal Stage?
The germinal stage of development is the first and shortest of the stages of the human lifespan. The main events in this stage of development are illustrated in Figure \(2\) and described in detail in the rest of this concept. The germinal stage lasts a total of eight to nine days. It begins in a Fallopian tube when an ovum is fertilized by a sperm to form a zygote (day 0). The germinal stage continues as the zygote undergoes several initial cell divisions to form a solid ball of cells called a morula (days 3-4). It then continues as the morula undergoes additional changes to become a hollow ball of cells called a blastocyst (days 5-7). The germinal stage ends when the blastocyst implants in the endometrium of the uterus (days 8-9). After implantation occurs, the blastocyst is called an embryo, and it will soon obtain nutrients from the mother’s blood via a temporary organ called the placenta. In the germinal stage, however, nutrients must be obtained from cell cytoplasm or secretions in the Fallopian tube or uterus.
Processes in the Germinal Stage
The germinal stage involves several different processes that change an egg and sperm first into a zygote, and then into an embryo. The processes include fertilization, cleavage, blastulation, and implantation.
Fertilization
Many sperm travel towards the egg due to chemical attraction. However, only one sperm will succeed in fertilizing the ovum (egg), by penetrating its cell membrane and depositing the genetic material into the egg, where the two nuclei fuse. The fertilized ovum (zygote) immediately becomes resistant to penetration by any other sperm arriving later. After fertilization occurs, the zygote remains in the fallopian tube for about 72 hours, and during this time it develops rapidly
Cleavage
By the second day after fertilization, the single-celled zygote undergoes mitosis to form two daughter cells. Mitosis continues to take place every 12 to 24 hours to produce the first four cells, then eight, and as many as sixteen cells by day 4. These early mitotic divisions are called cleavage. By day 4, the cells form a solid ball called a morula (see Figure \(4\)). Although cleavage results in more cells, the overall mass of cells making up the morula is still the same size as the initial zygote because the cells are confined within the zona pellucida. A large amount of cytoplasm in the original zygote becomes subdivided among the multiple cells of the morula.
Blastulation
Blastulation is the process of changing the morula into a blastocyst. It occurs from roughly day 5 to day 7 after fertilization. During blastulation, the morula changes from a solid ball of undifferentiated cells into a fluid-filled ball of differentiated cells, as shown in Figure \(4\). The major parts of the fully formed blastocyst are the embryoblast, trophoblast, and blastocoele.
• The embryoblast (inner cell mass) consists of a mass of cells inside the blastocyst. These cells will eventually develop into the embryo.
• The trophoblast is the outer cell layer of the blastocyst. Trophoblast cells will implant in the uterus and eventually develop into the placenta and other embryonic tissues.
• The blastocoele is a cavity formed by the migration of embryoblast cells to one pole of the blastocyst. The blastocoele fills with fluid secreted by trophoblast cells.
Implantation
Around day 8 or 9 after fertilization, implantation begins. Implantation is the process in which a blastocyst becomes embedded in the endometrium of the uterus (see Figure \(5\)). Implantation is triggered by contact between the blastocyst and endometrium. In response to this contact, trophoblast cells start to proliferate. The trophoblast cells start secreting enzymes that digest the mucosa covering the endometrium. These changes allow finger-like projections (called villi) of the trophoblast to penetrate into the endometrium. The projections pull the blastocyst — now called an embryo — into the endometrium until it is fully covered by endometrial epithelium.
Review
1. Define the germinal stage of human development.
2. Name four processes that occur during the germinal stage.
3. Describe three processes that enable successful fertilization after ovulation occurs and sperm enter the Fallopian tube.
4. What is cleavage? Where does it take place? What is its end result?
5. What is blastulation? How does the morula change during this process?
6. Identify the major parts of the blastocyst.
7. Define implantation. When and how does implantation occur?
8. List the stages of the developing human organism, in order, from fertilization to the end of implantation.
9. Explain why cells in the zygote are smaller on day 3-4 after fertilization than they were on day 2.
10. True or False: The zona pellucida disintegrates as the morula becomes a blastocyst.
11. True or False: Some cells in the blastocyst do not become part of the embryo.
12. Why do you think it is important that only one sperm fertilizes each egg? What mechanism helps ensure that this happens properly?
13. Put the following events in order of when they occur during early human development, from earliest to latest:
1. differentiation of cells
2. cleavage
3. implantation
4. adhesion
14. Which has the most cells?
1. blastocoele
2. embryoblast
3. morula
4. blastocyst
Explore More
bio.libretexts.org/link?17804#Explore_More
Attributions
1. Newborn by amsferguson, Pixabay license
2. Human fertilization by Ttrue12, CC BY-SA 3.0 via Wikimedia Commons
3. Sperm fertilization by CC BY-NC-SA 2.0 via Open Learn
4. Embryonic development by OpenStax College, CC BY 3.0
5. Implantation by OpenStax College, CC BY 3.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.2%3A_Germinal_Stage.txt |
The Most Important Time in Your Life?
In many cultures, marriage — along with birth and death — is considered the most pivotal life event. For pioneering developmental biologist Lewis Wolpert, however, these life events are overrated. According to Wolpert, "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Gastrulation is a major biological event that occurs early in the embryonic stage of human development.
Defining the Embryonic Stage
After a blastocyst implants in the uterus around the end of the first week after fertilization, its internal cell mass, which was called the embryoblast, is now known as the embryo. The embryonic stage lasts through the eighth week following fertilization, after which the embryo is called a fetus. The embryonic stage is short, lasting only about seven weeks in total, but developments that occur during this stage bring about enormous changes in the embryo. During the embryonic stage, the embryo becomes not only bigger but also much more complex. Figure \(2\) shows an eight to nine week old embryo. The embryo's finger, toes, head, eyes, and other structures are visible. It is no exaggeration to say that the embryonic stage lays the necessary groundwork for all of the remaining stages of life.
Embryonic Development
Starting in the second week after fertilization, the embryo starts to develop distinct cell layers, form the nervous system, make blood cells, and form many organs. By the end of the embryonic stage, most organs have started to form, although they will continue to develop and grow in the next stage (that of the fetus). As the embryo undergoes all of these changes, its cells continuously undergo mitosis, allowing the embryo to grow in size, as well as complexity.
Gastrulation
Late in the second week after fertilization, gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. A gastrula has 3 germ layers--the ectoderm, the mesoderm, and the endoderm. Some of the ectoderm cells from the blastula collapse inward and form the endoderm.
The final phase of gastrulation is the formation of the primitive gut that will eventually develop into the gastrointestinal tract. A tiny hole, called a blastopore, develops in one side of the embryo. The blastopore deepens and becomes the anus. The blastopore continues to tunnel through the embryo to the other side, where it forms an opening that will become the mouth. Whether this blastospore develops into a mouth or an anus determines whether the organism is a protostome or a deuterostome. With a functioning digestive tube, gastrulation is now complete.
Each of the three germ layers of the embryo will eventually give rise to different cells, tissues, and organs that make up the entire organism, which is illustrated in Figure \(4\). For example, the inner layer (the endoderm) will eventually form cells of many internal glands and organs, including the lungs, intestines, thyroid, pancreas, and bladder. The middle layer (the mesoderm) will form cells of the heart, blood, bones, muscles, and kidneys. The outer layer (the ectoderm) will form cells of the epidermis, nervous system, eyes, inner ears, and many connective tissues.
Table \(1\): The germ layers and what they give rise to
Germ Layer Gives rise to
Ectoderm The epidermis, glands of the skin, some cranial bones, pituitary and adrenal medulla, the nervous system, the mouth between cheek and gums, the anus
Mesoderm Connective tissues, bone, cartilage, blood endothelium of blood vessels, muscles, synovial membranes, serous membranes, kidneys, the lining of gonads
Endoderm The lining of the airways and digestive system, except the moth and distal part of the digestive system. Digestive, endocrine, and adrenal cortex glands.
Neurulation
Following gastrulation, the next major development in the embryo is neurulation, which occurs during weeks three and four after fertilization. This is a process in which the embryo develops structures that will eventually become the nervous system. Neurulation is illustrated in Figure \(4\). It begins when a structure of differentiated cells called a neural plate forms from the ectoderm. The neural plate then starts to fold inward until its borders converge. The convergence of the neural plate borders also results in the formation of a neural tube. Most of the neural tube will eventually become the spinal cord. The neural tube also develops a bulge at one end, which will later become the brain.
Organogenesis
In addition to neurulation, gastrulation is followed by organogenesis, when organs develop within the newly formed germ layers. Most organs start to develop during the third to eighth weeks following fertilization. They will continue to develop and grow during the following fetal period.
The heart is the first functional organ to develop in the embryo. The primitive blood vessels start to develop in the mesoderm during the third week after fertilization. A couple of days later, the heart starts to form in the mesoderm when two endocardial tubes grow. The tubes migrate toward each other and fuse to form a single primitive heart tube. By about day 21 or 22, the tubular heart starts to beat and pump blood, even as it continues to develop. By day 23, the primitive heart has formed five distinct regions. These regions will develop into the chambers of the heart and the septa (walls) that separate them by the end of the eighth week after fertilization.
Other Developments in the Embryo
Several other major developments that occur during the embryonic stage are summarized chronologically below, starting with the fifth week after fertilization.
Week Five
By week five after fertilization, the embryo measures about 4 mm (0.16 in.) in length and has begun to curve into a C shape. During this week, the following developments take place:
• Grooves called pharyngeal arches form. These will develop into the face and neck.
• The inner ears begin to form.
• Arm buds are visible.
• The liver, pancreas, spleen, and gallbladder start to form.
Week Six
By week six after fertilization, the embryo measures about 8 mm (0.31 in.) in length. During the sixth week, some of the developments that occur include:
• The eyes and nose start to develop.
• Leg buds form and the hands form as flat paddles at the ends of the arms.
• The precursors of the kidneys begin to form.
• The stomach starts to develop.
Week Seven
By week seven, the embryo measures about 13 mm (0.51 in.) in length. During this week, some of the developments that take place include:
• The lungs begin to form.
• The arms and legs have lengthened, and the hands and feet have started to develop digits.
• The lymphatic system starts to develop.
• The primary prenatal development of the sex organs begins.
Week Eight
By week eight — which is the final week of the embryonic stage — the embryo measures about 20 mm (0.79 in.) in length. During this week, some of the developments that occur include:
• Nipples and hair follicles begin to develop.
• External ears start to form.
• The face takes on a human appearance.
• Fetal heartbeat and limb movements can be detected by ultrasound.
• All essential organs have at least started to form.
Genetic and Environmental Risks to Embryonic Development
The embryonic stage is a critical period of development. Events that occur in the embryo lay the foundation for virtually all of the body’s different cells, tissues, organs, and organ systems. Genetic defects or harmful environmental exposures during this stage are likely to have devastating effects on the developing organism. They may cause the embryo to die and be spontaneously aborted (also called a miscarriage). If the embryo survives and goes on to develop and grow as a fetus, it is likely to have birth defects.
Environmental exposures are known to have adverse effects on the embryo include:
• Alcohol consumption: Exposure of the embryo to alcohol from the mother’s blood can cause fetal alcohol spectrum disorder. Children born with this disorder may have cognitive deficits, developmental delays, behavioral issues, and distinctive facial features.
• Infection by rubella virus: In adults, rubella (German measles) is a relatively mild disease, but if the virus passes from an infected mother to her embryo, it may have severe consequences. The virus may cause fetal death, or result in a diversity of birth defects, such as heart defects, microcephaly (abnormally small head), vision and hearing problems, cognitive deficits, growth problems, and liver and spleen damage.
• Radiation from diagnostic X-rays or radiation therapy in the mother: Radiation may damage DNA and cause mutations in embryonic germ cells. When mutations occur at such an early stage of development, they are passed on to daughter cells in many tissues and organs, which is likely to have severe impacts on the offspring.
• Nutritional deficiencies: A maternal diet lacking certain nutrients may cause birth defects. The birth defect called spina bifida is caused by a lack of folate when the nervous system is first forming, which happens early in the embryonic stage. In this disorder, the neural tube does not close completely and may lead to paralysis below the affected region of the spinal cord.
Extraembryonic Structures
Several structures form simultaneously with the embryo. These structures help the embryo grow and develop. These extraembryonic structures include the placenta, chorion, yolk sac, and amnion.
Placenta
The placenta is a temporary organ that provides a connection between a developing embryo (and later the fetus) and the mother. It serves as a conduit from the maternal organism to the offspring for the transfer of nutrients, oxygen, antibodies, hormones, and other needed substances. It also passes waste products (such as urea and carbon dioxide) from the offspring to the mother’s blood for excretion from the body of the mother.
The placenta starts to develop after the blastocyst has implanted in the uterine lining. The placenta consists of both maternal and fetal tissues. The maternal portion of the placenta develops from the endometrial tissues lining the uterus. The fetal portion develops from the trophoblast, which forms a fetal membrane called the chorion (described below). Finger-like villi from the chorion penetrate the endometrium. The villi begin to branch and develop blood vessels from the embryo.
As shown in Figure \(5\), maternal blood flows into the spaces between the chorionic villi, allowing the exchange of substances between the fetal blood and the maternal blood without the two sources of blood actually intermixing. The embryo is joined to the fetal portion of the placenta by a narrow connecting stalk. This stalk develops into the umbilical cord, which contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries, exchanges gases, and other substances with the mother’s blood, and travels back to the fetus through the umbilical vein.
Chorion, Yolk Sac, and Amnion
Besides the placenta, the chorion, yolk sac, and amnion also form around or near the developing embryo in the uterus. Their early development in the bilaminar embryonic disc is illustrated in Figure \(5\).
• Chorion: The chorion is a membrane formed by extraembryonic mesoderm and trophoblast. The chorion undergoes rapid proliferation and forms the chorionic villi. These villi invade the uterine lining and help form the fetal portion of the placenta.
• Yolk Sac: The yolk sac (or sack) is a membranous sac attached to the embryo and formed by cells of the hypoblast. The yolk sac provides nourishment to the early embryo. After the tubular heart forms and starts pumping blood during the third week after fertilization, the blood circulates through the yolk sac, where it absorbs nutrients before returning to the embryo. By the end of the embryonic stage, the yolk sac will have been incorporated into the primitive gut, and the embryo will obtain its nutrients from the mother’s blood via the placenta.
• Amnion: The amnion is a membrane that forms from extraembryonic mesoderm and ectoderm. It creates a sac, called the amniotic sac, around the embryo. By about the fourth or fifth week of embryonic development, amniotic fluid begins to accumulate within the amniotic sac. This fluid allows free movements of the fetus during the later stages of pregnancy and also helps cushion the fetus from potential injury.
Feature: My Human Body
Assume that you’ve been trying to conceive for many months and that you have just found out that you’re finally pregnant. You may be tempted to celebrate the good news with a champagne toast, but it’s not worth the risk. Alcohol can cross the placenta and enter the embryo’s (or fetus’s) blood. In essence, when a pregnant woman drinks alcohol, so does her unborn child. Alcohol in the embryo (or fetus) may cause many abnormalities in growth and development.
A child exposed to alcohol in utero may be born with a fetal alcohol spectrum disorder (FASD), the most severe of which is fetal alcohol syndrome (FAS). Signs and symptoms of FAS may include abnormal craniofacial appearance (Figure \(6\)), short height, low body weight, cognitive deficits, and behavioral problems, among others. The risk of FASDs and their severity if they occur depend on the amount and frequency of alcohol consumption, and also on the age of the embryo or fetus when the alcohol is consumed. Generally, greater consumption earlier in pregnancy is more detrimental. However, there is no known amount, frequency, or time at which drinking is known to be safe during pregnancy. The good news is that FASDs are completely preventable by abstaining from alcohol during pregnancy and while trying to conceive.
Review
1. When does the embryonic stage occur?
2. Name a few of the major developments that occur during the embryonic stage.
3. What is the embryonic disc? When and how does it form?
4. Define gastrulation. When does it occur?
5. Identify the three embryonic germ layers. Give examples of specific cell types that originate in each germ layer.
6. What happens during neurulation? When does it occur?
7. Define organogenesis. When does organogenesis take place in the embryo?
8. What is the first functional organ to develop in the embryo? When does this organ start to function?
9. Identify some of the developments that take place during weeks five through eight of the embryonic stage.
10. List three environmental exposures that may cause birth defects during the embryonic stage.
11. Identify extraembryonic structures that form at the same time as the embryo and help the embryo grow and develop. Give a function of each structure.
12. Put the following events in order of when they occur, from earliest to latest:
1. formation of the neural tube
2. formation of the three germ layers
3. formation of the primitive streak
4. incorporation of the yolk sac into the embryo
13. True or False: The nervous system develops from the same germ layer as skin cells do.
14. True or False: Leg buds are formed during gastrulation.
15. What are two tissues produced by the hypoblast?
Explore More
bio.libretexts.org/link?17805#Explore_More
Learn more about spina bifida here:
Attributions
1. Wedding couple in Kandy Sri Lanka by Peter van der Sluijs, CC BY-SA 3.0 via Wikimedia Commons
2. Human embryo by Anatomist 90, CC BY-SA 3.0 via Wikimedia Commons
3. Blastula and Gastrula by Abigail Pyne, public domain via
4. Neurulation by Stephen Walter Ranson, Public domain, via Wikimedia Commons
5. Placenta development by OpenStax College, CC BY 3.0 via Wikimedia Commons
6. FASD by Teresa Kellerman, CC BY-SA 3.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.3%3A_Embryonic_Stage.txt |
Hello, Baby
This mother-to-be is holding an ultrasound image of her fetus. She is nearly nine months pregnant, so the fetus is fully developed and almost ready to be born. The fetus has grown tremendously and changed in many other ways since it was a tiny embryo seven months previously.
Defining the Fetal Stage
A fetus is a prenatal human being between the embryonic stage and birth. The fetal stage extends from the beginning of the ninth week after fertilization to about 38 weeks after fertilization, which is the average time of birth. The fetal stage lasts a total of approximately 30 weeks. Figure \(2\) shows a seven-week-old embryo that is just getting ready to begin the fetal stage of development. At 7 weeks the embryo is about 10 mm long and has a big forehead. It is developing the inner ear but not the outer ear. The limb buds are visible.
Fetal Development
The image in Figure \(3\) shows a fetus at the start of week 9, the first week of the fetal stage. The fetus is shown larger than its actual size, which from crown to rump is only about 3.2 cm (1.3 in.) long. Even at this early age, however, the fetus has developed to the point of being recognizable as a human being. It possesses virtually all of the major body organs. However, most of the organs are not yet fully developed and functional, and some are not yet situated in their final anatomical locations. These final developments will occur during the remainder of the fetal stage.
Weeks 9 to 15
During weeks 9 to 15, the fetus’s reproductive organs develop rapidly. The external genitals of male and female fetuses are rather similar in appearance at first, but they will be clearly differentiated by week 12. At that point, the biological sex of the fetus can be determined with almost 100 percent accuracy using obstetric ultrasound. Figure \(4\) s shows a fetus at 11 weeks.
Other developments that usually occur in the fetus during weeks 9 to 15 after fertilization include the following:
• Facial development continues. The eyelids form, the ears move toward their final position on the sides of the head, and tooth buds appear.
• Fine, colorless hair called lanugo starts to grow on the fetus’s face. It will eventually cover most of the body until it is shed close to the time of birth.
• The thyroid gland matures and starts producing thyroid hormones. The liver and pancreas also start producing their secretions.
• The kidneys start functioning. The amniotic fluid the fetus swallows can now pass out of the body as urine.
• The fetus is very active. This is the result of uncontrolled movements and twitches that occur as the muscles, brain, and nervous pathways develop. The fetus may move its limb, make a fist with its fingers, and make sucking motions with its mouth. Generally, brain activity can be detected during these early weeks of the fetal stage.
Weeks 16 to 26
Many important changes occur in the fetus during weeks 16 to 26 after fertilization. Some of the specific developments that occur during weeks 16 to 26 include the following.
• The brain and sensory nerves develop to the point that the fetus has a sense of touch. This may lead to the fetus stroking its face, touching its limbs, and even sucking its thumb.
• The eyes and ears continue to develop. The eyes move to a forward-facing position, and the retinas develop. The ears also move to their final position, and the outer ears are now elevated above the surface of the head. Development of the middle ear and auditory nerve allows the fetus to hear. It can hear both internal sounds (such as the mother’s heartbeat) and external sounds (such as voices). The fetus may even be startled by loud noises.
• The fetus’s bones have already been developing, but they now start to ossify, beginning with the clavicles and bones in the legs. The bone marrow also develops and starts producing blood cells. Prior to this time, blood cells were produced by the liver and spleen.
• Alveoli form in the lungs. These functional units of the lungs must be fully developed before an infant can breathe air after birth.
• Considerable muscle development occurs. The fetus’s movements become more forceful, and the movements stimulate further development of the muscles.
• The intestines develop sufficiently that small amounts of sugars can be absorbed from the amniotic fluid that is swallowed. Virtually all of the fetus’s nutrients, however, still come from the mother’s blood via the placenta.
• The fetus develops a thick waxy coating called vernix. This coating protects the fetus’s skin from becoming chapped or irritated by amniotic fluid.
Weeks 27 to 38
During weeks 27 to 38 after fertilization, the bones of the fetus complete their development. The fetus also grows rapidly during these final weeks, and its body fat increases substantially. Its formerly wrinkled skin starts to plump out as layers of subcutaneous fat are deposited.
Additional changes that occur in the fetus during weeks 27 to 38 include the following:
• The fetus’s head hair grows thicker and coarser while the lanugo is shed. The waxy vernix covering the fetus becomes thicker at first, but most of it will disappear by birth.
• In preparation for breathing after birth, the fetus will repeatedly mimic breathing by moving the diaphragm. By about week 32, the lungs are likely to be fully developed so the fetus can breathe on its own, should it be born this early.
• The fetus can not only hear and feel touch, but its eyes can now detect light. In fact, the pupils can constrict and dilate in response to light.
• During this phase, the fetus sleeps much of the time. Its brain, however, is continuously active.
By the end of week 38, the fetus measures about 51 cm (20 in.) long. A 38-week fetus is pictured in Figure \(5\).
Fetal Circulation
The heart and blood vessels that form the cardiovascular system are among the earliest organs to develop in the embryo. They continue to develop in complexity and grow in size during the fetal stage. However, until birth, the circulation of blood in the fetus is different than the postnatal circulation will be, primarily because the lungs are not yet in use. The fetus cannot breathe the air because it is floating in amniotic fluid. Instead, the fetus obtains oxygen from the mother’s blood via the placenta and umbilical cord.
Prenatal Circulation
The fetal circulation before birth is illustrated in Figure \(6\). Oxygen-rich blood from the placenta is carried to the fetus by the umbilical vein. Some of the blood flows through a fetal vein called the ductus venosus, which carries the blood to the inferior vena cava. In turn, the vena cava carries blood to the right atrium of the heart. Throughout the fetal stage, there is an opening between the right and left atria, called the foramen ovale, which allows most of the blood reaching the right atrium to flow directly into the left atrium, thus bypassing the pulmonary circulation. Blood that enters the left atrium is pumped into the left ventricle, and from there through the aorta, the major artery that carries blood to the rest of the body. Blood that reaches the umbilical arteries flows back through the umbilical cord to the placenta, where carbon dioxide and other waste products from the fetus enter the maternal circulation.
Not all of the blood reaching the right atrium through the ductus venosus passes directly into the left atrium via the foramen ovale. A small amount of blood is pumped from the right atrium into the right ventricle, and from the right ventricle into the pulmonary arteries. A fetal artery called the ductus arteriosus directs most of this blood away from the nonfunctioning lungs by shunting blood from the pulmonary trunk to the aorta.
Postnatal Circulation
After birth, as the newborn takes the first breath, the blood circulation suddenly changes. There is decreased resistance in the lungs now that the infant is surrounded by air instead of amniotic fluid. The lowered resistance allows more blood to flow into the pulmonary arteries from the right atrium and ventricle, and less to flow through the foramen ovale into the left atrium. Blood now travels to the lungs through the pulmonary arteries and then back to the heart through the pulmonary veins to the left atrium. This produces an increase in pressure in the left atrium that forces the foramen ovale to close. Once the foramen ovale closes, blood can no longer flow through it and bypass the pulmonary circulation. The ductus arteriosus is no longer needed to shunt blood away from the lungs, and it normally closes within a day or two of birth. The ductus venosus usually closes within another couple of days.
Birth Weight
The fetal growth rate is one of two major factors that determine the weight of the fetus at birth, or birth weight, which averages about 3.4 kg (7.5 lb.) in a full-term infant. The other factor that determines birthweight is the length of gestation. Infants born before full term, which is defined as 36-40 weeks after fertilization, are usually smaller than full-term infants because they have spent less time growing in the uterus. Pre-term birth is one of the major causes of low birth weight, which is defined as a birth weight lower than 2.5 kg (5.5 lb.), regardless of gestational age. Low birth weight increases the risk of death shortly after birth. As many as 30 percent of deaths in the first month of life occur in preterm infants. Holding the length of gestation constant, a newborn may be classified as small for gestational age, appropriate for gestation age, or large for gestational age. Fetuses that did not grow adequately before birth may end up being small for gestational age, even when they are born at full term.
Viability of the Fetus
Fetal viability refers to the point in fetal development at which the fetus is likely to be able to survive outside the uterus. When babies are born too early, they have an elevated risk of dying within the first few hours to weeks of life. The main causes of early mortality in pre-term infants are inadequately developed respiratory and nervous systems. For babies born at 23 weeks of gestation, the chances of surviving are only between 20 and 35 percent, and survival is possible only with intensive, advanced medical care. For babies born at 25 weeks of gestation, the survival chances are much greater — as high as 70 percent — but again, intensive medical intervention is needed (see the newborn infant in Figure \(7\). The chances of survival are much better after 26 weeks of gestation. More than 90 percent of babies survive if they are born after 26 weeks and receive any necessary medical care. What a difference just three weeks makes!
Review
1. Define fetus. Delineate the fetal stage.
2. Describe the fetus at the beginning of the fetal stage.
3. ist some of the fetal developments that occur between weeks 9 and 15 after fertilization.
4. Give examples of fetal changes that occur during weeks 16 through 26 after fertilization.
5. Identify several developments that take place in the fetus between week 27 and birth.
6. How and why is fetal blood circulation different from postnatal circulation?
7. Compare and contrast fetal and adult hemoglobin.
8. Outline the typical pattern of fetal growth in size.
9. What is IUGR? What is its leading cause?
10. What is the average weight of a full-term infant at birth? How is low birth weight defined, and what are the two major causes of low birth weight?
11. Define fetal viability. At what age is a fetus likely to be viable?
12. Put the following events in order of when they occur, from earliest to latest:
1. The kidneys start functioning.
2. The ductus arteriosis closes.
3. The fetus begins to detect light.
4. The fetus begins to hear.
13. True or False: A fetus can produce urine.
14. True or False: The umbilical artery carries oxygenated blood to the fetus.
15. If a baby is born at 30 weeks, what is one type of medical intervention that might be necessary to keep the baby alive? Explain your answer.
