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ATP is critical for muscle contractions because it breaks the myosin-actin cross-bridge, freeing the myosin for the next contraction.
Learning Objectives
• Discuss how energy is consumed during movement
Key Points
• ATP prepares myosin for binding with actin by moving it to a higher- energy state and a “cocked” position.
• Once the myosin forms a cross-bridge with actin, the Pi disassociates and the myosin undergoes the power stroke, reaching a lower energy state when the sarcomere shortens.
• ATP must bind to myosin to break the cross-bridge and enable the myosin to rebind to actin at the next muscle contraction.
Key Terms
• M-line: the disc in the middle of the sarcomere, inside the H-zone
• troponin: a complex of three regulatory proteins that is integral to muscle contraction in skeletal and cardiac muscle, or any member of this complex
• ATPase: a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion, releasing energy that is often harnessed to drive other chemical reactions
ATP and Muscle Contraction
Muscles contract in a repeated pattern of binding and releasing between the two thin and thick strands of the sarcomere. ATP is critical to prepare myosin for binding and to “recharge” the myosin.
The Cross-Bridge Muscle Contraction Cycle
ATP first binds to myosin, moving it to a high-energy state. The ATP is hydrolyzed into ADP and inorganic phosphate (Pi) by the enzyme ATPase. The energy released during ATP hydrolysis changes the angle of the myosin head into a “cocked” position, ready to bind to actin if the sites are available. ADP and Pi remain attached; myosin is in its high energy configuration.
The muscle contraction cycle is triggered by calcium ions binding to the protein complex troponin, exposing the active-binding sites on the actin. As soon as the actin-binding sites are uncovered, the high-energy myosin head bridges the gap, forming a cross-bridge. Once myosin binds to the actin, the Pi is released, and the myosin undergoes a conformational change to a lower energy state. As myosin expends the energy, it moves through the “power stroke,” pulling the actin filament toward the M-line. When the actin is pulled approximately 10 nm toward the M-line, the sarcomere shortens and the muscle contracts. At the end of the power stroke, the myosin is in a low-energy position.
After the power stroke, ADP is released, but the cross-bridge formed is still in place. ATP then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site. ATP can then attach to myosin, which allows the cross-bridge cycle to start again; further muscle contraction can occur. Therefore, without ATP, muscles would remain in their contracted state, rather than their relaxed state.
38.18: Muscle Contraction and Locomotion - Regulatory Proteins
Tropomyosin and troponin prevent myosin from binding to actin while the muscle is in a resting state.
Learning Objectives
• Describe how calcium, tropomyosin, and the troponin complex regulate the binding of actin by myosin
Key Points
• Tropomyosin covers the actin binding sites, preventing myosin from forming cross-bridges while in a resting state.
• When calcium binds to troponin, the troponin changes shape, removing tropomyosin from the binding sites.
• The sarcoplasmic reticulum stores calcium ions, which it releases when a muscle cell is stimulated; the calcium ions then enable the cross-bridge muscle contraction cycle.
Key Terms
• tropomyosin: any of a family of muscle proteins that regulate the interaction of actin and myosin
• acetylcholine: a neurotransmitter in humans and other animals, which is an ester of acetic acid and choline
• sarcoplasmic reticulum: s smooth endoplasmic reticulum found in smooth and striated muscle; it contains large stores of calcium, which it sequesters and then releases when the muscle cell is stimulated
Regulatory Proteins
The binding of the myosin heads to the muscle actin is a highly-regulated process. When a muscle is in a resting state, actin and myosin are separated. To keep actin from binding to the active site on myosin, regulatory proteins block the molecular binding sites. Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation, which prevents contraction in a muscle without nervous input. The protein complex troponin binds to tropomyosin, helping to position it on the actin molecule.
Regulation of Troponin and Tropomyosin
To enable muscle contraction, tropomyosin must change conformation and uncover the myosin-binding site on an actin molecule, thereby allowing cross-bridge formation. Troponin, which regulates the tropomyosin, is activated by calcium, which is kept at extremely low concentrations in the sarcoplasm. If present, calcium ions bind to troponin, causing conformational changes in troponin that allow tropomyosin to move away from the myosin-binding sites on actin. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction. Cross-bridge cycling continues until Ca2+ ions and ATP are no longer available; tropomyosin again covers the binding sites on actin.
Calcium-Induced Calcium Release
The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm. Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the muscle cell to relax. During stimulation of the muscle cell, the motor neuron releases the neurotransmitter acetylcholine, which then binds to a post-synaptic nicotinic acetylcholine receptor.
A change in the receptor conformation causes an action potential, activating voltage-gated L-type calcium channels, which are present in the plasma membrane. The inward flow of calcium from the L-type calcium channels activates ryanodine receptors to release calcium ions from the sarcoplasmic reticulum. This mechanism is called calcium-induced calcium release (CICR). It is not understood whether the physical opening of the L-type calcium channels or the presence of calcium causes the ryanodine receptors to open. The outflow of calcium allows the myosin heads access to the actin cross-bridge binding sites, permitting muscle contraction. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.17%3A_Muscle_Contraction_and_Locomotion_-_ATP_and_Muscle_Contraction.txt |
Excitation–contraction coupling is the connection between the electrical action potential and the mechanical muscle contraction.
Learning Objectives
• Explain the process of excitation-contraction coupling and the role of neurotransmitters
Key Points
• A motor neuron connects to a muscle at the neuromuscular junction, where a synaptic terminal forms a synaptic cleft with a motor-end plate.
• The neurotransmitter acetylcholine diffuses across the synaptic cleft, causing the depolarization of the sarcolemma.
• The depolarization of the sarcolemma stimulates the sarcoplasmic reticulum to release Ca2+, which causes the muscle to contract.
Key Terms
• motor-end plate: postjunctional folds which increase the surface area of the membrane (and acetylcholine receptors) exposed to the synaptic cleft
• sarcolemma: a thin cell membrane that surrounds a striated muscle fiber
• acetylcholinesterase: an enzyme that catalyzes the hydrolysis of the neurotransmitter acetylcholine into choline and acetic acid
Excitation–Contraction Coupling
Excitation–contraction coupling is the physiological process of converting an electrical stimulus to a mechanical response. It is the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction.
Communication between Nerves and Muscles
A neural signal is the electrical trigger for calcium release from the sarcoplasmic reticulum into the sarcoplasm. Each skeletal muscle fiber is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. Electrical signals called action potentials travel along the neuron’s axon, which branches through the muscle, connecting to individual muscle fibers at a neuromuscular junction. The area of the sarcolemma on the muscle fiber that interacts with the neuron is called the motor-end plate. The end of the neuron’s axon is called the synaptic terminal; it does not actually contact the motor-end plate. A small space called the synaptic cleft separates the synaptic terminal from the motor-end plate.
Because neuron axons do not directly contact the motor-end plate, communication occurs between nerves and muscles through neurotransmitters. Neuron action potentials cause the release of neurotransmitters from the synaptic terminal into the synaptic cleft, where they can then diffuse across the synaptic cleft and bind to a receptor molecule on the motor end plate. The motor end plate possesses junctional folds: folds in the sarcolemma that create a large surface area for the neurotransmitter to bind to receptors. The receptors are sodium channels that open to allow the passage of Na+ into the cell when they receive neurotransmitter signal.
Depolarization in the Sarcolemma
Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate. Neurotransmitter release occurs when an action potential travels down the motor neuron’s axon, resulting in altered permeability of the synaptic terminal membrane and an influx of calcium. The Ca2+ ions allow synaptic vesicles to move to and bind with the presynaptic membrane (on the neuron) and release neurotransmitter from the vesicles into the synaptic cleft. Once released by the synaptic terminal, ACh diffuses across the synaptic cleft to the motor end plate, where it binds with ACh receptors.
As a neurotransmitter binds, these ion channels open, and Na+ ions cross the membrane into the muscle cell. This reduces the voltage difference between the inside and outside of the cell, which is called depolarization. As ACh binds at the motor end plate, this depolarization is called an end-plate potential. The depolarization then spreads along the sarcolemma and down the T tubules, creating an action potential. The action potential triggers the sarcoplasmic reticulum to release of Ca2+, which activate troponin and stimulate muscle contraction.
ACh is broken down by the enzyme acetylcholinesterase (AChE) into acetyl and choline. AChE resides in the synaptic cleft, breaking down ACh so that it does not remain bound to ACh receptors, which would cause unwanted extended muscle contraction. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.19%3A_Muscle_Contraction_and_Locomotion_-__ExcitationContraction_Coupling.txt |
Muscle tension is influenced by the number of cross-bridges that can be formed.
Learning Objectives
• Describe the factors that control muscle tension
Key Points
• The more cross-bridges that are formed, the more tension in the muscle.
• The amount of tension produced depends on the cross-sectional area of the muscle fiber and the frequency of neural stimulation.
• Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere; less tension is produced when the sarcomere is stretched.
• If more motor neurons are stimulated, more myofibers contract, and there is greater tension in the muscle.
Key Terms
• tension: condition of being held in a state between two or more forces, which are acting in opposition to each other
Control of Muscle Tension
Neural control initiates the formation of actin – myosin cross-bridges, leading to the sarcomere shortening involved in muscle contraction. These contractions extend from the muscle fiber through connective tissue to pull on bones, causing skeletal movement. The pull exerted by a muscle is called tension. The amount of force created by this tension can vary, which enables the same muscles to move very light objects and very heavy objects. In individual muscle fibers, the amount of tension produced depends primarily on the amount of cross-bridges formed, which is influenced by the cross-sectional area of the muscle fiber and the frequency of neural stimulation.
Cross-bridges and Tension
The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle fiber can produce. Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin. If more cross-bridges are formed, more myosin will pull on actin and more tension will be produced.
Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere. If a sarcomere at rest is stretched past an ideal resting length, thick and thin filaments do not overlap to the greatest degree so fewer cross-bridges can form. This results in fewer myosin heads pulling on actin and less muscle tension. As a sarcomere shortens, the zone of overlap reduces as the thin filaments reach the H zone, which is composed of myosin tails. Because myosin heads form cross-bridges, actin will not bind to myosin in this zone, reducing the tension produced by the myofiber. If the sarcomere is shortened even more, thin filaments begin to overlap with each other, reducing cross-bridge formation even further, and producing even less tension. Conversely, if the sarcomere is stretched to the point at which thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced. This amount of stretching does not usually occur because accessory proteins, internal sensory nerves, and connective tissue oppose extreme stretching.
The primary variable determining force production is the number of myofibers (long muscle cells) within the muscle that receive an action potential from the neuron that controls that fiber. When using the biceps to pick up a pencil, for example, the motor cortex of the brain only signals a few neurons of the biceps so only a few myofibers respond. In vertebrates, each myofiber responds fully if stimulated. On the other hand, when picking up a piano, the motor cortex signals all of the neurons in the biceps so that every myofiber participates. This is close to the maximum force the muscle can produce. As mentioned above, increasing the frequency of action potentials (the number of signals per second) can increase the force a bit more because the tropomyosin is flooded with calcium.
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Respiratory processes that help organisms exchange O2 and CO2 range from simple direct diffusion to complex respiratory systems.
Learning Objectives
• Review an overview of the functions of the respiratory system
Key Points
• Respiration ensures that cells, tissues, and major organs of the body receive an adequate supply of oxygen and that the carbon dioxide, a waste product, is efficiently removed; the exchange of oxygen and carbon dioxide occurs via diffusion across cell membranes.
• The mechanisms, processes, and structures used for respiration are dictated by the type, size, and complexity of the organism.
• Direct diffusion of gases through the outer membranes can be used by organisms such as flatworms as a means of respiration due to their small size and simplicity.
Key Terms
• deoxygenated: having removed the oxygen atoms from a molecule
• diffusion: The passive movement of a solute across a permeable membrane
• aerobic: living or occurring only in the presence of oxygen
Introduction
Breathing is an involuntary event. How often a breath is taken and how much air is inhaled or exhaled are tightly regulated by the respiratory center in the brain. Under normal breathing conditions, humans will breathe approximately 15 times per minute on average. A respiratory cycle consists of an inhalation and an exhalation: with every normal inhalation, oxygenated air fills the lungs, while with every exhalation, deoxygenated air rushes back out. The oxygenated air crosses the lung tissue, enters the bloodstream, and travels to organs and tissues. Oxygen (O2) enters the cells where it is used for metabolic reactions that produce ATP, a high-energy compound. At the same time, these reactions release carbon dioxide (CO2) as a by-product. CO2 is toxic and must be eliminated; thus, CO2 exits the cells, enters the bloodstream, travels back to the lungs, and is expired out of the body during exhalation.
The primary function of the respiratory system is to deliver oxygen to the cells of the body’s tissues and remove carbon dioxide. The main structures of the human respiratory system are the nasal cavity, the trachea, and the lungs. All aerobic organisms require oxygen to carry out their metabolic functions.
Along the evolutionary tree, different organisms have devised different means of obtaining oxygen from the surrounding atmosphere. The environment in which the animal lives greatly determines how an animal respires. The complexity of the respiratory system correlates with the size of the organism. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops. In unicellular (single-celled) organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell. Diffusion is a slow, passive transport process. In order to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake must match the rate of diffusion across the membrane. In other words, if the cell were very large or thick, diffusion would not be able to provide oxygen quickly enough to the inside of the cell. Therefore, dependence on diffusion as a means of obtaining oxygen and removing carbon dioxide remains feasible only for small organisms or those with highly-flattened bodies, such as flatworms (platyhelminthes). Larger organisms have had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages, accompanied by a complex circulatory system to transport oxygen throughout their entire body.
Direct Diffusion
For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist so that gases diffuse quickly via direct diffusion. Flatworms are small, literally flat worms, which ‘breathe’ through diffusion across the outer membrane. The flat shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen. If the flatworm had a cylindrical body, then the cells in the center would not be able to get oxygen. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.01%3A_Systems_of_Gas_Exchange_-_The_Respiratory_System_and_Direct_Diffusion.txt |
Respiration can occur using a variety of respiratory organs in different animals, including skin, gills, and tracheal systems.
Learning Objectives
• Describe how the skin, gills, and tracheal system are used in the process of respiration
Key Points
• Some animals, such as amphibians and earthworms, can use their skin (integument) to exchange gases between the external environment and the circulatory system due to the network of capillaries that lie below the skin.
• Fish and other aquatic organisms use gills to take up oxygen dissolved in the water and diffuse carbon dioxide out of the bloodstream.
• Some insects utilize a tracheal system that transports oxygen from the external environment through openings called spiracles.
Key Terms
• coelom: a fluid-filled cavity within the body of an animal; the digestive system is suspended within the cavity, which is lined by a tissue called the peritoneum
• gill: a breathing organ of fish and other aquatic animals
• spiracle: a pore or opening used (especially by spiders and some fish) for breathing
Skin and Gills
There are various methods of gas exchange used by animals. As seen in mammals, air is taken in from the external environment to the lungs. Other animals, such as earthworms and amphibians, use their skin (integument) as a respiratory organ. A dense network of capillaries lies just below the skin, facilitating gas exchange between the external environment and the circulatory system. The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell membranes.
Organisms that live in water also need a way to obtain oxygen. Oxygen dissolves in water, but at a lower concentration in comparison to the atmosphere, which has roughly 21 percent oxygen. Fish and many other aquatic organisms have evolved gills to take up the dissolved oxygen from water. Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in the water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid. Gills are found in mollusks, annelids, and crustaceans.
The folded surfaces of the gills provide a large surface area to ensure that fish obtain sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration). Similarly, carbon dioxide molecules diffuse from the blood (high concentration) to water (low concentration).
Tracheal Systems
Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct role in oxygen transport. Insects have a highly-specialized type of respiratory system called the tracheal system, which consists of a network of small tubes that carries oxygen to the entire body. The tracheal system, the most direct and efficient respiratory system in active animals, has tubes made of a polymeric material called chitin.
Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the tubular network, allowing oxygen to pass into the body, regulating the diffusion of CO2 and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the tracheal system with body movements.
39.03: Systems of Gas Exchange - Amphibian and Bird Respiratory Systems
Birds and amphibians have different oxygen requirements than mammals, and as a result, different respiratory systems.
Learning Objectives
• Differentiate among the types of breathing in amphibians and birds
Key Points
• Amphibians utilize gills for breathing early in life, and develop primitive lungs in their adult life; additionally, they are able to breathe through their skin.
• Birds have evolved a directional respiratory system that allows them to obtain oxygen at high altitudes: air flows in one direction while blood flows in another, allowing efficient gas exchange.
Key Terms
• gills: A breathing organ of fish, amphibians, and other aquatic animals.
Amphibian Respiration
Amphibians have evolved multiple ways of breathing. Young amphibians, like tadpoles, use gills to breathe, and they do not leave the water. As the tadpole grows, the gills disappear and lungs grow (though some amphibians retain gills for life). These lungs are primitive and are not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing through the lungs is forced. The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist. It has vascular tissues to make this gaseous exchange possible. This moist skin interface can be a detriment on land, but works well under water.
Avian Respiration
Birds are different from other vertebrates, with birds having relatively small lungs and nine air sacs that play an important role in respiration. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity. Gas exchange in birds occurs between air capillaries and blood capillaries, rather than in alveoli.
Flight poses a unique challenge with respect to breathing. Flying consumes a great amount of energy; therefore, birds require a lot of oxygen to aid their metabolic processes. Birds have evolved a respiratory system that supplies them with the oxygen needed to sustain flight. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs, and is then expelled during exhalation. The details of breathing between birds and mammals differ substantially.
In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, and gas exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles of air intake and exhalation to completely get the air out of the lungs. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.02%3A_Systems_of_Gas_Exchange_-_Skin_Gills_and_Tracheal_Systems.txt |
The mammalian respiratory system equilibrates air to the body, protects against foreign materials, and allows for gas exchange.
Learning Objectives
• Explain how air passes from the outside environment to the lungs, protecting them from particulate matter
Key Points
• The air that moves from the external environment into the body must pass through the nasal cavity where it is warmed, humidified, and surveyed for particulates.
• As air moves out of the nasal cavity, it moves into the pharynx, larynx, trachea, the primary bronchi (right and left lung), secondary and tertiary bronchi, bronchioles, terminal then respiratory bronchioles, alveolar ducts then alveolar sacs where gas exchange occurs with the capillaries.
• Components in the respiratory system allow for protection from foreign material; these include mucus production in the lungs and cilia in the bronchi and bronchioles to move matter out of the system.
• Components in the respiratory system that allow for protection from foreign material and include mucus production in the lungs and cilia in the bronchi and bronchioles.
Key Terms
• alveolus: a small air sac in the lungs, where oxygen and carbon dioxide are exchanged with the blood
• bifurcate: to divide or fork into two channels or branches
• bronchus: either of two airways, which are primary branches of the trachea, leading directly into the lungs
Mammalian Respiratory System
In mammals, pulmonary ventilation occurs via inhalation when air enters the body through the nasal cavity. Air passes through the nasal cavity and is warmed to body temperature and humidified. The respiratory tract is coated with mucus that is high in water to seal the tissues from direct contact with air. As air crosses the surfaces of the mucous membranes, it picks up water. This equilibrates the air to the body, reducing damage that cold, dry air can cause. Particulates in the air are also removed in the nasal passages. These processes are all protective mechanisms that prevent damage to the trachea and lungs.
From the nasal cavity, air passes through the pharynx and the larynx to the trachea. The function of the trachea is to funnel the inhaled air to the lungs and the exhaled air out of the body. The human trachea, a cylinder about 10-12cm long, 2cm in diameter found in front of the esophagus, extends from the larynx into the chest cavity. It is made of incomplete rings of hyaline cartilage and smooth muscle that divides into the two primary bronchi at the midthorax. The trachea is lined with mucus-producing goblet cells and ciliated epithelia that propel foreign particles trapped in the mucus toward the pharynx. The cartilage provides strength and support to the trachea to keep the passage open. The smooth muscle can contract, causing a decrease in the trachea’s diameter, which propels expired air upwards from the lungs at a great force. The forced exhalation helps expel mucus when we cough.
Lungs: Bronchi and Alveoli
The end of the trachea bifurcates to the right and left lungs, which are not identical. The larger right lung has three lobes, while the smaller left lung has two lobes. The muscular diaphragm, which facilitates breathing, is inferior to the lungs, marking the end of the thoracic cavity.
As air enters the lungs, it is diverted through bronchi beginning with the two primary bronchi. Each bronchus divides into secondary, then into tertiary bronchi, which further divide to create smaller diameter bronchioles that split and spread through the lung. The bronchi are made of cartilage and smooth muscle; at the bronchioles, the cartilage is replaced with elastic fibers. Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems that control muscle contraction or relaxation, respectively. In humans, bronchioles with a diameter smaller than 0.5 mm are the respiratory bronchioles. Since they lack cartilage, they rely on inhaled air to support their shape. As the passageways decrease in diameter, the relative amount of smooth muscle increases.
The terminal bronchioles then subdivide into respiratory bronchioles which subdivide into alveolar ducts. Numerous alveoli (sing. alveolus) and alveolar sacs surround the alveolar ducts. The alveolar ducts are attached to the end of each bronchiole; each duct ends in approximately 100 alveolar sacs. Each sac contains 20-30 alveoli that are 200-300 microns in diameter. Alveoli are made of thin-walled, parenchymal cells that are in direct contact with capillaries of the circulatory system. This ensures that oxygen will diffuse from alveoli into the blood and that carbon dioxide produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the relationship of the respiratory and circulatory systems. As there are so many alveoli (around 300 million per lung) within each alveolar sac and so many sacs at the end of each alveolar duct, the lungs have a sponge-like consistency. This organization produces a very large surface area that is available for gas exchange.
Protective Mechanisms
The air that organisms breathe contains particulate matter such as dust, dirt, viral particles, and bacteria that can damage the lungs. The respiratory system has protective mechanisms to avoid damage. In the nasal cavity, hairs and mucus trap small particles, viruses, bacteria, dust, and dirt to prevent entry. If particulates make it beyond the nose or enter via the mouth, the bronchi and bronchioles contain several protective devices. The lungs produce mucus that traps particulates. The bronchi and bronchioles contain cilia, small hair-like projections that line the walls of the bronchi and bronchioles. These cilia move mucus and particles out of the bronchi and bronchioles back up to the throat where it is swallowed and eliminated via the esophagus.
In humans, tar and other substances in cigarette smoke destroy or paralyze the cilia, making the removal of particles more difficult. In addition, smoking causes the lungs to produce more mucus, which the damaged cilia are unable to move. This causes a persistent cough, as the lungs try to rid themselves of particulate matter, making smokers more susceptible to respiratory ailments. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.04%3A_Systems_of_Gas_Exchange_-_Mammalian_Systems_and_Protective_Mechanisms.txt |
Gas pressures in the atmosphere and body determine gas exchange: both O2 and CO2 will flow from areas of high to low pressure.
Learning Objectives
• Describe how gas pressure influences the flow of gases during respiration
Key Points
• Atmospheric pressure is the sum of all the partial pressures of the gases in the atmosphere, including oxygen, carbon dioxide, nitrogen, and water vapor.
• In the atmosphere, the partial pressure of oxygen is much greater than the partial pressure of carbon dioxide.
• The partial pressure of oxygen in the atmosphere is much greater in comparison to the lungs, creating a pressure gradient; this allows oxygen to flow from the atmosphere into the lungs during inhalation.
Key Terms
• atmospheric pressure: the pressure caused by the weight of the atmosphere above an area
• partial pressure: the pressure one component of a mixture of gases would contribute to the total pressure
Gas Pressure and Respiration
The respiratory process can be better understood by examining the properties of gases. Gases move freely with their movement resulting in the constant hitting of particles against vessel walls. This collision between gas particles and vessel walls produces gas pressure.
Air is a mixture of gases: primarily nitrogen (N2; 78.6 percent), oxygen (O2; 20.9 percent), water vapor (H2O; 0.5 percent), and carbon dioxide (CO2; 0.04 percent). Each gas component of that mixture exerts a pressure. The pressure for an individual gas in the mixture is the partial pressure of that gas. Approximately 21 percent of atmospheric gas is oxygen. Carbon dioxide, however, is found in relatively small amounts (0.04 percent); therefore, the partial pressure for oxygen is much greater than that of carbon dioxide. The partial pressure of any gas can be calculated by: P = (Patm) (percent content in mixture).
Patm, the atmospheric pressure, is the sum of all of the partial pressures of the atmospheric gases added together: Patm = PN2 + PO2 + PH2O + PCO2= 760 mm Hg. The pressure of the atmosphere at sea level is 760 mm Hg. Therefore, the partial pressure of oxygen is: PO2 = (760 mm Hg) (0.21) = 160 mm Hg, while for carbon dioxide: PCO2 = (760 mm Hg) (0.0004) = 0.3 mm Hg. At high altitudes, Patmdecreases, but concentration does not change; the partial pressure decrease is due to the reduction in Patm .
When the air mixture reaches the lung, it has been warmed and humidified within the nasal cavity upon inhalation. The pressure of the water vapor in the lung does not change the pressure of the air, but it must be included in the partial pressure equation. For this calculation, the water pressure (47 mm Hg) is subtracted from the atmospheric pressure: 760 mm Hg 47 mm Hg = 713 mm Hg, and the partial pressure of oxygen is: (760 mm Hg 47 mm Hg) 0.21 = 150 mm Hg.
These pressures determine the gas exchange, or the flow of gas, in the system. Oxygen and carbon dioxide will flow according to their pressure gradient from high to low. Therefore, understanding the partial pressure of each gas will aid in understanding how gases move in the respiratory system. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.05%3A_Gas_Exchange_across_Respiratory_Surfaces_-_Gas_Pressure_and_Respiration.txt |
The purpose of respiration is to perform gas exchange, a process that involves ventilation and perfusion and that relies on the laws of partial pressure.
Learning Objectives
• Discuss how gas pressures influence the exchange of gases into and out of the body
Key Points
• The purpose of the respiratory system is to perform gas exchange.
• Gases tend to equalize their pressure in two regions that are connected; in such a situation, the respective pressure of each gas is known as that gas’s ” partial pressure.”
• A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower, and the greater the difference in pressure, the more rapidly the gases will move.
• Ventilation is the movement of air into and out of the lungs, and perfusion is the flow of blood in the pulmonary capillaries. For gas exchange to be efficient, the volumes involved in ventilation and perfusion should be compatible.
Key Terms
• oxyhaemoglobin: the form of hemoglobin, loosely combined with oxygen, present in arterial and capillary blood
• hemoglobin: iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin) and heme (a porphyrin ring with iron at its center)
• partial pressure: the pressure one component of a mixture of gases would contribute to the total pressure
The purpose of the respiratory system is to perform gas exchange. Pulmonary ventilation provides air to the alveoli for this gas exchange process. At the respiratory membrane, where the alveolar and capillary walls meet, gases move across the membranes, with oxygen entering the bloodstream and carbon dioxide exiting. It is through this mechanism that blood is oxygenated and carbon dioxide, the waste product of cellular respiration, is removed from the body.
In order to understand the mechanisms of gas exchange in the lung, it is important to understand the underlying principles of gases and their behavior. In addition to Boyle’s law, several other gas laws help to describe the behavior of gases.
Gas Laws and Air Composition
Gas molecules exert force on the surfaces with which they are in contact; this force is called pressure. In natural systems, gases are normally present as a mixture of different types of molecules. For example, the atmosphere consists of oxygen, nitrogen, carbon dioxide, and other gaseous molecules, and this gaseous mixture exerts a certain pressure referred to as atmospheric pressure (Table 2). Partial pressure (Px) is the pressure of a single type of gas in a mixture of gases. For example, in the atmosphere, oxygen exerts a partial pressure, and nitrogen exerts another partial pressure, independent of the partial pressure of oxygen (Figure 1). Total pressure is the sum of all the partial pressures of a gaseous mixture. Dalton’s law describes the behavior of nonreactive gases in a gaseous mixture and states that a specific gas type in a mixture exerts its own pressure; thus, the total pressure exerted by a mixture of gases is the sum of the partial pressures of the gases in the mixture.
Partial pressure is extremely important in predicting the movement of gases. Recall that gases tend to equalize their pressure in two regions that are connected. A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower. In addition, the greater the partial pressure difference between the two areas, the more rapid is the movement of gases.
Solubility of Gases in Liquids
Henry’s law describes the behavior of gases when they come into contact with a liquid, such as blood. Henry’s law states that the concentration of gas in a liquid is directly proportional to the solubility and partial pressure of that gas. The greater the partial pressure of the gas, the greater the number of gas molecules that will dissolve in the liquid. The concentration of the gas in a liquid is also dependent on the solubility of the gas in the liquid. For example, although nitrogen is present in the atmosphere, very little nitrogen dissolves into the blood, because the solubility of nitrogen in blood is very low. The exception to this occurs in scuba divers; the composition of the compressed air that divers breathe causes nitrogen to have a higher partial pressure than normal, causing it to dissolve in the blood in greater amounts than normal. Too much nitrogen in the bloodstream results in a serious condition that can be fatal if not corrected. Gas molecules establish an equilibrium between those molecules dissolved in liquid and those in air.