Attributions
1. Ultrasound by Public Domain Images, Pixabay license
2. Human Embryo (7th week of pregnancy) by Ed Uthman, licensed CC BY 2.0 via Flickr
3. 9-Week Human Embryo from Ectopic Pregnancy by Ed Uthman, licensed CC BY 2.0 via Flickr
4. fetus at the 11th gestational week - 3D HD live rendered image by Araujo Júnior E, Santana EF, Nardozza LM, Moron AF - Radiol Bras (2015 Jan-Feb) CC BY-NC 3.0 via SciELO
5. Eesti Tervishoiu Museum Estonian Health Care Museum Tallinn Estonia 2016 by A. Currell, licensed CC BY-NC 2.0 via Flickr
6. Fetal Circulation by OpenStax College, CC BY 3.0 via Wikimedia Commons
7. Premature infant by ceejayoz, licensed CC BY 2.0 via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.4%3A_Fetal_Stage.txt |
What a Life!
Sleep, cry, eat, repeat. Oh, the life of an infant. Newborn infants actually do spend most of their time in these three “pursuits.” However, by the end of their first year, they have greatly expanded their repertoire. By their first birthday, infants are typically starting to walk and talk, and they are spending about as much time awake as asleep. Clearly, infancy is a time of tremendous change.
Defining Infancy
Infancy refers to the first year of life after birth, and an infant is defined as a human being between birth and the first birthday. The term baby is usually considered synonymous with an infant, although it is commonly applied to the young of other animals, as well as humans. Human infants seem weak and helpless at birth, but they are actually born with a surprising range of abilities. Most of their senses are quite well developed, and they can also communicate their needs by crying, like the three-day-old baby in Figure \(2\). During their first year, infants develop many other abilities, some of which are described in this concept. They also grow more rapidly during their first year than they will at any other time during the rest of their life.
Neonate
A newborn infant is called a neonate up until the first four weeks after birth. A neonate, like the crying baby pictured in Figure \(2\), does not usually look like the plump, chubby-cheeked “Gerber baby” that most people envision when they hear the term "baby."
Status of the Newborn: Apgar Score
Immediately after birth, a simple test called an Apgar test, is administered to an infant to evaluate its transition from the uterus to the outside world (Figure \(3\)). The newborn is assessed on each of five easy-to-measure traits, and for each trait is given a score of 0, 1, or 2 (where 0 is the worst value and 2 is the best). After an infant is assessed on each trait, the values of all five traits are added together to yield the Apgar score. The highest (best) possible score is 10, but a score of 7 or higher is considered normal. A score of 4-6 is considered fairly low, and a score of 3 or lower is considered critically low. The purpose of the Apgar test is to determine quickly whether a newborn needs immediate medical care. It is not designed to predict long-term health issues.
The five traits that are assessed in an Apgar test are listed in Table \(1\). The table also shows how the acronym APGAR can be used to help remember the five traits.
Table \(1\): Apgar Test
Acronym (APGAR) Trait Score of 0 Score of 1 Score of 2
A = Appearance skin color blue or pale all over blue at extremities; body pink extremities and body both pink
P = Pulse heart rate absent <100 beats per minute >100 beats per minute
G = Grimace reflex irritability grimace no response to stimulation the grimace on suction or aggressive stimulation cry on stimulation
A = Activity activity none some flexion flexed arms and legs that resist extension
R = Respiration respiratory effort absent weak, irregular gasping strong, robust cry
Umbilical Cord
The umbilical cord of a newborn infant contains the umbilical artery and vein. The cord will normally be cut within seconds of birth, leaving a stub about 3-5 cm (1-2 in.) long (Figure \(4\)). The umbilical stub will dry out, shrivel, darken, and spontaneously fall off within about three weeks of birth. This will become the navel after it fully heals.
Physical Characteristics of the Neonate
Right after birth, a newborn’s skin is wet. It may be covered with streaks of blood, and coated with patches of waxy white vernix. The newborn may also have peeling skin on the wrists, hands, ankles, and feet. Some newborns still have the fine, colorless hair called lanugo, but it usually disappears within the first month after birth. Infants may be born with a full head of hair, or they may have very little hair, or even be bald. A newborn’s body proportions are distinctive, as well. The shoulders and hips are relatively wide, and the arms and legs are relatively long compared to the rest of the body. In addition, the abdomen protrudes slightly.
A newborn’s head, especially the cranium, is very large in proportion to the body. As shown in Figure \(5\), the newborn head makes up about one-quarter of the baby’s total body length, whereas the head of an adult makes up only about one-seventh of the adult’s total body length. The body is drawn to be the same length (height) at each age to make the differences in body proportion — and especially head size — more apparent.
Many regions of the neonate’s skull have not yet been converted to bone, leaving “soft spots” known as fontanels (Figure \(6\)). The two largest fontanels are the diamond-shaped anterior (frontal) fontanel, located at the top front of the skull, and the smaller triangular-shaped posterior (occipital) fontanel, located at the back of the head. During birth, the fontanels enable the bony plates of the skull to move and change shape, allowing the child's head to pass through the birth canal. Right after birth, the head may be temporarily misshapen for this reason. The ossification of the bones of the skull causes the posterior fontanel to close during the first two or three months after birth, and the anterior fontanel to close by nine to 18 months after birth.
Size and Growth of the Neonate
In the wealthier nations of the world, the total body length of a full-term infant at birth normally ranges between 46 and 56 cm (18 and 22 in.), with an average of 51 cm (20 in.). The birth weight of a full-term infant normally ranges from 2.5 to 4.5 kg (5.5 to 10 lb), with an average of 3.4 kg (7.5 lb). For pre-term infants, these numbers are likely to be lower, because these infants have had a shorter period of prenatal growth.
During the first week following birth, it is normal for the weight of a neonate to decrease by about three to seven percent of the birth weight. For example, a baby born at an average weight of 3.4 kg (7.5 lb) might weigh only 3.2 kg (7.1 lb) by the seventh day after birth. This loss of weight is a normal result of the resorption and urination of the fluid that initially fills the lungs. A contributing factor may be a delay of a few days before feeding becomes well established, which is also normal. After the first week, a healthy neonate should start to gain up to 20 g (0.7 oz) per day.
Neonate Senses
Some senses in newborns are already relatively well developed. Other senses are still immature and need to develop further after birth.
Sense of Touch in the Neonate
Newborns have a well-developed sense of touch, and they usually respond positively to soft stroking and cuddling. Gentle rocking back and forth, massages, and warm baths are also positively received by neonates, and they may calm a crying infant. Newborns can often comfort themselves by sucking their thumb, finger, or pacifier.
Vision in the Neonate
Newborns' vision is not yet fully developed. Both the retinas and the parts of the brain involved in vision are still immature. Most newborns are only able to focus on objects that are directly in front of their face and about 46 cm (18 in.) away. However, this is sufficient for the infant to see the mother’s face, as well as the areola and nipple. When a newborn is not feeding, sleeping, or crying, it is generally staring at objects within its visual range. Usually, anything that is shiny has sharp contrasting colors, or has a complex pattern will catch an infant’s eye. However, the neonate, like the infant pictured in Figure \(7\), has a clear preference for looking at human faces above all else.
Newborns have limited color perception. About three-quarters of newborns can distinguish red, but fewer than half can distinguish green, yellow, or blue. Color perception, however, improves quickly after birth. A newborn infant also lacks depth perception, which is the ability to see in three dimensions. This ability starts to develop only after an infant becomes mobile later in infancy. It continues to develop throughout early childhood.
Hearing in the Neonate
A sense of hearing is well developed at birth. Newborns usually respond more readily to female than male voices, and the sound of voices, especially the mother’s voice, may have a soothing effect on the infant. Sounds that the infant heard before birth — such as the parent’s breathing and heartbeat — are also comforting to the newborn.
Loud or sudden noises, on the other hand, are likely to startle and frighten a newborn. The neonate also responds to sounds of potential danger — such as angry voices of adults, thunder, or the cries of other infants — with greater attention. They may turn toward the sounds and blink their eyes.
Taste and Smell in the Neonate
Newborns can respond to different tastes, including sweet, sour, bitter, and salty tastes. They generally show a preference for sweet tastes. They also show a preference for the smell of foods that their mother ate regularly during pregnancy. Presumably, this occurs because amniotic fluid changes taste with different foods eaten by the mother.
Newborn Reflexes
Newborns have a number of instinctive behaviors, or reflexes, that help them survive. Crying is one example. It is instinctive in newborn infants, who may use it to express a variety of feelings, such as hunger, discomfort, overstimulation, or loneliness. The need to suckle is also instinctive. They have a sucking reflex that allows them to extract milk from the mother’s nipple or from the nipple on a bottle right after birth. In addition, infants have an instinctive behavior known as the rooting reflex that helps them find the nipple by touch. When an infant’s cheek is stroked or it rubs against an object, the baby automatically turns its head in that direction to find the nipple.
Infants are born with other reflexes that aid them in maintaining close physical contact with their caregiver. These reflexes help them hold onto the caregiver so they are less likely to fall, and also so they can satisfy their basic need for constant physical contact. Two of these reflexes are the Moro reflex and the grasping reflex.
• The Moro reflex is an instinctive behavior that is normally present in an infant from birth up until about three or four months of age. It occurs in response to a sudden loss of support when the infant feels as though it is falling. It involves three distinct components: suddenly spreading out the arms, bringing the arms back in toward the body, and, usually, crying. If the baby really were falling, these motions might help it reach out and grab its mother or another caregiver.
• The grasping reflex is the instinctive grasping of a finger or other object that is placed in the palm of an infant. This reflex actually arises before birth and is present until an infant is about five or six months of age. It may help an infant grip and hang on to the mother or another caregiver.
Milestones in Infant Development
Many developments occur during infancy. These include developments in several areas — motor skills, sensory abilities, and cognitive abilities. Infants vary in the exact timing of these developments, but the sequence of the developments is usually similar from one infant to another.
Two Months
During the first two months after birth, an infant normally develops the ability to hold their head erect and steady when they are held in an upright position. They will also develop the ability to roll from their side to their back. They are likely to start cooing and babbling at their parents and other people they know, and they will also start smiling at their parents (Figure \(9\)).
Four Months
By the end of the fourth month after birth, an infant can roll from front to side, lift their head 90 degrees while lying prone, sit up with support, and hold their head steady for brief periods. They will turn their head toward sounds and follow objects with their eyes. They will start to make vowel sounds and begin to laugh. They may even squeal with delight.
Six Months
Around six months of age, an infant is normally able to pick up objects and transfer them from hand to hand. They can also pull themselves into a sitting position. Their vision will have improved so it is now almost as acute as adult vision. The infant will also start noticing colors and start to show the ability to discriminate depth. They are likely to enjoy vocal play and may start making two-syllable sounds such as “mama” or “dada.” They may also start to show anxiety toward strangers.
Ten Months
By about ten months of age, an infant can wiggle and crawl, like the infant pictured in Figure \(10\), and can sit unsupported. If they drop a toy they will look for it, and they can now pick up objects with a pincer grasp (using the tips of the thumb and forefinger). They babble in a way that starts to resemble the cadences of speech. They are likely to exhibit fear around strangers.
Twelve Months
By the end of the first year, an infant normally can stand while holding onto furniture or someone’s hand. They may even be starting to walk, as the infant in Figure \(11\). When they drop toys, they watch where the toys go. The babies may cooperate with dressing, and they may wave goodbye. They may also babble a few words repeatedly and show that they understand simple commands.
Dental Development in the First Year
The deciduous (baby) teeth generally start to emerge around six months of age. The emergence of teeth is called teething. While the teeth are close to emerging through the gums, the gums may become red, swollen, and painful. The baby is likely to drool and be fussy during the few days it takes for the teeth to finally emerge. The baby might also refuse to eat or drink because of the discomfort. The two lower central incisors usually emerge first at about six months, followed by the two upper central incisors at about eight months. The four lateral incisors (two upper and two lower) emerge at roughly ten months.
Physical Growth in the First Year
Infancy is the period of most rapid growth after birth. Growth during infancy is even faster than it is during puberty when the adolescent growth spurt occurs, as shown in the graph in Figure \(12\).
Growth in Weight and Length
Following the initial weight loss right after birth, an infant normally gains an average of about 28 g (1 oz) per day during the first two months. Then, weight gain slows somewhat, and the infant normally gains about 0.45 kg (1 lb) per month during the remainder of the first year. At this rate of weight gain, an infant generally doubles its birth weight by six months after birth and triples its birth weight by 12 months after birth.
Growth in overall body length is also very rapid during infancy, especially in the first few months. Infants normally grow about 2.5 cm (1.0 in.) per month during the first six months. During the second six months, they normally grow about 1.2 cm (0.5 in.) per month. At this rate of growth in length, an infant may close to double its birth length by the end of the first year!
During doctor visits throughout the first year of life, a baby’s weight and length are measured. The baby’s values are compared to standard weight and length values for infants of the same age to assess whether the baby is growing normally. The actual weight and length are generally considered to be less important than evidence showing that the baby is failing to grow normally between visits. Babies that grow too slowly may have a health problem or maybe undernourished. If this goes uncorrected, it can produce permanent deficits in size. On the other hand, a faster-than-normal increase in weight may result in the infant becoming too heavy and being at greater risk of obesity later in life.
Feature: Reliable Sources
Sudden infant death syndrome (SIDS) is the unexplained death, usually during sleep, of a seemingly healthy infant. In the U.S., SIDS is one of the leading causes of death in the first year of life, with about two thousand infants dying in the U.S. each year from SIDS. The cause of SIDS is unknown, although scientists suspect that immaturity or abnormality of the part of the brain that controls arousal from sleep and breathing may be involved. Researchers have also identified several factors that increase the risk of SIDS. Some risks include male sex, pre-term birth, low birth weight, exposure to secondhand smoke, and sleeping on the stomach. Certain practices — such as placing an infant on its back to sleep and not using pillows or blankets in the crib — can help reduce the risk of SIDS.
Go online to learn more about SIDS. Find reliable sources that answer the following questions.
1. What current research is being undertaken to better understand the cause of SIDS? What risk factors or areas of concern are being investigated?
2. How can parents reduce the risk of SIDS in their infants? Which three reliable sources of information on SIDS would you recommend to new parents to raise their awareness of SIDS and how to reduce the risk of SIDS in their infants?
Review
1. Define infancy, infant, and neonate.
2. What is an Apgar test? When and why is it administered?
3. Describe what happens to the umbilical cord after birth.
4. What are some physical characteristics of a neonate?
5. What are the average length and weight of a well-nourished, full-term newborn?
6. Why do newborns typically lose some weight in the first week after birth?
7. Describe newborn sensory abilities.
8. Identify some of the reflexes that are present in newborn infants, and how they help the newborn survive.
9. Identify a milestone in infant development that typically occurs by each of the ages below. In general, how does the timing of developmental milestones vary among infants at the ages of two months, four months, six months, ten months, and one year
10. Outline dental development in the first year.
11. Describe growth during infancy.
12. Define the infant mortality rate, and explain its significance.
13. A mother brings her six-month-old to visit the pediatrician. She is concerned that he does not weigh nearly as much as his cousin, who is the same age. What is one piece of information that the pediatrician would likely want to know in order to help assess whether the infant’s weight is a concern?
14. A baby is born and a nurse immediately records the observations below. What is this baby’s APGAR score? Is this score considered normal? Explain your answer.
1. Its skin is blue at the extremities, but the body is pink.
2. Its heart rate is 98 beats per minute.
3. Baby cries on stimulation.
4. Baby has flexed arms and legs that resist extension.
5. Baby has a strong, robust cry.
Explore More
Blood in the umbilical cord is a rich source of stem cells that could potentially cure diseases, but cord blood is usually disposed of after birth. Watch the video below to learn about a public cord blood banking program that stores donated cord blood so that these valuable stem cells can be used to save lives.
Attributions
1. Baby sleeping by ULOVInteractive, Pixabay license
2. Newborn crying by Evan-Amos, public domain via Wikimedia Commons
3. Newborn by Bigroger27509, public domain via Wikimedia Commons
4. Doorknippen navelstreng by Mech, CC BY-SA 3.0 via Wikimedia Commons
5. Male figures showing proportions in five ages by http://wellcomeimages.org/Wellcome, CC BY 4.0 via Wikimedia Commons
6. Skull at birth by Henry Gray, public domain via Wikimedia Commons
7. Breastfeeding by capsulanudes via Pixabay license
8. Child's hand by Tembinkosi Sikupela via Unsplash license
9. Infant smiling, public domain via pxhere
10. Baby crawling by Bualong Pata, public domain via Wikimedia Commons
11. Learning to walk by Shaun MItchem, CC BY 2.0 via Wikimedia Commons
12. Human height growth per month by Cantus, CC0 via Wikimedia Commons
13. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.5%3A_Infancy.txt |
Child Labor
The kids in Figure \(1\) from 1911 are just children. All of them are between eight and 12 years old — years you no doubt spent in elementary and middle school. For the children in the picture, those years were spent as coal workers in a Pennsylvania mine. Their job was to separate impurities from coal by hand. For ten hours each day, six days a week, they would sit on wooden seats, perched over chutes and conveyor belts, picking impurities out of the coal. The use of children to do this work began in the mid-1860s, before mandatory child education and child labor laws had been passed in the United States. Although public disapproval of the employment of children as coal workers existed by the mid-1880s, the practice did not end in the United States until the 1920s.
Defining Childhood
This example of child labor from the early 1900s shows how greatly attitudes toward children and childhood have changed over the past century in the United States. Children used to be thought of as small versions of adults who should work to help support the family or at least “earn their keep.” We now know that children differ from adults in many ways besides their size, and we generally think of childhood as an idyllic time dominated by play, fun, and learning. However, even now, the definition of childhood varies. To start, childhood can be defined legally or biologically.
• Legally, childhood is defined as the period of minority, which lasts from birth until adulthood (majority). The age of maturity varies by place and purpose. For example, in the United States, at age 18, you are considered an adult for military service, but a minor for buying alcohol.
• Biologically, childhood is defined as the stage of a human organism between birth and adolescence. The first year of life is called infancy and is covered in the concept Infancy. The remaining years until adolescence are loosely divided into early childhood (one to five years), middle childhood (six to ten years), and pre-adolescence (11-12 years).
Early Childhood
Early childhood follows infancy, which ends at the first birthday. The first part of early childhood is toddlerhood when a child begins speaking and taking steps independently. Toddlerhood ends around age three when the child becomes less dependent on caretakers for basic needs. Early childhood continues with the preschool stage, which ends at about age five.
Toddlers
A toddler is a child between the ages of one and three years old. The toddler years are a time of great physical, cognitive, and psychosocial development. The deciduous dentition continues to erupt during these years, and growth in size is still fairly rapid, especially between the ages of one and two years, although it is considerably slower than it was during infancy. The children in Figure \(2\) show physical and motor development that is typical of toddlers at this age: walking with help.
Physical Development in Toddlers
By their first birthday, most toddlers can pull themselves up to a standing position and walk with help, if not alone. They can also sit down without assistance. They have the motor skills needed to bang two blocks together, turn through the pages of a book by flipping several pages at a time, and use a pincer grasp to pick up objects. They may be able to drink from a cup, but probably not without frequent spills. They may also be able to play with a ball by rolling or tossing it.
By the age of two years, toddlers can typically walk sideways and backward. They can also run, although they are likely to fall down often like the kids in Figure \(2\). Two-year-old toddlers can walk up and down stairs on both feet, one step at a time, while holding on to a rail or someone’s hand. They have the fine motor skills needed to build a tower of blocks that is six blocks high. They have also mastered drinking from a cup and can control a spoon well enough to feed themselves. In addition, they may be toilet trained, at least during waking hours.
By the age of three years, children have reached the end of the toddler stage. Their gross motor skills have progressed to the point that they are good at climbing, and they can now climb stairs one foot per step. They have the fine motor skills needed to handle small objects and to do simple puzzles. They can copy a circle and build a tower of blocks nine blocks high. They are also able to undress with assistance.
Cognitive and Psychosocial Development in Toddlers
One-year-old toddlers can use one- and two-syllable words (such as “ball” and “mama") and they can understand several other words. They can follow simple commands, especially if the commands are given with associated gestures. They can probably bring you a toy if you point at it and say, “Please bring me the toy.” They understand that objects continue to exist even when they are out of sight. They connect names with objects and use gestures or words to refer to objects or actions, such as pointing at a book, raising their arms to be picked up, or saying “cup.” In addition, they can mimic actions, such as covering the eyes while playing peekaboo. They experience separation anxiety and may cling to their parents. Children at this age do not play with other children, although they may play alongside them, like the toddlers pictured in Figure \(4\).
Two-year-old toddlers can use as many as 50 words, and they generally understand at least a couple of hundred more words. They can obey simple verbal commands and help dress and undress. They understand physical relationships, such as flipping a switch to turn on a light. They can search for hidden objects, solve problems through trial and error, and mimic adult behaviors, for example, by feeding a doll. They also demonstrate self-recognition (in a mirror), attachment to parents, and anxiety when separated from parents. On the other hand, they may start showing signs of independence. They frequently use the word “no” and may throw temper tantrums. Although temper tantrums may be a way of expressing strong emotions toddlers do not yet have the ability to express verbally, they may also be a way of showing growing independence and testing boundaries
Three-year-old toddlers are able to speak in short simple sentences and ask questions. They also easily learn new words, including people’s names. They attempt to sing along with songs. They can anticipate routines and are well on their way to being completely toilet trained. By this age, toddlers can show preferences for toys or foods and know how to play games with simple rules. This is also the age at which many children start to have imaginary companions.
Toddler Dentition
By the age of one year, toddlers have up to eight deciduous teeth — generally the four upper and four lower incisors. Between about 12 months and 15 months of age, the lower lateral incisors usually emerge. The four first molars typically emerge between ten and 16 months and the four canines between 16 and 20 months. The remaining deciduous dentition — the four second molars — generally emerge between 20 and 30 months of age. There is considerable individual variation in the ages at which the deciduous teeth emerge, but the sequence at which they emerge is similar in most children.
Growth of Toddlers
Table \(5\) shows typical weight and height ranges for toddlers at ages one, two, and three years old. The values are for well-nourished, healthy children in the United States. Children who are smaller than these values may still be well nourished and healthy because stature is determined by genes, as well as the environment. Children of small parents tend to be small as well.
Table \(5\): Weight and Height of Toddlers
Age (years) Weight (kg) Weight (lbs) Height (cm) Height (in.)
1 8.3 – 10.4 18.3 - 22.9 72.4 – 77.5 28.2 - 30.2
2 10.6 – 13.1 23.3 - 28.8 84.3 – 89.9 32.9 - 35.1
3 12.9 – 15.6 28.4 - 34.3 91.4 – 98.0 35.6 - 38.2
Toddlers generally experience about a 52 percent increase in weight from age one to age three, which is a much slower gain in weight than the nearly 200 percent increase in weight from birth to age one year. In terms of height, there is about a 26 percent increase from age one to age three. Again, this is a much slower rate of growth than the nearly 100 percent increase in height during the first year after birth.
Preschoolers
The preschool stage of early childhood generally refers to the ages four to five years — however, in some disciplines, such as psychology, the preschool stage may be extended to age six or seven.
Physical Development in Preschoolers
By the age of four, children typically can go downstairs one foot per step, skip on one foot, and pedal and steer a wheeled toy such as a tricycle. They can climb ladders and playground equipment, and they can run around obstacles with ease. They are able to build a tower with ten or more blocks and thread small wooden beads on a string. They can also reproduce some shapes and letters, and they can hold a crayon or marker with a tripod grasp as in Figure \(5\).
By the age of five, children can skip on alternate feet and may learn to walk on a balance beam and turn somersaults. They can catch a ball thrown from a meter away, touch their toes without bending the knees, and balance on either foot with good control for about ten seconds. They can also build a three-dimensional structure with blocks by copying from a picture or model, cut on a line with scissors, and demonstrate good control with a pencil or marker. By this age, children begin to color within the lines and can reproduce many shapes and letters. Some children are already learning to ride a bicycle, usually with training wheels.
Cognitive and Psychosocial Development in Preschoolers
The fourth year is generally the age at which young children ask the most questions. Their speech is almost entirely intelligible by this age, and they are beginning to use the correct past tense of verbs. They can talk about objects, people, and events that are not present. They can also recite simple songs and rhymes and state their first and last name (and sometimes their phone number!). They are likely to be able to count to 20 and may be starting to read very simple books with just a few words on each page. They can dress and undress with assistance, and attend to their own toilet needs. They may insist on trying to do things independently but tend to become easily frustrated when problems arise. They typically enjoy role-playing and make-believe activities, and they can cooperate with others and participate in group activities. By this age, they are beginning to establish close relationships with playmates.
Five-year-old children generally have a vocabulary of at least 1,500 words, produce sentences of at least five to seven words, and can define words by function (e.g., a bed is to sleep in). They can recognize the humor in simple jokes, make up jokes and riddles, and enjoy making other people laugh. They can identify and name several colors, sort objects on the basis of two qualities (such as color and shape), and place objects in order by size. They can count past 20 and often up to 100, and they may recognize the numerals one to ten. They know what calendars and clocks are for, and some may be starting to tell time. They can also recognize and identify coins and may be starting to count money. In addition, they are able to dress and undress alone. Children of this age often have just one or two “best” friends, and they may show affection and care toward others, such as another child who is hurt.
Growth of Preschoolers
By the time children pass their fourth birthday, their rate of growth has slowed considerably. During the preschool years, children typically gain about 1.8 to 2.7 kg (4 to 6 lb.) per year and grow about 5.1 to 7.6 cm (2 to 3 in.) per year. By the age of five, the majority of children weigh between 16.5 and 20.3 kg (36.3 and 44.7 lb.) and stand about 105 to 114 cm (41.3 to 44.2 in.) tall.
Middle Childhood
Middle childhood is the life stage between early childhood and pre-adolescence. It covers the ages of six to ten years when most children are in elementary school. Children within this age bracket are more independent and physically active than they were in the preschool years, but few have experienced any of the physical changes of puberty. Children in this range are more involved with friends and are learning to think in more complex ways than during their preschool years. Although progress in most major areas of development is relatively gradual during middle childhood, the cumulative differences between children aged six and those aged ten are substantial (Figure \(6\)).
Physical Development in Middle Childhood
Physically, the first several years of middle childhood are a time of steady development in abilities, such as agility, balance, and endurance. Muscle strength and coordination also improve from ages six to ten years, and movements become more controlled and graceful. During this period, children typically learn to ride a bicycle without training wheels (see Figure \(7\)). They also typically learn to jump rope, hit a baseball, and kick a soccer ball. They may learn more complex skills, as well, such as playing basketball, dancing, or playing a musical instrument.
Cognitive and Psychosocial Development in Middle Childhood
The period from six to ten years is a time of important cognitive changes. Generally, children in this age range develop more mature and logical ways of thinking. They gradually develop the ability to consider multiple parts of problems, although their thinking is still concrete, rather than abstract. Children in this age range also develop the ability to concentrate for increasing lengths of time. Whereas a six-year-old might be able to focus on a task for just 15 minutes and follow a series of only three commands, a ten-year-old might be able to focus on a task for more than an hour and follow a series of at least five commands. Language skills also improve during these years. A six-year-old typically uses complete but simple sentences with an average of only about six words. A ten-year-old uses much longer and more complex sentences with virtually the same grammar and pronunciation as adults.
Emotionally, children aged six to ten years may be somewhat fragile. Their self-esteem may change rapidly depending on how they think others perceive them, especially their peers, as peer acceptance becomes increasingly important during these years. If children aren’t chosen for a team or are snubbed by a friend, for example, their self-esteem may plummet. Children in this age range also develop body modesty and express an increasing desire for privacy. Although school-age children sometimes seem like small adults as they buckle down to schoolwork and take on new responsibilities at home, they can also revert to more immature emotions and behaviors. They may sometimes seem as stubborn and unreasonable as toddlers during the “terrible twos,” and the occasional tantrum may still occur.
From the ages of six to ten, children go from playing with same-gender friends to starting to play in mixed-gender groups. They may still like playing alone, but friends become increasingly important. They may enjoy playing on a sports team, like the Little League Baseball player in the picture below, or they may participate in scouts or other formal peer groups. They are generally good at cooperating and sharing, although they sometimes exhibit jealousy toward their peers or siblings.
Pre-Adolescence
The age range from about 11 to 12 years is called pre-adolescence or the “tweens,” and children in this age range are commonly called preteens. Pre-adolescence is considered a unique stage of development because it coincides with the start of puberty, although few of the obvious physical changes of puberty, such as sexual maturation, have yet to occur.
Pre-adolescence is also a time of significant cognitive and psychosocial development. This is typically when young people finally develop the ability to think abstractly. They can think beyond their personal experiences, and they can view the world less in terms of absolutes, such as right or wrong, black or white. Pre-adolescents also develop the ability to identify the cause and effect sequences, although they still may not be able to infer motives or to reason hypothetically. Relative to earlier ages in middle childhood, preteens tend to:
• Have a more realistic and less fantasy-based view of life: They may worry about a scary media event (such as kidnapping), but they no longer fear the monster under the bed. They may want to be an engineer — rather than a wizard— when they grow up.
• Think and act more reasonably and less emotionally: They might earn money to buy what they want, rather than throw a tantrum when it isn’t given to them on demand.
• Start developing a sense of identity: They may have increased feelings of independence and individuality, and no longer feel like just “one of the family.”
• Care more about their appearance and what they wear.
• Start to have romantic feelings toward a peer, and may experience “puppy love.”
Dentition in Middle Childhood and Pre-Adolescence
Middle childhood is the time when the deciduous teeth loosen and fall out, and most of the permanent teeth emerge. The first deciduous teeth to fall out are generally the eight incisors, which are typically lost between the ages of six and eight (Figure \(9\)). The incisors are followed by the eight premolars, which are generally lost between the ages of nine and 12, and by the four canines, which are generally lost between the ages of ten and 13. The second permanent molars generally emerge between the ages of 11 and 13. The only permanent teeth that emerge later are the third molars (or wisdom teeth) which generally emerge in late adolescence or early adulthood.
Growth in Middle Childhood and Pre-Adolescence
For most children, physical growth is slow and steady during middle childhood, although there may be a few brief growth spurts separated by periods of slower growth. In middle childhood, children generally gain an average of about 3.2 kg (7 lb.) per year and increase in height by an average of about 5.8 cm (2.3 in.) per year. By the age of 12, the average child weighs about 41 kg (91 lb.) and has a height of about 150 cm (59 in.). There is very little difference in size between different sexes at this age. Some of the weight gain during middle childhood generally reflects an increase in muscle mass relative to the muscle mass of the preschool child. Some children may begin the adolescent growth spurt during pre-adolescence, but most of this period of rapid growth occurs during adolescence.
Feature: Reliable Sources
In most of the United States, today’s kindergarten is yesterday’s first grade in terms of the academic skills that children are expected to master. This suggests that pre-K (pre-school) programs are the new kindergarten. Many studies have demonstrated that children who attend quality pre-K programs not only do better throughout the remainder of their school years but also tend to be more successful as adults. Such studies provided the impetus in many states to fund public pre-K education for all students who cannot afford private pre-K programs.
A study reported in 2016 has caused many people to reconsider the value of pre-K education. Undertaken in Tennessee by researchers at Vanderbilt University, the study found that the state’s newly funded voluntary program for low-income preschoolers had only fleeting benefits. Children who attended the program did better in the first year of school than control-group children who did not attend a pre-K program. However, by third grade, children in the control group had actually surpassed the children who had attended the state’s pre-K program.
Not surprisingly, the Tennessee study was very controversial, and it has raised a number of questions. Have the long-touted benefits of pre-K education been over-rated? Does pre-K education really have no lasting impact on children and their success in school and life? Is investing in public pre-K programs a waste of limited educational dollars? Or is the real issue high-quality pre-K versus low-quality pre-K? The Tennessee pre-K program that was the focus of the Vanderbilt study has been criticized for being poorly executed and under-funded. Would a higher-quality pre-K program have produced the positive results found in several other studies? And why did the children who attended the program initially do better than the control group, but then later perform worse? Was something about the education they were receiving at the kindergarten through third-grade level "undoing" the benefits of pre-K?
Research the issue of pre-K education and its pros and cons. Try to find the most up-to-date, evidence-based studies and discussions of the issue. After finding several reliable resources, form your own conclusions about the value of pre-K education. Keep in mind that a reasonable conclusion need not be definitive. In other words, it may be appropriate to conclude that there is not yet enough reliable evidence to assess whether or not pre-K programs are a good investment. Your conclusion should also be open to revision as new evidence becomes available.
Review
1. Contrast legal and biological definitions of childhood.
2. List the stages of biological childhood.
3. Define early childhood. What are its two main divisions?
4. Give examples of the range of developments that occur during early childhood.
5. How does the rate of physical growth during early childhood compare to the growth rate during infancy?
6. What changes in dentition occur during early childhood?
7. Define middle childhood and identify the range of years it covers.
8. Describe some of the changes that take place during middle childhood.
9. What is pre-adolescence, and when does it occur?
10. What are some of the main developmental events that happen in pre-adolescence?
11. Summarize changes in dentition and body size that occur during middle childhood and pre-adolescence.
12. If a toddler weighs 20 pounds at one year of age, which of the choices below is most likely her weight at age three, assuming typical growth?
1. 22 pounds
2. 25 pounds
3. 30 pounds
4. 40 pounds
13. Describe how the interaction between peers changes from the beginning to the end of biological childhood.
14. True or False: By the age of ten, boys are significantly larger than girls, on average.
15. True or False: Tantrums only occur during the early childhood years.
Attributions
1. Child labor coal by Lewis Hine, public domain via Wikimedia Commons
2. Friends by Cheryl Holt, via Pixabay license
3. Toddler running and falling by Jamie Campbell, licensed CC BY 2.0 via Wikimedia Commons
4. Playing with Xylophones by Donnie Ray Jones, licensed CC BY 2.0 via Flickr
5. Child drawing by D. Sharon Pruitt, licensed CC BY 2.0 via Wikimedia Commons
6. Operation good heart by US Army, public domain via Wikimedia Commons
7. Boy on bicyle by cegoh via Pixabay license
8. Children playing released CC0 via Pxhere
9. Smile with missing tooth by crimfants, licensed CC BY 2.0 via Wikimedia Commons
10. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.6%3A_Childhood.txt |
Risk Takers
The surfing teens in Figure \(1\) are tempting fate by trying to surf as close to each other as they can. Collisions with other surfers or surfboards cause the greatest number of surfing-related injuries. Surfing is risky enough without making it more dangerous by doing stunts like this. Taking unnecessary risks is usually thought to be a hallmark of adolescence.
Defining Adolescence
Adolescence is the period of transition between childhood and adulthood. It is generally considered to start with puberty, during which sexual maturation occurs and adolescents go through a spurt in growth. In many children, however, puberty actually begins during the stage called pre-adolescence, which covers the ages 11 to 12 years. Puberty may begin before adolescence, but it usually continues for several years, well into the adolescent stage, which ends during the late teens. Besides the physical changes of puberty, adolescence is also a time of significant cognitive and psychosocial changes. Many of these changes continue through the end of adolescence after most of the physical changes of puberty have already taken place.
Puberty
Puberty is the period during which humans become sexually mature. Besides maturation of the primary sex organs (those involved directly in reproduction), secondary sex characteristics also emerge during puberty. Adolescents with a high level of testosterone in their blood develop masculine traits (such as facial hair), and adolescents with a high amount of estrogen in their blood develop feminine traits (such as breasts). In addition, there is a period of rapid body growth during puberty, which results in sexual dimorphism in adult body size, composition, and shape.
When does puberty occur? The timing of puberty depends in part on biological sex, with puberty typically occurring earlier in the female sex than male sex. Besides biological sex, the timing of puberty is influenced by genetic and environmental factors. Although there is considerable individual variation in the age of onset, duration, and tempo of the physical changes of puberty, the sequence of these changes is relatively consistent among individuals.
Hormonal Control of Puberty
As shown in Figure \(2\), the changes of puberty are triggered by the hypothalamus in the brain. For unknown reasons, the hypothalamus starts releasing pulses of gonadotropin-releasing hormone (GnRH). This hormone travels directly to the anterior pituitary gland and stimulates it to secrete hormones that target the gonads (testes and ovaries). The main pituitary hormones are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH stimulates the testes to produce sperm and follicles in the ovaries to mature and secrete estrogen. LH stimulates the testes to secrete testosterone and the ovaries to secrete estrogen. Testosterone and estrogen, in turn, stimulate the development of primary and secondary sex characteristics and contribute to the spurt in physical growth.
Puberty in Male sexes
In the United States, puberty in the biological male sex generally begins between the ages of 11 and 12 years and is usually over by the age of 18. During puberty, the testes and scrotum start to increase in size first, followed by the penis. At the same time that the penis is growing, the seminal vesicles, prostate, and bulbourethral glands are also growing and developing. Secondary sex characteristics, such as pubic hair, also develop. Additional physical changes that occur in boys during puberty include the appearance of facial and body hair and deepening of the voice as the vocal cords increase in size. Visible changes in the external male genitalia are illustrated in Figure \(3\). The stages show the sequence in which the changes occur. Stage I represents the pre-pubertal stage at about age 11, and stage V represents the adult stage after the completion of puberty at about ages 16 to 18 years. The first ejaculation generally occurs by the age of 13 years. Even this early in puberty, the semen may contain some sperm. Although full fertility may not be gained for another year or two, boys are generally fertile before they have completed their adolescent growth and achieved an adult appearance.
Puberty in Females
Puberty in the biological female sexes typically begins a couple of years earlier than puberty in the biological males. In the United States, females begin puberty between the ages of nine and ten. Visible, external changes begin first, including the growth and development of the breasts and pubic hair. As shown in Figure \(4\), changes in these traits are traditionally divided into five stages, where stage I is the child stage prior to the start of puberty (around age eight or nine years) and stage V is the adult stage at the end of puberty (around age 14 to 16 years). About two years after breast development begins, the internal reproductive organs — including the uterus and vagina — start to grow and develop.
One of the most significant changes in females during puberty is menarche, which is the first menstrual period. It marks the beginning of menstruation. In the United States, menarche occurs at an average age of 12.4 years. However, there is considerable variation in this age, with menarche at any age between eight and 16 considered normal.
Adolescent Growth Spurt
The period of rapid growth in body size that occurs during puberty is called the adolescent growth spurt (AGS). Both height and weight increase at a rate that is faster than at any time since early childhood. There are also significant changes in body composition and body proportions. The adolescent growth spurt is controlled by hormones, including growth, thyroid, and sex hormones.
Growth in Height
Average growth rates in height for boys and girls are represented by the graph in Figure \(5\). The average boy and girl do not differ significantly from each other in growth rate before the AGS begins. However, by the time they have attained their final adult height, the average female is about 13 cm (5.1 in.) shorter than the average male. One reason is that the AGS occurs earlier in girls than in boys, so girls experience a shorter period of childhood growth, making them shorter, on average, when they begin the AGS in height. Another reason is that the peak height velocity (maximum rate of growth in height) is lower for the average girl than it is for the average boy.
In boys, the AGS in height usually starts at about the age of 11 years. The peak height velocity in boys occurs at about age 13.5 when growth in height is about 10.3 cm (4 in.) per year on average. Growth in height in boys ceases by about age 18 (or a bit later) when the ends of the long bones finally ossify at the epiphyses, so additional growth in height is no longer possible.
In girls, the AGS in height usually starts by the age of roughly 9.5 years. The peak height velocity in girls occurs at about age 11.5 years when growth in height is about 9 cm (3.5 in.) per year on average. Growth in height in girls is completed by about 16 years (if not earlier) when the closure of the epiphyses prevents any additional growth in height.
The accelerated rate of growth during the AGS happens at different times for various parts of the body, but it occurs in the same predictable sequence for both sexes. Generally, the extremities — including the head, hands, and feet — experience rapid growth first, followed by the arms and legs, and then by the trunk and shoulders. This non-uniform growth may make the adolescent body seem awkward and disproportionate until growth is completed.
Growth in Weight
Growth in weight shows a similar spurt during adolescence as growth in height. Growth in weight occurs partly because of the growth in height, but growth in muscle, bone, and (for girls especially) body fat also contributes to the growth in weight. In boys, the AGS in weight lags behind the AGS in height by about three months, whereas in girls the lag time is about six months.
Development of Sexual Dimorphism in Adult Body Composition and Shape
During the adolescent growth spurt, greater growth in muscles and bones occurs in males than females, and especially in the upper body. In males, the shoulders and chest broaden relative to the hips, whereas the reverse occurs in females: the pelvis and hips widen relative to the shoulders and chest. Male muscles may continue to grow and gain in strength for a year or more after growth in height is finished. Females also experience a major increase in body fat during adolescence, especially in the breasts and hips. All of these sex differences in growth during puberty account for the sexual dimorphism in adult human body composition and shape. Please note that the terms males and females refer to only biological sexes. A biological female may identify as a male and a biological male may identify as a female. Most children at this age fully understand their gender identity (see Chapter 22). Some start transitioning before puberty and never go through biological puberty.
Growth in Other Body Systems
The circulatory and respiratory systems also undergo rapid growth and development during the adolescent growth spurt. Both the heart and lungs increase in size and capacity. These and other changes lead to increased strength and tolerance for exercise and tend to occur to a greater degree in males than females.
Cognitive and Psychosocial Changes During Adolescence
Most of the physical changes of puberty occur relatively early in adolescence. Many other changes — including cognitive and psychosocial changes — occur throughout adolescence.
Changes in the Brain and Cognition
The brain does not increase in size very much during adolescence. Instead, most of the increase in brain size after birth occurs early in childhood. By the age of six, the brain has already attained about 90 percent of its adult size. The brain does, however, become significantly more complex during adolescence. In particular, the number of folds in the cerebral cortex of the brain increases. A process called “synaptic pruning” also occurs. In this process, unused pathways are eliminated. At the same time, myelination increases. Overall, the brain becomes more efficient and functional during adolescence, which in turn brings about major cognitive changes.
Adolescence is a time of rapid cognitive development. By the age of 15 or so, many adolescents have basic thinking abilities comparable to those of adults. They demonstrate similar levels of attention, memory, processing speed, and organization. Cognitive development may continue into the early 20s, as increasing capacity for insight and judgment develops through experience.
Some of the most significant changes in the brain during adolescence occur in the prefrontal cortex (PFC), which is the part of the cerebral cortex that covers the front part of the frontal lobes (see Figure \(6\)). This part of the brain is involved in such functions as decision making, information processing, abstract reasoning, problem-solving, evaluating risks and rewards, planning ahead, and controlling impulses. These are the so-called executive functions of the brain, and they mature throughout adolescence as the PFC develops.
Psychosocial Changes
The psychological and social changes that occur during adolescence are almost as marked as the physical changes associated with puberty. During adolescence, most teens develop a stronger sense of personal identity and start to develop their own system of moral and ethical values. Teens also generally develop a greater perception of their feelings of self-esteem and an increased awareness of body image. Most teens become more separated emotionally from their parents, and they may try to test the limits on their independence by breaking rules. At the same time, they generally spend much more time with their peers, and peer influence and acceptance are very important, especially early in adolescence. As a consequence, most teens exhibit a strong desire to conform to their peer group.
During adolescence, as sexual maturation progresses, there is an increased awareness of sexuality. This is typically the time when young people (like the couple in Figure \(7\)) become involved in romantic relationships for the first time. By late adolescence, a romantic relationship with a significant other may become more important than relationships with other peers.
The age at which adolescents go through the physical changes of puberty may have an important influence on their psychosocial development. Going through puberty early can lead to poor body image, low self-esteem, and unhealthy behaviors (such as frequent dieting). They are more likely to engage in other unhealthy behaviors, such as smoking, drinking alcohol, and early involvement in sexual activity. Late puberty tends to be more difficult than early puberty. Those who mature later than their peers may feel physically inferior and develop poor body image and low self-esteem.
Why Are Teens Risk-Takers?
During adolescence, teens develop the ability to think like adults, including the ability to evaluate risks and rewards in similar ways as adults. If this is the case, then why do adolescents tend to be risk-takers? One possible answer is that adolescents have different values than adults, and therefore make different decisions about risky behaviors. For example, they may give more weight to social rewards and peer pressure when evaluating risks and rewards.
Another possible answer is that adolescents are genetically programmed to be risk-takers. Some scientists have suggested that there might be an evolutionary benefit to an increased propensity for risk-taking in adolescence. The scientists argue that without a willingness to take risks, adolescents might not have the motivation or confidence to leave their family of origin and start a family of their own.
Feature: Myth vs. Reality
There are many commonly held ideas about teens that are not backed up by scientific evidence. It is important for teens, their parents, and their teachers and coaches to be aware of these misconceptions.
Myth: Teens can eat anything and still not gain weight because they are growing so rapidly.
Reality: Many teens eat too much food or the wrong foods and end up gaining too much weight. In fact, the rate of obesity in teenagers has tripled since 1980.
Myth: Teens listen only to their friends.
Reality: Teens actually report that their parents or the other caring adults in their lives are the greatest influences on their behavior. This is especially the case when it comes to sexual behavior.
Myth: Teens engage in arguments with their parents because they like to “push their buttons.”
Reality: Adults tend to take arguments personally and therefore interpret teen behavior in this way. However, adolescents are more likely to view arguments as a means of self-expression. Teens may argue with their parents in order to help establish their own sense of identity, rather than to annoy the adults.
Review
1. Define adolescence.
2. What is puberty, and what happens during puberty?
3. What causes puberty to begin, and what causes most of the physical changes of puberty?
4. When does puberty begin in boys and girls? What are some of the obvious physical changes that occur first?
5. What is the adolescent growth spurt?
6. Relate sex differences in the adolescent growth spurt to adult sexual dimorphism.
7. Describe changes in the brain that occur during adolescence, and relate these changes to cognitive development in teens.
8. Outline psychosocial changes that occur during adolescence.
9. How does early puberty tend to affect girls, and how does late puberty tend to affect boys?
10. If adolescents develop the ability to evaluate risks and rewards as adults do, why might teens be more likely than adults to take risks?
11. True or False: Sex hormones are involved in sexual maturity, but not overall physical growth.
12. True or False: In both sexes, the hands are some of the first areas to undergo the adolescent growth spurt.
13. What is one secondary sex characteristic that develops during puberty in both males and females?
14. Do you think that an individual boy can start puberty earlier than an individual girl? Why or why not?
15. When does puberty generally end in males and females?
Explore More
Adolescence is often a time of intense emotions. Watch this video to learn more about the biology behind this phenomenon and other features of the teenage brain that make it different from that of children and adults.
Attributions:
1. Two surfing teens by Mike Baird, licensed CC BY 2.0 via Wikimedia Commons
2. Hormonal Control of Human Reproduction by Lumen Learning, licensed CC BY 4.0
3. Tanner scale - male by J.McHardy, CC BY-SA 3.0 via Wikimedia Commons
4. Tanner scale - female by J.McHardy, CC BY-SA 3.0 via Wikimedia Commons
5. Human height graph by Cantus, CC0 via Wikimedia Commons
6. Prefrontal cortex by Zahr and Sullivan, public domain via Wikimedia Commons
7. Me and my girlfriend by Andrebortoli, public domain via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.7%3A_Adolescence_and_Puberty.txt |
Multi-generations
This family image includes an elderly woman and her young-adult daughters and granddaughters from the Hmong ethnic group in Laos. Grandmother and daughters are adults, but they are obviously far apart in age. What ages define the beginning and end of adulthood?
Defining Adulthood
Adulthood is the life stage between adolescence and death, but assigning exact ages to the beginning and end of adulthood is not easy. The event that marks the end of adulthood (death) is clear cut, but the age at which it occurs varies considerably. The beginning of adulthood is equally difficult to identify exactly. A person may be physically mature and a biological adult by age 16 or so, but not defined as an adult by law until older ages. For example, in the U.S., you cannot join the armed forces or vote until age 18, and you cannot take on many legal and financial responsibilities until age 21.
Stages of Adulthood
Adulthood is generally the longest stage of life, potentially lasting for up to 80 years, or even longer. The man in Figure $2$, for example, is pictured celebrating his 110$^{\text{th}}$ birthday, so he has already experienced over nine decades of adulthood! Although most physical growth and maturation are finished by the time adulthood starts, many changes occur during these decades of life. As a result, adults of different ages may be quite different from one another. Therefore, it makes sense to divide the long period of adulthood into stages, such as the stages of early adulthood, middle adulthood, and old age.
Early Adulthood
Early adulthood coincides more or less with the 20s and early 30s. During early adulthood, people generally form intimate relationships, both in friendship and in love. Many people become engaged or marry during this time. Often, they are completing their education and becoming established in a career.
Strength and physical performance typically reach their peak between 20 and 35 years of age (see Figure $3$). All sexes reach their peak fertility in the 20s, and for females, fertility starts declining in the 30s. Health problems in young adults tend to be relatively minor. Cancer is rare in this stage of adulthood, but there are a few exceptions, notably testicular cancer, cervical cancer, and Hodgkin’s lymphoma. The most common causes of death in young adulthood are homicides, car crashes, and suicides.
Middle Adulthood
Middle adulthood lasts from roughly the mid-30s to the mid-60s. Note that this age range is longer than the stage of life commonly called “middle age,” which is usually considered to range from about 45 to 65. During middle adulthood, many people raise a family and strive to attain career goals. Community involvement is also common in this life stage.
Middle adulthood is the stage when most people start showing physical signs of aging, such as wrinkles and gray hair, like the gray-haired woman in Figure $4$. Typically, vision, strength, aerobic performance, maximal heart rate, and reaction time also start to decline during middle adulthood, although there is great individual variation in the ages at which these changes occur.
Fertility continues to decline until menopause occurs, typically around age 52, after which they are is no longer fertile. Fertility in male sexes starts to decline after age 40. Most middle adults start to diminish in height — especially females who have osteoporosis, which is common after menopause. Up to 1 cm of height per decade may be lost. Some people experience a small degree of cognitive loss during middle adulthood, but this usually goes unnoticed, because life experiences and accumulating wisdom generally compensate for the loss. Middle adulthood is also the time when many people develop chronic diseases such as type 2 diabetes, cardiovascular disease, or cancer. These diseases are the chief causes of death in middle adulthood.
Old Age
Old age begins in the mid-60s and lasts until the end of life. Most people over 65 years of age have retired from work, freeing up their time for hobbies, grandchildren, and other interests. On the other hand, retirement may lead to less social contact and loneliness. Many elderly adults may also be exposed to prejudicial treatment because of their age (ageism). For these and other reasons, depression is very common during old age, and people over the age of 65 have the highest rate of suicide.
Physical and Cognitive Changes in Old Age
Physical declines that started in middle age continue during old age. Declines generally occur in stamina, strength, reflex times, and the senses. As they grow older, most people become increasingly frail, with loss of muscle mass and lessened mobility. However, there are many exceptions. Some people remain fit and active in old age. The man in Figure $5$ is an outstanding example.
Many people suffer from multiple chronic health conditions in old age. The immune system becomes less efficient, increasing the risk of serious illnesses, such as cancer and pneumonia. As people age, the number of brain cells also decreases. Nearly half of those over the age of 85 years exhibit at least mild cognitive impairment. Diseases such as Alzheimer’s disease that cause serious and permanent losses of mental function also become more common.
Age at Death
Old age ends at death, but when is death likely to occur? The average age at death is reflected in the statistical measure known as life expectancy. Life expectancy is defined as the average time an individual is expected to live. It is based on the year of their birth and their current age and gender. In the United States in 2015, life expectancy at birth was 77 years for males and 82 years for females.
Life expectancy is just an average, and many people outlive the life expectancy value for their year of birth and gender. In fact, by the year 2050, a projected half a million Americans will be at least 100 years old, thanks to a large number of baby boomers and advances in health care. Is there an upper limit on old age? In 2016, scientists identified the maximum human lifespan as 115 years on average, with an absolute upper limit of 125 years. These numbers may increase if scientists learn how to slow down aging. This requires understanding the causes of aging.
Causes of Aging
Why do we decline in so many ways as we age? Why is there an upper limit on the human lifespan? The causes of aging (and ultimately death) are not known for certain, but a number of factors have been proposed. These factors fall into two general categories: programmed factors and damage-related factors.
Programmed Factors
Programmed factors follow a biological timetable and maybe a continuation of the timetable that regulates childhood growth and development. An example of a programmed factor is the shortening of telomeres. Telomeres are regions of repetitive nucleotide sequences at the ends of chromosomes (Figure $6$). They may normally serve a variety of functions, such as protecting chromosomes from fusion with neighboring chromosomes. Telomeres become shorter each time a cell divides, and when telomeres become too short, the cell stops dividing and dies.
Damage-Related Factors
Damage-related factors include internal and external assaults on the organism that produce cumulative damage to DNA or cells. Many damage-related factors have been proposed, including the following:
• Exposure to environmental mutagens: Mutagens may damage DNA, and DNA damage can prevent cells from dividing. There are several checkpoints in the cell cycle where cell division is halted if DNA damage is detected.
• Accumulation of waste products in cells: Waste products may interfere with normal cellular metabolism. The amount of waste might reach a level at which cells can no longer function.
• excessive amounts of highly reactive chemicals called free radicals (for example, OH-): Free radicals can damage DNA and cells, and contribute to diseases such as cancer and cardiovascular disease. These diseases are major causes of death in the latter decades of life. Some causes of excess free radicals include exposure to environmental pollutants, drinking alcohol, eating trans fats, and smoking tobacco.