The composition of air in the atmosphere and in the alveoli differs. In both cases, the relative concentration of gases is nitrogen > oxygen > water vapor > carbon dioxide. The amount of water vapor present in alveolar air is greater than that in atmospheric air (Table 3). Recall that the respiratory system works to humidify incoming air, thereby causing the air present in the alveoli to have a greater amount of water vapor than atmospheric air. In addition, alveolar air contains a greater amount of carbon dioxide and less oxygen than atmospheric air. This is no surprise, as gas exchange removes oxygen from and adds carbon dioxide to alveolar air. Both deep and forced breathing cause the alveolar air composition to be changed more rapidly than during quiet breathing. As a result, the partial pressures of oxygen and carbon dioxide change, affecting the diffusion process that moves these materials across the membrane. This will cause oxygen to enter and carbon dioxide to leave the blood more quickly.
Ventilation and Perfusion
Two important aspects of gas exchange in the lung are ventilation and perfusion. Ventilation is the movement of air into and out of the lungs, and perfusion is the flow of blood in the pulmonary capillaries. For gas exchange to be efficient, the volumes involved in ventilation and perfusion should be compatible. However, factors such as regional gravity effects on blood, blocked alveolar ducts, or disease can cause ventilation and perfusion to be imbalanced. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.06%3A_Gas_Exchange_across_Respiratory_Surfaces_-_Basic_Principles_of_Gas_Exchange.txt |
Lung volumes measure the amount of air for a specific function, while lung capacities are the sum of two or more volumes.
Learning Objectives
• Distinguish between lung volume and lung capacity
Key Points
• The lung volumes that can be measured using a spirometer include tidal volume (TV), expiratory reserve volume (ERV), and inspiratory reserve volume (IRV).
• Residual volume (RV) is a lung volume representing the amount of air left in the lungs after a forced exhalation; this volume cannot be measured, only calculated.
• The lung capacities that can be calculated include vital capacity (ERV+TV+IRV), inspiratory capacity (TV+IRV), functional residual capacity (ERV+RV), and total lung capacity (RV+ERV+TV+IRV).
Key Terms
• tidal volume: the amount of air breathed in or out during normal respiration
• residual volume: the volume of unexpended air that remains in the lungs following maximum expiration
• spirometry: the measurement of the volume of air that a person can move into and out of the lungs
Lung Volumes and Capacities
Different animals exhibit different lung capacities based on their activities. For example, cheetahs have evolved a much higher lung capacity than humans in order to provide oxygen to all the muscles in the body, allowing them to run very fast. Elephants also have a high lung capacity due to their large body and their need to take up oxygen in accordance with their body size.
Human lung size is determined by genetics, gender, and height. At maximal capacity, an average lung can hold almost six liters of air; however, lungs do not usually operate at maximal capacity. Air in the lungs is measured in terms of lung volumes and lung capacities. Volume measures the amount of air for one function (such as inhalation or exhalation) and capacity is any two or more volumes (for example, how much can be inhaled from the end of a maximal exhalation).
Lung Volumes
The volume in the lung can be divided into four units: tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual volume. Tidal volume (TV) measures the amount of air that is inspired and expired during a normal breath. On average, this volume is around one-half liter, which is a little less than the capacity of a 20-ounce drink bottle. The expiratory reserve volume (ERV) is the additional amount of air that can be exhaled after a normal exhalation. It is the reserve amount that can be exhaled beyond what is normal. Conversely, the inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled after a normal inhalation. The residual volume (RV) is the amount of air that is left after expiratory reserve volume is exhaled. The lungs are never completely empty; there is always some air left in the lungs after a maximal exhalation. If this residual volume did not exist and the lungs emptied completely, the lung tissues would stick together. The energy necessary to re-inflate the lung could be too great to overcome. Therefore, there is always some air remaining in the lungs. Residual volume is also important for preventing large fluctuations in respiratory gases (O2 and CO2). The residual volume is the only lung volume that cannot be measured directly because it is impossible to completely empty the lung of air. This volume can only be calculated rather than measured..
Lung volumes are measured by a technique called spirometry. An important measurement taken during spirometry is the forced expiratory volume (FEV), which measures how much air can be forced out of the lung over a specific period, usually one second (FEV1). In addition, the forced vital capacity (FVC), which is the total amount of air that can be forcibly exhaled, is measured. The ratio of these values (FEV1/FVC ratio) is used to diagnose lung diseases including asthma, emphysema, and fibrosis. If the FEV1/FVC ratio is high, the lungs are not compliant (meaning they are stiff and unable to bend properly); the patient probably has lung fibrosis. Patients exhale most of the lung volume very quickly. Conversely, when the FEV1/FVC ratio is low, there is resistance in the lung that is characteristic of asthma. In this instance, it is difficult for the patient to get the air out of his or her lungs. It takes a long time to reach the maximal exhalation volume. In either case, breathing is difficult and complications arise.
Lung Capacities
The lung capacities are measurements of two or more volumes. The vital capacity (VC) measures the maximum amount of air that can be inhaled or exhaled during a respiratory cycle. It is the sum of the expiratory reserve volume, tidal volume, and inspiratory reserve volume. The inspiratory capacity (IC) is the amount of air that can be inhaled after the end of a normal expiration. It is, therefore, the sum of the tidal volume and inspiratory reserve volume. The functional residual capacity (FRC) includes the expiratory reserve volume and the residual volume. The FRC measures the amount of additional air that can be exhaled after a normal exhalation. The total lung capacity (TLC) is a measurement of the total amount of air that the lung can hold. It is the sum of the residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume.. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.07%3A_Gas_Exchange_across_Respiratory_Surfaces_-__Lung_Volumes_and_Capacities.txt |
Differences in partial pressures of O2 create a gradient that causes oxygen to move from the alveoli to the capillaries and into tissues.
Learning Objectives
• Explain the process of gas exchange across the alveoli
Key Points
• The change in partial pressure from the alveoli (high concentration) to the capillaries (low concentration) drives the oxygen into the tissue and the carbon dioxide into the blood (high concentration) from the tissues (low concentration), which is then returned to the lungs and exhaled.
• Once in the blood of the capillaries, the O2 binds to the hemoglobin in red blood cells which carry it to the tissues where it dissociates to enter the cells of the tissues.
• The lungs never fully deflate, so air that is inhaled mixes with the residual air left from the previous respiration, resulting in a lower partial pressure of oxygen within the alveoli.
Key Terms
• hemoglobin: iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin) and heme (a porphyrin ring with iron at its center)
• mole: in the International System of Units, the base unit of amount of substance
Gas Exchange across the Alveoli
In the human body, oxygen is used by cells of the body’s tissues to produce ATP, while carbon dioxide is produced as a waste product. The ratio of carbon dioxide production to oxygen consumption is referred to as the respiratory quotient (RQ), which typically varies between 0.7 and 1.0. If glucose alone were used to fuel the body, the RQ would equal one, as one mole of carbon dioxide would be produced for every mole of oxygen consumed. Glucose, however, is not the only fuel for the body; both proteins and fats are used as well. Since glucose, proteins, and fats are used as fuel sources, less carbon dioxide is produced than oxygen is consumed; the RQ is, on average, about 0.7 for fat and about 0.8 for protein.
The RQ is a key factor because it is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung: the alveolar PO2 (PALVO2). The lungs never fully deflate with an exhalation; therefore, the inspired air mixes with this residual air, lowering the partial pressure of oxygen within the alveoli. This results in a lower concentration of oxygen in the lungs than is found in the air outside the body. When the RQ is known, the partial pressure of oxygen in the alveoli can be calculated: alveolar PO2 = inspired PO2−((alveolar PO2)/RQ)
In the lungs, oxygen diffuses out of the alveoli and into the capillaries surrounding the alveoli. Oxygen (about 98 percent) binds reversibly to the respiratory pigment hemoglobin found in red blood cells. These red blood cells carry oxygen to the tissues where oxygen dissociates from the hemoglobin, diffusing into the cells of the tissues. More specifically, alveolar PO2 is higher in the alveoli (PALVO2=100mmHg) than blood PO2 in the capillaries (40mmHg). Since this pressure gradient exists, oxygen can diffuse down its pressure gradient, moving out of the alveoli and entering the blood of the capillaries where O2 binds to hemoglobin. At the same time, alveolar PCO2 is lower (PALV CO2=40mmHg) than blood PCO2 (45mmHg). Due to this gradient, CO2 diffuses down its pressure gradient, moving out of the capillaries and entering the alveoli.
Oxygen and carbon dioxide move independently of each other; they diffuse down their own pressure gradients. As blood leaves the lungs through the pulmonary veins, the venous PO2=100mmHg, whereas the venous PCO2=40mmHg. As blood enters the systemic capillaries, the blood will lose oxygen and gain carbon dioxide because of the pressure difference between the tissues and blood. In systemic capillaries, PO2=100mmHg, but in the tissue cells, PO2=40mmHg. This pressure gradient drives the diffusion of oxygen out of the capillaries and into the tissue cells. At the same time, blood PCO2=40mmHg and systemic tissue PCO2=45mmHg. The pressure gradient drives CO2 out of tissue cells and into the capillaries. The blood returning to the lungs through the pulmonary arteries has a venous PO2=40mmHg and a PCO2=45mmHg. The blood enters the lung capillaries where the process of exchanging gases between the capillaries and alveoli begins again.
In short, the change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs and oxygen into the blood. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.08%3A_Gas_Exchange_across_Respiratory_Surfaces_-_Gas_Exchange_across_the_Alveoli.txt |
Both inhalation and exhalation depend on pressure gradients between the lungs and atmosphere, as well as the muscles in the thoracic cavity.
Learning Objectives
• Describe how the structures of the lungs and thoracic cavity control the mechanics of breathing
Key Points
• The mechanics of breathing follow Boyle’s Law which states that pressure and volume have an inverse relationship.
• The process of inhalation occurs due to an increase in the lung volume (diaphragm contraction and chest wall expansion) which results in a decrease in lung pressure in comparison to the atmosphere; thus, air rushes in the airway.
• The process of exhalation occurs due to an elastic recoil of the lung tissue which causes a decrease in volume, resulting in increased pressure in comparison to the atmosphere; thus, air rushes out of the airway.
• There is no contraction of muscles during exhalation; it is considered a passive process.
• The lung is protected by layers of tissue referred to as the visceral pleura and parietal pleura; the intrapleural space contains a small amount of fluid that protects the tissue by reducing friction.
Key Terms
• visceral pleura: the portion of protective tissue that is attached directly to the lungs
• parietal pleura: the portion of the protective tissue that lines the inner surface of the chest wall and covers the diaphragm
The Mechanics of Human Breathing
The relationship between gas pressure and volume helps to explain the mechanics of breathing. Boyle’s Law is the gas law which states that in a closed space, pressure and volume are inversely related. As volume decreases, pressure increases and vice versa. When discussing the detailed mechanics of breathing, it is important to keep this inverse relationship in mind.
Inhalation and Exhalation
The thoracic cavity, or chest cavity, always has a slight, negative pressure which aids in keeping the airways of the lungs open. During the process of inhalation, the lung volume expands as a result of the contraction of the diaphragm and intercostal muscles (the muscles that are connected to the rib cage), thus expanding the thoracic cavity. Due to this increase in volume, the pressure is decreased, based on the principles of Boyle’s Law. This decrease of pressure in the thoracic cavity relative to the environment makes the cavity pressure less than the atmospheric pressure. This pressure gradient between the atmosphere and the thoracic cavity allows air to rush into the lungs; inhalation occurs. The resulting increase in volume is largely attributed to an increase in alveolar space because the bronchioles and bronchi are stiff structures that do not change in size.
During this process, the chest wall expands out and away from the lungs. The lungs are elastic; therefore, when air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs. These outward and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil to force the air out of the lungs. The intercostal muscles relax, returning the chest wall to its original position. During exhalation, the diaphragm also relaxes, moving higher into the thoracic cavity. This increases the pressure within the thoracic cavity relative to the environment. Air rushes out of the lungs due to the pressure gradient between the thoracic cavity and the atmosphere. This movement of air out of the lungs is classified as a passive event since there are no muscles contracting to expel the air.
Protection of the Lung
Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and dips into spaces is called the visceral pleura. A second layer of parietal pleura lines the interior of the thorax. The space between these layers, the intrapleural space, contains a small amount of fluid that protects the tissue by reducing the friction generated from rubbing the tissue layers together as the lungs contract and relax. If these layers of tissues become inflamed, this is categorized as pleurisy: a painful inflammation that increases the pressure within the thoracic cavity, reducing the volume of the lung. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.09%3A_Breathing_-_The_Mechanics_of_Human_Breathing.txt |
Types of breathing in humans include eupnea, hyperpnea, diaphragmatic, and costal breathing; each requires slightly different processes.
Learning Objectives
• Differentiate among the types of breathing in humans, amphibians, and birds
Key Points
• Eupnea is normal quiet breathing that requires contraction of the diaphragm and external intercostal muscles.
• Diaphragmatic breathing requires contraction of the diaphragm and is also called deep breathing.
• Costal breathing requires contraction of the intercostal muscles and is also called shallow breathing.
• Hyperpnea is forced breathing and requires muscle contractions during both inspiration and expiration such as contraction of the diaphragm, intercostal muscles, and accessory muscles.
• Amphibians utilize gills for breathing early in life and later develop primitive lungs in their adult life; additionally, they are able to breathe through their skin.
• Birds have evolved a directional respiratory system that allows them to obtain oxygen at high altitudes: air flows in one direction while blood flows in another, allowing efficient gas exchange.
Key Terms
• eupnea: normal, relaxed breathing; healthy condition of inhalation and exhalation
• hyperpnea: deep and rapid respiration that occurs normally after exercise or abnormally with fever or various disorders
• intercostal: between the ribs of an animal or person
Types of Breathing
There are different types, or modes, of breathing that require a slightly different process to allow inspiration and expiration. All mammals have lungs that are the main organs for breathing. Lung capacity has evolved to support the animal’s activities. During inhalation, the lungs expand with air and oxygen diffuses across the lung’s surface, entering the bloodstream. During exhalation, the lungs expel air and lung volume decreases. The various types of breathing, specifically in humans, include:
1) Eupnea: a mode of breathing that occurs at rest and does not require the cognitive thought of the individual. During eupnea, also referred to as quiet breathing, the diaphragm and external intercostals must contract.
2) Diaphragmatic breathing: a mode of breathing that requires the diaphragm to contract. As the diaphragm relaxes, air passively leaves the lungs. This type of breathing is also known as deep breathing.
3) Costal breathing: a mode of breathing that requires contraction of the intercostal muscles. As the intercostal muscles relax, air passively leaves the lungs. This type of breathing is also known as shallow breathing.
4) Hyperpnea: a mode of breathing that can occur during exercise or actions that require the active manipulation of breathing, such as singing. During hyperpnea, also known as forced breathing, inspiration and expiration both occur due to muscle contractions. In addition to the contraction of the diaphragm and intercostal muscles, other accessory muscles must also contract. During forced inspiration, muscles of the neck, including the scalenes, contract and lift the thoracic wall, increasing lung volume. During forced expiration, accessory muscles of the abdomen, including the obliques, contract, forcing abdominal organs upward against the diaphragm. This helps to push the diaphragm further into the thorax, pushing more air out. In addition, accessory muscles (primarily the internal intercostals) help to compress the rib cage, which also reduces the volume of the thoracic cavity.
Types of Breathing in Amphibians and Birds
In animals such as amphibians, there have been multiple ways of breathing that have evolved. In young amphibians, such as tadpoles that do not leave the water, gills are used to breathe. There are some amphibians that retain gills for life. As the tadpole grows, the gills disappear and lungs grow. These lungs are primitive and not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing via lungs is forced. The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist.
Other animals, such as birds, must face a unique challenge with respect to breathing, which is that they fly. Flying consumes a large amount of energy; therefore, birds require a lot of oxygen to aid their metabolic processes. They have evolved a respiratory system that supplies them with the oxygen needed to enable flying. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, while carbon dioxide diffuses from the blood into the lungs and is expelled during exhalation. However, the details of breathing between birds and mammals differ substantially.
In addition to lungs, birds have air sacs inside their body that are attached to the lungs. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, which allows efficient gas exchange. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles of air intake and exhalation to completely remove the air from the lungs. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.10%3A_Breathing_-_Types_of_Breathing.txt |
Breathing includes several components, including flow-resistive and elastic work; surfactant production; and lung resistance and compliance.
Learning Objectives
• Explain the roles played by surfactant, flow-resistive and elastic work, and lung resistance and compliance in breathing
Key Points
• Both flow-resistive and elastic work are conducted during the act of respiration; flow-resistive work involves the alveoli and tissues, while elastic work involves the intercostal muscles, chest wall, and diaphragm.
• These types of work function in an inverse relationship; for example, increasing the rate of respiration results in an increase in the flow-resistive work and a decrease in the elastic work.
• Surfactant is a phospholipid and lipoprotein substance produced in the lungs that functions similarly to a detergent: it reduces the surface tension between alveoli tissue and air within the alveoli, thereby reducing the work needed for airway inflation.
• Lung resistance plays a key role in the ability to efficiently exchange gases; if there is obstruction (resistance) within the airways, the result will be decreased gas exchange.
• Lung compliance plays a key role in the ability to efficiently exchange gases; if there is too much of an increase or decrease in elasticity of the lung, the result will be disruption of gas exchange, which will cause obstructive or restrictive diseases.
Key Terms
• surfactant: a lipoprotein in the tissues of the lung that reduces surface tension and permits more efficient gas transport
• tidal volume: the amount of air breathed in or out during normal respiration
The Work of Breathing
The number of breaths per minute is the respiratory rate; under non-exertion conditions, the human respiratory rate averages around 12–15 breaths/minute. The respiratory rate contributes to the alveolar ventilation, or how much air moves into and out of the alveoli, which prevents carbon dioxide buildup in the alveoli. There are two ways to keep the alveolar ventilation constant: increase the respiratory rate while decreasing the tidal volume of air per breath (shallow breathing), or decrease the respiratory rate while increasing the tidal volume per breath. In either case, the ventilation remains the same, but the work done and type of work needed are quite different. Both tidal volume and respiratory rate are closely regulated when oxygen demand increases.
There are two types of work conducted during respiration: flow-resistive and elastic work. Flow-resistive work refers to the work of the alveoli and tissues in the lung, whereas elastic work refers to the work of the intercostal muscles, chest wall, and diaphragm. When the respiratory rate is increased, the flow-resistive work of the airways is increased and the elastic work of the muscles is decreased. When the respiratory rate is decreased, the flow-resistive work is decreased and the elastic work is increased.
Surfactant
The air-tissue/water interface of the alveoli has a high surface tension, which is similar to the surface tension of water at the liquid-air interface of a water droplet that results in the bonding of the water molecules together. Surfactant is a complex mixture of phospholipids and lipoproteins that works to reduce the surface tension that exists between the alveoli tissue and the air found within the alveoli. By lowering the surface tension of the alveolar fluid, it reduces the tendency of alveoli to collapse.
Surfactant works like a detergent to reduce the surface tension, allowing for easier inflation of the airways. When a balloon is first inflated, it takes a large amount of effort to stretch the plastic and start to inflate the balloon. If a little bit of detergent were applied to the interior of the balloon, then the amount of effort or work needed to begin to inflate the balloon would decrease; it would become much easier. This same principle applies to the airways. A small amount of surfactant on the airway tissues reduces the effort or work needed to inflate those airways and is also important for preventing collapse of small alveoli relative to large alveoli. Sometimes, in babies that are born prematurely, there is lack of surfactant production; as a result, they suffer from respiratory distress syndrome and require more effort to inflate the lungs.
Lung Resistance and Compliance
In pulmonary diseases, the rate of gas exchange into and out of the lungs is reduced. Two main causes of decreased gas exchange are compliance (how elastic the lung is) and resistance (how much obstruction exists in the airways). A change in either can dramatically alter breathing and the ability to take in oxygen and release carbon dioxide.
Examples of restrictive diseases are respiratory distress syndrome and pulmonary fibrosis. In both diseases, the airways are less compliant and stiff or fibrotic, resulting in a decrease in compliance because the lung tissue cannot bend and move. In these types of restrictive diseases, the intrapleural pressure is more positive and the airways collapse upon exhalation, which traps air in the lungs. Forced or functional vital capacity (FVC), which is the amount of air that can be forcibly exhaled after taking the deepest breath possible, is much lower than in normal patients; the time it takes to exhale most of the air is greatly prolonged. A patient suffering from these diseases cannot exhale the normal amount of air.
Obstructive diseases and conditions include emphysema, asthma, and pulmonary edema. In emphysema, which mostly arises from smoking tobacco, the walls of the alveoli are destroyed, decreasing the surface area for gas exchange. The overall compliance of the lungs is increased, because as the alveolar walls are damaged, lung elastic recoil decreases due to a loss of elastic fibers; more air is trapped in the lungs at the end of exhalation. Asthma is a disease in which inflammation is triggered by environmental factors, obstructing the airways. The obstruction may be due to edema, smooth muscle spasms in the walls of the bronchioles, increased mucus secretion, damage to the epithelia of the airways, or a combination of these events. Those with asthma or edema experience increased occlusion from increased inflammation of the airways. This tends to block the airways, preventing the proper movement of gases. Those with obstructive diseases have large volumes of air trapped after exhalation. They breathe at a very high lung volume to compensate for the lack of airway recruitment. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.11%3A_Breathing_-_The_Work_of_Breathing.txt |
Dead space is a broken down or blocked region of the lung that produces a mismatch of air and blood in the lungs (V/Q mismatch).
Learning Objectives
• Compare and contrast anatomical and physiological dead space and their role in V/Q mismatch
Key Points
• At times, there is a mismatch between the amount of air (ventilation, V) and the amount of blood (perfusion, Q) in the lungs, referred to as ventilation/perfusion (V/Q) mismatch.
• The two major types of V/Q mismatch that result in dead space include: anatomical dead space (caused by an anatomical issue) and physiological dead space (caused by a functional issue with the lung or arteries ).
• Anatomical dead space can occur due to changes in gravity (i.e. posture positions: sitting, standing, lying); it will affect both ventilation (V) and perfusion (Q).
• Physiological dead space can occur due to changes in function, such as in cases of infection of the lung; it will typically affect ventilation if the infection is in the lung and will affect perfusion if the functional impairment is in the arteries.
• In a normal, healthy individual, changes in either ventilation or perfusion will result in correction of the other factor to ensure an appropriate V/Q ratio.
Key Terms
• perfuse: to force a fluid to flow over or through something, especially through an organ of the body
• dead space: air that is inhaled by the body in breathing, but does not partake in gas exchange
• hydrostatic: of or relating to fluids, especially to the pressure that they exert or transmit
• pulmonary circulation: the part of blood circulation which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart
• systemic circulation: the part of blood circulation which carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart
Dead Space: V/Q Mismatch
The pulmonary circulation pressure is very low compared to that of the systemic circulation; it is also independent of cardiac output. Recruitment is the process of opening airways that normally remain closed when cardiac output increases. As cardiac output increases, the number of capillaries and arteries that are perfused (filled with blood) increases. These capillaries and arteries are not always in use, but are ready if needed. However, at times, there is a mismatch between the amount of air (ventilation, V) and the amount of blood (perfusion, Q) in the lungs. This is referred to as ventilation/perfusion (V/Q) mismatch.
There are two types of V/Q mismatch that produce dead space. Dead space is characterized by regions of broken down or blocked lung tissue. Dead spaces can severely impact breathing due to the reduction in surface area available for gas diffusion. As a result, the amount of oxygen in the blood decreases, whereas the carbon dioxide level increases. Dead space is created when no ventilation and/or perfusion takes place. Anatomical dead space, or anatomical shunt, arises from an anatomical failure, while physiological dead space, or physiological shunt, arises from a functional impairment of the lung or arteries.
An example of an anatomical shunt is the effect of gravity on the lungs. The lung is particularly susceptible to changes in the magnitude and direction of gravitational forces. When someone is standing or sitting upright, the pleural pressure gradient leads to increased ventilation further down in the lung. As a result, the intrapleural pressure is more negative at the base of the lung than at the top; more air fills the bottom of the lung than the top. Likewise, it takes less energy to pump blood to the bottom of the lung than to the top when in a prone position (lying down). Perfusion of the lung is not uniform while standing or sitting. This is a result of hydrostatic forces combined with the effect of airway pressure. An anatomical shunt develops because the ventilation of the airways does not match the perfusion of the arteries surrounding those airways. As a result, the rate of gas exchange is reduced. Note that this does not occur when lying down because in this position, gravity does not preferentially pull the bottom of the lung down. When a healthy individual stands up quickly after lying down for a while, both ventilation and perfusion increase.
A physiological shunt can develop if there is infection or edema in the lung that obstructs an area. This will decrease ventilation but not affect perfusion; therefore, the V/Q ratio changes and gas exchange is affected.
The lung has the capability to compensate for mismatches in ventilation and perfusion. If ventilation is greater than perfusion, the arterioles dilate and the bronchioles constrict, increasing perfusion while reducing ventilation. Likewise, if ventilation is less than perfusion, the arterioles constrict while the bronchioles dilate to correct the imbalance. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.12%3A_Breathing_-_Dead_Space-_V_Q_Mismatch.txt |
The majority of oxygen in the body is transported by hemoglobin, which is found inside red blood cells.
Learning Objectives
• Describe how oxygen is bound to hemoglobin and transported to body tissues
Key Points
• Hemoglobin is made up of four subunits and can bind up to four oxygen molecules.
• Carbon dioxide levels, blood pH, body temperature, environmental factors, and diseases can all affect oxygen’s carrying capacity and delivery.
• A decrease in the oxygen-carrying ability of hemoglobin is observed with an increase in carbon dioxide and temperature, as well as a decrease in pH within the body.
• Sickle cell anemia and thalassemia are two hereditary diseases that decrease the blood’s oxygen-carrying capacity.
Key Terms
• thalassemia: an inherited disorder in which the person produces a high number of red blood cells, but the cells have lower levels of hemoglobin
• sickle cell anemia: a hereditary blood disorder, characterized by red blood cells that assume an abnormal, rigid, sickle shape
• heme: the component of hemoglobin responsible for binding oxygen; consists of an iron ion that binds oxygen and a porphyrin ring that binds the globin molecules; one molecule binds one molecule of oxygen
Transport of Oxygen in the Blood
Although oxygen dissolves in blood, only a small amount of oxygen is transported this way. Only 1.5 percent of oxygen in the blood is dissolved directly into the blood itself. Most oxygen, 98.5 percent, is bound to a protein called hemoglobin and carried to the tissues.
Hemoglobin
Hemoglobin, or Hb, is a protein molecule found in red blood cells (erythrocytes) made of four subunits: two alpha subunits and two beta subunits. Each subunit surrounds a central heme group that contains iron and binds one oxygen molecule, allowing each hemoglobin molecule to bind four oxygen molecules. Molecules with more oxygen bound to the heme groups are brighter red. As a result, oxygenated arterial blood where the Hb is carrying four oxygen molecules is bright red, while venous blood that is deoxygenated is darker red.
It is easier to bind a second and third oxygen molecule to Hb than the first molecule. This is because the hemoglobin molecule changes its shape, or conformation, as oxygen binds. The fourth oxygen is then more difficult to bind. The binding of oxygen to hemoglobin can be plotted as a function of the partial pressure of oxygen in the blood (x-axis) versus the relative Hb-oxygen saturation (y-axis). The resulting graph, an oxygen dissociation curve, is sigmoidal, or S-shaped. As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen.
Factors That Affect Oxygen Binding
The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition, other environmental factors and diseases can also affect oxygen-carrying capacity and delivery; the same is true for carbon dioxide levels, blood pH, and body temperature. When carbon dioxide is in the blood, it reacts with water to form bicarbonate (HCO3) and hydrogen ions (H+). As the level of carbon dioxide in the blood increases, more H+ is produced and the pH decreases. The increase in carbon dioxide and subsequent decrease in pH reduce the affinity of hemoglobin for oxygen. The oxygen dissociates from the Hb molecule, shifting the oxygen dissociation curve to the right. Therefore, more oxygen is needed to reach the same hemoglobin saturation level as when the pH was higher. A similar shift in the curve also results from an increase in body temperature. Increased temperature, such as from increased activity of skeletal muscle, causes the affinity of hemoglobin for oxygen to be reduced.
Diseases such as sickle cell anemia and thalassemia decrease the blood’s ability to deliver oxygen to tissues and its oxygen-carrying capacity. In sickle cell anemia, the shape of the red blood cell is crescent-shaped, elongated, and stiffened, reducing its ability to deliver oxygen. In this form, red blood cells cannot pass through the capillaries. This is painful when it occurs. Thalassemia is a rare genetic disease caused by a defect in either the alpha or the beta subunit of Hb. Patients with thalassemia produce a high number of red blood cells, but these cells have lower-than-normal levels of hemoglobin. Therefore, the oxygen-carrying capacity is diminished. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.13%3A_Transport_of_Gases_in_Human_Bodily_Fluids_-_Transport_of_Oxygen_in_the_Blood.txt |
Dissolution, hemoglobin binding, and the bicarbonate buffer system are ways in which carbon dioxide is transported throughout the body.