Feature: Reliable Sources
Popular media outlets sell books, diets, and programs that promote the calorie restriction theory of anti-aging. They make money by telling people how and why to eat less in order to live longer. Is this just hype or wishful thinking? Or are there real longevity benefits to calorie restriction? Research reliable sources to find answers to these questions.
Review
1. Define adulthood.
2. Why is it difficult to give exact ages for the beginning and end of adulthood?
3. List the stages of adulthood.
4. Describe the stage of early adulthood.
5. What is the age range of people in middle adulthood, and what are some of the changes that typically occur during this life stage?
6. Define old age, and describe this stage of life.
7. What does life expectancy measure? Identify two factors that influence life expectancy. What was the life expectancy of Americans born in 2015?
8. What is the maximum human lifespan?
9. Discuss possible causes of aging.
10. A 40-year-old person is typically considered to be
1. in old age
2. middle-aged
3. in middle adulthood
4. B and C
11. Compare the chief causes of death between early adulthood and middle adulthood.
12. Why do you think scientists are studying how to lengthen telomeres?
13. Free radicals and mutagens both cause damage to what structures?
14. True or False: Once a person reaches adulthood, their height stays constant.
15. True or False: Life expectancy is generally lower in females than in males.
Explore More
Dr. David Sinclair is a researcher who studies the causes of aging, and how we may be able to inhibit the process. Watch the talk below to learn more about this exciting research.
Attributions
1. Black H'mong family by Bob Tubbs, public domain via Wikimedia Commons
2. Shelby Harris by Rikeshia Davidson, public domain via Wikimedia Commons
3. Serena Williams by Wikigo, CC BY 3.0 via Wikimedia Commons
4. Ellen Langer by Robert Scoble CC BY 2.0 via Wikimedia Commons
5. Fauja Singh by Mithrandirthewise, public domain via Wikimedia Commons
6. Telomere caps by U.S. Department of Energy Human Genome Program, public domain via Wikimedia Commons
7. Male borderline underweight by Fredrik public domain via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.8%3A_Adulthood.txt |
Case Study Conclusion: Lead Danger
The Michigan National Guard Members in Figure \(1\) are on a mission, but they are defending residents from a different type of threat than you might expect—lead in their drinking water. This picture was taken in January 2016 in Flint, Michigan, during what is widely known as the “Flint water crisis.” The crisis started when the city’s water source was switched to a new source that had highly corrosive water. The corrosive water caused lead in pipes to be leached into residents’ drinking water. As you learned at the beginning of this chapter, lead is highly toxic and particularly dangerous to young children, so this was (and continues to be) a major public health crisis. In order to help protect residents’ health, the National Guard was deployed to distribute clean water, water filters, and lead test kits door to door.
Contaminated water is just one source of possible lead exposure. There are many other sources, and one of the most common is lead-based paint. Lead-based paint was banned by the U.S. government for use in housing in 1978, but many older homes still have this paint under newer layers, as shown in the graph in Figure \(2\). Lead paint and associated dust can be exposed when paint deteriorates, or when renovations occur. In fact, in 2014, the CDC estimated that 24 million housing units in the U.S. had deteriorating lead-based paint and associated dust.
Vanessa, Paul, and their 18-month-old son Lucas—whom you learned about in the beginning of this chapter—live in a home built before 1978. Therefore, exposure to lead paint or lead-containing dust might be the source of Lucas’ elevated blood lead level. Lucas’ pediatrician Dr. Morrison recommended that they get their home inspected for lead. The results show that their home does have lead paint underneath other layers. Even though the paint is not currently cracking or peeling, it did when they first moved in, and they had sanded and painted over many areas. Dry sanding over lead paint can create large amounts of lead dust. The inspectors believe this was likely the source of Lucas’ lead exposure since their water supply and other possible sources were ruled out. Since Lucas is a toddler and was crawling around, playing on the floor, and putting hands and objects in his mouth during and after the period of sanding—all of which are normal toddler behaviors, but ones that can make children at this age more susceptible to lead poisoning.
What about Vanessa, Lucas’ mother, who is three months pregnant? Fortunately, her blood lead level turned out to be relatively low—3 µg/dL. Under 5 µg/dL is considered normal, although there is no known safe level. As you have learned, her developing offspring is considered a fetus at this stage. It has recently completed the embryonic stage, which is when the major events of gastrulation, neurulation, and organogenesis occur to create the three germ layers, the nervous system, and most of the organs of the human body. Additionally, limbs, facial features, and sex organs have started to develop by this time, as well as the extraembryonic structures (such as the placenta and amnion) that protect and nourish the developing embryo and fetus. Because so many major developmental events occur during the early stages of pregnancy, exposure to toxic substances during this period can cause severe damage or even death of the embryo or fetus. Lead exposure during pregnancy can cause impaired growth, impaired neural development, low birth weight, spontaneous abortion (miscarriage), and stillbirth.
This is why Vanessa, Paul, and Dr. Morrison were concerned that Vanessa might have been exposed to lead. Lead can cross the placenta from the blood of the mother to the fetus. Also, lead stored in the mother’s bones from prior exposure can become liberated during pregnancy as her body breaks down the bone for calcium to form the bones of the fetus. This effect can be minimized if the pregnant woman gets enough calcium from her diet instead. Finally, lead can be transmitted through breast milk, so it is very important that pregnant and lactating women be tested for lead if they are at risk of being exposed to it.
What about Paul, Lucas’ father? While lead exposure can cause a variety of adverse health effects in adults—including problems in memory and concentration; headache; high blood pressure; and abdominal, joint, and muscle pain—these symptoms typically only occur at higher lead levels than those that are dangerous for children. Because the bodies and brains of adults have generally completed their development by early adulthood, there is usually less of an impact of poisons like lead that affects developmental processes than there is for infants and young children who are still undergoing rapid development. However, lead exposure can cause lowered sperm count and increase the production of abnormal sperm in adult men, so if Paul wants to have more children, he may want to get his lead level checked.
Because Vanessa’s lead level is not high, it is likely that Paul’s level is not high either, although it still would be a good idea for him to get tested. In general, adults are less exposed to lead than children, because they are less likely to engage in behaviors such as putting unwashed hands or non-food objects into their mouths. Physiologically, adults also absorb less lead into their bloodstream than children, due to differences in their breathing rate and gut absorption. One exception is adults who have jobs that expose them to lead, such as lead smelter workers, automotive repair workers, construction workers, and plumbers. Adults in high-risk occupations should take precautions to limit their exposure, and they may need to get tested on a periodic basis — particularly if they are pregnant or plan to get pregnant. They should also take safety measures such as changing clothes and showering before coming home since people who work with lead can unwittingly bring lead dust home on their clothes, shoes, and skin, and expose children and other family members.
As you may recall, Lucas’s lead level was 10 µg/dL, which is a cause for concern. Now that Vanessa and Paul have identified the most likely source of exposure—paint dust generated during sanding—they can take steps to prevent future exposure. This includes frequent mopping and wet-wiping of flat surfaces (such as window sills) to remove dust; cleaning around doors and other areas where friction can generate paint dust; frequently inspecting paint for any damage or deterioration, and using only lead-safe certified professionals for any future painting or repairs. They will also make sure that Lucas washes his hands frequently and that he eats a balanced diet. Nutrients such as iron, calcium, and vitamin C can inhibit the absorption of lead or help protect the body from the damaging effects of lead, so it is particularly important that children eat a nutritious diet when there is lead in the home. There are many other steps you can take to prevent lead exposure, which you can find online from the U.S. EPA, the CDC, and other reliable sources.
Dr. Morrison will closely monitor Lucas to make sure his blood lead level goes down and that his development is occurring normally. Lead can cause developmental delays. As you have learned, as children grow, they go through typical stages of development, including the acquisition of specific motor, language, and cognitive skills around certain ages. When these milestones are not achieved within a typical timeframe, it is considered a developmental delay. By assessing Lucas’ developmental milestones, Dr. Morrison can help monitor whether Lucas’ early lead exposure has affected his development. Many of the impacts of lead exposure in infancy and toddlerhood are not obvious until later in life as the brain develops further and starts carrying out more complex tasks. Early lead exposure can lead to later learning disabilities, behavioral problems, and lower IQ. The effects on the nervous system are generally irreversible, although therapies such as speech or behavioral therapies may help the child function better. Lead can also cause other types of medical problems such as anemia, hearing problems, and delayed puberty. Figure \(3\) illustrates some of the problems that lead exposure can cause in children.
Lead exposure in children is a serious public health problem. You can go online to read about the current status of the Flint water crisis, as well as the problem of childhood lead exposure more generally. Many experts agree that preventing lead exposure and more widespread blood lead level screening is critical to prevent permanent damage to children’s health. Infancy and early childhood is a wonderful time of tremendous growth and change in a person’s lifespan, but it is also a time that is highly vulnerable to damage—with potential lifelong consequences.
Chapter Summary
In this chapter, you learned about the growth and development of humans, from fertilization to old age. Specifically, you learned that:
• The germinal stage of development is the first and shortest of the stages of the human lifespan. It lasts between eight and nine days, beginning with fertilization and ending with implantation in the endometrium of the uterus, after which the developing organism is called an embryo.
• The germinal stage involves several different processes that change an egg and sperm first into a zygote, and then into an embryo. The processes include fertilization, cleavage, blastulation, and implantation.
• Fertilization takes place when a haploid sperm successfully enters a haploid egg and results in a single diploid cell called a zygote. This usually occurs in a Fallopian tube. Successful fertilization is enabled by the processes of chemotaxis, adhesion, and digestion.
• Cleavage refers to the first several mitotic cell divisions of the zygote. It takes place in the Fallopian tube and results in a solid ball of undifferentiated cells called a morula. The morula forms by about the fourth day after fertilization.
• Blastulation is the process in which the morula changes into a fluid-filled ball of differentiated cells called a blastocyst.
• Implantation is the process in which the blastocyst becomes embedded in the endometrium of the uterus. It occurs around day 8 or 9 after fertilization when trophoblast cells “hatch” from the zona pellucida and penetrate the endometrium.
• The embryonic stage of human development lasts from the time of implantation until the end of the eighth week after fertilization. Besides an increase in size, some of the changes that occur in the embryo include the formation of three cell layers, development of the nervous system, and the initial formation of most organs.
• During the second week after fertilization, the embryoblast differentiates into two groups of cells, called the epiblast and the hypoblast. Cell migration results in the formation of a two-layered (bilaminar) embryonic disc.
• By the end of the second week after fertilization, gastrulation occurs. The three cell layers are germ layers that will give rise to different cells throughout the body. The endoderm (inner layer) will eventually develop into cells of most internal glands and organs; the mesoderm (middle layer) will develop into cells of organs such as the bones, muscles, and heart; and the ectoderm (outer layer) will later develop into the skin and nervous system cells.
• Neurulation begins in the third week after fertilization. In this process, which takes about two weeks, the embryo forms structures that will eventually become the nervous system. A structure called the neural tube forms that will later develop into the spinal cord and brain, and a structure called the neural crest forms that will later develop into peripheral nerves.
• Organogenesis, or the formation of organs, also begins during the third week after fertilization. It continues through the end of the embryonic stage, by which time most organs have at least started to develop. The heart is the first functional organ to develop in the embryo. The heart starts to beat and pump blood by the end of the third week, but it continues to develop for several more weeks.
• Other developments that occur in the embryo during the fifth through eighth weeks after fertilization include limb and digit formation; formation of ears, eyes, and other facial features; and the main prenatal development of the sex organs.
• The embryonic stage is a critical period of development. Genetic defects or harmful environmental exposures (such as alcohol or radiation) during this stage are likely to have devastating effects.
• Several extraembryonic structures form at the same time as the embryo and help the embryo grow and develop. They include the placenta, chorion, yolk sac, and amnion.
• A fetus is a prenatal human being between the embryonic stage and birth. The fetal stage extends from the beginning of week 9 after fertilization to about 38 weeks after fertilization, which is the average time of birth.
• At the start of the fetal stage, the fetus is recognizable as a human being and possesses virtually all of the major body organs, although most of them are not yet fully developed and functional. The organs will continue to grow and develop during the fetal stage.
• Fetal developments that occur between weeks 9 and 15 after fertilization include differentiation of the reproductive organs. The thyroid, liver, pancreas, and kidneys also start functioning. The fetus is very active during this period, but the movements are mostly uncontrolled. Fine hair called lanugo starts to grow on the face and will eventually cover the body as well.
• Fetal developments that occur between weeks 16 and 26 after fertilization include the development of the senses of touch and hearing, the initial formation of alveoli in the lungs, beginning of ossification of the bones, and considerable muscle development. The bone marrow also starts producing blood cells, and waxy vernix develops to cover the fetus’s skin.
• Fetal developments that occur between weeks 27 and 38 include further development of the skeletal system, rapid body growth, and a substantial increase in body fat. Head hair grows thicker and coarser while the lanugo is shed. Vernix first increases and then disappears, usually before birth. The eyes develop to the point that the fetus can detect light.
• The heart and blood vessels are among the earliest organs to develop and function, but the circulation of blood in the fetus is different than the postnatal circulation will be because the lungs are not yet in use. Fetal hemoglobin is also different than adult hemoglobin. Fetal hemoglobin can bind with oxygen at lower pressures, which enables it to bind with oxygen from the mother’s blood in the placenta.
• The size of the fetus generally increases linearly during the fetal stage up until the last week or two before birth, when the rate of growth typically tapers off. Fetal growth deficit, called intrauterine growth restriction (IUGR), may occur because of maternal, fetal, or placental factors. Placental insufficiency is the leading cause of IUGR.
• The average weight of a full-term infant at birth is 3.4 kg (7.5 lb). Low birthweight is defined as a weight at birth of less than 2.5 kg (5.5 lb). Low birth weight is a major cause of mortality shortly after birth. It may occur because of IUGR or pre-term birth.
• Viability of the fetus refers to the point in fetal development at which the fetus is likely to survive outside the uterus. More than 90 percent of babies survive if they are born after 26 weeks and receive any necessary medical care. Babies born even a few weeks earlier have a much lower chance of surviving, mainly due to inadequately developed respiratory and nervous systems.
• Immediately after birth, an Apgar test is administered to determine whether the newborn needs urgent medical care. The baby is scored on five traits, including skin color and heart rate. The umbilical cord is also cut within seconds of birth, leaving a stub that will eventually dry out and fall off, forming the naval.
• Infancy refers to the first year of life after birth. An infant is defined as a human being between birth and the first birthday. A newborn baby is called a neonate up until the first four weeks after birth.
• Newborns may or may not have vernix or lanugo covering the skin, and they may or may not have head hair. Their body proportions are distinctive, and the head is very large relative to the body. Soft spots in the skull called fontanels — which allow the head to change shape slightly to fit through the birth canal — gradually ossify after birth.
• A well-nourished, full-term newborn averages about 51 cm (20 in.) in length and has an average birth weight of 3.4 kg (7.5 lb). A newborn typically loses a small amount of weight in the first week, but after that, a healthy neonate should start gaining weight rapidly.
• Newborns have well-developed senses of touch and hearing, and they can respond to different tastes and smells. However, their sense of vision is not yet fully developed, their visual acuity is poor, and they have limited color and depth perception.
• Infants are born with several reflexes that help them survive the first few months of life. They include crying for communication, suckling, and the rooting reflex, which helps them find a nipple. The Moro and grasping reflexes help them maintain close physical contact with the mother or another caregiver.
• Many important developments in motor, sensory, and cognitive abilities occur during infancy. There is variation among infants in the exact timing of these developments, but the sequence in which they occur is usually similar.
• The deciduous teeth generally start to emerge around six months of age. This is called teething, and it may cause discomfort and fussiness. Typically, all of the upper and lower incisors emerge during the first year.
• Infancy is the period of most rapid growth after birth. A healthy, well-nourished infant generally triples his birth weight and doubles his birth length by the first birthday. The head and trunk normally grow most rapidly, allowing rapid growth and development of the brain, heart, and lungs.
• Infancy is associated with a higher risk of death than any other life stage except old age. The infant mortality rate — defined as the number of infant deaths per one thousand live births — is an important measure of the level of health in a nation. It tends to be inversely correlated with a nation’s wealth.
• Legally, childhood is defined as the period of minority, from birth to the age of majority, or adulthood. Biologically, childhood is defined as the stage of a human organism between birth and adolescence. Following infancy, biological childhood includes early childhood, middle childhood, and pre-adolescence.
• Early childhood is the life stage between infancy and middle childhood. It is divided into toddlerhood (ages one to three) and the preschool years (ages three to five).
• Early childhood is a time of great physical, cognitive, and psychosocial development. Children go from just starting to walk and using a word or two at age one to riding a bike and using 1,500 words at age five. While the one-year-old toddler clings to her parents, the five-year-old preschooler runs off to play with her friends.
• Physical growth is slower in early childhood than it was in infancy, but still relatively rapid, especially in the toddler years. The remaining deciduous dentition also erupts during early childhood.
• Middle childhood is the life stage between early childhood and pre-adolescence. It covers the ages of six to ten when most children are in elementary school. During middle childhood, children are more independent and physically active than they were in the preschool years. They become more involved with friends and develop more sophisticated language and cognitive abilities.
• Pre-adolescence is the life stage between middle childhood and adolescence or ages 11 to 12 years. It coincides with the start of puberty, although few of the obvious physical changes of puberty have yet to occur. It is also a time of significant cognitive and psychosocial development. Pre-adolescents develop the ability to think abstractly, start to develop a sense of identity, and may experience “puppy love.”
• During both middle childhood and pre-adolescence, the deciduous teeth are lost and replaced by most of the permanent dentition. Physical growth and motor development are usually slow but steady during these two stages of childhood, but some children may begin the adolescent growth spurt during pre-adolescence.
• Adolescence is the period of transition between childhood and adulthood. It is generally considered to start with puberty, although in many children, puberty actually begins during pre-adolescence. Adolescence includes significant cognitive and psychosocial changes, some of which continue past the physical changes of puberty and into the late teens.
• Puberty is the period during which children become sexually mature. It includes maturation of the primary sex organs, the development of secondary sex characteristics, and the adolescent growth spurt.
• Puberty starts when the hypothalamus starts releasing pulses of gonadotropin-releasing hormone, which triggers the pituitary gland to secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH, in turn, stimulate the gonads to develop and release sex hormones. Sex hormones bring about most of the other changes of puberty.
• In boys, puberty generally begins between the ages of 11 and 12, when the external genitals and pubic hair start to develop. In girls, puberty generally begins between the ages of nine and ten, when the breasts and pubic hair start to develop.
• The adolescent growth spurt (AGS) is a period of rapid growth in height and weight. It also includes significant changes in body composition and shape. Girls start the growth spurt earlier than boys and typically have a somewhat lower peak growth rate. Sex differences in the AGS result in sexual dimorphism in adult size, body composition, and shape. Other body systems also undergo rapid growth and development during the AGS.
• Although the brain does not increase very much in size during adolescence, it does become more complex. The cerebral cortex becomes more folded, and unused pathways are eliminated. The most significant changes occur in the prefrontal cortex, which controls executive functions, such as decision making, abstract reasoning, and impulse control. By the age of 15 or so, many adolescents have basic thinking abilities comparable to those of adults.
• Psychosocial changes that occur during adolescence include the development of a stronger sense of personal identity and a personal system of moral and ethical values. Adolescents also become more emotionally separated from their parents, while peers and peer influence become more important to them. Many teens also become involved in romantic relationships for the first time.
• Adulthood is the stage of life between adolescence and death. The age at which death occurs varies considerably. The age at which adulthood starts varies depending on whether adulthood is defined biologically or legally. Adulthood is usually the longest stage of life, potentially lasting for many decades. It is generally divided into stages, such as the stages of early adulthood, middle adulthood, and old age.
• Early adulthood coincides more-or-less with the 20s and early 30s. During this stage, many people complete their education, start a career, and form intimate relationships. They may marry and start a family. Strength and physical performance typically reach their peak during early adulthood, as does fertility in females.
• Middle adulthood lasts from about the mid-30s to the mid-60s. Many adults raise a family and attain career goals during this stage. This is also the stage when most people start showing physical signs of aging and experience physical declines, including a decline in fertility. Some cognitive loss may also occur, and many people develop chronic diseases such as type 2 diabetes during middle adulthood.
• Old age begins in the mid-60s and lasts until death. Most people over the age of 65 have retired and have more free time. Some may have less social contact and experience loneliness, and many experience ageism. Physical declines that started in middle adulthood continue during old age. Most people become increasingly frail and have a greater risk of serious illnesses such as cancer. Cognitive impairment is common in old age and may become very serious in cases of disorders such as Alzheimer’s disease.
• The average age at death is given by the statistical measure called life expectancy, which varies by birth year and gender. For people born in the United States in 2015, life expectancy was 77 years for males and 82 years for females. Increasing numbers of Americans are living to be older than 100. The average maximum human lifespan is estimated to be 115 years, with an absolute upper limit of 125 years.
• The causes of aging (and ultimately death) are not known for certain. Factors that have been proposed fall into two general categories: programmed factors and damage-related factors. An example of the former is the shortening of telomeres; an example of the latter is mutations in DNA due to exposure to environmental mutagens.
In this chapter, you learned about the stages of human development and some of the factors — such as exposure to poisons like lead — that can damage the human organism. A disease is clearly a major factor that affects human health and lifespan. Read the next chapter to learn more about diseases in human populations, including causes, effects, treatments, and prevention of major types of diseases.
Chapter Summary Review
1. At which stages in the human lifespan do deciduous teeth, permanent teeth, and wisdom teeth generally emerge?
2. What is the shortest stage in the human lifespan? How long is it, and when does it occur?
3. What is generally the longest stage in the human lifespan? Why might this stage not be the longest for a given individual?
4. Which implants into the lining of the uterus?
1. zygote
2. gastrula
3. blastocyst
4. morula
5. Compare and contrast blastulation and gastrulation.
6. What is the term that refers to the first several mitotic divisions of the zygote?
7. At what point does the prenatal human start getting nutrients from its mother? How did it get nutrients before this point?
8. True or False: Blood does not normally mix between the mother and the fetus.
9. True or False: Adolescents have the highest rate of suicide of any age group.
10. True or False: The adolescent growth spurt is the most rapid period of growth that occurs after birth.
11. Hypothetically, what do you think would happen if a fetus had adult hemoglobin instead of fetal hemoglobin? Explain your answer.
12. For each of the stages or events below, identify approximately when they occur:
1. neonate
2. fetus
3. middle childhood
4. menopause
13. True or False: A fetus can usually survive if it is born at 26 weeks and without any medical intervention.
14. True or False: FSH and LH are involved in the onset of puberty in males.
15. Which cells in the blastocyst ultimately become muscle cells? Trace the development of these cells through gastrulation, naming each tissue they develop into along the way.
16. The chorion:
1. helps form the fetal portion of the placenta
2. is a sac that contains fluid that surrounds and protects the fetus
3. protects the developing zygote
4. is the lining of the uterus where implantation occurs
17. Pituitary tumors can affect the timing of the onset of puberty, causing it to occur abnormally early or late. Why do you think this might happen?
18. Why is it particularly important for a pregnant woman to avoid exposure to toxins during the embryonic stage of her pregnancy?
19. The neural tube develops into the:
1. peripheral nerves
2. spinal cord
3. brain
4. B and C
20. What do you think would be more concerning—a five percent weight loss of an infant in the first week after birth, or a five percent weight loss between the infant's third and fourth weeks after birth? Explain your answer.
21. What factors would you need to take into consideration to determine whether a 16-year-old should be considered an adult?
22. Name a disease that is more common in old age than in other stages of life.
23. Describe some of the cognitive changes that occur between the ages of six and ten.
24. Telomeres:
1. develop into extraembryonic tissues such as the placenta
2. are regions at the ends of chromosomes
3. are regions of bone growth during childhood and adolescence
4. turn into the primitive streak in the embryo
25. What are two characteristics of a newborn’s head that differ from that of adults?
26. The zona pellucida:
1. surrounds the cell membrane of the egg
2. surrounds the developing zygote
3. surrounds the developing fetus
4. A and B
27. Put the following events in order of when they typically occur during the human lifespan, from earliest to latest:
1. menarche
2. loss of deciduous teeth
3. loss of muscle mass
4. beginning of organogenesis
5. loss of grasping reflex
Attributions
1. Michigan National Guard by The National Guard, CC BY 2.0 via Wikimedia Commons
2. Lead-based paints graph by Environmental Protection Agency, public domain
3. Prevent lead poisoning by Centers for Disease Control, public domain
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/23%3A_Human_Growth_and_Development/23.9%3A_Case_Study_Conclusion%3A_Lead_Danger_and_Chapter_Summary.txt |
This chapter introduces the fundamentals of ecology, describes terrestrial and aquatic biomes, and outlines ecosystem processes and their value to humans. The chapter also describes interspecific relationships in communities, how energy flows through ecosystems, and how matter is recycled through ecosystems.
• 24.1: Case Study: The Web of Life
Camille grew up in a rural farming community, and both of her parents worked on a local farm. When pesticides were being applied to the crops, her parents had to use special protective equipment such as coveralls, gloves, and respirators. This is because many pesticides, which are substances that protect plants from damage and destruction by pests such as insects, can be hazardous to human health.
• 24.2: Introduction to Ecology
Ecology is the study of how living things interact with each other and with their environment. Although it is a science in its own right, ecology has areas of overlap with many other sciences, including biology, geography, geology, and climatology.
• 24.3: Ecosystems
An ecosystem is a set of interacting components that form a complex whole including all of its living things and its nonliving environment. The nonliving environment includes abiotic factors such as temperature, water, sunlight, and minerals in the soil. A community is the biotic part of an ecosystem.
• 24.4: Community Relationships
A community is the biotic part of an ecosystem and consists of all the populations of all the species that live and interact in the ecosystem. Populations of different species generally interact in a complex web of relationships.
• 24.5: Energy in Ecosystems
There are two basic types of organisms in terms of how they obtain energy: autotrophs and heterotrophs. Autotrophs (a.k.a producers) are organisms that use energy directly from the sun or from chemical bonds. Heterotrophs (a.k.a consumers) are organisms that obtain energy from other living things.
• 24.6: Cycles of Matter
The water and chemical elements that organisms need continuously cycle through ecosystems, passing repeatedly through their biotic and abiotic components. These cycles are called biogeochemical cycles because they are cycles of chemicals that include both organisms (bio) and abiotic components such as the ocean or rocks (geo).
• 24.7: Introduction to Human Populations
We know more about the human population and how it has grown than we know about the population of any other species thanks to demography, which is the scientific study of human populations. Demography encompasses the size, distribution, and structure of populations.
• 24.8: Population Dynamics
Populations are dynamic. They are continuously gaining individuals through births and losing individuals through deaths. Populations may also gain or lose significant numbers of individuals through migration, when people either enter or leave a population. All of these factors together determine whether and how quickly a population grows.
• 24.9: Climate Change
There is no longer any doubt that our planet is growing warmer and that human actions are the primary cause. There is also no question that if we don't do something about it soon, the consequences will be devastating.