Learning Objectives
• Explain how carbon dioxide is transported from body tissues to the lungs
Key Points
• Carbon dioxide is more soluble in blood than is oxygen; about 5 to 7 percent of all carbon dioxide is dissolved in the plasma.
• Carbon dioxide has the ability to attach to hemoglobin molecules; it will be removed from the body once they become dissociated from one another.
• In the bicarbonate buffer system, the most common form of carbon dioxide transportation in the blood, carbon dioxide is finally expelled from the body through the lungs during exhalation.
• Importantly, the bicarbonate buffer system allows little change to the pH of the body system; it allows for people to travel and live at high altitudes because the system can adjust itself to regulate carbon dioxide while maintaining the correct pH in the body.
Key Terms
• carbaminohemoglobin: a compound made up of hemoglobin and carbon dioxide; one of the forms in which carbon dioxide exists in the blood
• carbonic anhydrase: a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons
• carbon monoxide: a colorless, odourless, flammable, highly toxic gas
Transport of Carbon Dioxide in the Blood
Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods:
1. Dissolution directly into the blood
2. Binding to hemoglobin
3. Carried as a bicarbonate ion
Several properties of carbon dioxide in the blood affect its transport. First, carbon dioxide is more soluble in blood than is oxygen. About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Second, carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin. This form transports about 10 percent of the carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin is formed. Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide can freely dissociate from the hemoglobin and be expelled from the body.
Third, the majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buffer system. In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase (CA) within the red blood cells quickly converts the carbon dioxide into carbonic acid (H2CO3). Carbonic acid is an unstable, intermediate molecule that immediately dissociates into bicarbonate ions (HCO3) and hydrogen (H+) ions. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood, down its concentration gradient. It also results in the production of H+ ions. If too much H+ is produced, it can alter blood pH. However, hemoglobin binds to the free H+ ions, limiting shifts in pH. The newly-synthesized bicarbonate ion is transported out of the red blood cell into the liquid component of the blood in exchange for a chloride ion (Cl-); this is called the chloride shift. When the blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. The H+ ion dissociates from the hemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced is expelled through the lungs during exhalation.
The benefit of the bicarbonate buffer system is that carbon dioxide is “soaked up” into the blood with little change to the pH of the system. This is important because it takes only a small change in the overall pH of the body for severe injury or death to result. The presence of this bicarbonate buffer system also allows for people to travel and live at high altitudes. When the partial pressure of oxygen and carbon dioxide change at high altitudes, the bicarbonate buffer system adjusts to regulate carbon dioxide while maintaining the correct pH in the body.
Carbon Monoxide Poisoning
While carbon dioxide can readily associate and dissociate from hemoglobin, other molecules, such as carbon monoxide (CO), cannot. Carbon monoxide has a greater affinity for hemoglobin than does oxygen. Therefore, when carbon monoxide is present, it binds to hemoglobin preferentially over oxygen. As a result, oxygen cannot bind to hemoglobin, so very little oxygen is transported throughout the body. Carbon monoxide is a colorless, odorless gas which is difficult to detect. It is produced by gas-powered vehicles and tools. Carbon monoxide can cause headaches, confusion, and nausea; long-term exposure can cause brain damage or death. Administering 100 percent (pure) oxygen is the usual treatment for carbon monoxide poisoning as it speeds up the separation of carbon monoxide from hemoglobin. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/39%3A_The_Respiratory_System/39.14%3A_Transport_of_Gases_in_Human_Bodily_Fluids_-_Transport_of_Carbon_Dioxide_in_the_Blood.txt |
The circulatory systems is a network of blood vessels supplying the body with oxygen and nutrients, while removing carbon dioxide and waste.
Learning Objectives
• Describe the basic properties of the circulatory systems
Key Points
• The heart is central to the circulatory system as it is the fist-sized pump that circulates the blood throughout the body.
• As animals became more complex and multicellular, the circulatory system evolved because simple diffusion was insufficient to supply all of the cells with nutrients.
• The coordination of the circulatory system and the respiratory system to ensure proper gas exchange is very important in animals that have lungs and gills.
Key Terms
• respiration: the process by which cells obtain chemical energy by the consumption of oxygen and the release of carbon dioxide; the process of inhaling and exhaling; breathing
• cardiac: pertaining to the heart
Most animals are complex, multicellular organisms that require a mechanism for transporting nutrients throughout their bodies and for removing waste products. The circulatory system has evolved over time from simple diffusion through cells, in the early evolution of animals, to a complex network of blood vessels that reach all parts of the human body. This extensive network supplies the cells, tissues, and organs with oxygen and nutrients, while removing carbon dioxide and waste, the byproducts of respiration. The circulatory system can be thought of as a highway system that runs throughout the body.
At the core of the human circulatory system is the heart. The size of a clenched fist, the human heart is protected beneath the rib cage. Made of specialized and unique cardiac muscle, it pumps blood throughout the body and to the heart itself. Heart contractions are driven by intrinsic electrical impulses that the brain and endocrine hormones help to regulate. Understanding the heart’s basic anatomy and function is important to understanding the body’s circulatory and respiratory systems.
Gas exchange is one essential function of the circulatory system. A circulatory system is not needed in organisms with no specialized respiratory organs, such as unicellular organisms, because oxygen and carbon dioxide diffuse directly between their body tissues and the external environment. However, in organisms that possess lungs and gills, oxygen must be transported from these specialized respiratory organs to the body tissues via a circulatory system. Therefore, circulatory systems have had to evolve to accommodate the great diversity of body sizes and body types present among animals.
40.02: Overview of the Circulatory System - Open and Closed Circulatory Systems
The circulatory system can either be open or closed, depending on whether the blood flows freely in a cavity or is contained in vessels.
Learning Objectives
• Summarize circulatory system architecture
Key Points
• A closed circulatory system, found in all vertebrates and some invertebrates, circulates blood unidirectionally from the heart, around the body, and back to the heart.
• An open circulatory system, found in arthropods, pumps blood into a cavity called a hemocoel where it surrounds the organs and then returns to the heart(s) through ostia (openings).
• The blood found in arthropods, a mix of blood and interstitial fluid, is called hemolymph.
Key Terms
• ostium: a small opening or orifice, as in a body organ or passage
• hemolymph: a circulating fluid in the bodies of some invertebrates that is the equivalent of blood
• hemocoel: the system of cavities between the organs of arthropods and mollusks through which the blood circulates
Circulatory System Architecture
The circulatory system is effectively a network of cylindrical vessels (the arteries, veins, and capillaries) that emanate from a pump (the heart). In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system in which the blood is not moving freely in a cavity. In a closed circulatory system, blood is contained inside blood vessels, circulating unidirectionally (in one direction) from the heart around the systemic circulatory route, then returning to the heart again.
In contrast to a closed system, arthropods (including insects, crustaceans, and most mollusks) have an open circulatory system. In an open circulatory system, the blood is not enclosed in the blood vessels, but is pumped into a cavity called a hemocoel. The blood is called hemolymph because it mixes with the interstitial fluid. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity, reentering the heart through openings called ostia (singular: ostium). This movement allows for gas and nutrient exchange. An open circulatory system does not use as much energy to operate and maintain as a closed system; however, there is a trade-off with the amount of blood that can be moved to metabolically-active organs and tissues that require high levels of oxygen. In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today is probably because they were outmatched by the arrival of birds 150 million years ago. Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to obtain food faster and possibly to prey on the insects. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.01%3A_Overview_of_the_Circulatory_System_-_The_Role_of_the_Circulatory_System.txt |
The circulatory systems of animals differ in the number of heart chambers and the number of circuits through which the blood flows.
Learning Objectives
• Describe how circulation differs between fish, amphibians, reptiles, birds, and mammals
Key Points
• Fish have a single systemic circuit for blood, where the heart pumps the blood to the gills to be re-oxygenated (gill circulation), after which the blood flows to the rest of the body and back to the heart.
• Other animals, such as amphibians, reptiles, birds, and mammals, have a pulmonary circuit, where blood is pumped from the heart to the lungs and back, and a second, systemic circuit where blood is pumped to the body and back.
• Amphibians are unique in that they have a third circuit that brings deoxygenated blood to the skin in order for gas exchange to occur; this is called pulmocutaneous circulation.
• The number of heart chambers, atria and ventricles, mitigates the amount of mixing of oxygenated and deoxygenated blood in the heart as more chambers usually mean more separation between the systemic and pulmonary circuits.
• Warm-blooded animals require the more-efficient system of four chambers that has the oxygenated blood completely separated from the deoxygenated blood.
Key Terms
• atrium: an upper chamber of the heart that receives blood from the veins and forces it into a ventricle
• ventricle: a lower chamber of the heart
Simple Circulatory Systems
The circulatory system varies from simple systems in invertebrates to more complex systems in vertebrates. The simplest animals, such as the sponges (Porifera) and rotifers (Rotifera), do not need a circulatory system because diffusion allows adequate exchange of water, nutrients, and waste, as well as dissolved gases (figure a). Organisms that are more complex, but still have only two layers of cells in their body plan, such as jellies (Cnidaria) and comb jellies (Ctenophora), also use diffusion through their epidermis and internally through the gastrovascular compartment. Both their internal and external tissues are bathed in an aqueous environment and exchange fluids by diffusion on both sides (figure b). Exchange of fluids is assisted by the pulsing of the jellyfish body.
For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body; therefore, more complex circulatory systems evolved. Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptation during evolution and associated differences in anatomy.
Fish Circulatory Systems
Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle (figure a). The atrium collects blood that has returned from the body, while the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated; this is called gill circulation. The blood then continues through the rest of the body before arriving back at the atrium; this is called systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish’s systemic circuit. The result is a limit in the amount of oxygen that can reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish.
Amphibian Circulatory Systems
In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart (pulmonary circulation) and the other throughout the rest of the body and its organs, including the brain (systemic circulation).
Amphibians have a three-chambered heart that has two atria and one ventricle rather than the two-chambered heart of fish (figure b). The two atria receive blood from the two different circuits (the lungs and the systems). There is some mixing of the blood in the heart’s ventricle, which reduces the efficiency of oxygenation. The advantage to this arrangement is that high pressure in the vessels pushes blood to the lungs and body. The mixing is mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit where gas exchange occurs in the lungs and through the skin. For this reason, amphibians are often described as having double circulation.
Reptile Circulatory Systems
Most reptiles also have a three-chambered heart similar to the amphibian heart that directs blood to the pulmonary and systemic circuits (figure c). The ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood. Some reptiles (alligators and crocodiles) are the most primitive animals to exhibit a four-chambered heart. Crocodilians have a unique circulatory mechanism where the heart shunts blood from the lungs toward the stomach and other organs during long periods of submergence; for instance, while the animal waits for prey or stays underwater waiting for prey to rot. One adaptation includes two main arteries that leave the same part of the heart: one takes blood to the lungs and the other provides an alternate route to the stomach and other parts of the body. Two other adaptations include a hole in the heart between the two ventricles, called the foramen of Panizza, which allows blood to move from one side of the heart to the other, and specialized connective tissue that slows the blood flow to the lungs. Together, these adaptations have made crocodiles and alligators one of the most successfully-evolved animal groups on earth.
Mammal and Bird Circulatory Systems
In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles (figure d). The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. The four-chambered heart of birds and mammals evolved independently from a three-chambered heart.
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The many roles of blood include delivering nutrients and oxygen to cells, transporting waste from cells, and maintaining homeostasis.
Learning Objectives
• Identify the variety of roles played by blood in the body
Key Points
• Blood plays an important role in regulating the body’s systems and maintaining homeostasis.
• Other functions include supplying oxygen and nutrients to tissues, removing waste, transporting hormones and other signals throughout the body, and regulating body pH and core body temperature.
• Blood is composed of plasma, red blood cells, white blood cells, and platelets.
• Blood platelets play a role in coagulation (the clotting of blood to stop bleed from an open wound); white blood cells play an important role in the immune system; red blood cells transport oxygen and carbon dioxide.
• Blood is considered a type of connective tissue because it is made in the bones.
Key Terms
• hydraulic: pertaining to water
• coagulation: the process by which blood forms solid clots
• homeostasis: the ability of a system or living organism to adjust its internal environment to maintain a stable equilibrium
The Role of Blood in the Body
Blood is a bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. The components of blood include plasma (the liquid portion, which contains water, proteins, salts, lipids, and glucose ), red blood cells and white blood cells, and cell fragments called platelets.
Blood plays an important role in regulating the body’s systems and maintaining homeostasis. It performs many functions within the body, including:
• Supplying oxygen to tissues (bound to hemoglobin, which is carried in red cells)
• Supplying nutrients such as glucose, amino acids, and fatty acids either dissolved in the blood or bound to plasma proteins (e.g., blood lipids)
• Removing waste such as carbon dioxide, urea, and lactic acid
• Immunological functions, including circulation of white blood cells and detection of foreign material by antibodies
• Coagulation, which is one part of the body’s self-repair mechanism (blood clotting by the platelets after an open wound in order to stop bleeding)
• Messenger functions, including the transport of hormones and the signaling of tissue damage
• Regulating body pH
• Regulating core body temperature
• Hydraulic functions, including the regulation of the colloidal osmotic pressure of blood
Medical terms related to blood often begin with hemo- or hemato- (also spelled haemo- and haemato-), which is from the Greek word α (haima) for “blood”. In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its origin in the bones.
40.05: Components of the Blood - Red Blood Cells
Red blood cells, made from bone marrow stem cells, are crucial for the exchange of oxygen and carbon dioxide throughout the body.
Learning Objectives
• Explain the structure and function of red blood cells
Key Points
• Red blood cells, or erythrocytes, get their color from the iron-containing protein hemoglobin that carries oxygen from the lungs to the body and carbon dioxide back to the lungs.
• In most mammals, erythrocytes do not have any organelles (e.g. nucleus, mitochondria ); this frees up room for the hemoglobin molecules and prevents the cell from using the oxygen it is carrying.
• Invertebrates use different pigments, such as hemocyanin (a blue-green, copper-containing protein), chlorocruorin (a green-colored, iron-containing pigment), and hemerythrin (a red, iron-containing protein), to bind and carry oxygen.
• Red blood cells have a variety of surface glycoproteins and glycolipids that result in the different blood types A, B, and O.
• The average life span of a red blood cell is 120 days, at which time the liver and spleen break them down for recycling.
Key Terms
• hemoglobin: iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin) and heme (a porphyrin ring with iron at its center)
• hemolymph: a circulating fluid in the bodies of some invertebrates that is the equivalent of blood
• anucleate: of a cell which does not have a nucleus
• erythrocyte: an anucleate cell in the blood involved with the transport of oxygen called a red blood cell because of the red coloring of hemoglobin
Red Blood Cells
Red blood cells, or erythrocytes (erythro- = “red”; -cyte = “cell”), specialized cells that circulate through the body delivering oxygen to other cells, are formed from stem cells in the bone marrow. In mammals, red blood cells are small, biconcave cells that, at maturity, do not contain a nucleus or mitochondria; they are only 7–8 µm in size. In birds and non-avian reptiles, red blood cells contain a nucleus.
The red coloring of blood comes from the iron-containing protein hemoglobin (see [a] in ) The principal job of this protein is to carry oxygen, but it transports carbon dioxide as well. Hemoglobin is packed into red blood cells at a rate of about 250 million molecules of hemoglobin per cell. Each hemoglobin molecule binds four oxygen molecules so that each red blood cell carries one billion molecules of oxygen. There are approximately 25 trillion red blood cells in the five liters of blood in the human body, which could carry up to 25 sextillion (25 × 1021) molecules of oxygen at any time. In mammals, the lack of organelles in erythrocytes leaves more room for the hemoglobin molecules. The lack of mitochondria also prevents use of the oxygen for metabolic respiration. Only mammals have anucleated red blood cells; however, some mammals (camels, for instance) have nucleated red blood cells. The advantage of nucleated red blood cells is that these cells can undergo mitosis. Anucleated red blood cells metabolize anaerobically (without oxygen), making use of a primitive metabolic pathway to produce ATP and increase the efficiency of oxygen transport.
Not all organisms use hemoglobin as the method of oxygen transport. Invertebrates that utilize hemolymph rather than blood use different pigments containing copper or iron to bind to the oxygen. Hemocyanin, a blue-green, copper-containing protein is found in mollusks, crustaceans, and some of the arthropods ( b). Chlorocruorin, a green-colored, iron-containing pigment, is found in four families of polychaete tubeworms. Hemerythrin, a red, iron-containing protein, is found in some polychaete worms and annelids ( c). Despite the name, hemerythrin does not contain a heme group; its oxygen-carrying capacity is poor compared to hemoglobin.
The small size and large surface area of red blood cells allow for rapid diffusion of oxygen and carbon dioxide across the plasma membrane. In the lungs, carbon dioxide is released while oxygen is taken in by the blood. In the tissues, oxygen is released from the blood while carbon dioxide is bound for transport back to the lungs. Studies have found that hemoglobin also binds nitrous oxide (NO). Nitrous oxide is a vasodilator: an agent that causes dilation of the blood vessels, thereby reducing blood pressure. It relaxes the blood vessels and capillaries which may help with gas exchange and the passage of red blood cells through narrow vessels. Nitroglycerin, a heart medication for angina and heart attacks, is converted to NO to help relax the blood vessels, increasiing oxygen flow throughout the body.
A characteristic of red blood cells is their glycolipid and glycoprotein coating; these are lipids and proteins that have carbohydrate molecules attached. In humans, the surface glycoproteins and glycolipids on red blood cells vary between individuals, producing the different blood types, such as A, B, and O. Red blood cells have an average life span of 120 days, at which time they are broken down and recycled in the liver and spleen by phagocytic macrophages, a type of white blood cell. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.04%3A_Components_of_the_Blood_-_The_Role_of_Blood_in_the_Body.txt |
White blood cells, also called leukocytes, play an important role in the body’s immune response by identifying and targeting pathogens.
Learning Objectives
• Explain the structure and function of white blood cells
Key Points
• White blood cells contain nuclei; they can be divided into granulocytes (e.g. neutrophils, eosinophils, and basophils) and agranulocytes (e.g. monocytes and lymphocytes ).
• White blood cells can become macrophages at sites of infection or inflammation or they can circulate in the bloodstream searching for damaged tissue or foreign particles.
• Lymphocytes make up the majority of the cells in the immune system; they include B cells, T cells, and natural killer cells, all of which attack foreign particles or cells such as viruses, fungi, bacteria, transplanted cells, and cancer cells.
Key Terms
• macrophage: a white blood cell that phagocytizes necrotic cell debris and foreign material, including viruses, bacteria, and tattoo ink; part of the innate immune system
• pathogen: any organism or substance, especially a microorganism, capable of causing disease, such as bacteria, viruses, protozoa, or fungi
• leukocyte: a white blood cell
• granule: a small structure in a cell
White Blood Cells
White blood cells, also called leukocytes (leuko = white), make up approximately one percent, by volume, of the cells in blood. The role of white blood cells is very different from that of red blood cells. They are primarily involved in the immune response to identify and target pathogens, such as invading bacteria, viruses, and other foreign organisms. White blood cells are formed continually; some live only for hours or days, while some live for years.
The morphology of white blood cells differs significantly from red blood cells. They have nuclei and do not contain hemoglobin. The different types of white blood cells are identified by their microscopic appearance after histologic staining. Each has a different, specialized function. One of the two main groups are the granulocytes, which contain granules in their cytoplasm, and include the neutrophils, eosinophils, and basophils ( a). The second main group is the agranulocytes, which lack granules in their cytoplasm, and include the monocytes and lymphocytes ( b).
Some white blood cells become macrophages that either stay at the same site or move through the blood stream and gather at sites of infection or inflammation where they are attracted by chemical signals from foreign particles and damaged cells. Lymphocytes are the primary cells of the immune system. They include B cells, T cells, and natural killer cells. B cells destroy bacteria and inactivate their toxins; they also produce antibodies. T cells attack viruses, fungi, some bacteria, transplanted cells, and cancer cells. Natural killer cells attack a variety of infectious microbes and certain tumor cells.
One reason that HIV poses significant management challenges is because the virus directly targets T cells by gaining entry through a receptor. Once inside the cell, HIV then multiplies using the T cell’s own genetic machinery. After the HIV virus replicates, it is transmitted directly from the infected T cell to macrophages. The presence of HIV can remain unrecognized for an extensive period of time before full disease symptoms develop. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.06%3A_Components_of_the_Blood_-_White_Blood_Cells.txt |
Learning Objectives
• Describe the roles played by platelets and coagulation factors
Blood must form clots to heal wounds and prevent excess blood loss. Small cell fragments called platelets (thrombocytes) are formed from the disintegration of larger cells called megakaryocytes ( a). For each megakaryocyte, 2000–3000 platelets are formed with 150,000 to 400,000 platelets present in each cubic millimeter of blood. Each platelet is disc shaped and 2–4 μm in diameter. They contain many small vesicles, but do not contain a nucleus.
The inner surface of blood vessels is lined with a thin layer of cells (endothelial cells) that under normal situations produce chemical messengers that inhibit platelet activation. When the endothelial layer is injured, collagen is exposed, releasing other factors to the bloodstream which attracts platelets to the wound site. When the platelets are activated, they clump together to form a platelet plug (fibrin clot) ( b), releasing their contents. The released contents of the platelets activate other platelets and also interact with other coagulation factors. Coagulation factors (clotting factors) are proteins in the blood plasma that respond in a complex cascade to convert fibrinogen, a water-soluble protein present in blood serum, into fibrin, a non-water soluble protein, which strengthens the platelet plug. Many of the clotting factors require vitamin K to function. Vitamin K deficiency can lead to problems with blood clotting. The plug or clot lasts for a number of days, stopping the loss of blood.
Outside of the body, platelets can also be activated by a negatively-charged surface, such as glass. Non-physiological flow conditions (especially high values of shear stress) caused by arterial stenosis or artificial devices (e.g. mechanical heart valves or blood pumps) can also lead to platelet activation.
Key Points
• Platelets (thrombocytes) are small, anucleated cell fragments that result from the disintegration of megakaryocytes.
• Under normal conditions, blood vessel walls produce chemical messengers that inhibit platelet activation, but, when injured, they expose collagen, releasing factors that attract platelets to the wound site.
• Activated platelets stick together to form a platelet plug, which activates coagulation factor proteins found in the blood to further enhance the response to injury by strengthening the plug with fibrin.
• Vitamin K is necessary for the proper function of many coagulation factors; a deficiency is detrimental to blood clotting.
• Platelets can become activated and form clots in situations with non-physiological flow caused by disease or artificial devices.
Key Terms
• collagen: Any of more than 28 types of glycoprotein that forms elongated fibers, usually found in the extracellular matrix of connective tissue.
• clot: a solidified mass of blood
• stenosis: an abnormal narrowing or stricture in a blood vessel or other tubular organ
40.08: Components of the Blood - Plasma and Serum
Plasma is the liquid component of blood after all of the cells and platelets are removed; serum is plasma after coagulation factors have been removed.
Learning Objectives
• Explain the structure and function of plasma and serum
Key Points
• Plasma, the liquid component of blood, comprises 55 percent of the total blood volume.
• Plasma is composed of 90 percent water with antibodies, coagulation factors, and other substances such as electrolytes, lipids, and proteins required for maintaining the body.
• The removal of coagulation factors from plasma leaves a fluid similar to interstitial fluid, known as serum.
• Albumin, a protein produced in the liver, comprises about one-half of the blood serum proteins; it functions to maintain osmotic pressures and to transport hormones and fatty acids.
• Immunoglobin is a protein antibody produced in the mucosal lining; it plays an important role in antibody mediated immunity.
Key Terms
• interstitial fluid: a solution found in tissue spaces that inundates and moistens cells in multicellular animals
• electrolyte: any of the various ions (such as sodium or chloride) that regulate the electric charge on cells and the flow of water across their membranes
• viscosity: a quantity expressing the magnitude of internal friction in a fluid, as measured by the force per unit area resisting uniform flow
Plasma and Serum
Plasma, the liquid component of blood, comprises 55 percent of the total blood volume. It can separated by artificially spinning or centrifuging the blood at high rotations of 3000 rpm or higher. The blood cells and platelets that make up about 45 percent of the blood are separated by centrifugal forces to the bottom of a specimen tube, leaving the plasma as the upper layer. Plasma consists of 90 percent water along with various substances required for maintaining the body’s pH, osmotic load, and for protecting the body. The plasma also contains the coagulation factors and antibodies.
Serum, the plasma component of blood which lacks coagulation factors, is similar to interstitial fluid in which the correct composition of key ions acting as electrolytes is essential for normal functioning of muscles and nerves. Other components in the serum include proteins, which assist with maintaining pH and osmotic balance while giving viscosity to the blood; antibodies, or specialized proteins that are important for defense against viruses and bacteria; lipids, including cholesterol, which are transported in the serum; and various other substances including nutrients, hormones, metabolic waste, and external substances, such as drugs, viruses, and bacteria.
Human serum albumin, the most abundant protein in human blood plasma, is synthesized in the liver. Albumin, which constitutes about one-half of the blood serum protein, transports hormones and fatty acids, buffers pH, and maintains osmotic pressures. Immunoglobin, a protein antibody produced in the mucosal lining, plays an important role in antibody mediated immunity. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.07%3A_Components_of_the_Blood_-_Platelets_and_Coagulation_Factors.txt |
The heart pumps blood through the body with the help of structures such as ventricles, atria, and valves.
Learning Objectives
• Diagram the anatomical structure of the heart
Key Points
• The heart is divided into four chambers consisting of two atria and two ventricles; the atria receive blood, while the ventricles pump blood.
• The right atrium receives blood from the superior and inferior vena cavas and the coronary sinus; blood then moves to the right ventricle where it is pumped to the lungs.
• The lungs re-oxygenate the blood and send it to the left atrium.
• Blood moves from the left atrium to the left ventricle via the bicuspid valve; blood is pumped out of the left ventricle to the aorta, which sends blood to the organs and muscles of the body.
• The heart is composed of three layers: the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer).
Key Terms
• aorta: the largest artery in the human body which carries the blood from the heart to all parts of the body except the lungs
• inferior vena cava: large vein that carries deoxygenated blood from the lower half of the body to the right atrium of the heart
• superior vena cava: large vein that carries deoxygenated blood from the upper half of the body to the right atrium of the heart
Structure of the Heart
The heart is a complex muscle that pumps blood through the three divisions of the circulatory system: the coronary (vessels that serve the heart), pulmonary (heart and lungs), and systemic (systems of the body). Coronary circulation intrinsic to the heart takes blood directly from the main artery (aorta) coming from the heart. For pulmonary and systemic circulation, the heart has to pump blood to the lungs or the rest of the body, respectively.
The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit, it is smaller than the left side, which must send blood out to the whole body in the systemic circuit.
In humans, the heart is about the size of a clenched fist. It is divided into four chambers: two atria and two ventricles. There are one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood while the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the veins of the upper organs and arms. The right atrium also receives blood from the inferior vena cava, which drains blood from the veins of the lower organs and legs. In addition, the right atrium receives blood from the coronary sinus, which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the right atrioventricular valve (tricuspid valve), a flap of connective tissue that opens in only one direction to prevent the backflow of blood. After it is filled, the right ventricle pumps the blood through the pulmonary arteries to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close, preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. The valve separating the chambers on the left side of the heart is called the biscuspid or mitral valve (left atrioventricular valve).The blood passes through the bicuspid valve to the left ventricle where it is pumped out through the aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes, preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.
Layers of the Heart
The heart is composed of three layers: the epicardium, the myocardium, and the endocardium. The inner wall of the heart is lined by the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, the second layer of which is a membranous layered structure (the pericardium) that surrounds and protects the heart; it allows enough room for vigorous pumping, but also keeps the heart in place, reducing friction between the heart and other structures.
Blood Vessels
The heart has its own blood vessels that supply the heart muscle with blood. The coronary arteries branch from the aorta, surrounding the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the coronary veins to take the deoxygenated blood back to the right atrium, where the blood will be re-oxygenated through the pulmonary circuit.
Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. The heart muscle will die without a steady supply of blood; because of the narrow size of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowing of blood flow and subsequent oxygen deprivation can cause severe pain, known as angina. Complete blockage of the arteries will cause myocardial infarction—death of cardiac muscle tissue—which is commonly known as a heart attack. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.09%3A_Mammalian_Heart_and_Blood_Vessels_-_Structures_of_the_Heart.txt |
Blood vessels include arteries, capillaries, and veins which are responsible for transporting blood throughout the body.
Learning Objectives
• Explain the structure of arteries, veins, and capillaries and how blood flows through the body
Key Points
• Arteries carry blood away from the heart; the main artery is the aorta.
• Smaller arteries called arterioles diverge into capillary beds, which contain 10-100 capillaries that branch among the cells and tissues of the body.
• Capillaries carry blood away from the body and exchange nutrients, waste, and oxygen with tissues at the cellular level.
• Veins are blood vessels that bring blood back to the heart and drain blood from organs and limbs.
• Capillaries have one layer of cells (the endothelial tunic or tunica intima) where diffusion and exchange of materials takes place.