• 24.10: Case Study Conclusion: Organic and Chapter Summary
Camille, who you read about in the beginning of the chapter, asks herself questions like this whenever she goes food shopping. If organic agricultural practices are significantly better for the environment, she would like to buy organic food products at least some of the time. But are they better? And if so, how?
Thumbnail: Bumblebee pollinating Aquilegia vulgaris. (CC BY-SA 3.0 Unported; Roo72).
24: Ecology
Case Study: Farming for Balance
Camille grew up in a rural farming community, and both of her parents worked on a local farm. Camille uses she/her/hers pronouns. When pesticides were being applied to the crops, as in Figure \(1\), her parents had to use special protective equipment such as coveralls, gloves, and respirators. This is because many pesticides, which are substances that protect plants from damage and destruction by pests such as insects, can be hazardous to human health if inhaled, consumed, or absorbed through the skin. Camille began to wonder—if pesticides can be dangerous to humans, do they have negative effects on other animals and the rest of the environment?
As an adult, Camille notices that some food items in the grocery store are labeled with a seal indicating they are organic—like the seal in Figure \(3\). Camille knows that organic food is generally grown without synthetic (man-made) pesticides, and she likes the idea of buying organic food to support this kind of agriculture. However, she also notices that organic food tends to be slightly more expensive than conventionally-produced food, so she wants to learn more about what “organic” means and whether organic agricultural practices are really better for the environment as a whole.
Camille does some research online and finds out that organic food products in the U.S. are regulated by the U.S. Department of Agriculture (USDA). The USDA has a detailed set of requirements that describe the practices that farmers must use in order to label their food products as organic. These include not using most synthetic chemical substances, such as pesticides or fertilizer, on crops. Organic meat must come from animals that are fed organic feed and meet various other criteria. On the USDA’s National Organic Program website, Camille notices the statement: “Organic is a labeling term for food or other agricultural products that have been produced using cultural, biological, and mechanical practices that support the cycling of on-farm resources, promote ecological balance, and conserve biodiversity in accordance with the USDA organic regulations.” This statement indicates to Camille that organic agricultural practices are beneficial for the environment, but how exactly do they provide these benefits?
As you read this chapter, you will learn about the science of ecology—that is, the study of the complex relationships between living organisms and the environment around them. You will see how interactions between different species and non-living components of the environment can come into balance, and how drastic changes can occur when this balance is altered. By the end of the chapter, you will have a better understanding of what is meant by resource recycling, ecological balance, and biodiversity, and the importance of these concepts. Then in the conclusion to this case study, you will learn specifically how these concepts relate to organic agricultural practices. This information can help you, as well as Camille, make a more informed decision about whether to choose organic foods.
Chapter Overview: Ecology
In this chapter, you will learn about ecology and how it relates to humans. Specifically, you will learn about:
• How the biological world is organized into nested ecological hierarchies including the individual, population, community, ecosystem, biome, and biosphere.
• Basic concepts in ecology including the ecosystem, niche, habitat, and the competitive exclusion principle.
• The goods and services ecosystems provide to humans, including food and oxygen production.
• The types of relationships between species in a community, including different types of symbiotic relationships, predation, and competition. The effect of community relationships on the evolution of adaptations and the extinction of species.
• The two basic types of organisms in terms of how they obtain energy: autotrophs (producers) and heterotrophs (consumers). Types of heterotrophs including herbivores, omnivores, carnivores, and decomposers.
• How energy flows through ecosystems from producers up through food chains and webs.
• Trophic levels, which are the feeding positions in a food chain or web, starting with producers at the first trophic level and moving up through higher levels of consumers.
• How the amounts of energy and biomass change from lower to higher trophic levels.How matter cycles through ecosystems in the form of biogeochemical cycles of water, carbon, nitrogen, and phosphorus.
• Demography, which is the scientific study of human populations including size, geographic distribution, and structure, such as the number of people in different age groups in the population.
• The age-sex structure of populations and how this may provide insight into political and socioeconomic change.
• Exponential growth compared to logistic growth, the latter of which slows as the population reaches a size called the carrying capacity.
• How the human population may currently be near the carrying capacity, which could result in environmental damage, disease, war, and famine; and how these negative effects are exacerbated by excessive use of resources by wealthier countries.
• How zero population growth and strategies to reduce the fertility rate can help mitigate problems caused by overpopulation.
• Projections of future human population growth and how these predictions are made.
• Global climate change, and how human activities have intensified the greenhouse effect, causing global warming. Projections of future global warming; potential impacts of global warming on humans such as increased coastal flooding, lack of food and water, and violence; and how we can lessen the threat by moving away from fossil fuels towards cleaner sources of energy.
As you read the chapter, think about the following questions:
1. The use of some pesticides has been shown to kill bees. How could this affect an ecosystem?
2. Synthetic fertilizers often have high amounts of nitrogen and phosphorus. Why do these fertilizers contain these elements? Can you predict any ecological disturbance that an excess amount of these elements might cause?
3. Organic agricultural methods generally increase soil quality. What is soil quality, and how does it relate to biodiversity and biogeochemical cycles?
Attributions
1. Pesticide application by Jeff Vanuga, public domain via Wikimedia Commons
2. Pesticide spraying by jetsandzepplins, licensed CC BY 2.0 via flickr
3. USDA organic seal, public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.01%3A_Case_Study_-_The_Web_of_Life.txt |
What the ...?
The red-tipped organisms in Figure \(1\) were discovered in 1977. Called tube worms, they live on the deep ocean floor, thousands of meters below the water’s surface. They cluster around hydrothermal vents that spew out hot mineral water. The environment around the vents would be deadly for most other organisms. Minerals in the vent water are toxic, the weight of ocean water above creates tremendous pressure, and it’s always very cold and completely dark. Without sunlight, photosynthesis isn’t even possible. Yet the vents support thriving communities of diverse species, many of which live nowhere else on Earth. Besides tube worms, they include equally strange ghost fish and crabs with eyes on their back. How have these organisms adapted to living in the harsh environment around hydrothermal vents? How do they obtain energy without sunlight and photosynthesis? How do the various vent species interact? Finding answers to questions such as these is the domain of ecology.
What Is Ecology?
Ecology is the study of how living things interact with each other and with their environment. Although it is a science in its own right, ecology has areas of overlap with many other sciences, including biology, geography, geology, and climatology. It is also closely related to genetics and ethology (the study of animal behavior). In addition, evolutionary concepts of adaptation and natural selection are the cornerstones of modern ecological theory.
Some of the phenomena that ecologists study include the interactions of organisms, the flow of energy and recycling of matter through living things, and the biodiversity and distribution of organisms relative to the environment. There are many practical applications of ecology. Among others, they include the conservation of endangered species (Figure \(2\)), natural resource management, urban planning, and human health.
Living Things and the Environment
Despite their tremendous diversity, all organisms have the same basic needs: energy and matter. These must be obtained from the environment. Therefore, organisms are not closed systems. They depend on and are influenced by their environment.
The environment of an organism includes two types of factors: biotic and abiotic.
• Biotic factors are the living aspects of the environment. They consist of other organisms, including members of the same and different species.
• Abiotic factors are the nonliving aspects of the environment. They include factors such as sunlight, soil, temperature, and water.
Consider as an example the relationship between leafhoppers and ants (Figure \(3\)). Ants “herd” leafhoppers and use their excretions for food, much as a dairy farmer herds cows and uses their milk. Leafhoppers suck sap from plants and excrete excess liquid as a sugary fluid called honeydew. As the honeydew passes out of a leafhopper’s anus, the ant “farmer” feeds on the fluid.
The leafhoppers in the “herd” also benefit from their relationship with the “ant.” The ant protects the leafhoppers from potential predators such as wasps. The amount of shade in the environment, which is an abiotic factor, is an important influence on the leafhoppers and ants. Environments with at least 50 percent shade are more densely populated by ants and leafhoppers than sunnier environments. Some species of “herder” ants even construct shelters to provide shade for their “herds.”
Ecological Hierarchy
Studying all living things and their environments would be a huge undertaking. Generally, the study of ecology is made more manageable by organizing the biological world into a nested hierarchy.
Ecology typically focuses on the living world at and above the level of the individual organism. These levels are illustrated in Figure \(4\) and defined as follows:
• A population consists of all the individual organisms of the same species that live and interact in the same area. For example, all of the angelfish living in the same area of the ocean make up the angelfish population.
• A community refers to all of the populations of different species that live and interact in the same area. The aquatic community that includes the angelfish also includes the populations of other species of fish, corals, and many other organisms.
• An ecosystem includes all the living things in a given area, together with the nonliving environment. The nonliving environment includes abiotic factors such as water, minerals, and sunlight.
• A biome is a group of similar ecosystems with the same general type of physical environment anywhere in the world. Terrestrial biomes are generally delineated by climate and major types of vegetation. Examples of terrestrial biomes include tropical rainforests and deserts. Aquatic biomes are generally defined by the distance from shore and the depth of water. Examples of aquatic biomes include the shallow water near shore (littoral zone) and the deepest water at the bottom of a body of water (benthic zone).
• The biosphere includes every part of Earth where life exists, including all the land, water, and air where living things can be found. The biosphere is the largest ecological category and consists of many different biomes.
Basic Ideas in Ecology
A number of concepts and principles are basic to the study of ecology. They include the ecosystem, niche, habitat, and competitive exclusion principle.
Ecosystem
The ecosystem is one of the most important concepts in ecology and often the focus of ecological studies. It consists of all the biotic and abiotic factors in an area and their interactions. While an ecosystem is a real system in nature, it is often artificially delineated by researchers. For example, depending on an ecologist’s research focus, a lake could be considered an ecosystem, but so could a dead log, like the one in Figure \(5\). Both the lake and the log contain a variety of species that interact with each other and with abiotic factors.
When it comes to energy, ecosystems are not closed. They need constant inputs of energy. Most ecosystems get energy from sunlight. A small minority, including hydrothermal vent ecosystems, get energy from chemical compounds. Unlike energy, the matter is not constantly added to ecosystems. Instead, it is recycled. Water and elements such as carbon and nitrogen are used over and over again.
Niche
One of the most important concepts associated with ecosystems is the niche. A niche refers to the role of a species in its ecosystem. It includes all the ways that the species interacts with the biotic and abiotic factors of the ecosystem. Two important aspects of any species’ niche are its sources of energy and nutrients and how it obtains them. For example, the jumping spider in Figure \(6\) is a carnivore (meat eater) that obtains food by preying on insects such as flies.
Habitat
Another fundamental aspect of a species’ niche is its habitat. The habitat is the natural environment in which a species lives and to which it is adapted. A species’ habitat includes any factors of the environment — including both biotic and abiotic factors — that are related directly or indirectly to the use of the environment by the species.
Species may have general or specific habitat requirements. For example, small white butterflies in the species Pieris rapae (Figure \(7\); on the left) are found on all continents of the world except Antarctica. Their larvae can feed on many different plant species, and the butterflies themselves thrive in any open location. In contrast, large blue butterflies in the species Phengaris arion (Figure \(7\); on the right) are found only in certain types of grassland areas. Their larvae can feed only on species in the plant genus Thymus. In addition, because of their complex life cycle, the butterflies can live only in areas in which certain ant species also reside.
Competitive Exclusion Principle
A given area may contain many different species, but each species must have a different niche. Two different species cannot occupy the same niche in the same place for very long. This is known as the competitive exclusion principle. If two species were to occupy the same niche, what would happen? The two species would compete with one another for the same food or other limiting resources in the environment. Eventually, one species might outcompete and replace the other. Alternatively, one species might evolve somewhat different adaptations to a similar but different niche so they could continue to live in the same area.
Review
1. Define ecology.
2. Why are individual organisms not closed systems?
3. Compare and contrast biotic and abiotic environmental factors, and give examples of each type of factor.
4. Describe the nested hierarchy by which ecologists organize the biological world.
5. What is the biosphere?
6. Define ecosystem.
7. Describe the niche concept in ecology.
8. How is the habitat of a species defined?
9. State the competitive exclusion principle.
10. Compare and contrast the roles of energy and matter in an ecosystem.
11. Which of the following can contain more than one species for an extended period of time? Explain your answer.
1. A niche
2. A community
3. A population
4. An ecosystem
12. Do you think there can be an ecosystem in an urban environment, such as a city? Why or why not?
13. True or False. The jumping spider and its prey occupy the same niche.
14. True or False. The same type of biome can exist in different locations on the planet.
15. Why is the study of climate-related to the study of ecology?
Explore More
Watch this video to learn about the importance of conservation.
Attributions
1. Riftia tube worm colony Galapagos 2011, NOAA Photo Library, public domain via Wikimedia Commons
2. Milne-Edwards' Sportive Lemur, Ankarafantsika, Madagascar by Frank Vassen, licensed CC BY 2.0 via Wikimedia Commons
3. Common jassid nymph and ant by Fir0002/Flagstaffotos, CC BY-NC 3.0 via Wikimedia Commons
4. Ecological levels by Christopher Auyeung via CK-12 licensed CC BY-NC 3.0
5. Nurse log by Nicholas A. Tonelli, licensed CC BY 2.0 via Wikimedia Commons
6. Jumping spider by James Niland, licensed CC BY 2.0 via Flickr
7. Pieris rapae adult by Christian Bauer, CC BY 2.0 via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.02%3A_Introduction_to_Ecology.txt |
Another Day, Another Dung Ball
The black beetle in Figure \(1\) has made a ball from the dung (feces) of a mammal and is rolling it away from the dung pile. After forming a ball from the dung, the beetle rolls the ball in a straight line, despite all obstacles, to a safe location so other dung beetles can’t steal it. Dung beetles can roll dung balls that are up to ten times their own weight. What do they do with all that dung? Generally, they bury it. Then, depending on the species of dung beetle, they either use the dung ball as a source of stored food or lay their eggs inside it. In the latter case, when larvae hatch from the eggs, they have a nutritious food source handy. Yum!
Why focus on this poop-consuming insect? Dung beetles play a very important role in agricultural areas as well as in many natural ecosystems. By burying and consuming dung, they improve nutrient recycling and soil structure. They also protect livestock from pests such as flies that would otherwise be attracted by dung. Dung beetles save the American cattle industry an estimated \$380 million a year by burying livestock feces. This is just one of a multitude of valuable services that are provided to humans by ecosystems and their organisms.
What Is an Ecosystem?
Like other systems, an ecosystem is a set of interacting components that form a complex whole. The interacting components of an ecosystem are all of its living things and its nonliving environment. The nonliving environment includes such abiotic factors as temperature, water, sunlight, and minerals in the soil. A community is the biotic part of an ecosystem. It consists of all the populations of all the species that live and interact in the ecosystem. The abiotic and biotic parts of an ecosystem are linked together by flows of energy and cycles of nutrients through the system.
There is no widely agreed upon way to delineate a specific ecosystem. Theoretically, ecosystems can vary tremendously in size. Consider a forest as an example. It might cover hundreds or even thousands of acres, forming a large ecosystem in which an individual tree is of little consequence. However, an individual tree can also be considered an ecosystem, with millions of organisms living in and on it, ranging from microbes to small mammals. Even a single leaf can be considered an ecosystem. Several generations of an aphid population can exist over the lifespan of the leaf, as in Figure \(2\). Each of the aphids, in turn, supports a diverse community of bacteria.
Ecosystem Processes
Ecosystem processes move energy and matter through the biotic and abiotic components of the system. These processes begin with primary production by producers. The energy that flows through almost all ecosystems is obtained primarily from the sun and enters ecosystems through the process of photosynthesis. This process is carried out by producers that may include plants, certain microbes, and/or algae. These producers capture energy from sunlight and use it to turn inorganic carbon dioxide (from the atmosphere) and water into organic carbon molecules and oxygen.
Mineral Nutrient Recycling
Ecosystems continually take in energy from the wider environment around them. Mineral nutrients, on the other hand, are mostly recycled within ecosystems among living things and abiotic components of ecosystems. Nitrogen in the atmosphere, for example, is taken up by certain soil bacteria, which change the nitrogen to a form that plants can use. From plants, nitrogen cycles to animals and eventually to decomposers, which return nitrogen to the soil. In most terrestrial ecosystems, nitrogen is a limiting factor in plant growth. A limiting factor is any factor that constrains the population size of one or more species in an ecosystem. Because most terrestrial ecosystems are nitrogen-limited, nitrogen cycling is an important control on ecosystem production. Other nutrients that are recycled within ecosystems include phosphorus, potassium, and magnesium.
Ecosystem Goods and Services
Ecosystems provide a variety of goods and services upon which people depend. Without healthy natural ecosystems, we could not survive as a species.
Ecosystem Goods
Ecosystem goods include tangible, material products of ecosystem processes, including foods such as wild game and fruits, construction materials such as wood and bamboo, and medicinal plants such as the willow tree pictured in Figure \(4\). Ecosystem goods also include less tangible things, such as ecosystem features that provide tourist attractions and recreational opportunities. The genes in wild plants and animals are another ecosystem good. These organisms provide a storehouse of genetic material that can be used to improve domestic species.
Ecosystem Services
The services ecosystems provide maybe even more important than the goods, yet they have traditionally been taken for granted. They include processes that maintain the water cycle, provide oxygen to the air, remove greenhouse gases from the atmosphere, filter pollutants from water, and pollinate crops. These services not only have economic value; they are also invaluable for the maintenance of human life.
As an example, consider the ecosystem service of pollination. Most flowering plants require help from pollinators such as insects and birds to produce fruit and seeds. Pollinators play an essential role in the production of more than 150 food crops in the United States, including almost all fruit and grain crops, from apples to alfalfa. The single most important pollinator of crops is the honey bee (like the one pictured in Figure \(5\)). Honey bees provide an estimated 1.6 billion dollars of natural pollinating services to agriculture in the United States alone.
Feature: Human Biology in the News
One of the biggest and potentially most devastating mysteries taking place in ecosystems in recent years is the decline of honey bees. Beekeepers normally expect some of their honey bees to die off from one season to the next, but recent losses are double of those from the past. The dramatic reduction in honey bee populations has made the news not only because it is so shocking in its devastation but also because of the extremely valuable ecosystem service that honey bees provide as pollinators. Beekeepers must spend more money to try to keep their bees alive and to start new colonies to replace those that have died out. This cost is passed on to farmers who make use of honey bee pollinators. Ultimately, the cost is passed on to consumers like you.
Researchers are desperately trying to find the answer to the mysterious demise of honey bees so they can devise plans to protect them. One possible cause that has been identified recently is a parasitic mite of the aptly named species Varroa destructor. Shown in Figure \(6\), this mite has been called the “vampire” of the bee world because it feeds on the insect equivalent of blood (called hemolymph) of both adult and juvenile honey bees. Imagine a blood-sucking mosquito the size of a dinner plate attacking you for your blood. That’s about how large the vampire mites are relative to their honey bee hosts. Worse yet, individual bees are often attacked by multiple mites. In addition, besides sucking the bee’s “blood,” the mites also transmit other pathogens that suppress the bee host’s immune system. No wonder the mites are taking such a toll on honey bee populations!
Chemicals called miticides are available that can kill Varroa mites. Unfortunately, honey bees are also adversely affected by the chemicals. The use of miticides may even be contributing to honey bee mortality. Another problem is that the mites are developing resistance to the most commonly used miticides. One potential strategy currently under investigation is to breed honey bees that are naturally resistant to the mites. However, that strategy may take a long period of time to come to fruition, if indeed it does.
In the meantime, another strategy can be implemented. That strategy is to strengthen honey bee immune systems by providing bees with a greater variety of easily accessible forage plants. Evidence suggests that when honey bees have optimal nutrition, they are better able to deal with mites and other potential causes of mortality. However, modern agribusiness and the growth of cities have decreased the amount and variety of natural flowering plants that bees need to thrive. You don’t have to be a beekeeper or scientist to help counter this trend and contribute to honey bee health and survival. If you are a homeowner, you can grow wildflowers in your yard, making sure to include species that bloom at different times of the year.
Review
1. Define ecosystem, and identify the components that make up an ecosystem.
2. How do ecosystems function?
3. What materials are recycled in ecosystems?
4. Define and give examples of ecosystem goods and services.
5. Why do you think farmers sometimes use a fertilizer that contains nitrogen on their crops?
6. Decomposers:
1. A. are primary producers.
2. B. do not play important roles in ecosystems.
3. C. help recycle nutrients in an ecosystem.
4. D. are usually abiotic.
Explore More
Watch this video to learn about the importance of biodiversity
Watch this video to learn more about the population decline of bees
Attributions
1. Dung beetle by Bayhaus via Pixabay license
2. Aphidoidea by Thomas Bresson, licensed CC BY 3.0 via Wikimedia Commons
3. Mushrooms by adeg via Pixabay license
4. Weeping willow by daledbe via Pixabay license
5. Honey bee by Orangeaurochs, licensed CC BY 2.0 via Wikimedia Commons
6. Vorroa mites, public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.03%3A_Ecosystems.txt |
Clowning Around
If you saw the movie Finding Nemo, then you probably recognize the colorful fish in Figure \(1\). The Marlin character in the movie was based on fish like these. Commonly referred to as clownfish, they are shown here swimming around the waving tentacles of animals called sea anemones. Sea anemones are predators that kill any prey that come too close by injecting poison with their tentacles. The anemones don’t harm the clownfish, perhaps because they are coated with mucus. But why do clownfish “hang out” with anemones? One reason is for food. The clownfish eat the remains of the anemones’ prey after they finish feeding. Another reason is for safety. Clownfish are safe near anemones because potential predators are scared off by the poison tentacles. Anemones also benefit from having clownfish nearby. Clownfish help anemones catch food by attracting prey with their bright colors, and clownfish feces provide nutrients to anemones. The relationship between clownfish and anemones is an example of a community relationship.
What Is a Community?
A community is the biotic part of an ecosystem. It consists of all the populations of all the species that live and interact in the ecosystem. It also includes their relationships with each other. All organisms in an ecosystem are connected in one way or another. In fact, populations of different species generally interact in a complex web of relationships. Relationships between species in communities are important factors in natural selection and help shape the evolution of the interacting species. There are three major types of community relationships: symbiosis, predation, and competition.
Symbiosis
Symbiosis is a close relationship between two organisms of different species in which at least one of the organisms benefits. For the other organism, the relationship may be beneficial or harmful, or it may have no effect. There are three basic types of symbiosis: mutualism, commensalism, and parasitism.
Mutualism
Mutualism is a symbiotic relationship in which individuals from both species benefit. The relationship between clownfish and anemones described above is an example of mutualism. Pollination of plants by pollinators such as bees is another example, as shown in Figure \(2\). Pollinators collect pollen from flowers for food. In the process of gathering the pollen, they disperse some of the pollen to other flowers and pollinate them. Humans have a mutualistic relationship with many species of intestinal bacteria. The bacteria gain a safe home with lots of available nutrients. In return, the bacteria provide their human host with vitamins, help with digestion, protection from harmful bacteria, or other goods or services.
Commensalism
Commensalism is a symbiotic relationship in which an individual from one of the species benefits while an individual from the other species is unaffected. For example, some types of tiny insects called mites attach themselves to larger flying insects for transportation. The mites benefit from the free ride, and the larger insects are unaffected. Various biting lice and fleas feed harmlessly on the feathers of birds and on sloughed-off flakes of skin from mammals. Numerous birds (such as the cattle egret in Figure \(3\)) feed on insects and small mammals that are disturbed by large grazing mammals or a farmer’s plow. Plasmodium protists that cause human malaria and the mosquitoes that transmit them have a commensal relationship. The protists need the mosquitoes to get from one human host to another, but the mosquitoes are not affected by the protists.
Parasitism
Parasitism is a symbiotic relationship in which an organism from one species, called the parasite, benefits, while an organism from the other species, called the host, is harmed. Many species of animals are parasites, at least during some stage of their life cycle. Most species are also hosts to one or more parasites. Some parasites live on the surface of their host. Others live inside their host. They may enter the host through a break in the skin or in food or water. For example, roundworms are parasites of mammals, including humans, cats, and dogs. Figure \(4\) shows adult roundworms clogging part of a human small intestine. The worms produce huge numbers of eggs, which are passed in the host’s feces to the environment. Other individuals may be infected by swallowing the eggs in contaminated food or water.
Some parasites kill their host. The roundworms in the human intestine in Figure \(4\) would most likely have killed their host were it not for surgical intervention. However, most parasites do not kill their host. It’s easy to understand why. If a parasite kills its host, the parasite is also likely to die. Instead, the majority of parasites cause relatively minor damage to their host.
Predation
Predation is a community relationship in which organisms in one species, called the predator, consume tissues of organisms in another species, called the prey. Often this means killing the prey and eating all or most of the prey organism. You can see a graphic example of this in Figure \(5\). In this example, a snake is a predator and the prey is a large lizard. The snake is swallowing the live lizard whole. Prey species are not always killed by their predators. For example, many animals such as deer and cattle graze on plants without usually killing them. Another example of this type of predation is a mosquito feeding on a human organism’s blood.
Predators are often prey on their own. For example, blue jays prey on insects and may, in turn, be preyed upon by snakes. Snakes may also have predators, such as hawks. Examples of the few predators that are not also prey include sperm whales, tigers, and crocodiles.
Predator-Prey Population Dynamics
A predator-prey relationship tends to keep the populations of both species in balance. Each population is a limiting factor on the other population. This is shown in the graph in Figure \(6\). As the prey population increases, there is more food for predators. Therefore, after a slight lag time, the predator population increases as well. As the number of predators increases, more prey is captured. As a result, the prey population starts to decrease. Then, as fewer prey become available, the predator population declines as well. This type of interaction might continue indefinitely.
Adaptations to Predation
Both predators and prey are likely to have adaptations to predation that evolve through natural selection. Predator adaptations help them capture prey, whereas prey adaptations help them avoid predators. A common adaptation in both predators and prey is camouflage. Several examples are shown in Figure \(7\). Camouflage in prey helps them hide from their predators. Camouflage in predators helps them sneak up on or entrap their prey.
Interspecific Competition
Interspecific competition is a community relationship in which organisms from different species rely on the same limiting resource in their ecosystem. The resource might be food, water, sunlight, or space, among others.
Outcomes of Interspecific Competition
Interspecific competition is the basis of the competitive exclusion principle, which states that two different species cannot occupy the same niche in the same place for very long. Interspecific competition is likely to have one of two possible evolutionary outcomes: extinction of one species or the evolution of greater specialization in both species.
Interspecific competition often leads to extinction. The species that is less well adapted may get fewer of the limiting resource that both species need. As a result, members of that species are less likely to survive, and the species may go extinct.
Instead of extinction, interspecific competition sometimes leads to greater specialization in both species. Specialization occurs when competing species evolve different adaptations. For example, they may evolve adaptations that allow them to use different food sources or to obtain food at different times of the day. Figure \(8\) illustrates an example of this outcome of interspecific competition.