• Veins and arteries have two more tunics that surround the endothelium: the middle tunica media is composed of smooth muscle that regulates blood flow, while the outer tunica externa is connective tissue that supports blood vessels.
Key Terms
• vasodilation: dilation of the blood vessels
• vasoconstriction: constriction of a blood vessel
• venule: small vein, especially one that connects capillaries to a larger vein
Arteries, Veins, and Capillaries
The blood from the heart is carried through the body by a complex network of blood vessels. Arteries take blood away from the heart. The main artery is the aorta that branches into other major arteries, which take blood to different limbs and organs. These major arteries include the carotid artery, which takes blood to the brain; the brachial arteries, which take blood to the arms; and the thoracic artery, which takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidneys, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then into smaller vessels called arterioles, to reach more deeply into the muscles and organs of the body.
Arterioles diverge into capillary beds. Capillary beds contain a large number (10 to 100) of capillaries that branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood cells in single-file lines and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into venules that connect to minor veins, which connect to major veins that take blood high in carbon dioxide back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system.
The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels. The inner, tunica intima is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. This tunic is continuous with the endocardium of the heart. Unlike veins and arteries, capillaries have only one tunic; this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction, narrowing of the blood vessels, and vasodilation, widening of the blood vessels; this is important in the overall regulation of blood pressure.
Veins and arteries both have two further tunics that surround the endothelium: the middle, tunica media is composed of smooth muscle, while the outer tunica externa is connective tissue (collagen and elastic fibers). The elastic, connective tissue stretches and supports the blood vessels, while the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly-pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different from arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.10%3A_Mammalian_Heart_and_Blood_Vessels_-_Arteries_Veins_and_Capillaries.txt |
The cardiac cycle uses mechanical actions and electrical signals to push blood in and out of the heart.
Learning Objectives
• Summarize the caridac cycle and explain the role of the SA node and the AV node in regulating the its rhythm
Key Points
• During the cardiac cycle, the heart contracts via systole, pushing blood out of the heart, and relaxes via diastole, filling the heart with blood.
• Cardiomyocytes, or cardiac cells, are striated and are responsible for the pumping of the heart; they are the only muscle cells with intercalated disks.
• The heart’s internal pacemaker regulates and times the beating of the heart via electrical signals.
• Electrical signals start at the SA node, causing atria contraction, and then move on to AV node, delaying electrical impulses to allow blood from the atrium to fill the ventricles.
• Signals move from the AV node to the bundle of His and then to the Prukinje fibers, which then allows the ventricles to contract.
Key Terms
• bundle of His: specialized heart muscle cells that transmit electrical impulses from the AV node in the heart to the muscle cells of the heart wall, which contract in response producing the heart beat
• sinoatrial (SA) node: impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm
• diastole: relaxation and dilation of the heart chambers, between contractions, during which they fill with blood
• systole: rhythmic contraction of the heart, by which blood is driven through the arteries
• atrioventricular (AV) node: part of the electrical control system of the heart that coordinates the top of the heart; electrically connects atrial and ventricular chambers
The Cardiac Cycle
The main purpose of the heart is to pump blood through the body; it does so in a repeating sequence called the cardiac cycle. The cardiac cycle is the coordination of the filling and emptying of blood by electrical signals that cause the heart muscles to contract and relax. The human heart beats over 100,000 times per day. In each cardiac cycle, the heart contracts (systole), pushing out the blood and pumping it through the body. This is followed by a relaxation phase (diastole), where the heart fills with blood. The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Closing of the atrioventricular valves produces a monosyllabic “lup” sound. Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the pulmonary artery (which transports blood to the lungs). Closing of the semilunar valves produces a monosyllabic “dup” sound.
The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that comprise the heart muscle. Cardiomyocytes are distinctive muscle cells that are striated like skeletal muscle, but pump rhythmically and involuntarily like smooth muscle; they are connected by intercalated disks exclusive to cardiac muscle. Cardiomyocytes are self-stimulated for a period of time; isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.
The autonomous beating of cardiac muscle cells is regulated by the heart’s internal pacemaker that uses electrical signals to time the beating of the heart. The electrical signals and mechanical actions are intimately intertwined. The internal pacemaker starts at the sinoatrial (SA) node, which is located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node, causing the two atria to contract in unison. The pulse reaches a second node, the atrioventricular (AV) node, between the right atrium and right ventricle, where it pauses for approximately 0.1 seconds before spreading to the walls of the ventricles. This pause allows the blood in the atria to empty completely into the ventricles before the ventricles pump out the blood. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, causing the ventricles to contract. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes. This information can be observed as an electrocardiogram (ECG): a recording of the electrical impulses of the cardiac muscle. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.11%3A_Mammalian_Heart_and_Blood_Vessels_-_The_Cardiac_Cycle.txt |
The heart pumps oxygenated and deoxygenated blood throughout the body in a complex system of arteries, veins, and capillaries.
Learning Objectives
• Explain the blood flow through the body
Key Points
• As blood is pumped away from the heart, it travels through the aorta to arteries, aterioles, and the capillary beds.
• Blood flow through the capillary beds reaches almost every cell in the body and is controlled to divert blood according to the body’s needs.
• After oxygen is removed from the blood, the deoxygenated blood flows to the lungs, where it is reoxygenated and sent through the veins back to the heart.
Key Terms
• arteriole: one of the small branches of an artery, especially one that connects with capillaries
• vein: a blood vessel that transports blood from the capillaries back to the heart
• artery: an efferent blood vessel from the heart, conveying blood away from the heart regardless of oxygenation status
• vena cava: either of the two large veins that take oxygen depleted blood from the upper body and lower body and return it to the right atrium of the heart
How Blood Flows Through the Body
As the heart pumps, blood is pushed through the body through the entire circulatory system. Oxygenated blood is pumped away from the heart to the rest of the body, while deoxygenated blood is pumped to the lungs where it is reoxygenated before returning to the heart.
Blood Flow Away from the Heart
With each rhythmic pump of the heart, blood is pushed under high pressure and velocity away from the heart, initially along the main artery, the aorta. In the aorta, the blood travels at 30 cm/sec. From the aorta, blood flows into the arteries and arterioles and, ultimately, to the capillary beds. As it reaches the capillary beds, the rate of flow is dramatically (one-thousand times) slower than the rate of flow in the aorta. While the diameter of each individual arteriole and capillary is far narrower than the diameter of the aorta, the rate is actually slower due to the overall diameter of all the combined capillaries being far greater than the diameter of the individual aorta.
The slow rate of travel through the capillary beds, which reach almost every cell in the body, assists with gas (especially oxygen and carbon dioxide) and nutrient exchange. Blood flow through the capillary beds is regulated depending on the body’s needs and is directed by nerve and hormone signals. For example, after a large meal, most of the blood is diverted to the stomach by vasodilation (widening) of vessels of the digestive system and vasoconstriction (narrowing) of other vessels. During exercise, blood is diverted to the skeletal muscles through vasodilation, while blood to the digestive system would be lessened through vasoconstriction. The blood entering some capillary beds is controlled by small muscles called precapillary sphincters. A sphincter is a ringlike band of muscle that surrounds a bodily opening, constricting and relaxing as required for normal physiological functioning. If the precapillary sphincters are open, the blood will flow into the associated branches of the capillary bed. If all of the sphincters are closed, then the blood will flow directly from the arteriole to the venule through the thoroughfare channel. These muscles allow the body to precisely control when capillary beds receive blood flow. At any given moment, only about 5-10 percent of our capillary beds actually have blood flowing through them.
Blood Flow to the Heart
After the blood has passed through the capillary beds, it enters the venules, veins, and finally the two main venae cavae (singular, vena cava) that take blood back to the heart. The flow rate increases again, but is still much slower than the initial rate in the aorta. Blood primarily moves in the veins by the rhythmic movement of smooth muscle in the vessel wall and by the action of the skeletal muscle as the body moves. Because most veins must move blood against the pull of gravity, blood is prevented from flowing backward in the veins by one-way valves. Thus, because skeletal muscle contraction aids in venous blood flow, it is important to get up and move frequently after long periods of sitting so that blood will not pool in the extremities. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.12%3A_Blood_Flow_and_Blood_Pressure_Regulation_-_Blood_Flow_Through_the_Body.txt |
Blood pressure is the pressure of blood against the blood vessel walls during the cardiac cycle; it is influenced by a variety of factors.
Learning Objectives
• Describe the process of blood pressure regulation
Key Points
• Normal blood pressure for a healthy adult is 120 mm Hg during systole (peak pressure in the arteries ) and 80 mm Hg during diastole (the resting phase).
• Blood pressure is regulated in the body by changes to the diameters of blood vessels in response to changes in the cardiac output and stroke volume.
• Factors such as stress, nutrition, drugs, exercise, or disease can invoke changes in the diameters of the blood vessels, altering blood pressure.
Key Terms
• cardiac output: the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute
• hydrostatic: of or relating to fluids, especially to the pressure that they exert or transmit
• stroke volume: the volume of blood pumped from one ventricle of the heart with each beat
Blood Pressure
Blood pressure is the pressure of the fluid (blood) against the walls of the blood vessels. Fluid will move from areas of high to low hydrostatic pressures. In the arteries, the hydrostatic pressure near the heart is very high. Blood flows to the arterioles (smaller arteries) where the rate of flow is slowed by the narrow openings of the arterioles. The systolic pressure is defined as the peak pressure in the arteries during the cardiac cycle; the diastolic pressure is the lowest pressure at the resting phase of the cardiac cycle. During systole, when new blood is entering the arteries, the artery walls stretch to accommodate the increase of pressure of the extra blood. During diastole, the walls return to normal because of their elastic properties.
Blood pressure values are universally stated in millimeters of mercury (mm Hg). The blood pressure of the systole phase and the diastole phase gives the two readings for blood pressure. For example, the typical value for a resting, healthy adult is 120/80, which indicates a reading of 120 mm Hg during the systole and 80 mm Hg during diastole.
Blood Pressure Regulation
Throughout the cardiac cycle, the blood continues to empty into the arterioles at a relatively even rate. However, these measures of blood pressure are not static; they undergo natural variations from one heartbeat to another and throughout the day. The measures of blood pressure also change in response to stress, nutritional factors, drugs, or disease. The body regulates blood pressure by changes in response to the cardiac output and stroke volume.
Cardiac output is the volume of blood pumped by the heart in one minute. It is calculated by multiplying the number of heart contractions that occur per minute (heart rate) times the stroke volume (the volume of blood pumped into the aorta per contraction of the left ventricle). Therefore, cardiac output can be increased by increasing heart rate, as when exercising. However, cardiac output can also be increased by increasing stroke volume, such as if the heart were to contract with greater strength. Stroke volume can also be increased by speeding blood circulation through the body so that more blood enters the heart between contractions. During heavy exertion, the blood vessels relax and increase in diameter, offsetting the increased heart rate and ensuring adequate oxygenated blood gets to the muscles. Stress triggers a decrease in the diameter of the blood vessels, consequently increasing blood pressure. These changes can also be caused by nerve signals or hormones; even standing up or lying down can have a great effect on blood pressure.
Contributions and Attributions
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44806/latest...ol11448/latest. License: CC BY: Attribution
• Human Physiology/The cardiovascular system. Provided by: Wikibooks. Located at: en.wikibooks.org/wiki/Human_P...gh_Capillaries. License: CC BY-SA: Attribution-ShareAlike
• artery. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/artery. License: CC BY-SA: Attribution-ShareAlike
• arteriole. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/arteriole. License: CC BY-SA: Attribution-ShareAlike
• vein. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/vein. License: CC BY-SA: Attribution-ShareAlike
• vena cava. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/vena_cava. License: CC BY-SA: Attribution-ShareAlike
• Blood circulation. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Blood_circulation. License: CC BY: Attribution
• OpenStax College, Blood Flow and Blood Pressure Regulation. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44806/latest...40_04_01ab.png. License: CC BY: Attribution
• Circulatory System en. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Ci..._System_en.svg. License: CC BY: Attribution
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44806/latest...ol11448/latest. License: CC BY: Attribution
• OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44806/latest...ol11448/latest. License: CC BY: Attribution
• Human Physiology/The cardiovascular system. Provided by: Wikibooks. Located at: en.wikibooks.org/wiki/Human_P...gh_Capillaries. License: CC BY-SA: Attribution-ShareAlike
• hydrostatic. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/hydrostatic. License: CC BY-SA: Attribution-ShareAlike
• cardiac output. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/cardiac%20output. License: CC BY-SA: Attribution-ShareAlike
• stroke volume. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/stroke%20volume. License: CC BY-SA: Attribution-ShareAlike
• Blood circulation. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Blood_circulation. License: CC BY: Attribution
• OpenStax College, Blood Flow and Blood Pressure Regulation. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44806/latest...40_04_01ab.png. License: CC BY: Attribution
• Circulatory System en. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Ci..._System_en.svg. License: CC BY: Attribution
• OpenStax College, Blood Flow and Blood Pressure Regulation. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44806/latest...e_40_04_03.jpg. License: CC BY: Attribution | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/40%3A_The_Circulatory_System/40.13%3A_Blood_Flow_and_Blood_Pressure_Regulation_-_Blood_Pressure.txt |
Learning Objectives
• Describe the process and purpose of osmoregulation
What is osmoregulation?
Doctors typically recommend drinking eight to ten glasses of water a day. This amount is necessary for the proper balance of electrolytes in the human body. The intake is balanced by more or less equal excretion of fluids by urination, defecation, sweating, and, to a lesser extent, respiration. The body’s organs and tissues are immersed in fluid at a constant temperature, pH, and solute concentration, each of which contributes to maintaining the body’s homeostasis. The solutes in body fluids are mainly mineral salts and sugars. Osmotic regulation, or osmoregulation, keeps these solutes at the ideal concentrations. Osmotic homeostasis is maintained despite the influence of external factors such as temperature, diet, and weather conditions.
Osmosis is the diffusion of water across a membrane in response to osmotic pressure caused by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance (osmotic balance) across membranes within the body’s fluids, which are composed of water plus electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved in water. A non-electrolyte, in contrast, does not dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance. The body’s fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of the body (such as the pleural, serous, and cell membranes) are semi-permeable: they allow passage of certain types of solutes and water, but not others. Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across the membrane.
A cell immersed in plain water tends to swell as water diffuses in from the hypotonic or “low salt” solution. In contrast, a cell shrivels when placed in a solution of high salt concentration. The cell loses water, which moves outside to the hypertonic or “high salt” environment. Isotonic cells have an equal concentration of solutes inside and outside the cell; this equalizes the osmotic pressure on either side of the semi-permeable membrane.
Osmoconformers are marine animals which, in contrast to osmoregulators, maintain the osmolarity of their body fluids such that it is always equal to the surrounding seawater. Osmoconformers decrease the net flux of water into or out of their bodies from diffusion. They maintain internal solute concentrations within their bodies at a level equal to the osmolarity of the surrounding medium.
The body is subject to a continual intake and loss of water and electrolytes. Excess electrolytes and wastes that result from osmoregulation are transported to the kidneys and excreted. The process of excretion helps the body maintain osmotic balance.
Need for Osmoregulation
Complex multicellular animals exchange water and nutrients with the environment by consuming food and water, and by excreting sweat, urine, and feces. When disease or injury damage the mechanisms that regulate osmotic pressure, toxic waste or water may accumulate, with potentially dire consequences.
Mammalian systems have evolved to regulate osmotic pressure by managing concentrations of electrolytes found in the three major fluids: blood plasma, extracellular fluid, and intracellular fluid. Water movement due to osmotic pressure across membranes may change the volume of these fluid compartments. Because blood plasma is one of the fluid components, osmotic pressure can directly influence blood pressure and other medical indicators.
Key Points
• Osmoregulation maintains the proper balance of electrolytes in the human body, despite external factors such as temperature, diet, and weather conditions.
• By diffusion of water or solutes, osmotic balance ensures that optimal concentrations of electrolytes and non-electrolytes are maintained in cells, body tissues, and in interstitial fluid.
• Solutes or water move across a semi-permeable membrane, causing solutions on either side of it to equalize in concentration.
• Cells in hypotonic solutions swell as water moves across the membrane into the cell, whereas cells in hypertonic solutions shrivel as water moves out of the cell.
• Water movement due to osmotic pressure across membranes may change the volume of the body’s fluid compartments; therefore, it can directly influence medical indicators, such as blood pressure.
Key Terms
• electrolyte: any of the various ions (such as sodium or chloride) that regulate the electric charge on cells and the flow of water across their membranes
• osmosis: The net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane
• osmotic pressure: the hydrostatic pressure exerted by a solution across a semipermeable membrane from a pure solvent
41.02: Osmoregulation and Osmotic Balance - Transport of Electrolytes across Cell Membranes
Learning Objectives
• Explain the relationship between osmotic pressure and the transport of electrolytes across cell membranes
Transport of Electrolytes across Cell Membranes
A teaspoon of table salt readily dissolves in water. The solubility of sodium chloride results from its capacity to ionize in water. Salt and other compounds that dissociate into their component ions are called electrolytes. In water, sodium chloride (NaCl) dissociates into the sodium ion (Na+) and the chloride ion (Cl). The most important ions, whose concentrations are very closely regulated in body fluids, are the cations sodium (Na+), potassium (K+), calcium (Ca+2),and magnesium (Mg+2); and the anions chloride (Cl-), carbonate (CO3-2), bicarbonate (HCO3-), and phosphate(PO3-). Electrolytes are lost from the body during urination and perspiration. For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration.
Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to the number of solute atoms or molecules and not dependent on the size of the solute molecules. Because electrolytes dissociate into ions, adding relatively more solute molecules to a solution, they exert a greater osmotic pressure per unit mass than non-electrolytes such as glucose.
Water passes through semi-permeable membranes by passive diffusion, moving along a concentration gradient and equalizing the concentration on either side of the membrane. Electrolyte ions may not be able to passively diffuse across a membrane, but may instead require special mechanisms to cross the semi-permeable membrane. The mechanisms that transport ions across membranes are facilitated diffusion and active transport. Facilitated diffusion of solutes occurs through protein-based channels. Active transport requires energy in the form of ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.
Transport across cell membranes: Paul Andersen describes how cells move materials across the cell membrane. All movement can be classified as passive or active. Passive transport, such as diffusion, requires no energy as particles move along their gradient. Active transport requires additional energy as particles move against their gradient. Specific examples, such as GLUT and the Na/K, pump are included.
Key Points
• Important ions cannot pass through membranes by passive diffusion; if they could, maintaining specific concentrations of ions would be impossible.
• Osmotic pressure is directly proportional to the number of solute atoms or molecules; ions exert more pressure per unit mass than do non- electrolytes.
• Electrolyte ions require facilitated diffusion and active transport to cross the semi-permeable membranes.
• Facilitated diffusion occurs through protein -based channels, which allow passage of the solute along a concentration gradient.
• In active transport, energy from ATP changes the shape of membrane proteins that move ions against a concentration gradient.
Key Terms
• facilitated diffusion: The spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins.
• passive diffusion: movement of water and other molecules across membranes along a concentration gradient
• active transport: movement of a substance across a cell membrane against its concentration gradient (from low to high concentration) facilitated by ATP conversion | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.01%3A_Osmoregulation_and_Osmotic_Balance_-_Introduction.txt |
Learning Objectives
• Describe osmolality and milliequivalent
Concept of osmolality and milliequivalent
In order to calculate osmotic pressure, it is necessary to understand how solute concentrations are measured. The unit for measuring solutes is the mole. One mole is defined as the molecular weight of the solute in grams. For example, the molecular weight of sodium chloride is 58.44; thus, one mole of sodium chloride weighs 58.44 grams. A solution’s molarity is the number of moles of solute per liter of solution. On the other hand, a solution’s molality is the number of moles of solute per kilogram of solvent. If the solvent is water, one kilogram of water is equal to one liter of water. Osmolarity is related to osmolality, but is affected by changes in water content, as well as temperature and pressure. In contrast, osmolality is unaffected by temperature and pressure.
Molarity and molality represent solution concentration, but electrolyte concentrations are usually expressed in terms of milliequivalents per liter (mEq/L). The mEq/L is the ion concentration, in millimoles, multiplied by the number of electrical charges on the ion. The milliequivalent unit incorporates both the ion concentration and the charge on the ions.
Thus, for ions that have a charge of one, such as sodium (Na+), one milliequivalent is equal to one millimole. For ions that have a charge of two, such as calcium (Ca2+), one milliequivalent is equal to 0.5 millimoles. Another unit of electrolyte concentration is the milliosmole (mOsm), which is the number of milliequivalents of solute per kilogram of solvent. Osmoregulation maintains body fluids in a range of 280 to 300 mOsm.
Concentration of solutions; part 2; moles, millimoles & milliequivalents by Professor Fink: Professor Fink reviews the use of moles, millimoles & milliquivalents in expressing concentration and dosage. Example problems are presented explaining how to prepare molar solutions and convert to percent concentration. In addition, Professor Fink explains how to convert from millimoles to milliequivalents, or convert milliequivalents back to millimoles.
Key Points
• Osmotic pressure is calculated from a solution’s molarity and the charge on the ions.
• A solution’s molarity is the number of moles of solute per liter of solution, while a solution’s molality is the number of moles of solute per kilogram of solvent.
• Osmolarity is related to osmolality, but osmolality is unaffected by temperature and pressure.
• Electrolyte concentrations are usually expressed in terms of milliequivalents per liter (mEq/L), which is the ion concentration, in millimoles, multiplied by the number of electrical charges on the ion.
Key Terms
• osmotic pressure: the hydrostatic pressure exerted by a solution across a semipermeable membrane from a pure solvent
• molarity: the number of moles of solute per liter of solution, giving a solution’s molar concentration
• molality: the concentration of a substance in solution, expressed as the number of moles of solute per kilogram of solvent
• mole: in the International System of Units, the base unit of amount of substance
41.04: Osmoregulation and Osmotic Balance - Osmoregulators and Osmoconformers
Learning Objectives
• Compare the ability of stenohaline and euryhaline organisms to adapt to external fluctuations in salinity
Persons lost at sea without any fresh water to drink are at risk of severe dehydration because the human body cannot adapt to drinking seawater, which is hypertonic (having higher osmotic pressure) in comparison to body fluids. Stenohaline organisms, such as goldfish, can tolerate only a relatively-narrow range of salinity. About 90 percent of bony fish species can live in either freshwater or seawater, but not both. These fish are incapable of osmotic regulation in the alternate habitat.
However, a few species, known as euryhaline organisms, spend part of their lifecycle in fresh water and part in seawater. These organisms, such as the salmon, are tolerant of a relatively-wide range of salinity. They evolved osmoregulatory mechanisms to survive in a variety of aquatic environments. In relatively hypotonic (low osmotic pressure) fresh water, their skin absorbs water (see [a] in ). The fish do not drink much water and balance electrolytes by passing dilute urine while actively taking up salts through the gills. When they move to a hypertonic marine environment, the salmon lose water, excreting the excess salts through their gills and urine (see [b] in ).
Most marine invertebrates, on the other hand, may be isotonic with sea water (osmoconformers). Their body fluid concentrations conform to changes in seawater concentration. The blood composition of cartilaginous fishes, such as sharks and rays, is similar to that of bony fishes. However, the blood of sharks contains urea and trimethylamine oxide (TMAO). The shark’s blood electrolyte composition is not similar to that of seawater, but maintains isotonicity with seawater by storing urea at high concentrations. Sharks are “ureotelic” animals that secrete urea to maintain osmotic balance. TMAO stabilizes proteins in the presence of high urea levels, preventing the disruption of peptide bonds that would otherwise occur at such high levels of urea.
Key Points
• Stenohaline organisms can tolerate only a relatively-narrow range of salinity.
• Euryhaline organisms are tolerant of a relatively-wide range of salinity.
• Osmoconformers are organisms that remain isotonic with seawater by conforming their body fluid concentrations to changes in seawater concentration.
Key Terms
• euryhaline: able to tolerate various saltwater concentrations
• osmoconformer: a marine organism (usually an invertebrate) that maintains its internal salinity such that it is always equal to the surrounding seawater
• stenohaline: tolerant of only a narrow range of saltwater concentrations | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.03%3A_Osmoregulation_and_Osmotic_Balance_-_Concept_of_Osmolality_and_Milliequivalent.txt |
Learning Objectives
• Discuss the urea cycle
Mammals, including humans, are the primary producers of urea. Because they secrete urea as the primary nitrogenous waste product, they are called ureotelic animals. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals. It is the main nitrogen-containing substance in the urine of mammals. Urea is a colorless, odorless solid, highly soluble in water, and practically non-toxic. Dissolved in water, it is neither acidic nor alkaline. The body uses it in many processes, the most notable one being nitrogen excretion. Urea is widely used in fertilizers as a convenient source of nitrogen. It is also an important raw material for the chemical industry.
Apart from mammals, urea is also found in the urine of amphibians, as well as some fish. Interestingly, tadpoles excrete ammonia, but shift to urea production during metamorphosis. In humans, apart from being a carrier of waste nitrogen, urea also plays a role in the countercurrent exchange system of the nephrons, which allows for re-absorption of water and critical ions from the excreted urine. This mechanism, controlled by an anti-diuretic hormone, allows the body to create hyperosmotic urine, which has a higher concentration of dissolved substances than the blood plasma. This mechanism is important to prevent the loss of water, to maintain blood pressure, and to maintain a suitable concentration of sodium ions in the blood plasmas.
The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The overall chemical reaction by which ammonia is converted to urea is
2 NH3 (ammonia) + CO2 + 3 ATP + H2O → H2N-CO-NH2 (urea) + 2 ADP + 4 Pi + AMP.
The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea. The amino acid L-ornithine is converted into different intermediates before being regenerated at the end of the urea cycle. Hence, the urea cycle is also referred to as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle. Its deficiency can lead to accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria, while the last three reactions occur in the cytosol.
Key Points
• Ureotelic animals, which includes mammals, produce urea as the main nitrogenous waste material.
• 2 NH3 + CO2 + 3 ATP + H2O → H2N-CO-NH2 + 2 ADP + 4 Pi + AMP is the chemical reaction by which toxic ammonia is converted to urea.
• The urea cycle involves the multi-step conversion (carried out by five different enzymes ) of the amino acid L- ornithine into different intermediates before being regenerated.
Key Terms
• ureotelic: animals that secrete urea as the primary nitrogenous waste material
• ornithine: an amino acid, which acts as an intermediate in the biosynthesis of urea
• urea: a water-soluble organic compound, CO(NH2)2, formed by the metabolism of proteins and excreted in the urine
41.06: Nitrogenous Wastes - Nitrogenous Waste in Birds and Reptiles- Uric Acid
Learning Objectives
• Compare the major byproduct of ammonia metabolism in mammals to that of birds and reptiles
Of the four major macromolecules in biological systems, both proteins and nucleic acids contain nitrogen. During the catabolism, or breakdown, of nitrogen-containing macromolecules, carbon, hydrogen, and oxygen are extracted and stored in the form of carbohydrates and fats. Excess nitrogen is excreted from the body. Nitrogenous wastes tend to form toxic ammonia, which raises the pH of body fluids. The formation of ammonia itself requires energy in the form of ATP and large quantities of water to dilute it out of a biological system.
While aquatic animals can easily excrete ammonia into their watery surroundings, terrestrial animals have evolved special mechanisms to eliminate the toxic ammonia from their systems. The animals must detoxify ammonia by converting it into a relatively-nontoxic form such as urea or uric acid.
Birds, reptiles, and most terrestrial arthropods, such as insects, are called uricothelic organisms because they convert toxic ammonia to uric acid or the closely-related compound guanine (guano), rather than urea. In contrast, mammals (including humans) produce urea from ammonia; however, they also form some uric acid during the breakdown of nucleic acids. In this case, uric acid is excreted in urine instead of in feces, as is done in birds and reptiles.
Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a white paste or powder. The production of uric acid involves a complex metabolic pathway that is energetically costly in comparison to processing of other nitrogenous wastes such as urea (from the urea cycle) or ammonia; however, it has the advantages of reducing water loss and, hence, reducing the need for water.
Uric acid is also less toxic than ammonia or urea. It contains four nitrogen atoms; only a small amount of water is needed for its excretion. Out of solute, it precipitates and forms crystals. The enzyme xanthine oxidase makes uric acid from xanthine and hypoxanthine, which in turn are produced from other purines. Xanthine oxidase is a large enzyme whose active site consists of the metal, molybdenum, bound to sulfur and oxygen. Uric acid is released in hypoxic conditions.
Key Points
• Nitrogenous wastes in the body tend to form toxic ammonia, which must be excreted.
• Mammals such as humans excrete urea, while birds, reptiles, and some terrestrial invertebrates produce uric acid as waste.
• Uricothelic organisms tend to excrete uric acid waste in the form of a white paste or powder.
• Conversion of ammonia into uric acid is more energy intensive than the conversion of ammonia into urea.
• Producing uric acid instead of urea is advantageous because it is less toxic and reduces water loss and the subsequent need for water.