Feature: Human Biology in the News
Schistosomiasis is a neglected tropical disease that affects millions of people worldwide and causes thousands of deaths each year. It is caused by parasitic flatworms called schistosomes. The disease is spread by contact with fresh water that is contaminated with the larval stage of the parasites. The parasite larvae are released into the water by infected freshwater snails, which are also hosts of the parasites. Infection occurs when the tiny larvae penetrate human skin.
After schistosome larvae gain entrance to the human body, they develop into adult worms in their host’s veins. Female adults release eggs inside the human host. Some of the eggs may become trapped in body tissues, causing an immune reaction and major damage to internal organs. Other eggs pass out of the human host through urine or feces. If the eggs enter a body of water and infect freshwater snails, the cycle of transmission and human infection is likely to be repeated. Schistosomiasis is especially common among children in developing countries (like the child in Figure \(9\)) because they are more likely to play in contaminated water. Other high-risk groups include farmers, fishermen, and people who must use contaminated water for household purposes.
Although drugs are available to cure schistosomiasis, most people who have been infected and cured are likely to be reinfected because of continued unavoidable contact with contaminated water. Researchers are trying to develop a vaccine to prevent transmission of the parasites but so far without success. That’s where a bioengineer at Stanford University comes in. Professor Manu Prakash has been studying schistosome parasites with novel approaches to understand how the parasites move through the water to infect human hosts.
At the end of 2016, Professor Prakash published preliminary results of his research. He found that schistosome larvae have a completely unique way of swimming. The larvae have a forked tail that they hold perpendicular to their body while they swim against gravity to reach water near the surface. Using mathematical modeling and robotics, Professor Prakash was able to show that this particular swimming technique is the optimal way for the larvae to quickly reach surface water where they are most likely to encounter a human being to infect. Schistosome larvae have no feeding organs, so they must find a human host within about 12 hours or they will die. Professor Prakash hopes that the detailed understanding of how the parasites swim may lead to a way to slow them down so they cannot reach and infect a human host within this 12-hour window.
Review
1. In the context of the ecosystem, what is a community?
2. Identify the three major types of community relationships.
3. What is symbiosis?
4. Name three different types of symbiosis.
5. Define mutualism and describe an example.
6. Why is the relationship between cattle egrets and grazing mammals an example of commensalism?
7. Define parasitism and give an example.
8. What is predation, and what is an example of predation?
9. Explain the relationship between predator and prey populations.
10. How can predators and prey influence each other’s evolution?
11. What is an interspecific competition?
12. What are two possible outcomes of interspecific competition?
13. True or False. In a symbiotic relationship, the two species have to physically touch each other.
14. True or False. The relationship between the protists that cause human malaria and humans is an example of commensalism.
Explore More
Watch this video to learn about biodiversity
Attributions
1. Amphiprion ocellaris by Michael arvedlund, public domain via Wikimedia Commons
2. Pollination by Louise Docker, licensed CC BY 2.0 via Wikimedia Commons
3. Cattle egrets by Jorge Láscar, licensed CC BY 2.0 via Wikimedia Commons
4. Intestine blocked by worms by SuSanA Secretariat, licensed CC BY 2.0 via Wikimedia Commons
5. Dolichophis jugularis by yigal gini, licensed CC BY 2.5 via Wikimedia Commons
6. Predator-prey graph by Hana Zavadska via CK-12 licensed CC BY-NC 3.0
7. Camouflage by Thomas Hubauer, licensed CC BY 2.0 via Flickr
8. Brown Anole by pondhawk, CC BY 2.0 via Flickr
9. Schistosomiasis in a child by SuSanA Secretariat, licensed CC BY 2.0 via Wikimedia Commons
10. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.04%3A_Community_Relationships.txt |
Sea Angel
It’s easy to see why the aquatic creature in Figure \(1\) is commonly called a sea angel. It has gossamer-like “wings” that flutter gently and help it swim, and its diaphanous body gives it an otherworldly appearance. Although it appears angelic, this tiny invertebrate is actually a vicious predator. It has a secret weapon in the form of six sharp tentacles hidden in its face. When an unsuspecting prey drifts by, the sea angel turns into a devilish killing machine. It lashes out its tentacles, grabs its prey, and then slowly eats it. Predators like sea angels obtain energy from prey organisms. This is just one of the ways that organisms obtain energy.
How Organisms Obtain Energy
There are two basic types of organisms in terms of how they obtain energy: autotrophs and heterotrophs.
Autotrophs
Autotrophs are organisms that use energy directly from the sun or from chemical bonds. Commonly called producers, they use energy and simple inorganic compounds to produce organic molecules. Autotrophs are vital to all ecosystems because all organisms need organic molecules and only autotrophs can produce them from inorganic compounds. There are two basic types of autotrophs: photoautotrophs and chemoautotrophs.
Photoautotrophs
Photoautotrophs are autotrophs that use energy from sunlight to make organic compounds by photosynthesis. Photoautotrophs include plants, algae, and many bacteria, as shown in Table \(1\). They are the primary producers in the vast majority of ecosystems on Earth.
Chemoautotrophs
Chemoautotrophs use energy from chemical bonds to make organic compounds by chemosynthesis. Chemoautotrophs include certain bacteria and archaeans. They are the primary producers in ecosystems that form around hydrothermal vents and in hot springs.
Table \(1\): Different types of photoautotrophs are important in different ecosystems
Type of Photoautotroph Type of Ecosystem(s) Example Example
Plants Terrestrial
Tree
Grasses
Algae Aquatic
Diatoms
Seaweed
Bacteria Aquatic and Terrestrial
Cyanobacteria
Purple Bacteria
Heterotrophs
Heterotrophs are organisms that obtain energy from other living things. Like sea angels, they take in organic molecules by consuming other organisms, so they are commonly called consumers. Heterotrophs include all animals and fungi as well as many protists and bacteria. Heterotrophs can be classified by what they usually eat as herbivores, carnivores, omnivores, or decomposers.
Herbivores
Herbivores are heterotrophs that directly consume producers such as plants or algae. They are a necessary link between producers and other heterotrophs such as carnivores. Examples of herbivores include deer, rabbits, sea urchins, grasshoppers, mice, and the larvae of many insects, like the caterpillar in Figure \(2\). Herbivorous animals typically have mouthparts or teeth adapted to grasping or grinding the tough materials in plants. Many herbivores have mutualistic intestinal microbes that help them break down hard-to-digest plant matter.
Carnivores
Carnivores are heterotrophs that consume animals; examples of heterotrophs include lions, polar bears, hawks, salmon, and spiders. Obligate carnivores (such as cats) are unable to digest plants so they can only eat animals. Facultative carnivores (such as dogs) can digest plant matter but plants are not an important food source for them. Most carnivores are predators that catch and kill live animals for consumption. Some carnivores, called scavengers, find and eat animals that have already died, such as the prey remnants left behind by predators. Examples of scavengers include vultures, hyenas, and blowflies, like those in Figure \(3\).
Omnivores
Omnivores are heterotrophs that consume both plants and animals. They include pigs, brown bears, gulls, crows, and humans. Omnivores actually fall on a continuum between herbivores and carnivores. Some omnivores eat more plants than animals, whereas other omnivores eat more animals than plants. Some organisms are seasonally omnivorous, meaning that they eat plants in some seasons and animals in other seasons. An example is a grizzly bear. When salmon or other fish are plentiful, the bears are primarily carnivorous; but when berries ripen and become plentiful, the bears are mainly herbivorous. Some omnivores eat animals during some life stages and plants during other life stages. For example, most tadpoles are herbivores that eat algae, whereas adult frogs are carnivores that eat insects and other invertebrates.
Decomposers
Decomposers are heterotrophs that break down and feed on the remains of dead organisms and other organic wastes such as feces. In the process, they release simple inorganic molecules back to the environment. Producers can then use the molecules to make new organic compounds. Decomposers are classified by the type of organic matter they break down. Two types are detritivores and saprotrophs.
• Detritivores are decomposers that ingest and digest detritus, which includes dead leaves, animal feces, and other organic debris that collects on the ground or at the bottom of a body of water. Terrestrial detritivores include earthworms and dung beetles. Aquatic detritivores include “bottom feeders” such as sea cucumbers and catfish.
• Saprotrophs are decomposers that feed on dead organic matter by secreting digestive enzymes and digesting it externally, rather than by ingesting the matter and digesting it internally. Saprotrophs include fungi and single-celled protozoa. Fungi, like those in Figure \(4\), are the only organisms that can decompose wood.
Models of Energy Flow
Energy enters all ecosystems from the sun or from inorganic chemicals. The energy then flows through ecosystems from producers, who can use inorganic forms of energy, to consumers, who can obtain energy only from organic compounds in other living things. Ecologists commonly represent this flow of energy through the organisms of an ecosystem with models such as food chains and food webs. These models represent feeding relationships, showing who eats whom. Although the models are generally oversimplifications of reality, they have proven useful for testing hypotheses about ecosystems and identifying common patterns that many ecosystems share.
Food Chains
A food chain is an ecological model that represents a single pathway through which energy flows in an ecosystem. Food chains are virtually always simpler than what really happens in nature because most organisms consume — and are consumed by — more than one species. Two examples of food chains, one terrestrial and one aquatic, are shown in Figure \(5\). In both food chains, the organisms at the bottom are producers. In the terrestrial food chain, the producers are grasses, and in the aquatic food chain, the producers are tiny plants called phytoplankton. The producers in each food chain are consumed by herbivores. The herbivores, in turn, are consumed by carnivores, which are themselves the prey of other carnivores. The top organism in each food chain is a predator — called an apex predator — that is not preyed upon by any other species.
Many food chains, including those pictured in Figure \(5\):, do not include decomposers. However, decomposers are a significant component of energy flow in every ecosystem. Decomposers break down any remaining organic matter (whether from producers or consumers), using some of the energy it contains and releasing excess nutrients back into the environment.
Food Webs
A food web is an ecological model that represents multiple pathways through which energy flows in an ecosystem. It generally includes many intersecting food chains. Although food webs, like food chains, are usually simplifications of reality, they do demonstrate that most organisms eat, and are eaten by, more than one species. Two examples of food webs, one terrestrial, and one aquatic, are shown in Figure \(6\). Consider the grasshopper in the terrestrial food web as an example. It is an herbivore that consumes only plants, but the grasshopper is consumed by multiple other consumers, including spiders, mice, birds, and frogs.
Trophic Levels
Table \(2\): Trophic Levels in Food Chains and Food Webs
Trophic Level How It Obtains Energy Example
1st trophic level: producers photosynthesis or chemosynthesis grass
2nd trophic level: primary consumers consumes producers rabbit
3rd trophic level: secondary consumers consumes primary consumers snake
4th trophic level: tertiary consumers consumes secondary consumers hawk
The different feeding positions in a food chain or food web are called trophic levels. The main trophic levels are defined in Table \(2\). All food chains and food webs have at least two or three trophic levels, one of which must be producers (1st trophic level). Generally, there is a maximum of four trophic levels, and only rarely are there five or more trophic levels. Most consumers actually feed at more than one trophic level. Humans, for example, are primary consumers when they eat plants such as vegetables. They are secondary consumers when they eat meat from herbivores such as cattle. They are tertiary consumers when they eat secondary consumers such as salmon, which eat smaller fish.
Trophic Levels and Energy
Energy is passed up a food chain or food web from lower to higher trophic levels. However, as shown in the energy pyramid in Figure \(7\), only about 10 percent of the energy at one trophic level is actually passed up to the next higher trophic level. The other 90 percent of energy at each trophic level is used by organisms at that level for metabolism, growth, and repair. Metabolism generates heat (thermal energy), which is the energy that is lost to the environment. Some energy is also lost as incompletely digested food that is excreted. The decline in energy from one trophic level to the next explains why there are rarely more than four trophic levels in a food chain or food web. There is generally inadequate energy remaining above four trophic levels to support organisms at additional trophic levels
Trophic Levels and Biomass
With less energy at higher trophic levels, it is generally the case that fewer organisms can be supported at higher levels. Although individual organisms tend to be larger in size at higher trophic levels, their smaller numbers result in less biomass at higher levels. Biomass is the amount of organic matter present in an individual organism or in all the organisms at a given trophic level. The decrease in numbers and biomass of organisms from lower to higher trophic levels is represented by the ecological pyramid in Figure \(7\).
Review
1. What are autotrophs? Name three types of organisms that are autotrophs.
2. Compare and contrast photoautotrophs and chemoautotrophs.
3. Define heterotroph.
4. What types of organisms are heterotrophs?
5. How are heterotrophs classified on the basis of what they consume?
6. What are food chains and food webs?
7. What are the trophic levels? Identify the different trophic levels in a food chain or food web.
8. Why are there rarely more than four trophic levels in an ecosystem?
9. How do the numbers and biomass of organisms usually change from lower to higher trophic levels?
10. Explain the phenomenon of bioaccumulation.
11. Herbivores are at which trophic level?
1. \(1^{st}\)
2. \(2^{nd}\)
3. \(3^{rd}\)
4. \(4^{th}\)
12. True or False. In some food chains, chemoautotrophs are the type of organism at the 1sttrophic level.
13. True or False. Apex predators are at the trophic level that contains the most energy.
14. Which of the following is not a heterotroph?
1. An apple tree
2. A mushroom
3. A tadpole
4. A and B
15. Which of the following terms apply to humans: autotroph; heterotroph; carnivore; omnivore; herbivore; producer; primary consumer; tertiary consumer
Explore More
Watch this video to learn more about pollution
Attributions
1. Clione by Kevin Raskoff (NOAA Photo Library), public domain via Wikimedia Commons
2. Raunkiærseg, public domain via Wikimedia Commons
3. Nematus ribesii feeding on leaf by Daniel Mietchen, licensed CC0 via Wikimedia Commons
4. The fly feast by LASZLO ILYES, CC BY 2.0 via Wikimedia Commons
5. Fungi in Borneo by Cayce, CC BY 2.0 via Wikimedia Commons
6. Simplified food chain by LadyofHats (Mariana Ruiz Villarreal), dedicated CC0 via Wikimedia Commons
7. Food web by LadyofHats (Mariana Ruiz Villarreal), dedicated CC0 via Wikimedia Commons
8. Ecological pyramid by CK-12 licensed CC BY-NC 3.0
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.05%3A_Energy_in_Ecosystems.txt |
Dino Water?
Did a dinosaur once drink the water in Figure \(1\)? You may have heard the claim — usually made in the context of the water cycle — that the same water exists on Earth now as existed tens of millions of years ago when dinosaurs roamed the planet. In fact, dinosaurs probably did not once drink the same water molecules that we consume today. Whereas the atoms that make up water molecules have existed for eons, individual water molecules are broken down and reformed too often to last that long. Nonetheless, unlike energy, which is continuously added to Earth by the sun, water is constantly recycled.
Biogeochemical Cycles
The water and chemical elements that organisms need continuously cycle through ecosystems, passing repeatedly through their biotic and abiotic components. These cycles are called biogeochemical cycles because they are cycles of chemicals that include both organisms (bio) and abiotic components such as the ocean or rocks (geo). As matter moves through a biogeochemical cycle, it may be held for various periods of time in different components of the cycle. A component of a biogeochemical cycle that holds an element or water for a long period of time is called a reservoir. For example, the deep ocean is a reservoir for water. It may hold water for thousands of years.
The rest of this concept takes a closer look at four particular biogeochemical cycles: the water, carbon, and nitrogen cycles
Water Cycle
Water is essential to all living things on Earth because virtually all biochemical reactions take place in water. Water can dissolve almost anything, so it also provides an efficient way to transfer substances between and within cells. The water cycle, also known as the hydrological cycle, describes the continuous movement of water on, above, and below Earth’s surface. As it cycles, water moves from one exchange pool or reservoir to another. In different parts of the cycle, water exists as a liquid (water), solid (ice), or gas (water vapor). Therefore, the water cycle includes several physical processes by which water changes state.
Movement through the water cycle
• Evaporation occurs when water on Earth’s surface changes to water vapor. When the sun heats water, it gives water molecules enough energy to escape into the atmosphere.
• Sublimation occurs when ice and snow change directly to water vapor without first melting to form liquid water. Sublimation occurs because of the heat from the sun.
• Transpiration occurs when plants release water vapor through leaf pores called stomata. Plants take up more water through their roots than they need for photosynthesis and other processes. Much of this excess water is given off via transpiration.
• Condensation is the process in which water vapor changes to liquid water, forming water droplets. If enough water droplets are present, they may form a visible cloud. If the droplets become large enough, they fall to Earth because of gravity as precipitation, such as rain, snow, sleet, or hail.
• Precipitation that falls on land may flow over the surface of the ground. This water is called runoff, and it may eventually flow into a body of water.
• Some of the precipitation that falls on land may soak into the ground and become groundwater. Groundwater may seep out of the ground at a spring or into a body of water such as a lake or the ocean. Some groundwater may be taken up by plant roots. Some may flow deeper underground to an aquifer.
Carbon Cycle
Carbon is the basis of life on Earth. Chains of carbon bonded together to form the backbone of many biochemical molecules. Carbon is also an important component of rocks and minerals, and carbon exists in the atmosphere in compounds such as carbon dioxide. The carbon cycle is the biogeochemical cycle in which carbon moves through the biotic and abiotic components of ecosystems. The carbon cycle is represented by the diagram in Figure \(3\).
Carbon cycles quickly between organisms and the atmosphere. Cellular respiration by living things releases carbon into the atmosphere as carbon dioxide. Photosynthesis by producers such as plants removes carbon dioxide from the atmosphere and uses it to make organic carbon compounds. Carbon in organic compounds moves through ecosystem communities from producers to consumers, as modeled by food chains and food webs that show feeding relationships. Carbon is also released back into the environment when organisms decompose.
Several human actions release huge amounts of additional carbon into the atmosphere. The most significant of these actions is the burning of fossil fuels. Large amounts of carbon in the gas methane are also released into the atmosphere from the decomposition of livestock manure and the wastes in landfills. Some natural events can also quickly add carbon to the atmosphere. Wildfires produce carbon dioxide as a product of combustion, and volcanic eruptions release carbon dioxide from molten rock (magma). Large volcanic eruptions (like the one in Figure \(4\)) can release enormous amounts of carbon dioxide in a short period of time.
Carbon generally cycles more slowly through other processes. For example, running water slowly dissolves carbon in rocks, and most of this carbon ends up in the ocean. The top layer of ocean water dissolves some carbon dioxide out of the atmosphere, and carbon also enters ocean water from the decomposition of aquatic organisms. Carbon from these sources may settle to the bottom of the ocean as sediment. Over millions of years, this carbon may form fossil fuels or carbon-containing rocks. Carbon can remain in these reservoirs for millions of years.
Nitrogen Cycle
Nitrogen makes up 78 percent of Earth’s atmosphere. It is also an important element in living things. Nitrogen is needed for proteins, nucleic acids, and many other organic molecules, including chlorophyll, without which plants and other photoautotrophs could not carry out photosynthesis. The nitrogen cycle is the biogeochemical cycle that recycles nitrogen through the biotic and abiotic components of ecosystems. Figure \(5\) shows how nitrogen cycles through a terrestrial ecosystem. Nitrogen passes through aquatic ecosystems in a similar cycle.
Plants cannot use nitrogen gas in the air to make organic compounds for themselves and the organisms that consume them. However, they can use nitrogen in the form of compounds such as nitrates, which they can absorb through their roots. The process of changing nitrogen gas to nitrates is called nitrogen fixation. It is carried out by bacteria, called nitrogen-fixing bacteria, that live in soil or on the roots of legumes such as peas. Nitrogen fixation is the primary source of nitrogen used by plants in most ecosystems.
When plants and other organisms die or release wastes, decomposers break down their organic compounds. In the process, they release nitrogen in the form of ammonium ions into the soil. The ammonium ions can be absorbed by plant roots. The ions can also be changed to nitrates by soil bacteria called nitrifying bacteria.
Not all of the nitrates produced by nitrogen-fixing and nitrifying bacteria are used by plants. Some of the nitrates are changed back to nitrogen gas by soil bacteria called denitrifying bacteria. This nitrogen returns to the atmosphere, thus completing the cycle.
Review
1. What are biogeochemical cycles? Give examples of matter that have such cycles.
2. Compare and contrast exchange pools and reservoirs in biogeochemical cycles. Give an example of each.
3. How does water change as it moves through the water cycle? Illustrate your answer with examples.
4. Most of Earth’s water is salt water in the ocean, and most precipitation falls back into the ocean. How is Earth’s supply of freshwater continuously renewed?
5. Describe what may happen to liquid precipitation that falls on land.
6. What is an aquifer? How can people access the water in an aquifer?
7. Name two reservoirs of frozen water.
8. How does the carbon cycle between organisms and the environment?
9. Explain how human actions can change the carbon cycle.
10. Identify some of the abiotic processes that cycle carbon slowly.
11. How do living things obtain nitrogen from the environment?
12. How is nitrogen returned to the atmosphere to complete the nitrogen cycle?
13. True or False. Plants are involved in the water cycle.
14. True or False. Most precipitation falls into the ocean because ocean water covers much of Earth’s surface.
Attributions
1. Water by drfuenteshernandez via Pixabay license
2. Water cycle by Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation, CC BY-NC 3.0
3. Carbon Cycle by Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation, CC BY-NC 3.0
4. Mount St. Helens eruption by Austin Post, Public domain, via Wikimedia Commons
5. Nitrogen cycle by Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation, CC BY-NC 3.0
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.06%3A_Cycles_of_Matter.txt |
growing like a weed
It’s been called the world’s most successful weed species because it has grown so quickly in numbers and spread so far geographically. Everywhere this species has gone, it has taken over local ecosystems. Is the species in question a weedy plant like the dandelions pictured in Figure \(1\)? No; that dubious honor has been given to our own species, Homo sapiens.
So Many People
Our species is relatively young, with the earliest modern humans dating back only to about 200,000 years ago. However, in a relatively short period of time since then, a total of about 108 billion people have lived on planet Earth. More than 7 percent of them, or an estimated 7.6 billion people, were alive in September 2019. The number of humans on the planet is projected to increase to at least 9 billion by 2050 and could reach 11 billion or more by 2100! The human species also currently lives on every continent. Six of Earth’s seven continents are permanently inhabited on a large scale. Only Antarctica is sparsely inhabited and without permanent settlements.
The rapid increase and spread of the human population have raised concerns about our continued existence as a species. Some thinkers have speculated that continued rapid increase in human numbers will sooner or later outstrip the resources available on planet Earth and lead to a population catastrophe. Some scientists think we have a long way to go before that happens. Others think we have already exceeded our limit, citing evidence of widespread environmental damage caused by human actions and the more than 1 billion people in the world who live in extreme poverty.
Studying the Human Population
We know more about the human population and how it has grown than we know about the population of any other species thanks to demography, which is the scientific study of human populations. Demography encompasses the size, distribution, and structure of populations. Population structure is the proportion of people by age, sex, and, often, by other parameters as well, such as ethnicity or education. Demography also encompasses population processes that change population size and structure, including births, deaths, and migration.
Demography is considered to be at the crossroads of several disciplines, including sociology, economics, epidemiology, anthropology, and history. Besides studying current populations, demographers reconstruct past population characteristics, such as estimating the world population size 10,000 years ago. Demographers also make predictions about populations in the future, such as how many people will live in cities in 2050. In addition, many demographers study relationships between population characteristics and other factors, such as economic, social, or cultural factors. An example is the youth bulge phenomenon described below.
Demographic Data
Demographic data are routinely collected by the governments of most countries. Important sources of demographic data include vital statistics registries. These registries track all births and deaths as well as certain changes in status, such as marriages, divorces, and migrations. Censuses are also important sources of demographic data. They are usually conducted by national governments every 10 years. For example, the United States government has been conducting a national census every 10 years since 1790. A census has the primary goal of counting every person in the country, but it also typically collects information on such variables as age, sex, marital status, education, employment status, and occupation.
Demographic Measures
Demographers use data from vital statistics registries and censuses, among other sources, to calculate measures of population characteristics. Some of these measures are useful to know as you read more about human populations below and in other concepts in this chapter. They include the following:
• birth rate — number of live births in 1 year per 1,000 people in the population
• death rate number of deaths in 1 year per 1,000 people in the population
• fertility rate — the average number of live births per woman by the end of the childbearing years
• replacement fertility rate — fertility rate at which women average only enough children to replace themselves and their partner in the population
• life expectancy — the average age of death in a population, or the average length of life
• population growth rate (r) — the net number of people added to a population in 1 year per 100 people already in the population
Age-Sex Structure
The age-sex structure of a population is a frequently measured population parameter. It refers to the number of individuals of each sex and age group in the population. The age-sex structure of a population is often represented by a special type of bar graph called a population pyramid. You can see two examples of population pyramids in Figure \(2\), the first, on the left, for the sub-Saharan African country of Nigeria and the second, on the right, for France. Both population pyramids represent the age-sex distribution in the year 2015. In each case, the population is distributed along the horizontal axis, with males shown on the left and females on the right. The male and female populations are broken down into 5-year age groups represented as horizontal bars along the vertical axis, with the youngest age groups at the bottom and the oldest at the top.
A great deal of information about a population can be gleaned from its population pyramid because its shape changes slowly over time based on births and deaths and, in some cases, international migration. Births add people to a population only in the youngest age group, whereas deaths remove people from all age groups of a population. The population pyramid for Nigeria, for example, is actually pyramidal in shape, with a broad base of young children and tapered sides showing rapidly decreasing numbers of people at older ages. This type of pyramid reflects a population that has high birth rates and relatively high death rates. The population pyramid for France, in contrast, has a nonpyramidal shape. The narrow base of children and young adults reflects a relatively low birth rate over the past several decades. The bulge of people in mid-to late-adulthood is evidence of higher birth rates in previous generations (the post-World War II baby boom) coupled with low death rates. The larger proportion of females than males at older ages, which is especially pronounced in the French population pyramid, is due to the higher rates of death of males than females, especially in older age groups. This trend is seen in most human populations.
Population pyramids may also provide insights into political and social stability and economic development. An example of this is the so-called “youth bulge,” which is a disproportionately large cohort of young adults, the age groups when people typically enter the labor force and electorate. As an example, you can see a youth bulge in the 2010 population pyramid for Egypt in Figure \(3\). A youth bulge may cause young adults to have high rates of unemployment and social and political alienation. These conditions, in turn, may result in a heightened risk of violence and political instability. A youth bulge has been posited as an important contributor to the rise of fascism in 20th-century Europe, the spread of communism during the Cold War, and the events of the Arab Spring, which began in Egypt in 2011.
Review
1. Why has the human species been called the world’s most successful weed species?
2. What is demography? What aspects of the population does it study?
3. Identify and define six common demographic measures.
4. What is the age-sex structure of a population?
5. How does a population pyramid represent the age-sex structure of a population?
6. Explain what can be learned about a population from its population pyramid.
Attributions
1. Dandelion by Jakub Kolář, dedicated CC0 via Wikimedia Commons
2. Population pyramid of Nigeria 2015 by The World Factbook, public domain via Wikimedia Commons
3. Egypt population by Delphi234, dedicated CC0 via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.07%3A_Introduction_to_Human_Populations.txt |
adding on
Population growth refers to changes in population size over time. Figure \(1\) illustrates one way that populations grow: adding babies through births. Can you think of other ways that populations change in size in a given area?