Key Terms
• urea: a water-soluble organic compound, CO(NH2)2, formed by the metabolism of proteins and excreted in the urine
• guano: the excrement of seabirds, cave-dwelling bats, pinnipeds, or birds more generally
• purine: any of a class of organic heterocyclic base containing fused pyrimidine and imidazole rings; they are components of nucleic acids
• xanthine: a precursor of uric acid found in many organs of the body
• hypoxanthine: an intermediate in the biosynthesis of uric acid
• uric acid: a bicyclic heterocyclic phenolic compound, formed in the body by the metabolism of protein and excreted in the urine | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.05%3A_Nitrogenous_Wastes_-_Nitrogenous_Waste_in_Terrestrial_Animals-_The_Urea_Cycle.txt |
Learning Objectives
• Describe the process of handling wastes in microorganisms
A contractile vacuole (CV) is an organelle, or sub-cellular structure, that is involved in osmoregulation and waste removal. Previously, a CV was known as a pulsatile or pulsating vacuole. CVs should not be confused with vacuoles which store food or water. A CV is found predominantly in protists and in unicellular algae. In freshwater environments, the concentration of solutes inside the cell is higher than outside the cell. Under these conditions, water flows from the environment into the cell by osmosis. Thus, the CV acts as a protective mechanism against cellular expansion (and possibly explosion) from too much water; it expels excess water from the cell by contracting. However, not all species that possess a CV are freshwater organisms; some marine and soil microorganisms also have a CV. The CV is predominant in species that do not have a cell wall, but there are exceptions. Through the process of evolution, the CV was mostly eliminated in multicellular organisms; however it still exists in the unicellular stage of several multicellular fungi and in several types of cells in sponges, including amoebocytes, pinacocytes, and choanocytes.
The CV’s phases of collecting water (expansion) and expelling water (contraction) are periodical. One cycle takes several seconds, depending on the species and the environment’s osmolarity. The stage in which water flows into the CV is called diastole. The contraction of the CV and the expulsion of water from the cell is called systole. Water always flows from outside the cell into the cytoplasm; and only then from the cytoplasm into the CV. Species that possess a CV always use it, even in very hypertonic (high concentration of solutes) environments, since the cell tends to adjust its cytoplasm to become even more hyperosmotic (hypertonic) than the environment. The amount of water expelled from the cell and the rate of contraction are related to the osmolarity of the environment. In hyperosmotic environments, less water will be expelled and the contraction cycle will be longer.
The number of CVs per cell varies, depending on the species. Amoeba have one; Dictyostelium discoideum, Paramecium aurelia, and Chlamydomonas reinhardtii have two; and giant amoeba, such as Chaos carolinensis, have many. In some unicellular eukaryotic organisms (e.g., amoeba), cellular wastes, such as ammonia and excess water, are excreted by exocytosis as the contractile vacuoles merge with the cell membrane, expelling wastes into the environment. In Paramecium, which, presumably, has the most-complex and highly-evolved CV, the vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water, it is pumped into the vacuole. When the vacuole is full, it expels the water through a pore in the cytoplasm that can be opened and closed.
Key Points
• Contractile vacuoles protect a cell from absorbing too much water and potentially exploding by excreting excess water.
• Wastes, such as ammonia, are soluble in water; they are excreted from the cell along with excess water by the contractile vacuoles.
• Contractile vacuoles function in a periodic cycle by expanding while collecting water and contracting to release the water.
Key Terms
• contractile vacuole: a vacuole that removes waste or excess water
• osmoregulation: the homeostatic regulation of osmotic pressure in the body in order to maintain a constant water content
• osmolarity: The osmotic concentration of a solution, normally expressed as osmoles of solute per litre of solution.
• hypertonic: having a greater osmotic pressure than another | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.07%3A__Excretion_Systems_-_Contractile_Vacuoles_in_Microorganisms.txt |
Learning Objectives
• Compare and contrast the way in which planaria and annelids handle waste products
As multi-cellular systems evolved to have organ systems that divided the metabolic needs of the body, individual organs evolved to perform the excretory function. Excretory cells known as flame cells developed in flatworms, while nephridia developed as excretory cells in annelids.
Flame Cells of Planaria
Planaria are flatworms that live in fresh water. Their excretory system consists of two tubules connected to a highly-branched duct system that leads to pores located all along the sides of the body. The filtrate is secreted through these pores. The cells in the tubules are called flame cells (or protonephridia) because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope. Flame cells function like a kidney, removing waste materials through filtration. The cilia propel waste matter down the tubules and out of the body through excretory pores that open on the body surface; cilia also draw water from the interstitial fluid, allowing for filtration. After excretion, any useful metabolites are reabsorbed by the cell. Flame cells are found in freshwater invertebrates, such as flatworms, including parasitic tapeworms and free-living planaria.
Nephridia of Worms
Earthworms (annelids) and some other invertebrates, such as arthropods and mollusks, have slightly-more-evolved excretory structures called nephridia. A pair of nephridia is present on each segment of the earthworm. They are similar to flame cells in that they have tubules with cilia and function like a kidney to remove wastes, but they often open to the exterior of the organism. The ciliated tubules filter fluid from the body cavity and carry waste, including excess ions, through openings called nephrostomes. From the nephrostomes, excretion occurs through a pore called the nephridiopore. A nephridium is more evolved than a flame cell in that it has a system for reabsorption of some useful waste products, such as metabolites and ions, by a capillary network before excretion (unlike planaria that can only reabsorb useful metabolites after excretion).
Key Points
• Nephridia are more evolved than flame cells because they can reabsorb useful metabolites before excretion of waste.
• Both nephridia and flame cells are ciliated tubules that filter fluids in the cell to remove waste.
• Flame cells are connected to a duct system of pores to expel wastes, while nephridia often are open to the exterior of the organism.
Key Terms
• flame cell: a specialized excretory cell found in the simplest freshwater invertebrates
• nephridium: a tubular excretory organ in some invertebrates
• nephridiopore: the external opening of a nephridium, where waste is excreted from the cell
• nephrostome: the funnel-shaped opening of a nephridium into the body cavity
41.09: Excretion Systems - Malpighian Tubules of Insects
Learning Objectives
• Explain how insects use malpighian tubules to excrete wastes and maintain osmotic balance
Malpighian tubules line the gut of some species of arthropods, such as bees. They are usually found in pairs in the posterior regions of arthropod alimentary canals; the number of tubules varies with the species of insect. The system of malpighian tubules consists of branching tubules, which increase their surface area, near the hemolymph (a mixture of blood and interstitial fluid that is found in insects, other arthropods, and most mollusks) and fat tissues. They are lined with microvilli for reabsorption and maintenance of osmotic balance. They contain actin for support.
Malpighian tubules work cooperatively with specialized glands in the wall of the rectum. Body fluids are not filtered, as in the case of nephridia. Instead, urine is produced by tubular secretion mechanisms by the cells lining the malpighian tubules that are bathed in hemolymph. Metabolic wastes, such as urea and amino acids, freely diffuse into the tubules, while ions are transported through active pump mechanisms. There are exchange pumps lining the tubules which actively transport H+ ions into the cell and K+ or Na+ ions out; water passively follows to form urine. The secretion of ions alters the osmotic pressure, which draws water, electrolytes, and nitrogenous waste (uric acid) into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low-water environments and uric acid is precipitated and excreted as a thick paste or powder. By not dissolving wastes in water, these organisms are able to conserve water; this is especially important for life in dry environments.
Key Points
• Malpighian tubules are found in the posterior regions of insects, where they work with glands in the rectum to excrete waste and maintain osmotic balance.
• Ions are transported through active pumps found in the malpighian tubules; as the ions are secreted, water and waste are drawn to the tubules due to the change in osmotic pressure.
• Nitrogenous wastes, such as uric acid, are precipitated as thick pastes or powder to be excreted.
Key Terms
• malpighian tubule: a tubule that extends from the alimentary canal to the exterior of the organism, excreting water and wastes in the form of solid nitrogenous compounds
• uric acid: a bicyclic heterocyclic phenolic compound, formed in the body by the metabolism of protein and excreted in the urine
• hemolymph: a circulating fluid in the bodies of some invertebrates that is the equivalent of blood
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, SHARED PREVIOUSLY | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.08%3A_Excretion_Systems_-_Flame_Cells_of_Planaria_and_Nephridia_of_Worms.txt |
Learning Objectives
• Explain how the kidneys serve as the main osmoregulatory organs in mammalian systems, using the functional properties of nephrons
Kidneys: The Main Osmoregulatory Organ
The kidneys are a pair of bean-shaped structures that are located just below and posterior to the liver in the peritoneal cavity. Adrenal glands, also called suprarenal glands, sit on top of each kidney. Kidneys regulate the osmotic pressure of a mammal’s blood through extensive filtration and purification in a process known as osmoregulation. All the blood in the human body is filtered many times a day by the kidneys. These organs use almost 25 percent of the oxygen absorbed through the lungs to perform this function. Oxygen allows the kidney cells to efficiently manufacture chemical energy in the form of ATP through aerobic respiration. Kidneys eliminate wastes from the body; urine is the filtrate that exits the kidneys.
Externally, the kidneys are surrounded by three layers. The outermost layer, the renal fascia, is a tough connective tissue layer. The second layer, the perirenal fat capsule, helps anchor the kidneys in place. The third and innermost layer is the renal capsule. Internally, the kidney has three regions: an outer cortex, a medulla in the middle, and the renal pelvis in the region called the hilum of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters.
Because the kidney filters blood, its network of blood vessels is an important component of its structure and function. The arteries, veins, and nerves that supply the kidney enter and exit at the renal hilum. Renal blood supply starts with the branching of the aorta into the renal arteries (which are each named based on the region of the kidney they pass through) and ends with the exiting of the renal veins to join the inferior vena cava. The renal arteries split into several segmental arteries upon entering the kidneys. Each segmental artery splits further into several interlobar arteries that enter the renal columns, which supply the renal lobes. The interlobar arteries split at the junction of the renal cortex and medulla to form the arcuate arteries. The arcuate, “bow shaped” arteries form arcs along the base of the medullary pyramids. Cortical radiate arteries, as the name suggests, radiate out from the arcuate arteries, branch into numerous afferent arterioles, and then enter the capillaries supplying the nephrons.
Key Points
• Kidneys regulate the osmotic pressure of a mammal’s blood through extensive filtration and purification, in a process known as osmoregulation.
• Kidneys filter the blood; urine is the filtrate that eliminates waste from the body via the ureter into the bladder.
• The kidneys are surrounded by three layers: renal fascia, perirenal fat capsule, and the renal capsule.
Key Terms
• renal: pertaining to the kidneys
41.11: Human Osmoregulatory and Excretory Systems - Nephron- The Functional Unit of the Kidney
Learning Objectives
• Explain the role of the nephron as the functional unit of the kidney
The nephron, the functional unit of the kidney, is responsible for removing waste from the body. Each kidney is composed of over one million nephrons that dot the renal cortex, giving it a granular appearance when sectioned sagittally (from front to rear). Eighty-five percent of nephrons are cortical nephrons, deep in the renal cortex; the remaining 15 percent are juxtamedullary nephrons, which lie in the renal cortex close to the renal medulla.
Diagram of a nephron
The nephron is the functional unit of the kidney. The glomerulus and convoluted tubules of the nephron are located in the cortex of the kidney, while the collecting ducts are located in the pyramids of the kidney’s medulla.
A nephron consists of three parts: a renal corpuscle, a renal tubule, and the associated capillary network, which originates from the cortical radiate arteries. The renal corpuscle, located in the renal cortex, is composed of a network of capillaries known as the glomerulus, as well as a cup-shaped chamber that surrounds it: the glomerular or Bowman’s capsule.
The renal tubule is a long, convoluted structure that emerges from the glomerulus. It can be divided into three parts based on function. The first part is called the proximal convoluted tubule (PCT), due to its proximity to the glomerulus. The second part is called the loop of Henle, or nephritic loop, because it forms a loop (with descending and ascending limbs) that goes through the renal medulla. The third part of the renal tubule is called the distal convoluted tubule (DCT); this part is also restricted to the renal cortex. This last part of the nephron connects with and empties its filtrate into collecting ducts that line the medullary pyramids. The collecting ducts amass contents from multiple nephrons, fusing together as they enter the papillae of the renal medulla.
As urine travels down the collecting duct system, it passes by the medullary interstitium, which has a high sodium concentration as a result of the loop of Henle’s countercurrent multiplier system. Urine leaves the medullary collecting ducts through the renal papillae, emptying into the renal calyces, the renal pelvis, and finally into the bladder via the ureter.
Key Points
• Kidneys contain two types of nephrons, each located in different parts of the renal cortex: cortical nephrons and juxtamedullary nephrons.
• A nephron comprises a renal corpuscle, a renal tubule, and the associated capillary network.
• Internally, kidneys are mainly composed of over one million nephrons and an extensive network of blood vessels and capillaries.
Key Terms
• glomerulus: A small intertwined group of capillaries within a kidney’s nephron that filters the blood to make urine.
• loop of Henle: A structure in a kidney’s nephron that connects the proximal convoluted tubule to the distal convoluted tubule. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.10%3A_Human_Osmoregulatory_and_Excretory_Systems_-_Kidney_Structure.txt |
Learning Objectives
• Outline the process by which kidneys filter blood, reabsorb nutrients and water, and produce urine
Blood Filtration and Nutrient and Water Reabsorption
Kidneys filter blood in a three-step process. First, the nephrons filter blood that runs through the capillary network in the glomerulus. Almost all solutes, except for proteins, are filtered out into the glomerulus by a process called glomerular filtration. Second, the renal tubules collect the filtrate. Most of the solutes are reabsorbed in the PCT by a process called tubular reabsorption. In the loop of Henle, the filtrate continues to exchange solutes and water with the renal medulla and the peritubular capillary network.
Finally, some substances, such as electrolytes and drugs, are removed from blood through the peritubular capillary network into the distal convoluted tubule or collecting duct. Urine is a collection of substances that have not been reabsorbed during glomerular filtration or tubular reabsorbtion.
Glomerular Filtration
The formation of urine occurs through three steps: glomerular filtration, tubular reabsorption, and tubular secretion. The process of glomerular filtration filters out most of the solutes due to the high blood pressure and specialized membranes in the afferent arteriole. The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure. The “leaky” connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily. All solutes in the glomerular capillaries, including sodium ions and negatively and positively charged ions, pass through by passive diffusion; the only exception is macromolecules such as proteins. There is no energy requirement at this stage of the filtration process. Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys. GFR is regulated by multiple mechanisms and is an important indicator of kidney function.
Tubular Reabsorption and Secretion
Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed; this occurs either by passive or active transport. Reabsorption of water and key electrolytes are regulated and influenced by hormones. Sodium (Na+) is the most abundant ion; most of it is reabsorbed by active transport and then transported to the peritubular capillaries. Because Na+ is actively transported out of the tubule, water follows to even out the osmotic pressure. Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT. This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries. Every solute, however, has a transport maximum; the excess solute is not reabsorbed. Kidneys’ osmolarity of body fluids is maintained at 300 milliosmole (mOsm).
In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb. Additionally, the loop of Henle invades the renal medulla, which is naturally high in salt concentration. It tends to absorb water from the renal tubule and concentrate the filtrate. The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, it acts as a countercurrent multiplier. The vasa recta around the loop of Henle acts as the countercurrent exchanger.
Additional solutes and wastes are secreted into the kidney tubules during tubular secretion, which is the opposite process to tubular reabsorption. The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae. From here, the papillae deliver the filtrate, now called urine, into the minor calyces that eventually connect to the ureters through the renal pelvis.
Key Points
• Glomerular filtration, tubular reabsorption, and tubular secretion are the three primary steps in which kidneys filter blood and maintain proper electrolyte balance.
• Glomerular filtration removes solutes from the blood; it is the first step of urine formation.
• In tubular reabsoption, the second step of urine formation, almost all nutrients are reabsorbed in the renal tubule by active or passive transport.
• Tubular secretion is the last step of urine formation, where solutes and waste are secreted into the collecting ducts, ultimately flowing to the bladder in the form of urine.
Key Terms
• arteriole: one of the small branches of an artery, especially one that connects with capillaries
• countercurrent: a current that flows against the prevailing one
• electrolyte: any of the various ions (such as sodium or chloride) that regulate the electric charge on cells and the flow of water across their membranes | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.12%3A__Human_Osmoregulatory_and_Excretory_Systems_-_Kidney_Function_and_Physiology.txt |
Learning Objectives
• Describe hormonal control by epinephrine and norepinephrine of osmoregulatory functions
Epinephrine
As a hormone and neurotransmitter, epinephrine acts on nearly all body tissues. Its actions vary by tissue type and tissue expression of adrenergic receptors. For example, high levels of epinephrine cause smooth muscle relaxation in the airways, but cause contraction of the smooth muscle that lines most arterioles. Epinephrine acts by binding to a variety of adrenergic receptors. Epinephrine is a nonselective agonist of all adrenergic receptors, including the major subtypes α1, α2, β1, β2, and β3. Epinephrine’s binding to these receptors triggers a number of metabolic changes. Binding to α-adrenergic receptors inhibits insulin secretion by the pancreas, stimulates glycogenolysis (the breakdown of glycogen) in the liver and muscle, and stimulates glycolysis (the metabolic pathway that converts glucose into pyruvate) in muscle. β-Adrenergic receptor binding triggers glucagon secretion in the pancreas, increased adrenocorticotropic hormone (ACTH) secretion by the pituitary gland, and increased lipolysis by adipose tissue. Together, these effects lead to increased blood glucose and fatty acids, providing substrates for energy production within cells throughout the body.
Norepinephrine
Norepinephrine is a catecholamine with multiple roles. It is the hormone and neurotransmitter most responsible for vigilant concentration in contrast to its most-chemically-similar hormone, dopamine, which is most responsible for cognitive alertness. Areas of the body that produce or are affected by norepinephrine are described as noradrenergic. One of the most important functions of norepinephrine is its role as the neurotransmitter released from the sympathetic neurons to affect the heart. An increase in norepinephrine from the sympathetic nervous system increases the rate of contractions in the heart. Norepinephrine also underlies the fight-or-flight response, along with epinephrine, directly increasing heart rate, triggering the release of glucose from energy stores, and increasing blood flow to skeletal muscle. When norepinephrine acts as a drug, it increases blood pressure by increasing vascular tone through α-adrenergic receptor activation. Norepinephrine is synthesized from dopamine by dopamine β-hydroxylase in the secretory granules of the medullary chromaffin cells and is released from the adrenal medulla into the blood as a hormone. It is also a neurotransmitter in the central nervous system and sympathetic nervous system, where it is released from noradrenergic neurons in the locus coeruleus. The actions of norepinephrine are carried out via the binding to adrenergic receptors.
Role of Epinephrine and Norepinephrine in Kidney Function
Epinephrine and norepinephrine are released by the adrenal medulla and nervous system respectively. They are the flight/fight hormones that are released when the body is under extreme stress. During stress, much of the body’s energy is used to combat imminent danger. Kidney function is halted temporarily by epinephrine and norepinephrine. These hormones function by acting directly on the smooth muscles of blood vessels to constrict them. Once the afferent arterioles are constricted, blood flow into the nephrons of the kidneys stops. These hormones go one step further and trigger the renin-angiotensin-aldosterone system, the hormone system that regulates blood pressure and water (fluid) imbalance.
Key Points
• Epinephrine, produced by the adrenal medulla, causes either smooth muscle relaxation in the airways or contraction of the smooth muscle in arterioles, which results in blood vessel constriction in the kidneys, decreasing or inhibiting blood flow to the nephrons.
• Norepinephrine, produced by the adrenal medulla, is a stress hormone that increases blood pressure, heart rate, and glucose from energy stores; in the kidneys, it will cause constriction of the smooth muscles, resulting in decreased or inhibited flow to the nephrons.
• Together, epinephrine and norepinephrine cause constriction of the blood vessels associated with the kidneys to inhibit flow to the nephrons.
Key Terms
• epinephrine: (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress
• norepinephrine: a neurotransmitter found in the locus coeruleus which is synthesized from dopamine
• catecholamine: any of a class of aromatic amines derived from pyrocatechol that are hormones produced by the adrenal gland
• adrenergic: containing or releasing adrenaline | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.13%3A_Hormonal_Control_of_Osmoregulatory_Functions_-_Epinephrine_and_Norepinephrine.txt |
Learning Objectives
• Describe hormonal control by the renin-angiotensin-aldosterone system
Renin-Angiotensin-Aldosterone
The renin-angiotensin-aldosterone system (RAAS) is a hormone system that regulates blood pressure and water (fluid) balance. This system proceeds through several steps to produce angiotensin II, which acts to stabilize blood pressure and volume. Renin is secreted by a part of the juxtaglomerular complex and produced by the granular cells of the afferent and efferent arterioles. Renin is a circulating enzyme that acts on angiotensinogen, which is made in the liver, converting it to angiotensin I. Defective renin production can cause a continued decrease in blood pressure and cardiac output. After renin facilitates the production of angiotensis I, angiotensin converting enzyme (ACE) then converts angiotensin I to angiotensin II. Angiotensin II raises blood pressure by constricting blood vessels and also triggers the release of the mineralocorticoid aldosterone from the adrenal cortex. This, in turn, stimulates the renal tubules to reabsorb more sodium. Angiotensin II also triggers the release of anti-diuretic hormone (ADH) from the hypothalamus, leading to water retention in the kidneys. It acts directly on the nephrons, decreasing glomerular filtration rate. Thus, via the RAAS, the kidneys control blood pressure and volume directly. Medically, blood pressure can be controlled by drugs that inhibit ACE (called ACE inhibitors).
Mineralocorticoids
Mineralocorticoids are hormones synthesized by the adrenal cortex that affect osmotic balance. One type of mineralocorticoid, known as aldosterone, regulates sodium levels in the blood. Almost all of the sodium in the blood is reclaimed by the renal tubules under the influence of aldosterone. As sodium is always reabsorbed by active transport and water follows sodium to maintain osmotic balance, aldosterone manages not only sodium levels, but also the water levels in body fluids. Aldosterone also stimulates potassium secretion concurrently with sodium reabsorption. By contrast, absence of aldosterone means that no sodium is reabsorbed in the renal tubules; all of it is excreted in the urine. In addition, the daily dietary potassium load is not secreted; retention of potassium ions (K+) can cause a dangerous increase in plasma K+ concentration. Patients who have Addison’s disease have a failing adrenal cortex and cannot produce aldosterone. They constantly lose sodium in their urine; if the supply is not replenished, the consequences can be fatal.
Antidiurectic Hormone
Antidiuretic hormone or ADH (also called vasopressin) helps the body conserve water when body fluid volume, especially that of blood, is low. It is formed by the hypothalamus, but is stored and released from the posterior pituitary gland. It acts by inserting aquaporins in the collecting ducts, promoting reabsorption of water. ADH also acts as a vasoconstrictor (constricting blood vessels) and increases blood pressure during hemorrhaging.
Atrial Natriuretic Peptide Hormone
The atrial natriuretic peptide (ANP) hormone lowers blood pressure by acting as a vasodilator (dilating or widening blood vessels). It is released by cells in the atrium of the heart in response to high blood pressure and in patients with sleep apnea. ANP affects salt release; because water passively follows salt to maintain osmotic balance, it also has a diuretic effect. ANP also prevents sodium reabsorption by the renal tubules, decreasing water reabsorption (thus acting as a diuretic) and lowering blood pressure. Its actions suppress the actions of aldosterone, ADH, and renin. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.14%3A_Hormonal_Control_of_Osmoregulatory_Functions_-_Other_Hormonal_Controls_for_Osmoregulation.txt |
Learning Objectives
• Explain the purpose of the immune system
The environment surrounding all of us consists of numerous pathogens: agents (usually microorganisms) that cause disease(s) in their hosts. A host is the organism that is invaded and often harmed by a pathogen. Pathogens, which include bacteria, protists, fungi, and other infectious organisms, can be found in food and water, on surfaces, and in the air. Concern over pathogens is one of the main reasons that we wash our hands after going to the bathroom or touching raw meat.
Mammalian immune systems evolved for protection from such pathogens. They are composed of an extremely-diverse array of specialized cells and soluble molecules that coordinate a rapid, flexible defense system capable of providing protection from a majority of these disease agents. Central to this goal, the immune system must be capable of recognizing “self” from “other” so that when it destroys cells, it destroys pathogen cells and not host cells.
The immune response that defends against pathogens can be classified as either innate or active. The innate immune response is present in its final state from birth and attempts to defend against all pathogens. Conversely, the adaptive immune response stores information about past infections and mounts pathogen-specific defenses. It expands over time, gaining more information about past targets so that it can respond quickly to future pathogens. The adaptive immune response functions throughout the body to combat specific pathogens that it has encountered before (a process known as reactivation). However, we are born with only innate immunity, developing our adaptive immune response after birth.
Components of both immune systems constantly search the body for signs of pathogens. When pathogens are found, immune factors are mobilized to the site of an infection. The immune factors identify the nature of the pathogen, strengthen the corresponding cells and molecules to combat it efficiently, and then halt the immune response after the infection is cleared to avoid unnecessary host cell damage. Features of the immune system (e.g., pathogen identification, specific response, amplification, retreat, and remembrance) are essential for survival against pathogens.
Key Points
• The mammalian immune system has evolved an extremely-diverse array of specialized cells and soluble molecules that allow it to defend against a variety of pathogens, including bacteria, protists, fungi, and other infectious organisms.
• The innate immune response serves as a general defense against all pathogens, but has no capacity to adapt or learn when new pathogens attack.
• The adaptive immune response has a “memory” about previously encountered pathogens and is able to mount pathogen-specific defenses based on this memory.
Key Terms
• pathogen: any organism or substance, especially a microorganism, capable of causing disease, such as bacteria, viruses, protozoa, or fungi
• immune system: the system that differentiates self from non-self and protects the body from foreign substances and pathogenic organisms by producing an immune response
42.02: Innate Immune Response - Physical and Chemical Barriers
Learning Objectives
• Describe physical and chemical barriers in the innate immune response
The immune system comprises both innate and adaptive immune responses. Innate immunity occurs naturally due to genetic factors or physiology. It is not induced by infection or vaccination, but is constantly available to reduce the workload for the adaptive immune response. The adaptive immune response expands over time, storing information about past infections and mounting pathogen-specific defenses. Both the innate and adaptive levels of the immune response involve secreted proteins, receptor-mediated signaling, and intricate cell -to-cell communication. From an historical perspective, the innate immune system developed early in animal evolution, roughly a billion years ago, as an essential response to infection. In the innate immune response, any pathogenic threat triggers a consistent sequence of events that can identify the type of pathogen and either clear the infection independently or mobilize a highly-specialized adaptive immune response.
Before any immune factors are triggered, the skin (also known as the epithelial surface) functions as a continuous, impassable barrier to potentially-infectious pathogens. The skin is considered the first defense of the innate immune system; it is the first of the nonspecific barrier defenses. Pathogens are killed or inactivated on the skin by desiccation (drying out) and by the skin’s acidity. In addition, beneficial microorganisms that coexist on the skin compete with invading pathogens, preventing infection. Desquamation, or peeling skin, also serves to dislodge organisms that have adhered to the surface of the body and are awaiting entry. Regions of the body that are not protected by skin (such as the eyes and mucous membranes ) have alternative methods of defense. These include tears in the eyes; mucous membranes that provide partial protection despite having to allow absorption and secretion; mucus secretions that trap and rinse away pathogens; and cilia (singular cilium) in the nasal passages and respiratory tract that push the mucus with the pathogens out of the body. Furthermore, tears and mucus secretions contain microbicidal factors that prevent many infections from entering via these routes.
Despite these barriers, pathogens may enter the body through skin abrasions or punctures, or by collecting on mucosal surfaces in large numbers that overcome the mucus or cilia. Some pathogens have evolved specific mechanisms that allow them to overcome physical and chemical barriers.
Once inside, the body still has many other defenses, including chemical barriers. Some of these include the low pH of the stomach, which inhibits the growth of pathogens; blood proteins that bind and disrupt bacterial cell membranes; and the process of urination, which flushes pathogens from the urinary tract. The blood-brain barrier also protects the nervous system from pathogens that have already entered the blood stream, but would do significantly more damage if they entered the central nervous system.
Key Points
• The skin, or epithelial surface, serves as the primary barrier to microbial entry into the body; skin peeling, drying out, and the skin’s acidity all serve to dislodge or kill foreign pathogens.
• Orifices such as the eyes and mouth, which are not covered by skin, have other mechanisms by which they prevent entry; tears wash away microbes, while cilia in the nasal passages and respiratory tract push mucus (which traps pathogens) out of the body.
• Many chemical barriers also exist once pathogens make it past the outer physical barriers; the acidity of the stomach ensures that few organisms arriving with food survive the digestive system.
Key Terms
• cilium: a hairlike organelle projecting from a eukaryotic cell (such as unicellular organism or one cell of a multicelled organism), which serves either for locomotion by moving or as sensors
• microbicidal: functioning to reduce the infectivity of microbes | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.01%3A_Innate_Immune_Response_-_Innate_Immune_Response.txt |
Learning Objectives
• Describe the role of PAMPs and PRRs, interferons, and other cytokines in innate immunity
When a pathogen enters the body, cells in the blood and lymph detect the specific pathogen-associated molecular patterns (PAMPs) on the pathogen’s surface. PAMPs are carbohydrate, polypeptide, and nucleic acid “signatures” that are expressed by viruses, bacteria, and parasites, but which differ from molecules on host cells. These PAMPs allow the immune system to recognize “self” from “other” so as not to destroy the host.
The immune system has specific cells with receptors that recognize these PAMPs. A macrophage is a large, phagocytic cell that engulfs foreign particles and pathogens. Macrophages recognize PAMPs via complementary pattern recognition receptors (PRRs). PRRs are molecules on macrophages and dendritic cells which are in contact with the external environment and can thus recognize PAMPs when present. A monocyte, a type of leukocyte (white blood cell) that circulates in the blood and lymph, differentiates into macrophages after it moves into infected tissue. Dendritic cells bind molecular signatures of pathogens, promoting pathogen engulfment and destruction.
Once a pathogen is detected, the immune system must also track whether it is replicating intracellularly (inside the cell, as with most viruses and some bacteria) or extracellularly (outside of the cell, as with other bacteria, but not viruses). The innate immune system must respond accordingly by identifying the extracellular pathogen and/or by identifying host cells that have already been infected.
Cytokine release affect
The binding of PRRs with PAMPs triggers the release of cytokines, which signal that a pathogen is present and needs to be destroyed along with any infected cells. A cytokine is a chemical messenger that regulates cell differentiation (form and function), proliferation (production), and gene expression to affect immune responses. At least 40 types of cytokines exist in humans that differ in terms of the cell type that produces them, the cell type that responds to them, and the changes they produce.
One subclass of cytokines is the interleukin (IL), which mediates interactions between leukocytes (white blood cells). Interleukins are involved in bridging the innate and adaptive immune responses. In addition to being released from cells after PAMP recognition, cytokines are released by the infected cells which bind to nearby uninfected cells, inducing those cells to release cytokines, resulting in a cytokine burst.
A second class of cytokines is interferons, which are released by infected cells as a warning to nearby uninfected cells. A function an interferons is to inhibit viral replication, making them particularly effective against viruses. They also have other important functions, such as tumor surveillance. Interferons work by signaling neighboring uninfected cells to destroy RNA (often a very important biomolecule for viruses) and reduce protein synthesis; signaling neighboring infected cells to undergo apoptosis (programmed cell death); and activating immune cells.
Cytokines also send feedback to cells of the nervous system to bring about the overall symptoms of feeling sick, which include lethargy, muscle pain, and nausea. These effects may have evolved because the symptoms encourage the individual to rest, preventing them from spreading the infection to others. Cytokines also increase the core body temperature, causing a fever, which causes the liver to withhold iron from the blood. Without iron, certain pathogens (such as some bacteria) are unable to replicate; this is called nutritional immunity.
Phagocytosis and inflammation
The first cytokines to be produced are pro-inflammatory; that is, they encourage inflammation, or the localized redness, swelling (edema), heat, loss of function, and pain that result from the movement of leukocytes and fluid through increasingly-permeable capillaries to a site of infection. The population of leukocytes that arrives at an infection site depends on the nature of the infecting pathogen. Both macrophages and dendritic cells engulf pathogens and cellular debris through phagocytosis. A neutrophil is also a phagocytic leukocyte that engulfs and digests pathogens. Neutrophils, the most-abundant leukocytes of the immune system, have a nucleus with two to five lobes and contain organelles (lysosomes) that digest engulfed pathogens. An eosinophil is a leukocyte that works with other eosinophils to surround a parasite. It is involved in the allergic response and in protection against helminthes (parasitic worms).
Neutrophils and eosinophils are particularly important leukocytes that engulf large pathogens, such as bacteria and fungi. A mast cell is a leukocyte that produces inflammatory molecules, such as histamine, in response to large pathogens. A basophil is a leukocyte that, like a neutrophil, releases chemicals to stimulate the inflammatory response. Basophils are also involved in allergy and hypersensitivity responses and induce specific types of inflammatory responses. Eosinophils and basophils produce additional inflammatory mediators to recruit more leukocytes. A hypersensitive immune response to harmless antigens, such as in pollen, often involves the release of histamine by basophils and mast cells; this is why many anti-allergy medications are anti-histamines.
Key Points
• Pathogens are recognized by a variety of immune cells, such as macrophages and dendritic cells, via pathogen-associated molecular patterns (PAMPs) on the pathogen surface, which interact with complementary pattern-recognition receptors (PRRs) on the immune cells’ surfaces.
• Upon binding of PRRs with PAMPs (pathogen recognition), immune cells release cytokines to tell other cells to start fighting back.
• One class of cytokines, interferons, warn nearby uninfected cells of impending infection, cause cells to start cleaving RNA and reduce protein synthesis, and signal nearby infected cells to undergo apoptosis.
• Another class of cytokines, called inerleukins, mediate interactions between white blood cells ( leukocytes ) and help bridge the innate and adaptive immune responses.
• Inflammation (hot, red, swollen, painful tissue associated with infection) is encouraged by cytokines that are produced immediately upon pathogen recognition; the increase in blood flow associated with inflammation allows more leukocytes (a type of innate immune cell) to reach the infected area.
Key Terms
• macrophage: a white blood cell that phagocytizes necrotic cell debris and foreign material, including viruses, bacteria, and tattoo ink; part of the innate immune system
• phagocytosis: the process where a cell incorporates a particle by extending pseudopodia and drawing the particle into a vacuole of its cytoplasm
• cytokine: any of various small regulatory proteins that regulate the cells of the immune system; they are released upon binding of PRRs to PAMPS | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.03%3A_Innate_Immune_Response_-_Pathogen_Recognition.txt |
Learning Objectives
• Describe the role of natural killer cells in the immune response
Lymphocytes are leukocytes (white blood cells) that are histologically identifiable by their large, darkly-staining nuclei; they are small cells with very little cytoplasm. After a pathogen enters the body, infected cells are identified and destroyed by natural killer (NK) cells, which are a type of lymphocyte that can kill cells infected with viruses or tumor cells (abnormal cells that uncontrollably divide and invade other tissue). While NK cells are part of the innate immune response, they are best understood relative to their counterparts in the adaptive immune response,T cells, which are also classified as lymphocytes.
T cells are lymphocytes that mature in the thymus gland and identify intracellular infections, especially from viruses, by the altered expression of major histocompatibility class (MHC) I molecules on the surface of infected cells. MHC I molecules are proteins on the surfaces of all nucleated cells which help the immune system distinguish between “self” and “non-self.” If the cell is infected, the MHC I molecules display fragments of proteins from the infectious agents to T-cells. Healthy cells do not display any proteins and will be ignored by the immune system, while the cells identified as “non-self” by foreign proteins will be attacked by the immune system.
An infected cell (or a tumor cell) is often incapable of synthesizing and displaying MHC I molecules appropriately. The metabolic resources of cells infected by some viruses produce proteins that interfere with MHC I processing and/or trafficking to the cell surface. The reduced MHC I on host cells varies from virus to virus and results from active inhibitors being produced by the viruses. This process can deplete host MHC I molecules on the cell surface, which prevents T-cells from recognizing them, but which NK cells detect as “unhealthy” or “abnormal” while searching for cellular MHC I molecules. As such, NK cells offer a complementary check for unhealthy cells, relative to T cells. Similarly, the dramatically-altered gene expression of tumor cells leads to expression of extremely- deformed or absent MHC I molecules that also signal “unhealthy” or “abnormal.”
NK cells are always active; an interaction with normal, intact MHC I molecules on a healthy cell disables the killing sequence, causing the NK cell to move on. After the NK cell detects an infected or tumor cell, its cytoplasm secretes granules comprised of perforin: a destructive protein that creates a pore in the target cell. Granzymes are released along with the perforin in the immunological synapse. A granzyme, a protease that digests cellular proteins, induces the target cell to undergo programmed cell death, or apoptosis. Phagocytic cells then digest the cell debris left behind. NK cells are constantly patrolling the body. They are an effective mechanism for controlling potential infections and preventing cancer progression.
Key Points
• Natural killer (NK) cells are lymphocytes (a subclass of white blood cells) that recognize infected or tumorogenic cells and kill them.
• Unlike the related T cells, NK cells do not recognize fragments of the infecting particle, but rather the incorrect display of major histocompatibility complex ( MHC ) I molecules.
• NK cells are always active, but will not perform their killing function on cells with intact MHC I molecules.
• When NK cells detect an infected or tumor cell, they secrete granules that contain perforin, creating a pore in the target cell; granzymes then pass through these pores, degrading cellular proteins, causing cells to undergo apoptosis.
Key Terms
• lymphocyte: a type of white blood cell or leukocyte that is divided into two principal groups and a null group: B-cells, T-cells, and natural killer (NK) cells
• major histocompatibility complex: a protein present on the extracellular surface of the cell that displays portions of the proteins that are degraded inside the cell
• T cell: a lymphocyte, from the thymus, that can recognize specific antigens and can activate or deactivate other immune cells
42.05: Innate Immune Response - The Complement System
Learning Objectives
• Explain how the complement system aids antibody response
Complement
The innate immune system serves as a first responder to pathogenic threats that bypass natural physical and chemical barriers of the body. Using a combination of cellular and molecular attacks, the innate immune system identifies the nature of a pathogen and responds with inflammation, phagocytosis (where a cell engulfs a foreign particle), cytokine release, destruction by NK cells, and/or a complement system. In this concept, we will discuss the complement system.
An array of approximately 20 types of soluble proteins, called a complement system, functions to destroy extracellular pathogens. Cells of the liver and macrophages synthesize complement proteins continuously. These proteins are abundant in the blood serum and are capable of responding immediately to infecting microorganisms. The complement system is so named because it is complementary to the antibody response of the adaptive immune system. Complement proteins bind to the surfaces of microorganisms and are particularly attracted to pathogens that are already bound by antibodies. Binding of complement proteins occurs in a specific and highly-regulated sequence, with each successive protein being activated by cleavage and/or structural changes induced upon binding of the preceding protein(s). After the first few complement proteins bind, a cascade of sequential binding events follows in which the pathogen rapidly becomes coated in complement proteins.
Complement proteins perform several functions. They serve as a marker to indicate the presence of a pathogen to phagocytic cells, such as macrophages and B cells, to enhance engulfment. This process is called opsonization. Certain complement proteins can combine to form attack complexes that open pores in microbial cell membranes. These structures destroy pathogens by causing their contents to leak. When innate mechanisms are insufficient to clear an infection, the adaptive immune response is informed and mobilized.
Key Points
• The complement system is so named because it is complementary to the antibody response of the adaptive immune system.
• The complement system proteins are produced continuously by the liver and macrophages, are abundant in the blood serum, and are capable of immediate response to infecting microorganisms.
• The complement system works by first having several proteins bind to a target; this binding event then begins a series of highly-specific and regulated sequences wherein successive proteins are activated by cleavage and/or structural changes of the preceding proteins.
• The complement system serves as a marker to indicate targets for phagocytic cells; complement proteins can also combine to form attack complexes capable of opening pores in microbial cell membranes.
Key Terms
• opsonization: the process by which a pathogen is marked for ingestion and destruction by a phagocyte
• complement system: an aspect of the innate immune system that supplements the actions of the antibodies and phagocytic cells in clearing out pathogens from an organism | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.04%3A_Innate_Immune_Response_-_Natural_Killer_Cells.txt |
Learning Objectives
• Explain the role played by B and T cells in the adaptive immune system
Introduction
The adaptive, or acquired, immune response to an initial infection takes days or even weeks to become established, much longer than the innate response. However, adaptive immunity is more specific to an invading pathogen and can fight back much more quickly than the innate response if it has seen the pathogen before. Adaptive immunity occurs after exposure to an antigen either from a pathogen or a vaccination. An antigen is a molecule that binds to a specific antibody, often stimulating a response in the immune system as a result.
The adaptive immune response activates when the innate immune response insufficiently controls an infection. In fact, without information from the innate immune system, the adaptive response could not be mobilized. There are two types of adaptive responses: the cell-mediated immune response, which is controlled by activated T cells, and the humoral immune response, which is controlled by activated B cells and antibodies. Upon infection, activated T and B cells that have surface binding sites with specificity to the molecules on the pathogen greatly increase in number and attack the invading pathogen. Their attack can kill pathogens directly or they can secrete antibodies that enhance the phagocytosis of pathogens and disrupt the infection. Adaptive immunity also involves a memory, which gives the host long-term protection from reinfection by the same type of pathogen; upon re-exposure, this host memory will facilitate a rapid and powerful response.
B and T Cells
Lymphocytes, which are white blood cells, are formed with other blood cells in the red bone marrow found in many flat bones, such as the shoulder or pelvic bones. The two types of lymphocytes of the adaptive immune response are B and T cells. Whether an immature lymphocyte becomes a B cell or T cell depends on where in the body it matures. The B cells remain in the bone marrow to mature (hence the name “B” for “bone marrow”), while T cells migrate to the thymus, where they mature (hence the name “T” for “thymus”).
B Cell Receptors
The maturation of a B or T cell involves becoming immunocompetent, meaning that it can recognize and bind to a specific molecule or antigen. This recognition, which is central to the functioning of the adaptive immune response, results from the presence of highly specific receptors on the surfaces of B and T cells. On B cells, these receptors contain antibodies, which are responsible for antigen binding. An antibody is specific for one particular antigen; typically, it will not bind to anything else. Upon antigen binding to a B cell receptor, a signal is sent into the B cell to turn on an immune response.
T Cell Receptors
Meanwhile, T cell receptors are responsible for the recognition of pathogenic antigens by T cells. Unlike B cells, T cells do not directly recognize antigens. Instead, they recognize antigens presented on major histocompatibility complexes ( MHCs ) that cells use to display which proteins are inside of them. If a cell is infected, it will present antigenic portions of the infecting pathogen on its MHC for recognition by T cells, which will then mount an appropriate immune response. Unlike antibodies, which can typically bind one and only one antigen, T cell receptors have more flexibility in their capacity to recognize antigens presented by MHCs.
It is the specific pathogen recognition (via binding antigens) of B and T cells that allows the adaptive immune response to adapt. During the maturation process, B and T cells that bind too strongly to the body’s own cells’ antigens are eliminated in order to minimize an immune response against the body’s own tissues. Only those cells that react weakly to the body’s own cells will remain. This process occurs during fetal development and continues throughout life. Once they are immunocompetent, the T and B cells migrate to the spleen and lymph nodes where they remain until they are called on during an infection. B cells are involved in the humoral immune response, which targets pathogens loose in blood and lymph, while T cells are involved in the cell-mediated immune response, which targets infected cells.
Key Points
• The adaptive immune response is slower to develop than the innate immune response, but it can act much more powerfully and quickly than the innate immune response against pathogens that it has seen before.
• B and T cells are lymphocytes, or white blood cells, which are able to recognize antigens that distinguish “self” from “other” in the body.
• B and T cells that recognize “self” antigens are destroyed before they can mature; this helps to prevent the immune system from attacking its own body.
Key Terms
• B cell: a lymphocyte, developed in the bursa of birds and the bone marrow of other animals, that produces antibodies and is responsible for the immune system
• T cell: a lymphocyte, from the thymus, that can recognize specific antigens and can activate or deactivate other immune cells
• antigen: a substance that binds to a specific antibody; may cause an immune response | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.06%3A_Adaptive_Immune_Response_-_Antigen-presenting_Cells-_B_and_T_cells.txt |
Learning Objectives
• Summarize the humoral immune response
The humoral immune response fights pathogens that are free in the bodily fluids, or “humours”. It relies on antigens (which are also often free in the humours) to detect these pathogens. An antigen is a biomolecule, such as a protein or sugar, that binds to a specific antibody. An antibody/antigen interaction may stimulate an immune response. Not every biomolecule is antigenic and not all antigens produce an immune response. B cells are the major cell type involved in the humoral immune response. When a foreign antigen (one coming from a pathogen, for example) is detected, B cells in the body that recognize that antigen will begin to produce antibodies as a means of fighting off the foreign invader.
B cell maturation
During maturation, B cells gain antigen receptor molecules, termed B cell receptors (BCRs), which are displayed in large numbers, extracellularly on their membrane. These membrane-bound protein complexes contain antibodies, which enable specific antigen recognition. Each B cell initially produced has only one kind of antibody (antigen receptor), which makes every B cell unique. It is the immense number of B cells in the body, each of which produces a unique antibody, that allows the immune system to detect such a wide variety of pathogenic antigens. B cells containing antibodies that recognize “self” antigens are destroyed before they can mature, preventing the immune system from attacking the host. Once B cells mature in the bone marrow, they migrate to lymph nodes or other lymphatic organs, where they may begin to encounter pathogens.
B cell activation
When a B cell encounters the antigen that binds to its receptor, the antigen molecule is brought into the cell by endocytosis, reappearing on the surface of the cell bound to an MHC class II molecule. When this process is complete, the B cell is sensitized. In most cases, the sensitized B cell must then encounter a specific kind of T cell, called a helper T cell, before it is activated. This activation of the helper T cell occurs when a dendritic cell presents an antigen on its MHC II molecule, allowing the T cell to recognize it and mature.
The helper T cell binds to the antigen-MHC class II complex and is induced to release cytokines that induce the B cell to divide rapidly, making thousands of identical (clonal) cells. These daughter cells become either plasma cells or memory B cells. The memory B cells remain inactive at this point. A later encounter with the antigen, caused by a reinfection by the same bacteria or virus, will result in them dividing into a new population of plasma cells. The plasma cells, on the other hand, produce and secrete large quantities, up to 100 million molecules per hour, of antibody molecules. An antibody, also known as an immunoglobulin (Ig), is a protein that is produced by plasma cells after stimulation by an antigen. Antibodies are the agents of humoral immunity. Antibodies occur in the blood, in gastric and mucus secretions, and in breast milk. Antibodies in these bodily fluids can bind pathogens and mark them for destruction by phagocytes before they are able to infect cells.
Antibodies
These antibodies circulate in the blood stream and lymphatic system, binding with the antigen whenever it is encountered. The binding can fight infection in several ways. Antibodies can bind to viruses or bacteria, which interferes with the chemical interactions required for them to infect or bind to other cells. The antibodies may create bridges between different particles containing antigenic sites, clumping them all together and preventing their proper functioning. Antibody neutralization can prevent pathogens from entering and infecting host cells. The neutralized antibody-coated pathogens can then be filtered by the spleen to be eliminated in urine or feces. The antigen-antibody complex stimulates the complement system described previously, destroying the cell bearing the antigen. Antibodies also opsonize pathogen cells, wherein they mark them for destruction by phagocytic cells, such as macrophages or neutrophils. Additionally, antibodies stimulate inflammation, while their presence in mucus and on the skin prevents pathogen attack.
The production of antibodies by plasma cells in response to an antigen is called active immunity. This describes the host’s active response of the immune system to an infection or to a vaccination. There is also a passive immune response wherein antibodies are introduced into the host from an outside source, instead of the individual’s own plasma cells. For example, antibodies circulating in a pregnant woman’s body move across the placenta into the developing fetus. The child benefits from the presence of these antibodies for up to several months after birth. In addition, a passive immune response is possible by injecting antibodies into an individual in the form of an antivenom to a snake-bite toxin or antibodies in blood serum to help fight a hepatitis infection, giving immediate relief.
Key Points
• Antigens are proteins and other macromolecules that bind to a specific antibody and are used by the immune system to recognize pathogens.
• B cells express receptors (BCRs) on their membrane which contain antibodies; these antibodies allow B cells to detect pathogens and release further antibodies to fight the infection.
• Antibodies fight infections in three ways: they mark pathogens for destruction by phagocytic cells in a process known as opsonization, they coat key sites on pathogens necessary for infection, and they induce the complement cascade to occur against antibody-bound pathogens.
• Once the adaptive immune response has encountered an antigen, B cells will divide to produce plasma cells, which rapidly secrete antibodies to that antigen in a process called active immunity.
Key Terms
• antibody: a protein produced by B-lymphocytes that binds to a specific antigen
• opsonize: to make (bacteria or other cells) more susceptible to the action of phagocytes by use of opsonins
• MHC: an acronym for major histocompatibility complex; these extracellular protein receptors display antigens derived from extracellular (class I) or intracellular (class II) proteins and other biomolecules | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.07%3A_Adaptive_Immune_Response_-_Humoral_Immune_Response.txt |
Learning Objectives
• Summarize the cell-mediated immune response
T cells
Just as the humoral immune response has B cells which mediate its response, the cellular immune response has T cells, which recognize infected cells and destroy them before the pathogen inside can replicate and spread to infect other cells. Unlike B cells, T lymphocytes (T cells) are unable to recognize pathogens without assistance. First, an antigen-presenting cell (APC, such as a dendritic cell or a macrophage ) detects, engulfs (via phagocytosis in the case of macrophages or by entry of the pathogen of its own accord in the case of dendritic cells), and digests pathogens into hundreds or thousands of antigen fragments. These fragments are then transported to the surface of the APC, where they are presented on proteins known as Major Histocompatibility Complexes class II (MHC II, see ). T cells become activated towards a certain antigen once they encounter it displayed on an MHC II. After a virus or bacteria enters a cell, it can no longer be detected by the humoral immune response. Instead, the cellular immune response must take over. To do so, a T cell will become activated by interacting with an antigen of the infecting cell or virus presented on the MHC II of an APC.
Cytotoxic T cells mediate one arm of the cellular immune response
There are two main types of T cells: helper T lymphocytes (TH) and the cytotoxic T lymphocytes (TC). The TH lymphocytes function indirectly to tell other immune cells about potential pathogens, while cytotoxic T cells (TC) are the key component of the cell-mediated part of the adaptive immune system which attacks and destroys infected cells. TC cells are particularly important in protecting against viral infections because viruses replicate within cells where they are shielded from extracellular contact with circulating antibodies. Once activated, the TC creates a large clone of cells with one specific set of cell-surface receptors, similar to the proliferation of activated B cells. As with B cells, the clone includes active TC cells and inactive memory TC cells. The resulting active TC cells then identify infected host cells.
TC cells attempt to identify and destroy infected cells by triggering apoptosis (programmed cell death) before the pathogen can replicate and escape, thereby halting the progression of intracellular infections. To recognize which cells to pursue, TC recognize antigens presented on MHC I complexes, which are present on all nucleated cells. MHC I complexes display a current readout of intracellular proteins inside a cell and will present pathogen antigens if the pathogen is present in the cell. TC cells also support NK lymphocytes to destroy early cancers.
Cytokines released by TH cells recruit NK cells and phagocytes
Cytokines are signaling molecules secreted by a TH cell in response to a pathogen-infected cell; they stimulate natural killer cells and phagocytes such as macrophages. Phagocytes will then engulf infected cells and destroy them. Cytokines are also involved in stimulating TC cells, enhancing their ability to identify and destroy infected cells and tumors. A summary of how the humoral and cell-mediated immune responses are activated appears in. B plasma cells and TC cells are collectively called effector cells because they are involved in “effecting” (bringing about) the immune response of killing pathogens and infected host cells.
Key Points
• Once a pathogen enters a cell, it can no longer be detected by the humoral immune response; instead, the cell-mediated immune response must take over to kill the infected cell before it can allow the virus or bacteria to replicate and spread.
• T cells recognize infected cells by interacting with antigen present on their MHC II molecules; before a T cell can do so, it must be activated via interaction with an antigen presenting cell, or APC.
• Once a cytotoxic T cell (TC) is activated, it will clone itself, producing many TC cells with the correct receptors; some portion of the cells are active and will help destroy infected cells, while others are inactive memory cells that will create more active TC cells if the infection returns.
• Helper T cells (TH cells) also aid in cell-mediated immunity by releasing signaling molecules known as cytokines which can recruit natural killer cells and phagocytes to destroy infected cells and further activate TC cells; they do not directly destroy pathogens.
Key Terms
• cytotoxic T cell: a subgroup of lymphocytes (white blood cells) that are capable of inducing death to infected somatic or tumor cells; part of cell-mediated immunity
• cytokine: any of various small regulatory proteins that regulate the cells of the immune system; they are released upon binding of PRRs to PAMPS | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.08%3A_Adaptive_Immune_Response_-_Cell-Mediated_Immunity.txt |
Learning Objectives
• Describe the features of the lymphatic system as they relate to the immune response
Lymphatic system
Lymph, the watery fluid that bathes tissues and organs, contains protective white blood cells, but does not contain erythrocytes (red blood cells). Lymph moves about the body through the lymphatic system, which is made up of vessels, lymph ducts, lymph glands, and organs such as tonsils, adenoids, thymus, and spleen. Although the immune system is characterized by circulating cells throughout the body, the regulation, maturation, and intercommunication of immune factors occur at specific sites that are known as lymph nodes.
The blood circulates immune cells, proteins, and other factors through the body. Approximately 0.1 percent of all cells in the blood are leukocytes, which include monocytes (the precursor of macrophages) and lymphocytes. Most cells in the blood are red blood cells. Cells of the immune system can travel between the distinct lymphatic and blood circulatory systems, which are separated by interstitial space, by a process called extravasation (passing through to surrounding tissue).
Recall that cells of the immune system originate from stem cells in the bone marrow. B cell maturation occurs in the bone marrow, whereas progenitor cells migrate from the bone marrow and develop and mature into naïve T cells in the organ called the thymus. On maturation, T and B lymphocytes circulate to various destinations. Lymph nodes scattered throughout the body house large populations of T and B cells, dendritic cells, and macrophages. Lymph gathers antigens as it drains from tissues. These antigens are filtered through lymph nodes before the lymph is returned to circulation. Antigen-presenting cells (APCs) in the lymph nodes capture and process antigens, informing nearby lymphocytes about potential pathogens.
The spleen houses B and T cells, macrophages, dendritic cells, and NK cells. The spleen is also the site where APCs that have trapped foreign particles in the blood can communicate with lymphocytes. Antibodies are synthesized and secreted by activated plasma cells in the spleen, which filters foreign substances and antibody-complexed pathogens from the blood. Functionally, the spleen is to the blood as lymph nodes are to the lymph.
Key Points
• The lymphatic system contains lymph: a fluid that bathes tissues and organs and contains white blood cells (not red blood cells).
• Once B and T cells mature, the majority of them enter the lymphatic system, where they are stored in lymph nodes until needed.
• Lymph nodes also store dendritic cells and macrophages; as antigens are filtered through the lymphatic system, these cells collect them so as to present them to B and T cells.
• The spleen, which is to blood what lymph nodes are to lymph, filters foreign substances and antibody -complexed pathogens from the blood.
Key Terms
• lymph: a colorless, watery, bodily fluid carried by the lymphatic system, consisting mainly of white blood cells
42.10: Adaptive Immune Response - Immunological Memory
Learning Objectives
• Describe the role of memory B and T cells in immunological memory
The adaptive immune system has a memory component that allows for a rapid and large response upon re-invasion of the same pathogen. During the adaptive immune response to a pathogen that has not been encountered before, known as the primary immune response, plasma cells secreting antibodies and differentiated T cells increase, then plateau over time. As B cells and T cells mature into effector cells, a subset of the naïve populations differentiates into B and T memory cells with the same antigen specificities. A memory cell is an antigen-specific B or T lymphocyte that does not differentiate into an effector cell during the primary immune response, but that can immediately become an effector cell on re-exposure to the same pathogen. As the infection is cleared and pathogenic stimuli subside, the effector cells are no longer needed; they undergo apoptosis. In contrast, the memory cells persist in circulation.
If the pathogen is not encountered again during the individual’s lifetime, B and T memory cells will circulate for a few years or even several decades, gradually dying off, having never functioned as effector cells. However, if the host is re-exposed to the same pathogen type, circulating memory cells will immediately differentiate into plasma cells and TC cells without input from APCs or TH cells. This is known as the secondary immune response. One reason why the adaptive immune response is delayed is that it takes time for naïve B and T cells with the appropriate antigen specificities to be identified, activated, and proliferate. On reinfection, this step is skipped. The result is a more rapid production of immune defenses. Memory B cells that differentiate into plasma cells output ten to hundred-fold greater antibody amounts than were secreted during the primary response. This rapid and dramatic antibody response may stop the infection before it can even become established. Individuals may not realize they had been exposed.
Vaccination is based on the knowledge that exposure to noninfectious antigens, derived from known pathogens, generates a mild primary immune response. The immune response to vaccination may not be perceived by the host as illness, but still confers immune memory. When exposed to the corresponding pathogen to which an individual was vaccinated, the reaction is similar to a secondary exposure. Because each reinfection generates more memory cells and increased resistance to the pathogen, some vaccine courses involve one or more booster vaccinations to mimic repeat exposures.
Key Points
• During the adaptive immune response to a pathogen that the body has not previously encountered (the primary immune response), the level of plasma cells that secrete antibodies to the new pathogen and T cells that recognize the pathogen will increase at a modest pace.
• If a pathogen is re-encountered, memory B and T cells can immediately differentiate into plasma cells and cytotoxic T cells without input from APCs or TH cells; this secondary immune response occurs much more rapidly than the primary immune response.
• Vaccination can be used to generate a mild, primary immune response against an inactivated pathogen, which will allow the secondary immune response to function the first time the immune system encounters the actual pathogen.