Patterns of Population Growth
Populations are dynamic. They are continuously gaining individuals through births and losing individuals through deaths. Populations may also gain or lose a significant number of individuals through migration when people either enter or leave a population. All of these factors together determine whether and how quickly a population grows. Population growth rates may change over time. Two well-studied patterns of change in population growth rates are exponential and logistic growth.
Exponential and Logistic Growth
Under ideal conditions, populations of most species, including Homo sapiens, have the potential for exponential growth, represented by curve A in Figure \(2\). With exponential growth, the population starts out growing slowly, but as population size increases, the growth rate also increases. The larger the population becomes, the more quickly it grows.
Almost no populations live under ideal conditions. Therefore, most do not grow exponentially, at least not indefinitely. They may start outgrowing exponentially, but sooner or later, something will limit their growth. Many factors may limit population growth so it slows down or even stops. Often, the factors are density-dependent. Density-dependent factors are those that cause population growth to slow down when the population becomes too large and crowded. For example, the population may start to run out of food, or crowding may lead to infectious disease epidemics. More deaths may occur or more people may emigrate, causing population growth to slow and population size to level off. Curve B inFigure \(2\) represents this pattern of growth, which is called logistic growth.
At what size does a population stop growing in the logistic growth pattern? That depends on the population’s carrying capacity (Figure \(2\)). The carrying capacity (K) is the largest population size that can be supported by available resources without harming the environment. Population growth hits a ceiling at that size in the logistic growth model.
Exponential Growth of the Human Population
For most of our species’ existence, the global population grew very slowly. Then, starting a few centuries ago, the human population started to grow exponentially. You can see this clearly in the graph in Figure \(3\). It took the human population many millennia to reach 1 billion people, which occurred around 1800 CE. After that, it took only a little over a century for the number to reach 2 billion. In less than another century, we added another 5 billion people, reaching a total of 7 billion people by 2012. Today, the human population is rapidly approaching the 8 billion mark. At a global growth rate of 1.03 percent in 2021, we are adding a net number of more than 80 million people each year. If that growth rate were to continue, the total human population would double in just 58 years.
Is it possible for the human population to keep growing at 1.2 percent? The late-18th century economist Thomas Malthus predicted that the continued rapid growth of the human population would soon outstrip food production, leading to increasing famine and higher death rates. This would be evidence that the population had reached its carrying capacity and could no longer keep growing. Unless population growth was reigned in before the carrying capacity was reached, Malthus argued, there would be a population crash caused by heightened warfare, malnutrition, and disease.
Overpopulation
Since Malthus made his dire warning, the human population has grown from just under a billion to more than 7 billion people. Has the human population already reached or surpassed its carrying capacity? Is the planet overpopulated with people? Do we have a human overpopulation problem?
Human Carrying Capacity
Attempts to calculate the carrying capacity for the global human population have produced widely varying estimates. However, a meta-analysis of 69 such studies concluded that the best estimate of the human carrying capacity is 7.7 billion people. The human population is projected to reach 10.88 billion people by the year 2100. If these estimates are correct, it suggests that the human population is at the tipping point and must stop growing. Some human populations already suffer shortages of food, water, and other resources; and our use and acquisition of resources have already damaged the environment. Such evidence suggests that we have reached our carrying capacity and there really is an overpopulation problem.
Not Just Overpopulation
Although many environmental problems are aggravated by the size of the human population, some experts think that over-consumption and waste by populations in wealthy nations are worse problems than sheer human population numbers. People in the more-developed nations use resources at a rate more than 30 times greater than the rate in less-developed nations, where the majority of people live today. If everyone used resources at the rate of people in developed countries, the total human population would need more than one planet Earth to supply their demands. Reducing profligate consumption of resources and our ecological footprint are clearly needed to help solve the overpopulation problem.
Slowing Human Population Growth
Environmental problems are not caused solely by human overpopulation. However, having so many people on the planet certainly makes problems worse, so it is important to reduce the rate of human population growth. A widely accepted goal is an overall zero growth rate for the human population. Zero population growth (ZPG) occurs when the birth rate equals the death rate (assuming no net migration for the human population as a whole). ZPG can be achieved if women average only enough children to replace themselves and their partners in the population. This is called the replacement fertility rate. It ranges from just over 2 to almost 3 children per woman, depending on the death rate. At a higher death rate, the replacement fertility rate is higher because fewer children survive to adulthood to replace their parents in the population. Even if the fertility rate falls to the replacement level, however, there will still be a time lag before the population growth rate levels off. That’s because populations that have recently had high birth rates have a youthful age distribution, with a large proportion of women at peak childbearing ages. With so many young women, the population birth rate will remain high for at least another generation.
Childbearing is a deeply important and personal decision that is influenced by many socioeconomic and cultural factors. Obviously, trying to influence how many children women have is a complex problem. A top-down approach was instituted in China in 1979 when it enacted a one-child-per-woman policy. The Chinese government has credited the policy with reducing China’s population by 400 million people. However, China’s fertility rate was already falling when the policy was put into effect, so the impact of the policy is disputed. The actual growth of the Chinese population is shown in the graph in Figure \(4\). The tiny dip in the curve starting around 1979 suggests the policy’s impact on population growth was minimal.
Unlike in China, most countries do not have direct policies to limit fertility rates. However, evidence from many populations shows that women start having fewer children when there are more educational and economic opportunities for females, advances in gender equality, greater knowledge of family planning, and better access to contraception. Needless to say, such society-wide changes are often very difficult to achieve and require multiple approaches, but the future of our species may depend on them.
Feature: Human Biology in the News
Just days into his presidency on January 23, 2017, and surrounded solely by male members of his administration, President Donald Trump signed an executive order reinstating the so-called “Mexico City Policy.” This policy also referred to as the “global gag rule,” was first put into effect by President Ronald Reagan in 1984. The policy withholds U.S. government funding from any international non-governmental organization that performs or promotes abortions as a family planning option. The Mexico City Policy was rescinded by the Clinton Administration in 1993, reinstated by the Bush Administration in 2001, and rescinded again by the Obama Administration in 2009.
While Trump’s reinstatement of the Mexico City Policy was praised by Republican politicians and anti-abortion activists, Democratic politicians and abortion-rights activists reacted to it with newsworthy alarm. They called the reinstatement a “catastrophe” for women in less-developed countries, arguing that it will lead to large increases in unintended pregnancies, unsafe abortions, and neonatal and maternal deaths.
The Mexico City Policy shows how politically charged the control of fertility can be. However, promoting family planning in high-fertility populations is an important part of the solution to the overpopulation problem. Unless fertility is reduced to replacement levels, human populations will continue to grow.
Review
1. Why are populations dynamic?
2. What factors determine the growth rate of the human population?
3. Describe variation in recent human population growth rates.
4. Compare and contrast exponential and logistic patterns of population growth.
5. Define carrying capacity.
6. Briefly summarize how the population of the human species has grown.
7. Discuss evidence for and concerns about human overpopulation.
8. Define the replacement fertility rate, and explain why it depends on the death rate.
9. Assuming the death rate remains constant, why is there a time lag between a decrease in fertility and the slowing of population growth?
10. Identify evidence-based factors that lead to fertility decline.
Explore More
Watch this video to learn more about population dynamics.
Attributions
1. Kids at daycare by Grant Barrett, CC BY 2.0 via Wikimedia Commons
2. Population growth patterns by Hana Zavadska via CK-12 licensed CC BY-NC 3.0
3. Growth of the human population by Hana Zavadska via CK-12 CC BY-NC 3.0
4. China Demography by Quilokos, Demmo, CC BY 3.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.08%3A_Population_Dynamics.txt |
Can we take the heat?
This image represents the biggest problem that the human species has ever faced: the current trend in global warming. There is no longer any doubt that our planet is growing warmer and that human actions are the primary cause. There is also no question that if we don’t do something about it soon, the consequences will be devastating. Of the 17 warmest years on record, 16 of them have occurred since the year 2000. While record-breaking years attract the most public attention, individual years are less significant than the overall trend, and the upward trend in Earth’s temperature is alarming. The average temperature of the planet has been rising for more than a century, and its rate of increase is accelerating. The most significant reason for Earth’s rising temperature is the human impact on the greenhouse effect.
The Greenhouse Effect
The greenhouse effect is the process by which the atmosphere of a planet such as Earth warms the planet’s surface to a temperature above what it would be without the atmosphere. The atmosphere raises the surface temperature if it contains certain gases — called greenhouse gases — that can radiate energy down to the planet’s surface.
How does the greenhouse effect work? As shown in Figure $2$, of the total amount of solar energy available at the top of the atmosphere, some of the energy is reflected back into space by the atmosphere or Earth, and some is absorbed by the atmosphere and clouds. However, most of the energy is absorbed by Earth’s surface, and much of this energy is radiated back into the atmosphere as infrared radiation, which we feel as heat. If this radiation is absorbed by greenhouse gas molecules in the atmosphere, it is re-emitted by the molecules in all directions. The effect is to warm Earth’s surface and the lower atmosphere.
Earth’s natural greenhouse effect is critical to supporting life on the planet. Without the greenhouse effect, Earth’s average temperature would be about -18 degrees C (0 degrees F), as compared with the present average of 15 degrees C (59 degrees F). However, human activities have intensified the natural greenhouse effect by increasing greenhouse gas concentrations in the atmosphere. This is known as the enhanced or anthropogenic greenhouse effect, and it is causing global warming.
Greenhouse Gases
Greenhouse gases are radiatively active gases, meaning they can absorb and re-radiate infrared energy. They include all gases with three or more atoms. As shown in Table $1$, the top three greenhouse gases in Earth’s atmosphere are carbon dioxide (CO2), hydrofluorocarbons (HFCs), and methane (CH4). The table also lists the main human activities that contribute to greater concentrations of these and other greenhouse gases in the atmosphere.
Table $1$: Greenhouse gases ranked on the basis of their effect on the enhanced greenhouse effect and climate change. Starred items are natural greenhouse gases
Gas Production
60% $CO_2 \nonumber$ Carbon dioxide * Burning fossil fuels, deforestation
16% $HFCs \nonumber$ Hydroflurocarbons Aerosols, refrigerants
15% $CH_4 \nonumber$ Methane * Organic waste, cattle, fuel production
5% $N_2O \nonumber$ Nitrous oxide Fertilizer, soil, fuels
2% $PFCs \nonumber$ Perfluorcarbons Paint, textile and aluminum production
1% $SF_6 \nonumber$ Sulfur hexafluoride Electrical industry, rubber/MG production
1% $H_2O \nonumber$ Water vapour * Irrigation, evaporation, ice melting
The Importance of Carbon Dioxide
As Table $1$ shows, the major greenhouse gas in terms of its effect on climate is carbon dioxide. How much a greenhouse gas contributes to the enhanced greenhouse effect and global warming depends on how well it radiates heat and also on its abundance and persistence in the atmosphere. Although HFCs and methane are much more potent radiators of heat than carbon dioxide, carbon dioxide is much more abundant and lasts far longer in the atmosphere.
Sources and Sinks of Carbon Dioxide
Carbon dioxide enters Earth’s atmosphere largely through the burning of fossil fuels. Coal produces twice as much carbon dioxide as natural gas, with oil being intermediate. Currently, about half of the carbon dioxide released from the burning of fossil fuels remains in the atmosphere. The rest is taken up by plants (for photosynthesis) or dissolved by seawater. Deforestation reduces the total amount of carbon dioxide that is absorbed by plants, thereby increasing the amount of carbon dioxide that remains in the atmosphere.
Global Climate Change
Climate change, in general, refers to any change in average weather conditions on Earth that lasts for at least several decades. Short-term perturbations in climate, such as El Niño events, are not considered to be climate change. Earth’s climate has repeatedly changed in earlier epochs because of natural disturbances, such as massive volcanic eruptions and movements of continents. Over the past couple of centuries, the most important cause of climate change has been human actions, which are causing an enhanced greenhouse effect and global warming.
Evidence of Global Climate Change
Evidence for global warming comes from a variety of sources. Reasonably complete direct measurements of global surface temperatures are available beginning in the late 1800s. The graph in Figure $3$ shows how the annual mean temperature of Earth’s land and the ocean has deviated from baseline values (set at the mid-20th century). Relative to this baseline, earlier temperatures were generally cooler, and more recent temperatures have been consistently warmer. Overall, the graph shows a long-term warming trend.
Less direct but equally convincing evidence of recent global warming is the shrinking of glaciers and polar ice fields. As an example, Figure $4$ shows changes in the McCarty Glacier in Alaska. The bottom photo shows the large McCarty Glacier as it appeared in the summer of 1909. The top photo shows that the glacier had completely disappeared in the summer of 2004.
Projections of Future Climate Change
Unless energy policies change substantially, the world will continue to depend on fossil fuels and greenhouse gas emissions will remain high. Greenhouse gases already present in the atmosphere will persist for decades or even centuries, so they will continue to influence Earth’s climate long into the future. This will be the case, regardless of any steps that are taken to reduce greenhouse gas emissions going forward.
Depending on assumptions about future greenhouse gas emissions, climate models predict that the mean global surface temperature is likely to rise by 0.3-4.8 degrees C (0.5-8.6 degrees F) before the temperature levels off. Future climate change is also predicted to differ from region to region around the globe (Figure $5$). Warming has been and will continue to be greater over land than over the ocean because water has a greater capacity than land to absorb heat. Warming also has been and will continue to be greater in the Arctic than anywhere else on Earth for reasons that are not yet fully known.
Impacts of Future Global Warming
Future global warming is projected to have a wide range of impacts besides increases in temperature alone. They include continued retreat of glaciers and polar ice sheets, greater frequency, and severity of extreme weather events such as droughts, expansion of deserts, and changes in agricultural production. The increase in temperature will also cause a shift toward the poles in terrestrial plant and animal ranges as well as irreversible impacts on particular ecosystems, including tundra and coral reefs. Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems. Even the frequency of human violence, including violent crimes and warfare, is expected to increase as Earth’s temperature rises. Increased violence is likely because of the worsening of many underlying problems — greater water shortages and crop failures, worse poverty and hunger, higher rates of disease, and displacement of people due to weather disasters and habitat destruction.
One reason habitats will be destroyed is a rise in sea level. Sea level is already rising and will probably continue to rise for centuries because of an increase in the volume of water in the ocean. The water volume will increase because of the continued melting of ice sheets and glaciers as well as the natural expansion of water as it warms. The average sea level could rise by as much as 2.3 meters for each degree Celsius of temperature increase. The map in Figure $6$ shows coastal areas that would be underwater if the sea level were to rise by an average of 6 meters, which would occur if the mean global surface temperature rose by about 2.6 degrees C. Many of the world’s largest and most densely populated cities are located in low-lying coastal areas. If these areas were inundated by the ocean, it would cause the displacement of hundreds of millions of people and nearly unfathomable economic losses.
Potential Solutions to Climate Change
Most climate experts agree that future global warming should be limited to less than 2.0 degrees C. Otherwise, greater global warming would have catastrophic impacts and eventually exceed the capacity of natural and human systems to adapt. Controlling global warming requires first and foremost deep cuts in greenhouse gas emissions. International organizations and national governments have adopted resolutions and policies with this goal, the most important of which is promoting the use of renewable energy resources instead of fossil fuels. Other approaches to greenhouse gas control include increasing the amount of carbon removed from the atmosphere, for example, through the protection of forests and reforestation programs.
Review
1. Define the greenhouse effect.
2. Explain how the greenhouse effect works.
3. Why is the natural greenhouse effect necessary for life on Earth?
4. What are greenhouse gases?
5. How have human actions enhanced the natural greenhouse effect?
6. List the top three greenhouse gases in Earth’s atmosphere in terms of their effect on climate.
7. Why is carbon dioxide the most significant cause of the enhanced greenhouse effect? How does carbon dioxide enter the atmosphere?
8. Describe how atmospheric carbon dioxide concentrations have changed over the past millennium.
9. Define climate change.
10. Identify the main cause of recent and ongoing climate change on Earth.
11. Describe direct and indirect evidence for global warming.
12. Outline spatial variation in global warming.
13. How much is the global surface temperature predicted to increase in the future? Give the range of values based on climate models. What factor is mainly responsible for the variation in model projections? What is the greatest increase in temperature that most climate experts recommend avoiding the most catastrophic effects of global warming?
14. Identify several potential impacts of future global warming.
15. What is the single most important way to control global warming?
16. Which activity causes the most carbon dioxide to be emitted into the atmosphere?
1. The burning of coal
2. Exhalation by mammals
3. The burning of natural gas
4. The burning of oil
17. The sea level could rise by approximately how much if the global temperature increases by 4.8 degrees.?
1. 4.8 meters
2. 6 meters
3. 8 meters
4. 11 meters
Explore More
Watch these videos to learn about sustainability
Attributions
1. Earth on stove by Lesserland, CC0 via Wikimedia Commons
2. Greenhouse Effect by US EPA, public domain via CK-12 Foundation.
3. Global temperature anomaly by NASA Goddard Institute for Space Studies, public domain via Wikimedia Commons
4. Comparison photos of McCarty Glacier in Kenai Fjords National Park, Alaska. by USGS, public domain via Wikimedia Commons
5. GISS temperature 2000-09 by NASA image, public domain via Wikimedia Commons
6. 6m Sea Level Rise by NASA, public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.09%3A_Climate_Change.txt |
Case Study Conclusion: Farming for Balance
These organic tomatoes look delicious, but is it worth choosing them over less expensive conventionally-produced tomatoes? Camille, who you read about in the beginning of the chapter, asks herself questions like this whenever she goes food shopping. If organic agricultural practices are significantly better for the environment, she would like to buy organic food products at least some of the time. But are they better? And if so, how?
For foods to be officially labeled as organic in the U.S., they must meet detailed requirements from the USDA about how they are produced. One major aspect of organic agriculture is that most synthetic pesticides and fertilizers cannot be used on crops, unlike in conventional agriculture. As you have learned, pesticides can be harmful to human health if appropriate safety precautions are not taken. But besides direct exposure to humans, pesticides can also have negative effects on the broader ecosystem. For instance, some pesticides are toxic to bees. In fact, in addition to the varroa mite, which you learned about in this chapter, scientists think that pesticides are one of the factors that have contributed to the recent dramatic reduction of honey bee populations.
What effect does the loss of honey bees have on the ecosystem? As you have learned, bees and flowering plants have a mutualistic symbiotic relationship, where bees benefit by obtaining food from flowers, and the plants benefit from pollination by bees. Recall that the honey bee is the most important pollinator of crops, providing more than one billion dollars of pollinating services to agriculture in the U.S. By contributing to the loss of bee populations, pesticides can disturb the ecological balance between plants and their pollinators. When pollinators are reduced, so is food yield, which affects humans and other animals in the food chain that rely on foods such as fruits, vegetables, and nuts.
This is such a serious issue that in 2017, the U.S. Environmental Protection Agency (EPA) implemented a policy to reduce the risk of pesticides on bee populations, including banning the use of certain pesticides when bees are most likely to be present. Although organic agriculture prohibits the use of most synthetic pesticides, the use of some naturally derived pesticides is allowed. Some of these compounds may also be toxic to bees, other pollinators, and natural pest predators such as ladybugs. So before Camille decides to purchase an organic food product over a conventional one because of concerns about the negative effects of pesticides on the environment, she should research whether naturally derived pesticides have been used on that product, and if so, whether they have any negative effects.
Organic agriculture also generally prohibits the use of synthetic fertilizers. These fertilizers are concentrated sources of chemical elements such as nitrogen and phosphorus. Why would farmers want to add these elements to their crops? As you have learned, both nitrogen and phosphorus are nutrients that plants need to produce organic compounds. In fact, nitrogen is a limiting factor of plant growth in most terrestrial ecosystems. Therefore, adding these elements can increase plant growth and crop yield.
But there can be too much of a good thing. Recall that nitrogen and phosphorus are recycled through the biotic and abiotic factors in the ecosystem, as part of their respective biogeochemical cycles. Ecosystems have a delicate balance of complex interactions, and when one component changes significantly in an ecosystem, it usually causes a variety of other effects. In the case of synthetic fertilizers, the excess nutrients can run off into waterways from irrigation and rain. This is called nutrient pollution and it can seriously upset the balance in aquatic biomes. For instance, excess nitrogen and phosphorus in bodies of water such as rivers and lakes can cause the overgrowth of algae. The overgrowth of algae can clog waterways, block sunlight to deeper levels, and use up dissolved oxygen. In turn, this can kill other aquatic organisms such as fish. This process is called eutrophication, and you can see an example in Figure \(3\). Besides seriously disrupting the ecosystem, eutrophication can directly harm human health because some large overgrowths of algae (algal blooms) can produce potent toxins. These toxins become increasingly concentrated as they move up the food chain, and people can become ill or even die when they consume fish or shellfish from areas where there are algal blooms.
What about organic fertilizers? Organic fertilizers are generally better for the ecosystem than synthetic fertilizers. Organic fertilizers come directly from plant or animal sources, such as compost or manure. They tend to contain lower concentrations of nutrients such as nitrogen and phosphorus than synthetic fertilizers. Organic fertilizer often needs to be broken down by decomposers before many of its nutrients can be used by plants, which limits the speed at which these nutrients become introduced to the environment and allows them to be retained by the soil for longer periods of time, increasing soil quality. It also encourages biodiversity in the soil by providing food for a variety of decomposers, which, as you have learned, are critical to matter recycling in ecosystems. Organic fertilizer also helps maintain soil structure, makes the soil more resistant to erosion, and helps with water infiltration. In these ways, organic fertilizers can help keep nutrient and water cycling balanced in the ecosystem.
There are many other aspects of organic agriculture beyond the types of pesticides and fertilizers used. For instance, organic agriculture promotes biodiversity through techniques such as crop rotation, the planting of cover crops, and encouraging the growth of plants and maintenance of habitats that attract beneficial pest predators and pollinators. These techniques add nutrients to the soil, improve soil structure, reduce pest damage, and promote pollination while also providing benefits to many species in the ecosystem. In general, organic agriculture tends to promote more natural ecosystem interactions than conventional agriculture, but as you have seen from the example of organic pesticides, “organic” isn’t always necessarily better in all respects. Therefore, there isn’t one easy answer about whether Camille should choose organic over conventional foods, but by learning about balance in ecosystems and the impact of specific farming practices, she—and you—can make more informed decisions at the grocery store.
Chapter Summary
In this chapter, you learned about the science of ecology and the ecosystems and biomes on Earth. Specifically, you learned that:
• Ecology is the study of how living things interact with each other and with their environment. All organisms need energy and matter that must be obtained from the environment, so organisms are not closed systems. The environment of an organism includes biotic factors, which are the living aspects of the environment, and abiotic factors, which are the nonliving aspects of the environment.
• Ecologists generally organize the biological world in a nested hierarchy. Above the level of the individual organism, from the least to most inclusive, the levels of this hierarchy are the population, community, ecosystem, biome, and biosphere. The biosphere consists of every part of Earth where life exists, including the land, water, and air.
• Basic ideas in ecology include the ecosystem, niche, habitat, and competitive exclusion principle. An ecosystem consists of all the biotic and abiotic factors in an area and their interactions. A niche is the role of a species in its ecosystem, and a habitat is a natural environment in which a species lives and to which it is adapted.
• An ecosystem is a set of interacting components that form a complex whole, including its community of living things (biotic components) and nonliving environmental factors such as water and sunlight (abiotic components).
• Ecosystem processes move energy and matter through the biotic and abiotic components of an ecosystem, starting with primary production by producers such as plants. Through photosynthesis, plants capture energy from sunlight and make organic molecules from inorganic compounds.
• Nutrients, including carbon and nitrogen, are recycled among the biotic and abiotic components of ecosystems. In most terrestrial ecosystems, nitrogen is a limiting factor in plant growth and primary production. A limiting factor is any factor that constrains the population size of one or more of an ecosystem’s species.
• Ecosystems provide a variety of goods and services upon which our species depends. Ecosystem goods range from foods to recreational opportunities. Ecosystem services range from providing oxygen to the air to pollinating crops.
• A community is the biotic part of an ecosystem. It consists of all the populations of all the species that live in the ecosystem and their relationships with each other. There are three major types of community relationships: symbiosis, predation, and competition.
• Symbiosis is a close relationship between two organisms of different species in which at least one of the organisms benefits. Types of symbiosis include mutualism, commensalism, and parasitism.
• Mutualism is a symbiotic relationship in which individuals from both species benefit. An example is a relationship between clownfish and sea anemones.
• Commensalism is a symbiotic relationship in which an individual from one of the species benefits while the individual from the other species is unaffected. An example is a relationship between cattle egrets and grazing mammals, in which the egrets benefit and the mammals are unaffected.
• Parasitism is a symbiotic relationship in which an organism from one species, called the parasite, benefits, while the organism from the other species, called the host, is harmed. An example is a relationship between parasitic roundworms and human hosts.
• Predation is a community relationship in which an organism of one species, called the predator, consumes tissues of an organism in another species, called the prey. An example is snake predators that consume prey animals such as lizards.
• A predator-prey relationship tends to keep the populations of both species in the balance because each population is a limiting factor on the other population.
• Both predators and prey are likely to have adaptations to predation such as camouflage that evolves through natural selection.
• Interspecific competition is a community relationship in which organisms from different species rely on the same limiting resource.
• Interspecific competition is the basis of the competitive exclusion principle and may lead to the extinction of one species or greater specialization in both species.
• All organisms need energy. There are two basic types of organisms in terms of how they obtain energy: autotrophs and heterotrophs.
• Autotrophs (producers) use energy directly from the sun or from chemicals to produce organic molecules. Photoautotrophs such as plants use energy from sunlight to make organic compounds by photosynthesis. Chemoautotrophs such as certain bacteria use energy from chemicals to make organic compounds by chemosynthesis.
• Heterotrophs (consumers) obtain energy by consuming other organisms. Heterotrophs include all animals and fungi as well as many protists and bacteria. They can be classified on the basis of what they consume as carnivores, which eat animals; herbivores, which eat plants; omnivores, which eat both animals and plants; and decomposers, which consume organic wastes and dead organisms.
• The flow of energy in an ecosystem can be represented with a food chain or food web. A food chain represents a single pathway through which energy flows in an ecosystem. A food web represents multiple pathways through which energy flows in an ecosystem.
• Feeding positions in a food chain or food web are called trophic levels. The first trophic level is producers; the second trophic level is consumers that eat producers; the third and higher trophic levels are consumers that eat organisms from the trophic level below them. There are rarely more than four trophic levels. Most consumers actually feed at more than one trophic level.