Key Terms
• vaccination: inoculation in order to protect against a particular disease or strain of disease; causes a primary immune response without illness, allowing the secondary response to destroy subsequent infection
• effector cell: a plasma B cell or cytotoxic T cell, which are the main types of cells responsible for the humoral and cellular immune responses, respectively | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.09%3A_Adaptive_Immune_Response_-_Cytotoxic_T_Lymphocytes_and_Mucosal_Surfaces.txt |
Learning Objectives
• Describe the process by which immune tolerance is acquired
Mucosal surfaces
The innate and adaptive immune responses discussed thus far comprise the systemic immune system (affecting the whole body), which is distinct from the mucosal immune system. Mucosa are another name for mucous membranes. Mucosal immunity is formed by mucosa-associated lymphoid tissue, or MALT, which functions independently of the systemic immune system; it has its own innate and adaptive components. MALT is a collection of lymphatic tissue that combines with epithelial tissue lining the mucosa throughout the body. This tissue functions as the immune barrier and immune response in areas of the body in direct contact to the external environment. The systemic and mucosal immune systems use many of the same cell types. Foreign particles that make their way to MALT are taken up by absorptive epithelial cells called M cells and delivered to APCs (antigen-presenting cells) located directly below the mucosal tissue. M cells are located in the Peyer’s patch, which is a lymphoid nodule. APCs of the mucosal immune system are primarily dendritic cells, with B cells and macrophages playing minor roles. Processed antigens displayed on APCs are detected by T cells in the MALT and at various mucosal induction sites, such as the tonsils, adenoids, appendix, or the mesenteric lymph nodes of the intestine. Activated T cells then migrate through the lymphatic system and into the circulatory system to mucosal sites of infection.
MALT is a crucial component of a functional immune system because mucosal surfaces, such as the nasal passages, are the first tissues onto which inhaled or ingested pathogens are deposited. This allows the immune system to detect and deal with pathogens very quickly after they enter the body through various mucous membranes. The mucosal tissue includes the mouth, pharynx, and esophagus, along with the gastrointestinal, respiratory, and urogenital tracts.
Immune tolerance
The immune system has to be regulated to prevent wasteful, unnecessary responses to harmless substances and, more importantly, so that it does not attack “self.” The acquired ability to prevent an unnecessary or harmful immune response to a detected foreign substance known not to cause disease or to self-antigens is described as immune tolerance. The primary mechanism for developing immune tolerance to self-antigens occurs during the selection for weakly, self-binding cells during T and B lymphocyte maturation. Any T or B lymphocytes that recognize harmless foreign or “self” antigens are deleted before they can fully mature into immunocompetent cells.
There are populations of T cells that suppress the immune response to self-antigens. They also suppress the immune response after the infection has cleared to minimize host cell damage induced by inflammation and cell lysis. Immune tolerance is especially well developed in the mucosa of the upper digestive system because of the tremendous number of foreign substances (such as food proteins) that APCs of the oral cavity, pharynx, and gastrointestinal mucosa encounter. Immune tolerance is brought about by specialized APCs in the liver, lymph nodes, small intestine, and lung that present harmless antigens to a diverse population of regulatory T (Treg) cells: specialized lymphocytes that suppress local inflammation and inhibit the secretion of stimulatory immune factors. The combined result of Treg cells is to prevent immunologic activation and inflammation in undesired tissue compartments, allowing the immune system to focus on hazardous pathogens instead.
Key Points
• Mucosal surfaces are those that are in contact with air, but, unlike the skin, allow fluid to flow in and out of them.
• Due to the location of mucosal surfaces, they are often the first sites to encounter new antigens.
• The immune system has to be regulated to prevent wasteful, unnecessary responses to harmless substances and, more importantly, so that it does not attack “self”.
• Mucosal-associated lymphoid tissue, or MALT, is involved in immune tolerance to harmless, foreign antigens.
Key Terms
• self-antigen: antigens (substances that bind to antibodies) that are usually well tolerated by the immune system, which has been educated to non-reactivity against the structures
• mucous membrane: a membrane which secretes mucus; it forms the lining of body passages that contact the air, such as the respiratory and genitourinary tracts including the mouth, nasal passages, vagina and urethra | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.11%3A_Adaptive_Immune_Response_-_Regulating_Immune_Tolerance.txt |
Learning Objectives
• Differentiate among the classes of antibodies
An antibody is a molecule that recognizes a specific antigen; this recognition is a vital component of the adaptive immune response. Antibodies are composed of four polypeptides: two identical heavy chains (large peptide units) that are partially bound to each other in a “Y” formation, which are flanked by two identical light chains (small peptide units). The area where the antigen is recognized on the antibody is known as the variable domain or variable region. This is why there are numerous antibodies that can each recognize a different antigen. The antibody base is known as the constant domain or constant region. The portion of an antigen that is recognized by the antibody is known as the epitope.
Antibody variation
In B cells, the variable region of the light chain gene has 40 variable (V) and five joining (J) segments. An enzyme called DNA recombinase randomly excises most of these segments out of the gene, splicing one V segment to one J segment. During RNA processing, all but one V and J segment are spliced out. Recombination and splicing may result in over 106 possible VJ combinations. As a result, each differentiated B cell in the human body typically has a unique variable chain. The constant domain, which does not bind to an antibody, is the same for all antibodies. The large diversity of antibody structure translates into the large diversity of antigens that antibodies can bind and recognize.
Similar to TCRs (T cell receptors) and BCRs (B cell receptors), antibody diversity is produced by the mutation and recombination of approximately 300 different gene segments encoding the light and heavy chain variable domains in precursor cells that are destined to become B cells. The variable domains from the heavy and light chains interact to form the binding site through which an antibody can bind a specific epitope on an antigen. The numbers of repeated constant domains in Ig classes (discussed below) are the same for all antibodies corresponding to a specific class. Antibodies are structurally similar to the extracellular component of the BCRs. The maturation of B cells into plasma cells occurs when the cells gain the ability to secrete the antibody portion of its BCR in large quantities.
Antibody Classes
Antibodies can be divided into five classes (IgM, IgG, IgA, IgD, and IgE) based on their physiochemical, structural, and immunological properties. Ig stands for immunoglobulin, another term for an antibody. IgGs, which make up about 80 percent of all antibodies in circulation, have heavy chains that consist of one variable domain and three identical constant domains. IgA and IgD also have three constant domains per heavy chain, whereas IgM and IgE each have four constant domains per heavy chain. The variable domain determines binding specificity, while the constant domain of the heavy chain determines the immunological mechanism of action of the corresponding antibody class. It is possible for two antibodies to have the same binding specificities, but be in different classes and, therefore, to be involved in different functions.
After an adaptive defense is produced against a pathogen, typically plasma cells first secrete IgM into the blood. BCRs on naïve B cells are of the IgM class and, occasionally, the IgD class. IgM molecules comprise approximately ten percent of all antibodies. Prior to antibody secretion, plasma cells assemble IgM molecules into pentamers (five individual antibodies) linked by a joining (J) chain. The pentamer arrangement means that these macromolecules can bind ten identical antigens. However, IgM molecules released early in the adaptive immune response do not bind to antigens as stably as do IgGs, which are one of the possible types of antibodies secreted in large quantities upon re-exposure to the same pathogen. The properties of immunoglobulins and their basic structures are shown in the table.
IgAs populate the saliva, tears, breast milk, and mucus secretions of the gastrointestinal, respiratory, and genitourinary tracts. Collectively, these bodily fluids coat and protect the extensive mucosa (4000 square feet in humans). The total number of IgA molecules in these bodily secretions is greater than the number of IgG molecules in the blood serum. A small amount of IgA is also secreted into the serum in monomeric form. Conversely, some IgM is secreted into bodily fluids of the mucosa. Similarly to IgM, IgA molecules are secreted as polymeric structures linked with a J chain. However, IgAs are secreted mostly as dimeric molecules, not pentamers.
IgE is present in the serum in small quantities and is best characterized in its role as an allergy mediator. IgD is also present in small quantities. Similarly to IgM, BCRs containing the IgD class of antibodies are found on the surface of naïve B cells. This class supports antigen recognition and subsequent maturation of B cells to plasma cells.
Key Points
• Antibodies contain four polypeptides: two identical (to each other) heavy chains in a “Y” formation and two idenitical (to each other) light chains on the outside of the top of the “Y” portion.
• Each antibody has a unique variable region, which is responsible for antigen detection and specificity.
• There are five classes of antibodies, each utilized by the body under different conditions, including IgM, IgG, IgA, IgD, and IgE; Ig stands for immunoglobulin.
• IgAs, secreted in the milk, tears and mucous, are the most numerous antibodies produced; inside of the body, circulating IgGs are the most abundant.
Key Terms
• immunoglobulin: any of the glycoproteins in blood serum that respond to invasion by foreign antigens and that protect the host by removing pathogens; also known as an antibody
• antigen: a substance that binds to a specific antibody; may cause an immune response
• B cell: a lymphocyte, developed in the bursa of birds and the bone marrow of other animals, that produces antibodies and is responsible for the immune system
• epitope: that part of a biomolecule (such as a protein) that is the target of an immune response; the part of the antigen recognized by the immune system | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.12%3A_Antibodies_-_Antibody_Structure.txt |
Learning Objectives
• Differentiate among affinity, avidity, and cross-reactivity in antibodies
Differentiated plasma cells are crucial players in the humoral immunity response. The antibodies they secrete are particularly significant against extracellular pathogens and toxins. Once secreted, antibodies circulate freely and act independently of plasma cells. Sometimes, antibodies can be transferred from one individual to another. For instance, a person who has recently produced a successful immune response against a particular disease agent can donate blood to a non-immune recipient, confering temporary immunity through antibodies in the donor’s blood serum. This phenomenon, called passive immunity, also occurs naturally during breastfeeding, which makes breastfed infants highly resistant to infections during the first few months of life.
Antibodies coat extracellular pathogens and neutralize them by blocking key sites on the pathogen that enhance their infectivity, such as receptors that “dock” pathogens on host cells. Antibody neutralization can prevent pathogens from entering and infecting host cells, as opposed to the cytotoxic T-cell-mediated approach of killing cells that are already infected to prevent progression of an established infection. The neutralized antibody-coated pathogens can then be filtered by the spleen and eliminated in urine or feces.
Antibodies also mark pathogens for destruction by phagocytic cells, such as macrophages or neutrophils, because they are highly attracted to macromolecules complexed with antibodies. Phagocytic enhancement by antibodies is called opsonization. In another process, complement fixation, IgM and IgG in serum bind to antigens, providing docking sites onto which sequential complement proteins can bind. The combination of antibodies and complement enhances opsonization even further, promoting rapid clearing of pathogens.
Affinity, avidity, and cross reactivity
Not all antibodies bind with the same strength, specificity, and stability. In fact, antibodies exhibit different affinities (attraction) depending on the molecular complementarity between antigen and antibody molecules. An antibody with a higher affinity for a particular antigen would bind more strongly and stably. It would be expected to present a more challenging defense against the pathogen corresponding to the specific antigen.
The term avidity describes binding by antibody classes that are secreted as joined, multivalent structures (such as IgM and IgA). Although avidity measures the strength of binding, just as affinity does, the avidity is not simply the sum of the affinities of the antibodies in a multimeric structure. The avidity depends on the number of identical binding sites on the antigen being detected, as well as other physical and chemical factors. Typically, multimeric antibodies, such as pentameric IgM, are classified as having lower affinity than monomeric antibodies, but high avidity. Essentially, the fact that multimeric antibodies can bind many antigens simultaneously balances their slightly-lower-binding strength for each antibody/antigen interaction.
Antibodies secreted after binding to one epitope on an antigen may exhibit cross reactivity for the same or similar epitopes on different antigens. Cross reactivity occurs when an antibody binds not to the antigen that elicited its synthesis and secretion, but to a different antigen. Because an epitope corresponds to such a small region (the surface area of about four to six amino acids), it is possible for different macromolecules to exhibit the same molecular identities and orientations over short regions.
Cross reactivity can be beneficial if an individual develops immunity to several related pathogens despite having been exposed to or vaccinated against only one of them. For instance, antibody cross reactivity may occur against the similar surface structures of various Gram-negative bacteria. Conversely, antibodies raised against pathogenic molecular components that resemble self molecules may incorrectly mark host cells for destruction, causing autoimmune damage. Patients who develop systemic lupus erythematosus (SLE) commonly exhibit antibodies that react with their own DNA. These antibodies may have been initially raised against the nucleic acid of microorganisms, but later cross-reacted with self-antigens. This phenomenon is also called molecular mimicry.
Key Points
• Antibodies are produced by plasma cells, but, once secreted, can act independently against extracellular pathogen and toxins.
• Antibodies bind to specific antigens on pathogens; this binding can inhibit pathogen infectivity by blocking key extracellular sites, such as receptors involved in host cell entry.
• Antibodies can also induce the innate immune response to destroy a pathogen, by activating phagocytes such as macrophages or neutrophils, which are attracted to antibody-bound cells.
• Affinity describes how strongly a single antibody binds a given antigen, while avidity describes the binding of a multimeric antibody to multiple antigens.
• A multimeric antibody may have individual arms with low affinity, but have high overall avidity due to synergistic effects between binding sites.
• Cross reactivity occurs when an antibody binds to a different-but-similar antigen than the one for which it was raised; this can increase pathogen resistance or result in an autoimmune reaction.
Key Terms
• avidity: the measure of the synergism of the strength individual interactions between proteins
• affinity: the attraction between an antibody and an antigen | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.13%3A_Antibodies_-_Antibody_Functions.txt |
Learning Objectives
• Explain the problems associated with immunodeficiency
Immunodeficiency
Failures, insufficiencies, or delays at any level of the immune response can allow pathogens or tumor cells to gain a foothold to replicate or proliferate to high enough levels that the immune system becomes overwhelmed, leading to immunodeficiency; it may be acquired or inherited. Immunodeficiency can be acquired as a result of infection with certain pathogens (such as HIV), chemical exposure (including certain medical treatments), malnutrition, or, possibly, by extreme stress. For instance, radiation exposure can destroy populations of lymphocytes, elevating an individual’s susceptibility to infections and cancer. Dozens of genetic disorders result in immunodeficiencies, including Severe Combined Immunodeficiency (SCID), bare lymphocyte syndrome, and MHC II deficiencies. Rarely, primary immunodeficiencies that are present from birth may occur. Neutropenia is one form in which the immune system produces a below-average number of neutrophils, the body’s most abundant phagocytes. As a result, bacterial infections may go unrestricted in the blood, causing serious complications.
HIV/AIDS
Human immunodeficiency virus infection / acquired immunodeficiency syndrome (HIV/AIDS), is a disease of the human immune system caused by infection with human immunodeficiency virus (HIV). During the initial infection, a person may experience a brief period of influenza-like illness. This is typically followed by a prolonged period without symptoms. As the illness progresses, it interferes more and more with the immune system. The person has a high probability of becoming infected, including from opportunistic infections and tumors that do not usually affect people who have working immune systems.
After the virus enters the body, there is a period of rapid viral replication, leading to an abundance of virus in the peripheral blood. During primary infection, the level of HIV may reach several million virus particles per milliliter of blood. This response is accompanied by a marked drop in the number of circulating CD4+ T cells, cells that are or will become helper T cells. The acute viremia, or spreading of the virus, is almost invariably associated with activation of CD8+ T cells (which kill HIV-infected cells) and, subsequently, with antibody production. The CD8+ T cell response is thought to be important in controlling virus levels, which peak and then decline, as the CD4+ T cell counts recover.
Ultimately, HIV causes AIDS by depleting CD4+ T cells (helper T cells). This weakens the immune system, allowing opportunistic infections. T cells are essential to the immune response; without them, the body cannot fight infections or kill cancerous cells. The mechanism of CD4+ T cell depletion differs in the acute and chronic phases. During the acute phase, HIV-induced cell lysis and killing of infected cells by cytotoxic T cells accounts for CD4+ T cell depletion, although apoptosis (programmed cell death) may also be a factor. During the chronic phase, the consequences of generalized immune activation coupled with the gradual loss of the ability of the immune system to generate new T cells appear to account for the slow decline in CD4+ T cell numbers.
Key Points
• If a pathogen is allowed to proliferate to certain levels, the immune system can become overwhelmed; immunodeficiency occurs when the immune system fails to respond sufficiently to a pathogen.
• Immunodeficiency can be caused by many factors, including certain pathogens, malnutrition, chemical exposure, radiation exposure, or even extreme stress.
• HIV is a virus that causes immunodeficiency by infecting helper T cells, causing cytotoxic T cells to destroy them.
Key Terms
• phagocyte: a cell of the immune system, such as a neutrophil, macrophage or dendritic cell, that engulfs and destroys viruses, bacteria, and waste materials
• lysis: the disintegration or destruction of cells
• immunodeficiency: a depletion in the body’s natural immune system, or in some component of it | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.14%3A_Disruptions_in_the_Immune_System_-_Immunodeficiency.txt |
Learning Objectives
• Distinguish between the disruptions to the immune system caused by allergies and autoimmunity
Maladaptive immune responses toward harmless foreign substances or self antigens that occur after tissue sensitization are termed hypersensitivities. This can potentially be very dangerous for an individual, as the immune response can be very powerful; it can destroy host tissue if not kept in check. The types of hypersensitivities include immediate, delayed, and autoimmunity hypersensitivities. A large proportion of the population is affected by one or more types of hypersensitivity.
Allergies
The immune reaction that results from immediate hypersensitivities, in which an antibody-mediated immune response occurs within minutes of exposure to a harmless antigen, is called an allergy. In the United States, 20 percent of the population exhibits symptoms of allergy or asthma, whereas 55 percent test positive against one or more allergens. Upon initial exposure to a potential allergen, an allergic individual synthesizes antibodies of the IgE class; this class of antibodies also mediates the immune response to parasitic worms. The constant domain of the IgE molecules interacts with mast cells embedded in connective tissues. This process primes, or sensitizes, the tissue. Upon subsequent exposure to the same allergen, IgE molecules on mast cells bind the antigen via their variable domains, stimulating the mast cell to release the modified amino acids histamine and serotonin. These chemical mediators then recruit eosinophils which mediate allergic responses. The effects of an allergic reaction range from mild symptoms such as sneezing and itchy, watery eyes, to more severe or even life-threatening reactions involving intensely-itchy welts known as hives, airway contraction with severe respiratory distress, and plummeting blood pressure. This extreme reaction is known as anaphylactic shock. If not treated with epinephrine to counter the blood pressure and breathing effects, this condition can be fatal.
Delayed hypersensitivity is a cell-mediated immune response that takes approximately one to two days after secondary exposure for a maximal reaction to be observed. This type of hypersensitivity involves the TH1 cytokine -mediated inflammatory response. It may manifest as local tissue lesions or contact dermatitis (rash or skin irritation). Delayed hypersensitivity occurs in some individuals in response to contact with certain types of jewelry or cosmetics. It also facilitates the immune response to poison ivy and is the reason why the skin test for tuberculosis results in a small region of inflammation on individuals who were previously exposed to Mycobacterium tuberculosis. Cortisone is typically used to treat such responses as it inhibits cytokine production.
Autoimmunity
Autoimmunity is a type of hypersensitivity to self antigens that affects approximately five percent of the population. Most types of autoimmunity involve the humoral immune response. Antibodies that inappropriately mark self components as foreign are termed autoantibodies. In patients with the autoimmune disease myasthenia gravis, muscle cell receptors that induce contraction in response to acetylcholine are targeted by antibodies. The result is muscle weakness that may include marked difficultly with fine and/or gross motor functions. In systemic lupus erythematosus, a diffuse autoantibody response to the individual’s own DNA and proteins results in various systemic diseases. Systemic lupus erythematosus may affect the heart, joints, lungs, skin, kidneys, central nervous system, or other tissues, causing tissue damage via antibody binding, complement recruitment, lysis, and inflammation.
Autoimmunity can develop with time; its causes may be rooted in molecular mimicry. Antibodies and TCRs may bind self antigens that are structurally similar to pathogen antigens, which the immune receptors first raised. As an example, infection with Streptococcus pyogenes (bacterium that causes strep throat) may generate antibodies or T cells that react with heart muscle, which has a similar structure to the surface of S. pyogenes. These antibodies can damage heart muscle with autoimmune attacks, leading to rheumatic fever. Insulin -dependent (Type 1) diabetes mellitus arises from a destructive inflammatory TH1 response against insulin-producing cells of the pancreas. Patients with this autoimmunity must be injected with insulin that originates from other sources.
Key Points
• While the immune system is normally very tightly controlled to disregard “self” and harmless antigens, a condition known as hypersensitivity can override this control, causing illness and injury to an individual.
• Allergies result when the immune system recognizes harmless antigens, such as pollen or dust; they are characterized by red, swollen eyes, sneezing and itching, and can also encompass more serious symptoms such as anaphylactic shock.
• Autoimmunity occurs when the immune system recognizes “self” antigens and begins attacking them; antibodies that recognize self are termed autoantibodies.
Key Terms
• autoantibody: an antibody formed in response to an agent (autoantigen) produced by the organism itself
• histamine: an amine that causes dilatation of capillaries, contraction of smooth muscle, and stimulation of gastric acid secretion; it is released during allergic reactions | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/42%3A_The_Immune_System/42.15%3A_Disruptions_in_the_Immune_System_-_Hypersensitivities.txt |
Learning Objectives
• Describe reproduction in animals
Animal Reproduction
Reproduction (or procreation) is the biological process by which new “offspring” (individual organisms) are produced from their “parents. ” It is a fundamental feature of all known life that each individual organism exists as the result of reproduction. Most importantly, reproduction is necessary for the survival of a species. The known methods of reproduction are broadly grouped into two main types: sexual and asexual.
In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a bacterial cell into two daughter cells is an example of asexual reproduction. This type of reproduction produces genetically-identical organisms (clones), whereas in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically different from their parents.
During sexual reproduction, the male gamete (sperm) may be placed inside the female’s body for internal fertilization, or the sperm and eggs may be released into the environment for external fertilization. Humans provide an example of the former, while seahorses provide an example of the latter. Following a mating dance, the female seahorse lays eggs in the male seahorse’s abdominal brood pouch where they are fertilized. The eggs hatch and the offspring develop in the pouch for several weeks.
Asexual versus Sexual Reproduction
Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of the species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller number of offspring, but the large amount of variation in their genes makes them less susceptible to disease.
Many organisms can reproduce sexually as well as asexually. Aphids, slime molds, sea anemones, and some species of starfish are examples of animal species with this ability. When environmental factors are favorable, asexual reproduction is employed to exploit suitable conditions for survival, such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH, or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources. When food sources have been depleted, the climate becomes hostile, or individual survival is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction.
Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. In addition, sexual reproduction usually results in the formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts, or other “over-wintering” stages of sexual reproduction ensure the survival during unfavorable times as the organism can “wait out” adverse situations until a swing back to suitability occurs.
Key Points
• Reproduction (or procreation) is the biological process by which new “offspring” are produced from their “parents”.
• Asexual reproduction yields genetically-identical organisms because an individual reproduces without another.
• In sexual reproduction, the genetic material of two individuals from the same species combines to produce genetically-different offspring; this ensures mixing of the gene pool of the species.
• Organisms that reproduce through asexual reproduction tend to grow exponentially and rely on mutations for DNA variation, while those that reproduce sexually yield a smaller number of offspring, but have larger genetic variation.
Key Terms
• reproduction: the act of producing new individuals biologically
• clone: a living organism produced asexually from a single ancestor, to which it is genetically identical | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.01%3A_Reproduction_Methods/43.1A%3A_Methods_of_Reproducing.txt |
Learning Objectives
• Discuss sexual and asexual reproduction methods
Asexual Reproduction
Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent. This type of reproduction occurs in prokaryotic microorganisms (bacteria) and in some eukaryotic single-celled and multi-celled organisms. Animals may reproduce asexually through fission, budding, fragmentation, or parthenogenesis.
Fission
Fission, also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates, forming a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps also reproduce through fission.
Budding
Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals. Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult, which breaks away from the main body; whereas in coral budding, the bud does not detach and multiplies as part of a new colony.
Fragmentation
Fragmentation is the breaking of the body into two parts with subsequent regeneration. If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow.
Many sea stars reproduce asexually by fragmentation. For example, if the arm of an individual sea star is broken off it will regenerate a new sea star. Fishery workers have been known to try to kill the sea stars that eat their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams. Fragmentation also occurs in annelid worms, turbellarians, and poriferans.
Note that in fragmentation, there is generally a noticeable difference in the size of the individuals, whereas in fission, two individuals of approximately the same size are formed.
Parthenogenesis
Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees use parthenogenesis to produce haploid males (drones) and diploid females (workers). If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced.
Some vertebrate animals, such as certain reptiles, amphibians, and fish, also reproduce through parthenogenesis. Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos. Two Komodo dragons, a bonnethead shark, and a blacktip shark have produced parthenogenic young when the females have been isolated from males.
Sexual Reproduction
Sexual reproduction is the combination of (usually haploid, or having a single set of unpaired chromosomes) reproductive cells from two individuals to form a third (usually diploid, or having a pair of each type of chromosome) unique offspring. Sexual reproduction produces offspring with novel combinations of genes. This can be an adaptive advantage in unstable or unpredictable environments. As humans, we are used to thinking of animals as having two separate sexes, male and female, determined at conception. However, in the animal kingdom, there are many variations on this theme.
Hermaphroditism
Hermaphroditism occurs in animals where one individual has both male and female reproductive parts. Invertebrates, such as earthworms, slugs, tapeworms and snails, are often hermaphroditic. Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams.
Key Points
• Asexual reproduction includes fission, budding, fragmentation, and parthenogenesis, while sexual reproduction is achieved through the combination of reproductive cells from two individuals.
• The ability of a species to reproduce through fragmentation depends on the size of part that breaks off, while in binary fission, an individual splits off and forms two individuals of the same size.
• Budding may lead to the production of a completely new adult that forms away from the original body or may remain attached to the original body.
• Observed in invertebrates and some vertebrates, parthenogenesis produce offspring that may be either haploid or diploid.
• Sexual reproduction, the production of an offspring with a new combination of genes, may also involve hermaphroditism in which an organism can self-fertilize or mate with another individual of the same species.
Key Terms
• binary fission: the process whereby a cell divides asexually to produce two daughter cells
• hermaphroditism: having sexual organs of both sexes
• parthenogenesis: a form of asexual reproduction where growth and development of embryos occur without fertilization
43.1C: Sex Determination
Learning Objectives
• Differentiate among the various ways animals determine the sex of offspring
Mammalian sex is determined genetically by the presence of X and Y chromosomes. Individuals homozygous for X (XX) are female, while heterozygous individuals (XY) are male. The presence of a Y chromosome causes the development of male characteristics, while its absence results in female characteristics. The XY system is also found in some insects and plants.
Avian sex determination is dependent on the presence of Z and W chromosomes. Homozygous for Z (ZZ) results in a male, while heterozygous (ZW) results in a female. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use this system.
The sex of some species is not determined by genetics, but by some aspect of the environment. Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods of egg development. This is referred to as environmental sex determination or, more specifically, as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males, while warm temperatures produce females. In some crocodiles, moderate temperatures produce males, while both warm and cool temperatures produce females. In some species, sex is both genetic- and temperature-dependent.
Individuals of some species change their sex during their lives, alternating between male and female. If the individual is female first, it is termed protogyny or “first female;” if it is male first, it is termed protandry or “first male.” Oysters, for example, are born male, grow, become female, and lay eggs; some oyster species change sex multiple times.
Key Points
• Mammals, birds, and some other animal species depend on heterozygous or homozygous chromosome combinations for sex determination.
• Cool or warm temperatures affect sex determination in species such as crocodiles and turtles.
• Some species, such as oysters, have the capability of alternating their sex several times within their life span.
Key Terms
• protandry: the condition in which an organism begins life as a male and then changes into a female
• protogyny: the condition in which an organism begins life as a female and then changes into a male
• homozygous: of an organism in which both copies of a given gene have the same allele
• heterozygous: of an organism which has two different alleles of a given gene | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.01%3A_Reproduction_Methods/43.1B%3A_Types_of_Sexual_and_Asexual_Reproduction.txt |
Learning Objectives
• Compare and contrast external and internal methods of fertilization
External Fertilization
External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization can then take place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. The release of the reproductive material may be triggered by water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers). Pairs of fish that are not broadcast spawners may exhibit courtship behavior. This allows the female to select a particular male. The trigger for egg and sperm release (spawning) causes the egg and sperm to be placed in a small area, enhancing the possibility of fertilization.
External fertilization in an aquatic environment protects the eggs from drying out. Broadcast spawning can result in a greater mixture of the genes within a group, leading to higher genetic diversity and a greater chance of species survival in a hostile environment. For sessile aquatic organisms such as sponges, broadcast spawning is the only mechanism for fertilization and colonization of new environments. The presence of the fertilized eggs and developing young in the water provides opportunities for predation, resulting in a loss of offspring. Therefore, millions of eggs must be produced by individuals. The offspring produced through this method must mature rapidly. The survival rate of eggs produced through broadcast spawning is low.
Internal Fertilization
Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization: oviparity, ovoviparity, and viviparity.
In oviparity, fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. These animals are classified as oviparous.