• Only about 10 percent of the energy at one trophic level actually passes on to the next higher trophic level. The rest of the energy is used up at the lower trophic level or lost to the environment as heat or incompletely digested food. Generally, there are fewer organisms and less biomass at higher trophic levels.
• Water and the chemical elements that organisms need continuously cycle through ecosystems. Cycles of matter are called biogeochemical cycles because they include both biotic and abiotic components and processes. Components that hold matter for short periods of time are called exchange pools, and components that hold matter for long periods of time are called reservoirs.
• The water cycle involves a water changing state as it moves from one exchange pool or reservoir to another.
• Carbon cycles quickly between organisms and the environment through cellular respiration and photosynthesis. Carbon in organic compounds moves through food chains and webs and some is released back to the environment by decomposers. Human actions such as burning fossil fuels release huge amounts of additional carbon dioxide into the atmosphere.
• Most natural processes cycle carbon more slowly. Running water slowly dissolves carbon in rocks and carries it to the ocean, and the top layer of ocean water dissolves carbon dioxide out of the atmosphere. Carbon in ocean water may gradually settle to the bottom, and some of this carbon may eventually be changed to fossil fuels or sedimentary rocks that can store carbon for millions of years.
• Nitrogen gas in the atmosphere cannot be used by plants, but nitrogen-fixing bacteria in soil or on plant roots change nitrogen gas to nitrates, which plant roots can absorb. Decomposition of organic matter releases nitrogen as ammonium ions that plants can also absorb or that nitrifying bacteria in soil can change to nitrates for use by plants. Denitrifying bacteria release nitrogen gas from unused nitrates, and this nitrogen enters the atmosphere and completes the cycle.
• Since our species first evolved 200,000 years ago, a total of 108 billion human beings have lived on Earth, with 7.4 billion of them alive in 2017 and many more predicted in the future. People permanently live on a large scale on every continent except Antarctica. The rapid increase and spread of the human population raise concerns over our species’ continued existence.
• The scientific study of human populations is called demography. It includes the study of population size, distribution, and structure. It also includes the study of population dynamics, including population growth and changes in population structure.
• The age-sex structure of a population is the number of individuals of each sex and age group in the population, typically represented by a bar graph called a population pyramid. The shape of a population pyramid reflects past births, deaths, and migrations; and it may provide insights into political and socio-economic change.
• In 2015, the global human population had an average growth rate of 1.2 percent, but the growth rate varied among nations from less than zero to greater than 3 percent. A 3 percent growth rate is high for human populations, leading to a doubling time of just 23 years. Rates of population growth much higher than 3 or 4 percent or much lower than zero are generally caused by high rates of migration.
• Population growth rates may change over time. With exponential growth, the larger the population becomes, the faster it grows. With logistic growth, population growth slows and levels off as the population size reaches its carrying capacity (K), which is the largest population size that can be supported by available resources without harming the environment.
• For most of its existence, the human population grew very slowly. It started growing exponentially a few centuries ago. It is currently adding more than 80 million people per year.
• Since the time of Malthus, there has been a concern that the human population is growing so rapidly that it will soon outstrip food production and crash due to increased warfare, famine, and disease. Estimates place the human population carrying capacity at 7.7 billion people, which we are expected to reach by 2020, suggesting that there is an overpopulation problem.
• Many environmental problems are aggravated by the size of the human population, although over-consumption and waste by populations in wealthy nations may be worse problems than overpopulation per se. A widely accepted goal is zero population growth (ZPG), which occurs when birth rates match death rates because women average only enough children to replace themselves and their partners. This is called the replacement fertility rate, and it depends on the death rate.
• Climate change refers to any change in average weather conditions on Earth that lasts for at least several decades. The most important cause of recent and ongoing climate change is human actions, which cause an enhanced greenhouse effect and global warming.
• Direct evidence for global warming comes from measurements of global land and ocean temperatures, which show an overall warming trend since the late 1800s. Indirect evidence for global warming comes from observations of its effects, such as the shrinking of glaciers over the past century or so.
• Even if greenhouse gas emissions are reduced in the future, the climate will continue to get warmer because of the greenhouse gases already present in the atmosphere. Projections for future increases in the mean global temperature range from less than 1 to almost 5 degrees C, depending on future greenhouse gas emissions. Warming has been and will continue to be greater over land than the ocean and greater in the Arctic than anywhere else on Earth.
• Future global warming is projected to have a wide range of impacts, many of which are already occurring. Impacts are likely to include the continued retreat of glaciers, greater weather extremes, expansion of deserts, shifts toward the poles in natural habitats, loss of biodiversity, changes in food production, rising sea levels, displacements of human populations, and increases in human violence.
• Most climate experts agree that future global warming should be limited to less than 2.0 degrees C. Otherwise, human and natural systems may be unable to adapt. Controlling global warming requires first and foremost deep cuts in greenhouse gas emissions by phasing out fossil fuels and replacing them with energy resources that do not produce greenhouse gases.
Chapter Summary Review
1. Which of the following have abiotic components, in addition to biotic components? Choose all that apply. A. Community B. Ecosystem C. Population D. Biosphere E. Biome
2. True or False. Earth has several types of biospheres.
3. True or False. There is one species of plankton, and it lives near the surface of the water.
4. Is a niche the same thing as a habitat? Why or why not?
5. Which of the following are considered abiotic factors?
1. Primary producers
2. Soil
3. Fungi
4. All of the above
6. Are organisms and ecosystems open or closed systems? Explain your answer.
7. If there is an unusually cool week in the summer, does that mean there is a change in climate? Explain your answer.
8. The concept that two species cannot occupy the same niche in the same place for very long is called the ___________________ .
9. Where are aquatic organisms that are bioluminescent, meaning that they can produce light, most likely to be found? Explain your answer.
10. When an organism is said to be “marine,” what does this mean?
1. That it is an aquatic mammal
2. That it lives in running water
3. That it lives in the ocean
4. That it can swim
11. Can there be an ecosystem within an ecosystem? Why or why not?
12. Explain the importance of decomposers within ecosystems, and relate them to biogeochemical cycles.
13. True or False. A symbiotic relationship is a close relationship where both species benefit.
14. True or False. Most of the gross primary production that is not used by producers themselves are broken down by decomposers.
15. Is a mosquito that feeds on human blood a parasite or a predator? Explain your answer.
16. For each statement below, choose which type of bacteria best fits the description. Each type of bacteria is used only once. Types of bacteria: denitrifying; nitrifying; nitrogen-fixing
1. Takes a product of decomposition and turns it into a molecule that plants can use.
2. Takes nitrogen from the atmosphere and turns it into a molecule that plants can use.
3. Releases nitrogen gas back into the atmosphere.
17. When a prey population decreases, its predator population usually:
1. Decreases
2. Increases
3. Does not change
4. Becomes heterotrophic
18. What are two reasons why two locations that are at the same latitude might have different temperatures?
19. Species A and species B are living in the same location and eat the same prey. When members of species A encounter members of species B, they will attack them, often resulting in the death of members of species B. Answer the following questions about this scenario.
1. What kind of interspecific competition is this?
2. Over time, what are some possible outcomes of this interaction on species B?
20. True or False. Increased sublimation would cause more water vapor to be emitted into the atmosphere.
21. True or False. There is generally less biomass at higher trophic levels.
22. Identify a reservoir for carbon, and explain why it is considered a reservoir.
23. What does “eating low on the food chain” mean?
24. Fish are usually:
1. nekton
2. autotrophs
3. primary producers
4. aphotic
25. Give one example of how plants are involved in each of the biogeochemical cycles: the water cycle, carbon cycle, and nitrogen cycle.
26. What is the difference between dogs and cats in terms of how they obtain their energy and nutrients?
27. The U.S. population was approximately twice as high in 2010 as it was in 1950, based on census numbers. Use this information to answer the following questions.
1. Can you conclude that new births were solely responsible for the increase in population size? Explain your answer.
2. What other types of data would help you determine the reasons for this increase in population size?
28. True or False. Some countries have a negative growth rate.
29. What is the current population of the Earth?
1. Around 5 billion
2. Around 6 billion
3. Around 7 billion
4. Around 9 billion
30. Explain why population growth slows down as the population size approaches the carrying capacity when there is logistic growth.
31. True or False. Carbon monoxide is the most significant greenhouse gas.
Attributions
1. Organic Produce by Alanthebox, dedicated CC0 via Wikimedia Commons
2. Dead honey bees by Skinkie, dedicated CC0 via Wikimedia Commons
3. Algae and dead fish Dianchi Lake, Skinkie CC0 via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/24%3A_Ecology/24.10%3A_Case_Study_Conclusion%3A__Organic_and_Chapter_Summary.txt |
The following pages examine some of the principles of chemistry upon which an understanding of modern biology depends.
01: The Chemical Basis of Life
Mixtures are heterogeneous forms of matter. Mixtures are composed of variable proportions of molecules and atoms.
The composition of a mixture is variable with each components retaining its characteristic properties. Its components are easily separated. Examples of Mixtures: soil, ocean water and other solutions, air, the cytosol of a cell
n contrast, compounds are homogeneous forms of matter. Their constituent elements (atoms and/or ions) are always present in fixed proportions . Properties of compounds include
• The relative proportions of the elements in a compound are fixed.
• The components of a compound do not retain their individual properties. Both sodium and chlorine are poisonous; their compound, table salt (NaCl) is absolutely essential to life.
• It takes large inputs of energy to separate the components of a compound.
Examples of Compounds
• water (H2O)
• table salt (NaCl)
• sucrose (table sugar, C12H22O11)
Separating the Components of a Mixture
Most laboratory work in biology requires the use of techniques to separate the components of mixtures. This is done by exploiting some property that distinguishes the components, such as their relative
• size
• density
• solubility
• electrical charge
Dialysis
Dialysis is the separation of small solute molecules or ions (e.g., glucose, Na+, Cl-) from macromolecules (e.g., starch) by virtue of their differing rates of diffusion through a differentially permeable membrane.
As shown in Figure \(1\), the cellophane used to construct a bag is perforated with tiny pores that permit ions and small molecules to pass through, but exclude molecules with molecular weights greater than about 12,000. If a cellophane bag is mixed with a mixture of sugar and starch and place it in salt water, the sugar molecules (teal dots) will diffuse out into the water until equilibrium is reached; i.e., until their concentrations are equal on both sides of the membrane. Similarly, the salt (red dots) will diffuse into the bag. However, because of their large size, all the starch (big blue disks) will be retained within the tubing.
Chromatography
Chromatography is the term used for several techniques for separating the components of a mixture. The different types of chromatography techniques used are: paper chromatography, exclusion chromatography, and affinity chromatography.
Paper chromatography technique provides an easy way to separate the components of a mixture. A drop of mixture is placed in one corner of a square of absorbent paper.
• One edge of the paper is immersed in a solvent. (a)
• The solvent migrates up the sheet by capillary attraction.
• As it does so, the substances in the drop are carried along at different rates. (b)
• Each compound migrates at a rate that reflects
• the size of its molecule and
• its solubility in the solvent.
• After a second run at right angles to the first (often using a different solvent), the various substances will be spread out at distinct spots across the sheet, forming a chromatogram. (c)
• The identity of each spot can be determined by comparing its position with the position occupied by known substances under the same conditions.
• In many cases, a fragment of the paper can be cut away from the sheet and chemical analysis run on the tiny amount of substance in it.
Autoradiography
If the mixture contains molecules that have been labeled with a radioactive isotope, these can be located by placing the chromatogram next to a sheet of X-ray film. The location of dark spots on the developed film (because of radiation emitted by the isotope) can be correlated with the position of the substances on the chromatogram.
The above figures (courtesy of Dr. James A. Bassham) show autoradiograms of the type that were essential in working out the dark reactions of photosynthesis. The dark spots show the radioactive compounds produced after 10 secs (left) and 2 minutes (right) of photosynthesis by the green alga Scenedesmus. The alga was supplied with carbon dioxide labeled with 14C, a radioactive isotope of carbon.
• At 10 seconds, most of the radioactivity is found in 3-phosphoglyceric acid ("P-Glyceric").
• At 2 minutes, phosphorylated 6-carbon sugars (glucose and fructose) have been synthesized as well as a number of amino acids.
The small rectangle and circle (lower right-hand corners) mark the spots where the cell extract was applied.
Exclusion chromatography
One of the most common problems in biochemical research is to separate the many components — usually macromolecules — in cell extracts and the like. Methods for separating the components of a mixture exploit such differences as size, electrical charge, and solubility in different solvents. of the molecules in it. One example: Electrophoresis which separates such macromolecules as proteins and DNA by their charge (and sometimes size as well).
Exclusion chromatography separates molecules on the basis of size. A column is filled with semi-solid beads of a polymeric gel that will admit ions and small molecules (blue) into their interior but not large ones (shown in red). When a mixture of molecules and ions dissolved in a solvent is applied to the top of the column, the smaller molecules (and ions) are distributed through a larger volume of solvent than is available to the large molecules. Consequently, the large molecules move more rapidly through the column, and in this way the mixture can be separated (fractionated) into its components. The porosity of the gel can be adjusted to exclude all molecules above a certain size. Sephadex and sepharose are trade names for gels that are available commercially in a broad range of porosities.
Affinity chromatography
The goal of affinity chromatography is to separate all the molecules of a particular specificity from the whole gamut of molecules in a mixture such as a blood serum. For example, the antibodies in a serum sample specific for a particular antigenic determinant can be isolated by the use of affinity chromatography.
The following steps are performed to achieve that:
Step 1
An immunoadsorbent is prepared. This consists of a solid matrix to which the antigen (shown in blue) has been coupled (usually covalently). Agarose, sephadex, derivatives of cellulose, or other polymers can be used as the matrix.
Step 2
The serum is passed over the immunoadsorbent. As long as the capacity of the column is not exceeded, those antibodies in the mixture specific for the antigen (shown in red) will bind (noncovalently) and be retained. Antibodies of other specificities (green) and other serum proteins (yellow) will pass through unimpeded.
Step 3
Elution. A reagent is passed into the column to release the antibodies from the immunoadsorbent. Buffers containing a high concentration of salts and/or low pH are often used to disrupt the noncovalent interactions between antibodies and antigen. A denaturing agent, such as 8 M urea, will also break the interaction by altering the configuration of the antigen-binding site of the antibody molecule.
Another, gentler, approach is to elute with a soluble form of the antigen. These compete with the immunoadsorbent for the antigen-binding sites of the antibodies and release the antibodies to the fluid phase.
Step 4
Dialysis. The eluate is then dialyzed against, for example, buffered saline in order to remove the reagent used for elution.
Electrophoresis
Electrophoresis uses a direct electric current to separate the components of a mixture by the differing electrical charge.
Example of Electrophoresis
Proteins in blood serum can be separated by electrophoresis.
• A drop of serum is applied in a band to a thin sheet of supporting material, like paper, that has been soaked in a slightly-alkaline salt solution.
• At pH 8.6, which is commonly used, all the proteins are negatively charged, but some more strongly than others.
• A direct current can flow through the paper because of the conductivity of the buffer with which it is moistened.
• As the current flows, the serum proteins move toward the positive electrode.
• The stronger the negative charge on a protein, the faster it migrates.
• After a time (typically 20 min), the current is turned off and the proteins stained to make them visible (most are otherwise colorless).
• The separated proteins appear as distinct bands.
• The most prominent of these and the one that moves closest to the positive electrode is serum albumin.
• The other proteins are the various serum globulins.
Pure Substances
Some of the pure substances isolated from mixtures cannot be further broken down. Oxygen (O2) is an example. It is one of the elements; the fundamental building blocks of matter. Most pure substances are compounds. Table salt, sodium chloride (NaCl), is an example; water (H2O) is another. If we pass an electrical current through molten NaCl, two new substances will be formed:
• sodium, a shiny metal so reactive that it must be stored out of contact with the air
• chlorine, a yellowish poisonous gas.
In this operation, a compound has been decomposed into its constitutive elements. Note the differences between separating the components of a mixture and those of a compound. The decomposition of NaCl required a large input of energy since the strong ionic bonds holding the Na and Cl atoms together must be broken. The ratio of the weights of the two products are always 23 parts of sodium to 35.5 parts of chlorine. This reflects the invariance of the ratio (1:1 in this case) of the number of atoms in a compound and the relative weights (23:35.5) of the atoms in table salt. The properties of the components of the compound are not the same as those of the compound itself. Both sodium and chlorine are hazardous to life; their compound, sodium chloride, is a vital ingredient of all animal diets. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/01%3A_The_Chemical_Basis_of_Life/1.01%3A_Mixtures_and_Compounds.txt |
Elements
Elements consist of only one kind of atom and cannot be decomposed into simpler substances. Our planet is made up of some 90 elements. (Tiny amounts — sometimes only a few atoms — of additional elements have been made in nuclear physics laboratories, but they play no role in our story). Of these 90, only 25 or so are used to build living things. The table shows the 11 most prevalent elements in the lithosphere (the earth's crust) and in the human body.
Living matter
• uses only a fraction of the elements available to it
• but, as the table shows, the relative proportions of those it does acquire from its surroundings are quite different from the proportions in the environment
So,
• the composition of living things is not simply a reflection of the elements available to them
• For example, hydrogen, carbon, and nitrogen together represent less than 1% of the atoms found in the earth's crust but some 74% of the atoms in living matter.
• one of the properties of life is to take up certain elements that are scarce in the nonliving world and concentrate them within living cells.
Some sea animals accumulate elements like vanadium and iodine within their cells to concentrations a thousand or more times as great as in the surrounding sea water. It has even been proposed that uranium be "mined" from the sea by extracting it from certain algae that can take up uranium from sea water and concentrate it within their cells.
There is still some uncertainty about the exact number of elements required by living things. Some elements, e.g., aluminum, are found in tiny amounts in living tissue, but whether they are playing an essential role or are simply an accidental acquisition (aluminum probably is) is sometimes difficult to determine.
Atoms
Each element is made up of one kind of atom. We can define an atom as the smallest part of an element that can enter into combination with other elements.
Structure of the atom
Each atom consists of a small, dense, positively-charged nucleus surrounded by much lighter, negatively-charged electrons. The nucleus of the simplest atom, the hydrogen atom (H), consists of a single positively-charged proton. Because of its single proton, the atom of hydrogen is assigned an atomic number of 1 and a single electron. The charge of the electron is the same magnitude as that of the proton, so the atom as a whole is electrically neutral. Its proton accounts for almost all the weight of the atom.
The nucleus of the atom of the element helium (He) has two protons (hence helium has an atomic number of 2) and two neutrons. Neutrons have the same weight as protons but no electrical charge. The helium atom has two electrons so that, once again, the atom as a whole is neutral.
The structure of each of the other kinds of atoms follows the same plan. From Lithium (At. No. = 3) to uranium (At. No. = 92), the atoms of each element can be listed in order of increasing atomic number. There are no gaps in the list. Each element has a unique atomic number and its atoms have one more proton and one more electron than the atoms of the element that precedes it in the list.
Electrons
Atomic Number Element Energy Levels or "shells"
K L M N O
1 Hydrogen (H) 1
2 Helium (He) 2
3 Lithium (Li) 2 1
4 Beryllium (Be) 2 2
5 Boron (B) 2 3
6 Carbon (C) 2 4
7 Nitrogen (N) 2 5
8 Oxygen (O) 2 6
9 Fluorine (F) 2 7
10 Neon (Ne) 2 8
11 Sodium (Na) 2 8 1
12 Magnesium (Mg) 2 8 2
13 Aluminum (Al) 2 8 3
14 Silicon (Si) 2 8 4
15 Phosphorus (P) 2 8 5
16 Sulfur (S) 2 8 6
17 Chlorine (Cl) 2 8 7
18 Argon (Ar) 2 8 8
19 Potassium (K) 2 8 8 1
20 Calcium (Ca) 2 8 8 2
21 Scandium (Sc) 2 8 9 2
22 Titanium (Ti) 2 8 10 2
23 Vanadium (V) 2 8 11 2
24 Chromium (Cr) 2 8 13 1
25 Manganese (Mn) 2 8 13 2
26 Iron (Fe) 2 8 14 2
27 Cobalt (Co) 2 8 15 2
28 Nickel (Ni) 2 8 16 2
29 Copper (Cu) 2 8 18 1
30 Zinc (Zn) 2 8 18 2
31 Gallium (Ga) 2 8 18 3
32 Germanium (Ge) 2 8 18 4
33 Arsenic (As) 2 8 18 5
34 Selenium (Se) 2 8 18 6
35 Bromine (Br) 2 8 18 7
36 Krypton (Kr) 2 8 18 8
42 Molybdenum (Mo) 2 8 18 13 1
48 Cadmium (Cd) 2 8 18 18 2
50 Tin (Sn) 2 8 18 18 4
53 Iodine (I) 2 8 18 18 7
Electrons are confined to relatively discrete regions around the nucleus. The two electrons of helium, for example, are confined to a spherical zone surrounding the nucleus called the K shell or K energy level.
Lithium (At. No. = 3) has three electrons, two in the K shell and one located farther from the nucleus in the L shell. Being farther away from the opposite (+) charges of the nucleus, this third electron is held less tightly.
Each of the following elements, in order of increasing atomic number, adds one more electron to the L shell until we reach neon (At. No. = 10) which has eight electrons in the L shell.
Sodium places its eleventh electron in a still higher energy level, the M shell.
From sodium to argon, this shell is gradually filled with electrons until, once again, a maximum of eight is reached.
Note that after the K shell with its maximum of two electrons, the maximum number of electrons in any other outermost shell is eight.
As we shall see, the chemical properties of each element are strongly influenced by the number of electrons in its outermost energy level (shell).
This table shows the electronic structure of the atoms of elements 1 – 36 with those that have been demonstrated to be used by living things shown in red. Four elements of still higher atomic numbers that have been shown to be used by living things are also included.
The electronic structure of an atom plays the major role in its chemistry.
The pattern of electrons in an atom — especially those in the outermost shell — determines
• the valence of the atom; that is, the ratios with which it interacts with other atoms, and to a large degree,
• the electronegativity of the atom; that is, the strength with which it attracts other electrons.
Elements with the same number of electrons in their outermost shell show similar chemical properties.
Example 1: Fluorine, chlorine, bromine, and iodine each have 7 electrons in their outermost shell. These so-called halogens are also quite similar in their chemical behavior. When dissolved in water, for example, they all produce germicidal solutions.
Example 2: Those elements with 1, 2, or 3 electrons in their outermost shell are the metals.
Example 3: Those elements with 4, 5, 6, or 7 in their outermost shell are the nonmetals.
Example 4: Helium (with its 2), neon, argon, and krypton (each with 8) have "filled" their outermost shells. They are the so-called inert or "noble" gases. They have no chemistry at all. Under normal conditions they do not interact with other atoms. So, it is the number and arrangement of the electrons in the atoms of an element that establish the chemical behavior of that element.
This is how it works:
The atoms of an element interact with other atoms in such ways and ratios that they can "fill" their outermost shell with 8 electrons (2 for hydrogen). They may do this by
• acquiring more electrons from another atom
• losing electrons to another atom
• sharing electrons with another atom
The number of electrons that an atom must acquire, or lose, or share to reach a stable configuration of 8 (2 for hydrogen) is called its valence.
Hydrogen, lithium, sodium, and potassium atoms all have a single electron in their outermost shell. Fluorine, chlorine, bromine, and iodine atoms all have 7. Any atom of the first group will interact with a single atom of any of the second group forming, HCl, NaCl, KI, etc. The result of all of these interactions is a pair of atoms each with an outermost shell like that of one of the inert gases: 2 for hydrogen, 8 for the others.
The elements with 2 electrons in their outermost shell interact with chlorine and the other halogens to form, e.g., BeCl2, MgCl2, CaCl2. Again, the result is a pair of atoms each with a stable octet of electrons in its outermost shell.
The elements with 3 electrons in their outermost shell will interact with chlorine in a ratio of 1:3, forming BCl3, AlCl3.
Carbon atoms, with their 4 electrons in the L shell interact with chlorine to form CCl4.
Nitrogen, with its 5 outermost electrons, interacts with hydrogen atoms in a ratio of 1:3, forming ammonia (NH3).
Oxygen and sulfur, with their 6 outermost electrons react with hydrogen to form water (H2O) and hydrogen sulfide (H2S).
What determines whether a pair of atoms swap or share electrons?
The answer is their relative electronegativities. If two atoms differ greatly in their affinity for electrons; that is, in their electronegativity, then the strongly electronegative atom will take the electron away from the weakly electronegative one.
Example: Na (weakly electronegative) gives up its single electron to an atom of chlorine (strongly electronegative) to form NaCl. The sodium atom now has only 10 electrons but still 11 protons so there is a net positive charge of one on the atom. Similarly, chlorine now has one more electron than proton so its now has a net negative charge of 1. Electrically charged atoms are called ions. The mutual attraction of opposite electrical charges holds the ions together by ionic bonds.
Example: Carbon and hydrogen are both only weakly electronegative so neither can remove electrons from the other. Instead they achieve a stable configuration by sharing their outermost electrons forming covalent bonds of CH4.
Isotopes
The number of protons in the nucleus of its atoms, which is its atomic number, defines each element. However, the nuclei of a given element may have varying numbers of neutrons. Because neutrons have weight (about the same as that of protons), such atoms differ in the atomic weight.
Atoms of the same element that differ in their atomic weight are called isotopes.
Atomic weights are expressed in terms of a standard atom: the isotope of carbon that has 6 protons and 6 neutrons in its nucleus. This atom is designated carbon-12 or 12C. It is arbitrarily assigned an atomic weight of 12 daltons (named after John Dalton, the pioneer in the study of atomic weights). Thus a dalton is 1/12 the weight of an atom of 12C. Both protons and neutrons have weights very close to 1 dalton each. Carbon-12 is the most common isotope of carbon. Carbon-13 (13C) with 6 protons and 7 neutrons, and carbon-14 (14C) with 6 protons and 8 neutrons are found in much smaller quantities.
Isotopes as "tracers"
One can prepare, for example, a carbon compound used by living things that has many of its normal 12C atoms replaced by 14C atoms. Carbon-14 happens to be radioactive. By tracing the fate of radioactivity within the organism, one can learn the normal pathway of this carbon compound in that organism. Thus 14C serves as an isotopic "label" or "tracer".
The basis of this technique is that the weight of the nucleus of an atom has little or no effect on the chemical properties of that atom. The chemistry of an element and the atoms of which it is made — whatever their atomic weight — is a function of the atomic number of that element. As long as the atom had 6 protons, it is an atom of carbon irrespective of the number of neutrons. Thus while 6 protons and 8 neutrons produce an isotope of carbon, 14C, 7 protons and 7 neutrons produce a totally-different element, nitrogen-14.
Contributors and Attributions
John W. Kimball. This content is distributed under a Creative Commons Attribution 3.0 Unported (CC BY 3.0) license and made possible by funding from The Saylor Foundation. | textbooks/bio/Introductory_and_General_Biology/Biology_(Kimball)/01%3A_The_Chemical_Basis_of_Life/1.02%3A_Elements_and_Atoms.txt |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.