In ovoviparity, fertilized eggs are retained in the female, but the embryo obtains its nourishment from the egg’s yolk; the young are fully developed when they are hatched. This occurs in some bony fish (such as the guppy, Lebistes reticulatus), some sharks, some lizards, some snakes (such as the garter snake, Thamnophis sirtalis), some vipers, and some invertebrate animals (such as the Madagascar hissing cockroach, Gromphadorhina portentosa).
In viviparity, the young develop within the female, receiving nourishment from the mother’s blood through a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some cartilaginous fish, and a few reptiles, making these animals viviparous.
Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is isolated within the female, which limits predation on the young. Internal fertilization also enhances the fertilization of eggs by a specific male. Even though fewer offspring are produced through this method, their survival rate is higher than that for external fertilization.
Key Points
• External fertilization is characterized by the release of both sperm and eggs into an external environment; sperm will fertilize the egg outside of the organism, as seen in spawning.
• Internal fertilization is characterized by sperm fertilizing the egg within the female; the three methods include: oviparity (egg laid outside female body), ovoviparity (egg held within female), and viviparity (development within female followed by live birth).
• Internal fertilization protects the fertilized egg or embryo from predation and harsh environments, which results in higher survival rates than can occur with external fertilization.
• Ovoviparity is characterized by an organism retaining a fertilized egg inside the body where development occurs and nourishment is received from the yolk.
• Viparity is characterized by an organism which has its young develop within the female and nourishment is received directly from the mother via a placenta.
Key Terms
• oviparous: egg-laying; depositing eggs that develop and hatch outside the body as a reproductive strategy
• viviparous: being born alive, as are most mammals, some reptiles, and a few fish (as opposed to being laid as an egg)
• ovoviparity: eggs are retained in the female, but the embryo obtains its nourishment from the egg’s yolk | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.02%3A_Fertilization/43.2A%3A_External_and_Internal_Fertilization.txt |
Learning Objectives
• Differentiate among the types of reproductive systems that have evolved
Several competing scientific hypotheses have been proposed to explain the evolution of sexual reproduction. All sexually-reproducing eukaryotic organisms derive from a common ancestor that was a single-celled eukaryotic species. Many protists reproduce sexually, as do multicellular plants, animals, and fungi. However, there are a few species which have secondarily lost this feature. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction.
Once multicellular organisms evolved and developed specialized cells, some also developed tissues and organs with specialized functions. The evolution of reproductive organs arrived with the development of gonads that produced sperm and eggs. These cells develop through meiosis, an adaption of mitosis, which reduced the number of chromosomes in each reproductive cell by half, while increasing the number of cells through cell division. The development of specialized gonads to produce sperm and egg was a major step in the evolutionary process.
An early development in reproduction occurred in the Annelids. These organisms produce sperm and eggs from undifferentiated cells in their coelom, storing them in that cavity. When the coelom becomes filled, the cells are released through an excretory opening or by the body splitting open. Further evolution of reproductive systems resulted in the development of reproductive systems that are sex specific. In these more advanced systems, sperm is made in the testes and then travels through coiled tubes to the epididymis for storage. Additionally, in these more advanced systems, eggs are matured in the ovary; when released, they travel to the uterine tubes for fertilization. These types of reproductive systems developed in insects (compared to annelids which have a coelom for storage). Specifically, in the insect reproductive system, a specialized sac developed, called a spermatheca, which is used to store sperm for later use, sometimes up to a year. This was a key development since fertilization in insects can be timed with environmental or food conditions that are optimal for offspring survival.
Vertebrates have similar structures (i.e., gonads that specialize in sex cell production) with a few differences in their reproductive systems. Non-mammals, such as birds and reptiles, have a common body opening, called a cloaca, for the digestive, excretory, and reproductive systems. Coupling between birds usually involves positioning the cloaca openings opposite each other for transfer of sperm. In mammals, there are separate openings for the systems in the female and a uterus for support of developing offspring. Depending on the type of species, there are differences in the uterus. In species that produce large numbers of offspring, the uterus has two chambers. In other species that produce one offspring, such as in primates, there is a single uterus.
Another development in the evolution of reproduction is the means by which sperm is transferred. During reproduction, sperm transfer from the male to the female ranges from releasing the sperm into the watery environment for external fertilization, to the joining of cloaca in birds, to the development of a penis for direct delivery into the female’s vagina in mammals. All of these methods of sperm transfer represent the varying ways reproduction has evolved and become specialized to specific organisms.
Key Points
• Annelids undergo sexual reproduction by producing sperm or eggs within the coelom and storing them within the cavity until they are ready to be released through an excretory opening.
• Insects have developed complete reproductive systems for the separate sexes and will often have a specialized sac for sperm called the spermatheca.
• Non-mammals will utilize a common body opening called the cloaca to transfer sperm between animals.
• The means by which sperm is transferred varies and can include releasing sperm into the environment as well as direct delivery to the vagina.
Key Terms
• coelom: a fluid-filled cavity within the body of an animal; the digestive system is suspended within the cavity, which is lined by a tissue called the peritoneum
• gonad: a sex organ that produces gametes; specifically, a testicle or ovary
• cloaca: the common duct in fish, reptiles, birds, and some primitive mammals that serves as the anus as well as the genital opening
• spermatheca: a small sac within the reproductive tract of some female invertebrates, such as insects, which stores sperm until it is used to fertilize the ova
Contributions and Attributions
• OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44838/latest...ol11448/latest. License: CC BY: Attribution
• OpenStax College, Biology. December 4, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44838/latest...ol11448/latest. License: CC BY: Attribution
• viviparous. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/viviparous. License: CC BY-SA: Attribution-ShareAlike
• Boundless. Provided by: Boundless Learning. Located at: www.boundless.com//biology/de...on/ovoviparity. License: CC BY-SA: Attribution-ShareAlike
• oviparous. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/oviparous. License: CC BY-SA: Attribution-ShareAlike
• Anemone Fish protecting it's spawn. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:An...it's_spawn.jpg. License: CC BY: Attribution
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44838/latest...ol11448/latest. License: CC BY: Attribution
• Evolution of sexual reproduction. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Evoluti...l_reproduction. License: CC BY-SA: Attribution-ShareAlike
• gonad. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/gonad. License: CC BY-SA: Attribution-ShareAlike
• cloaca. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cloaca. License: CC BY-SA: Attribution-ShareAlike
• coelom. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/coelom. License: CC BY-SA: Attribution-ShareAlike
• spermatheca. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/spermatheca. License: CC BY-SA: Attribution-ShareAlike
• Anemone Fish protecting it's spawn. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:An...it's_spawn.jpg. License: CC BY: Attribution
• Sexual cycle. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Sexual_cycle.svg. License: Public Domain: No Known Copyright
• Sperm-egg. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/File:Sperm-egg.jpg. License: CC BY: Attribution | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.02%3A_Fertilization/43.2B%3A_The_Evolution_of_Reproduction.txt |
Learning Objectives
• Diagram the structures of human male reproductive anatomy
Human Reproductive Anatomy
The reproductive tissues of male and female humans develop similarly in utero until a low level of the hormone testosterone is released from male gonads. Testosterone causes the undeveloped tissues to differentiate into male sexual organs. Primitive gonads become testes; other tissues produce a penis and scrotum in males.
Male Reproductive Anatomy
In the male reproductive system, the scrotum houses the testicles or testes, providing passage for blood vessels, nerves, and muscles related to testicular function. The testes, a pair of male reproductive organs, produce sperm and male sex hormones, including the steroid testosterone. Coiled in each testis are seminiferous tubules that produce sperm.
Sperm
Sperm are immobile at body temperature; therefore, the scrotum and penis are external to the body so that a proper temperature is maintained for motility. In land mammals, the pair of testes must be suspended outside the body at about 2° C lower than body temperature to produce viable sperm.
Sperm develop in the seminiferous tubules that are coiled inside the testes. The walls of the seminiferous tubules are composed of the developing sperm cells, with the least-developed sperm at the periphery of the tubule and the fully-developed sperm in the lumen. The sperm cells are mixed with “nursemaid” cells called Sertoli cells which protect the germ cells and promote their development. Other cells mixed in the wall of the tubules are the interstitial cells of Leydig; these cells produce high levels of testosterone once the male reaches adolescence.
Sperm consist of a flagellum (as a tail), a neck that contains the cell’s energy-producing mitochondria, and a head that contains the genetic material. When the sperm have developed flagella, or lash-like appendages that protrude from the cell body, and are nearly mature, they leave the testicles and enter the epididymis. This structure lies along the top and posterior portion of the testes; it is the site of sperm maturation. The sperm leave the epididymis and enter the vas deferens, which is the duct in the testicle that carries sperm from the epididymis to the ejaculatory duct.
Semen is a mixture of sperm and spermatic duct secretions (about 10 percent of the total), along with fluids from accessory glands, that contribute most of the semen’s volume. An ejaculate will contain from two to five milliliters of fluid with from 50–120 million sperm per milliliter. The bulk of the semen comes from the accessory glands associated with the male reproductive system, including the seminal vesicles, the prostate gland, and the bulbourethral gland.
Seminal vesicles, penis, prostate, and bulbourethral gland
The seminal vesicles are a pair of glands that lie along the posterior border of the urinary bladder. The glands make a solution that is thick, yellowish, and alkaline. As sperm are only motile in an alkaline environment, a basic pH is important to reverse the acidity of the vaginal environment. The solution also contains mucus, fructose (a sperm mitochondrial nutrient), a coagulating enzyme, ascorbic acid, and local-acting hormones called prostaglandins.
The penis is an organ that drains urine from the renal bladder and functions as a copulatory organ during intercourse. The penis contains three tubes of erectile tissue running through the length of the organ. These consist of a pair of tubes on the dorsal side, called the corpus cavernosum, and a single tube of tissue on the ventral side, called the corpus spongiosum. This tissue, when engorged with blood, becomes erect and hard, in preparation for intercourse. The organ is inserted into the vagina, culminating with an ejaculation, which is the forcible ejection of semen from the urethra. An orgasm is a two-stage process: first, glands and accessory organs connected to the testes contract; second, semen (containing sperm) is expelled through the urethra during ejaculation. After intercourse, the blood drains from the erectile tissue and the penis becomes flaccid.
The walnut-shaped prostate gland surrounds the urethra, the connection to the urinary bladder. It has a series of short ducts that directly connect to the urethra. The gland is a mixture of smooth muscle and glandular tissue. The muscle provides much of the force needed for ejaculation to occur.
The bulbourethral gland, or Cowper’s gland, is an exocrine gland which secretes a clear fluid known as pre-ejaculate that is generated upon sexual arousal. This gland releases its secretion prior to the release of the bulk of the semen. It neutralizes any acid residue in the urethra left over from urine. This usually accounts for a couple of drops of fluid in the total ejaculate and may contain a few sperm. Withdrawal of the penis from the vagina before ejaculation to prevent pregnancy may not work if sperm are present in the bulbourethral gland secretions.
Key Points
• The male gonads, or testes, produce sperm within the seminiferous tubules; the sperm are housed here until they are nearly mature, at which point they enter the epidydimis for full maturation.
• The testes are housed in the scrotum, an external sac that keeps the sperm at a temperature lower than that of the body.
• At ejaculation, sperm leave the epidydimis and enter the vas deferens, a duct which carries the sperm out of the body through the urethra, along with the fluids of various glands of the male reproductive system.
• The seminal vesicles produce a thick fluid that is alkaline in order to protect sperm from the acidic nature of the female vagina; it also contains sugars to nourish the sperm.
• The prostate gland produces the force necessary to push the sperm out of the epididymis at ejaculation, while the bulbourethral gland emits a fluid just prior to ejaculation that neutralizes acid from any urine left over in the urethra.
• During sexual arousal, the spongy tissue inside the penis (the corpus spongiosum) fills with blood, causing the penis to become erect and hard; after ejaculation, the blood flows back out of the penis, leaving it flaccid.
Key Terms
• epididymis: a narrow, tightly-coiled tube connecting the efferent ducts from the rear of each testicle to its vas deferens, where sperm are stored during maturation
• prostate gland: a gland in male mammals surrounding the urethra just below the urinary bladder that controls the release of urine from the bladder and produces a secretion that is the fluid part of semen
• seminiferous tubule: any of many threadlike structures, located in the testes, that are the specialized areas of sperm production | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.03%3A_Human_Reproductive_Anatomy_and_Gametogenesis/43.3A%3A_Male_Reproductive_Anatomy.txt |
The female reproductive structures produce eggs, support a growing embryo, and provide a birth canal to the fetus.
Learning Objectives
• Diagram the structures of human female reproductive anatomy
Key Points
• The external anatomy of the female reproductive system is referred to as the vulva; it includes the labia minora, which protects the vagina and urethra, and the labia majora, which surrounds it.
• Internal female reproductive structures include ovaries, oviducts, the uterus, and the vagina.
• The eggs develop in structures called follicles, which are located on the surface of the ovaries; at maturity, one of the follicles will rupture and release the egg, which is captured by the fimbrae of the oviduct.
• If fertilization occurs, it generally does so in the oviduct; the fertilized egg then travels down the oviduct and enters the uterus, where it will implant in the lining of the uterus, known as the endometrium.
• If fertilization does not occur, the endometrium of the uterus will slough off at the end of the menstrual period and is shed through the vagina, which is also the opening through which the penis enters during intercourse and through which the baby will exit during birth.
Key Terms
• clitoris: a small sensitive elongated erectile organ at the anterior part of the vulva in female mammals, homologous with the penis
• ovary: a female reproductive organ, often paired, that produces ova and in mammals secretes the hormones estrogen and progesterone
• vagina: the passage leading from the opening of the vulva to the cervix of the uterus for copulation and childbirth in female mammals
• uterus: an organ of the female reproductive system in which the young are conceived and develop until birth; the womb
• vulva: the external female sexual organs, collectively
Female reproductive anatomy
Female reproductive anatomy includes both external and internal structures. Among the external structures are the vulva, which consists of the mons pubis, clitoris, labia majora, labia minora, and the vestibular glands. The vulva is an area associated with the vestibule that includes the structures found in the inguinal (groin) area of women. The mons pubis is a round, fatty area that overlies the pubic symphysis. The clitoris is a structure with erectile tissue that contains a large number of sensory nerves and serves as a source of stimulation during intercourse. The labia majora are a pair of elongated folds of tissue that run posterior from the mons pubis and enclose the other components of the vulva. The labia majora derive from the same tissue that produces the scrotum in a male. The labia minora are thin folds of tissue centrally located within the labia majora. These labia protect the openings to the vagina and urethra. The mons pubis and the anterior portion of the labia majora become covered with hair during adolescence; the labia minora is hairless. The greater vestibular glands are found at the sides of the vaginal opening and provide lubrication during intercourse.
Internal female reproductive structures include ovaries, oviducts, the uterus, and the vagina. An ovary consists of a medulla and cortex: the medulla contains nerves and blood vessels to supply the cortex with nutrients and remove waste. The outer layers of cells of the cortex are the functional parts of the ovaries. The cortex is made up of follicular cells that surround eggs. During the menstrual cycle, a batch of follicular cells develops, preparing the eggs for release. At ovulation, one follicle ruptures and one egg is released.
The oviducts, or fallopian tubes, extend from the uterus in the lower abdominal cavity to the ovaries, but they are not in contact with the ovaries. The lateral ends of the oviducts flare out into a trumpet-like structure and have a fringe of finger-like projections called fimbriae. When an egg is released at ovulation, the fimbrae help the non-motile egg enter into the tube, the passage to the uterus. The walls of the oviducts are ciliated (covered in cilia ) and are primarily smooth muscle. The cilia beat toward the middle, while the smooth muscle contracts in the same direction, moving the egg toward the uterus. Fertilization usually takes place within the oviducts. The embryo is moved toward the uterus for further development. It usually takes the egg or embryo a week to travel through the oviduct.
The uterus, a structure about the size of a woman’s fist, is lined with an endometrium that is rich in blood vessels and mucus glands. The uterus supports the developing embryo and fetus during gestation. The thickest portion of the wall of the uterus is made of smooth muscle. Contractions of the smooth muscle in the uterus aid in passing the baby through the vagina during labor. A portion of the lining of the uterus sloughs off during each menstrual period if an egg has not been fertilized; it builds up again in preparation for an implantation. Part of the uterus, called the cervix, protrudes into the top of the vagina, which functions as the birth canal.
The vagina is a muscular tube that serves several purposes. It allows menstrual flow to leave the body, is the receptacle for the penis during intercourse, and serves as the vessel for the delivery of offspring. It is lined by stratified squamous epithelial cells to protect the underlying tissue. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.03%3A_Human_Reproductive_Anatomy_and_Gametogenesis/43.3B%3A__Female_Reproductive_Anatomy.txt |
Spermatogenesis and oogenesis are both forms of gametogenesis, in which a diploid gamete cell produces haploid sperm and egg cells, respectively.
Learning Objectives
• Distinguish between spermatogenesis and oogenesis
Key Points
• Gametogenesis, the production of sperm (spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis.
• In oogenesis, diploid oogonium go through mitosis until one develops into a primary oocyte, which will begin the first meiotic division, but then arrest; it will finish this division as it develops in the follicle, giving rise to a haploid secondary oocyte and a smaller polar body.
• The secondary oocyte begins the second meiotic division and then arrests again; it will not finish this division unless it is fertilized by a sperm; if this occurs, a mature ovum and another polar body is produced.
• In spermatogenesis, diploid spermatogonia go through mitosis until they begin to develop into gametes; eventually, one develops into a primary spermatocyte that will go through the first meiotic division to form two haploid secondary spermatocytes.
• The secondary spermatocytes will go through a second meiotic division to each produce two spermatids; these cells will eventually develop flagella and become mature sperm.
Key Terms
• spermatocyte: a male gametocyte, from which a spermatozoon develops
• oocyte: a cell that develops into an egg or ovum; a female gametocyte
• polar body: one of the small cells that are by-products of the meiosis that forms an egg
• mitosis: the division of a cell nucleus in which the genome is copied and separated into two identical halves. It is normally followed by cell division
• meiosis: cell division of a diploid cell into four haploid cells, which develop to produce gametes
Gametogenesis (Spermatogenesis and Oogenesis)
Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis. During meiosis, two cell divisions separate the paired chromosomes in the nucleus and then separate the chromatids that were made during an earlier stage of the cell’s life cycle, resulting in gametes that each contain half the number of chromosomes as the parent. The production of sperm is called spermatogenesis and the production of eggs is called oogenesis.
Oogenesis
Oogenesis occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to one to two million cells in the embryo.
The cell starting meiosis is called a primary oocyte. This cell will begin the first meiotic division, but be arrested in its progress in the first prophase stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary. This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.
Spermatogenesis
Spermatogenesis occurs in the wall of the seminiferous tubules, with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia (singular: spermatagonium), go through mitosis with one offspring going on to differentiate into a sperm cell, while the other gives rise to the next generation of sperm.
Meiosis begins with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte. This haploid cell must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid. When it reaches the lumen of the tubule and grows a flagellum (or “tail”), it is called a sperm cell. Four sperm result from each primary spermatocyte that goes through meiosis.
Stem cells are deposited during gestation and are present at birth through the beginning of adolescence, but in an inactive state. During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of these cells and the production of viable sperm. This continues into old age. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.03%3A_Human_Reproductive_Anatomy_and_Gametogenesis/43.3C%3A__Gametogenesis_%28Spermatogenesis_and_Oogenesis%2.txt |
The onset of puberty is controlled by two major hormones: FSH initiates spermatogenesis and LH signals the release of testosterone.
Learning Objectives
• Explain the function of male hormones in reproduction
Key Points
• The onset of puberty is signaled by high pulses of GnRH secreted by the hypothalamus; this in turn signals the release of FSH and LH from the pituitary gland.
• FSH causes the Sertoli cells of the testes (which help nurse developing sperm cells) to begin the process of spermatogenesis in the testes.
• LH triggers the production of testosterone from the Leydig cells of the testis; testosterone causes the development of secondary sex characteristics in the male.
• As spermatogenesis and testosterone production increase, the Sertoli cells produce inhibin, which, together with rising levels of testosterone, inhibit the release of FSH and LH from the pituitary gland.
Key Terms
• puberty: the age at which a person is first capable of sexual reproduction
• Sertoli cell: a kind of sustentacular cell which serves as a “nurse” cell of the testes and which is part of a seminiferous tubule
• Leydig cell: one of the interstitial cells, located next to the seminiferous tubules inside the testicle, that produce testosterone
• follicle stimulating hormone: a gonadotropic glycoprotein hormone, secreted in the anterior pituitary, that stimulates the growth of ovarian follicles in female mammals, and induces spermatogenesis in male mammals
• luteinizing hormone: a hormone, produced by part of the pituitary gland, that stimulates ovulation and the development of the corpus luteum in female mammals, and the production of androgens by male mammals
• inhibin: a peptide hormone, secreted by the gonads, which inhibits the secretion of follicle-stimulating hormone
• testosterone: steroid hormone produced primarily in the testes of the male; it is responsible for the development of secondary sex characteristics in the male
Male Hormones
Puberty is a period of several years in which rapid physical growth and psychological changes occur, culminating in sexual maturity. The average onset of puberty is age 11 or 12 for boys. Some of the most significant parts of pubertal development involve distinctive physiological changes in individuals’ height, weight, body composition, and circulatory and respiratory systems. These changes are largely influenced by hormonal activity. Hormones play an organizational role, priming the body to behave in a certain way once puberty begins, and an activational role, referring to changes in hormones during adolescence that trigger behavioral and physical changes.
At the onset of puberty, the hypothalamus begins secreting high pulses of GnRH, or gonadotropin-releasing hormone. In response, the pituitary gland releases follicle stimulating hormone (FSH) and luteinizing hormone (LH) into the male system for the first time. FSH enters the testes, stimulating the Sertoli cells, which help to nourish the sperm cells that the testes produce, to begin facilitating spermatogenesis. LH also enters the testes, stimulating the interstitial cells, called Leydig cells, to make and release testosterone into the testes and the blood.
Testosterone, the hormone responsible for the secondary sexual characteristics that develop in the male during adolescence, stimulates spermatogenesis, or the process of sperm production in the testes. Secondary sex characteristics include a deepening of the voice, the growth of facial, axillary, and pubic hair, and the beginnings of the sex drive.
A negative feedback system occurs in the male with rising levels of testosterone acting on the hypothalamus and anterior pituitary to inhibit the release of GnRH, FSH, and LH. The Sertoli cells produce the hormone inhibin, which is released into the blood when the sperm count is too high. This inhibits the release of GnRH and FSH, which will cause spermatogenesis to slow down. If the sperm count reaches 20 million/ml, the Sertoli cells cease the release of inhibin, allowing the sperm count to increase. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.04%3A_Hormonal_Control_of_Human_Reproduction/43.4A%3A_Male_Hormones.txt |
The stages of the ovarian cycle in the female are regulated by hormones secreted by the hypothalamus, pituitary, and the ovaries.
Learning Objectives
• Explain the function of female hormones in reproduction
Key Points
• As in males, GnRH secreted by the hypothalamus triggers the release of FSH and LH from the pituitary; however, in females, this signals the ovaries to produce estradiol and progesterone.
• FSH stimulates the growth and maturation of follicles on the ovaries, which house and nourish the developing eggs; the follicle, in turn, releases inhibin, which inhibits the production of FSH.
• Progesterone stimulates the growth of the endometrial lining of the uterus in order to prepare it for pregnancy; a strong surge of LH at around day 14 of the cycle triggers ovulation of an egg from the most mature follicle.
• After ovulation, the ruptured follicle becomes a corpus luteum, which secretes progesterone to either regrow the uterine lining or to support the pregnancy if it occurs.
• During middle age, a woman’s ovaries become less sensitive to FSH and LH and, therefore, cease to mature follicles and undergo ovulation; this is known as menopause.
Key Terms
• corpus luteum: a yellow mass of cells that forms from an ovarian follicle during the luteal phase of the menstrual cycle in mammals; it secretes steroid hormones
• menopause: the ending of menstruation; the time in a woman’s life when this happens
• endometrium: the mucous membrane that lines the uterus in mammals and in which fertilized eggs are implanted
• estradiol: a potent estrogenic hormone produced in the ovaries of all vertebrates; the synthetic compound is used medicinally to treat estrogen deficiency and breast cancer
• menstruation: the periodic discharging of the menses, the flow of blood and cells from the lining of the uterus in females of humans and other primates
Female Hormones
The control of reproduction in females is more complex than that of the male. As with the male, the hypothalamic hormone GnRH (gonadotropin-releasing hormone) causes the release of the hormones FSH (follicle stimulating hormone) and LH (luteinizing hormone) from the anterior pituitary. In addition, estrogens and progesterone are released from the developing follicles, which are structures on the ovaries that contain the maturing eggs.
In females, FSH stimulates the development of egg cells, called ova, which develop in structures called follicles. Follicle cells produce the hormone inhibin, which inhibits FSH production. LH also plays a role in the development of ova, as well as in the induction of ovulation and stimulation of estradiol and progesterone production by the ovaries. Estradiol and progesterone are steroid hormones that prepare the body for pregnancy. Estradiol is the reproductive hormone in females that assists in endometrial regrowth, ovulation, and calcium absorption; it is also responsible for the secondary sexual characteristics of females. These include breast development, flaring of the hips, and a shorter period necessary for bone maturation. Progesterone assists in endometrial re-growth and inhibition of FSH and LH release.
The Ovarian Cycle and the Menstrual Cycle
The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle governs the preparation and maintenance of the uterine lining. These cycles occur concurrently and are coordinated over a 22–32 day cycle, with an average length of 28 days.
The first half of the ovarian cycle is the follicular phase. Slowly-rising levels of FSH and LH cause the growth of follicles on the surface of the ovary, which prepares the egg for ovulation. As the follicles grow, they begin releasing estrogens and a low level of progesterone. Progesterone maintains the endometrium, the lining of the uterus, to help ensure pregnancy. Just prior to the middle of the cycle (approximately day 14), the high level of estrogen causes FSH and, especially, LH to rise rapidly and then fall. The spike in LH causes ovulation: the most mature follicle ruptures and releases its egg. The follicles that did not rupture degenerate and their eggs are lost. The level of estrogen decreases when the extra follicles degenerate.
If pregnancy implantation does not occur, the lining of the uterus is sloughed off, a process known as menstruation. After about five days, estrogen levels rise and the menstrual cycle enters the proliferative phase. The endometrium begins to regrow, replacing the blood vessels and glands that deteriorated during the end of the last cycle.
Following ovulation, the ovarian cycle enters its luteal phase and the menstrual cycle enters its secretory phase, both of which run from about day 15 to 28. The luteal and secretory phases refer to changes in the ruptured follicle. The cells in the follicle undergo physical changes, producing a structure called a corpus luteum, which produces estrogen and progesterone. The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH. The uterus is again being prepared to accept a fertilized egg, should it occur during this cycle. The inhibition of FSH and LH prevents any further eggs and follicles from developing. The level of estrogen produced by the corpus luteum increases to a steady level for the next few days.
If no fertilized egg is implanted into the uterus, the corpus luteum degenerates and the levels of estrogen and progesterone decrease. The endometrium begins to degenerate as the progesterone levels drop, initiating the next menstrual cycle. The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH to start the cycles again.
Menopause
As women approach their mid-40s to mid-50s, their ovaries begin to lose their sensitivity to FSH and LH. Menstrual periods become less frequent and finally cease; this process is known as menopause. There are still eggs and potential follicles on the ovaries, but without the stimulation of FSH and LH, they will not produce a viable egg to be released. The outcome of this is the inability to have children.
Various symptoms are associated with menopause, including hot flashes, heavy sweating, headaches, some hair loss, muscle pain, vaginal dryness, insomnia, depression, weight gain, and mood swings. Estrogen is involved in calcium metabolism and, without it, blood levels of calcium decrease. To replenish the blood, calcium is lost from bone, which may decrease the bone density and lead to osteoporosis. Supplementation of estrogen in the form of hormone replacement therapy (HRT) can prevent bone loss, but the therapy can have negative side effects, such as an increased risk of stroke or heart attack, blood clots, breast cancer, ovarian cancer, endometrial cancer, gall bladder disease, and, possibly, dementia. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.04%3A_Hormonal_Control_of_Human_Reproduction/43.4B%3A_Female_Hormones.txt |
In fertilization, the sperm binds to the egg, allowing their membranes to fuse and the sperm to transfer its nucleus into the egg.
Learning Objectives
• Describe the process of fertilization
Key Points
• A mammalian egg is covered by a layer of glycoproteins called the zona pellucida, which the sperm must penetrate in order to fertilize the egg.
• Upon binding with the egg, the sperm initiates the acrosome reaction, in which it releases digestive enzymes that degrade the zona pellucida, allowing the plasma membrane of the sperm to fuse with that of the egg.
• Upon fusion of the two plasma membranes, the sperm’s nucleus enters the egg and fuses with the nucleus of the egg.
• Both the sperm and the egg each contain one half the normal number of chromosomes, so when they fuse the resulting zygote is a diploid organism with a complete set of chromosomes.
• When the egg is successfully fertilized, it releases proteins that prevent it from being fertilized by another sperm, a condition known as polyspermy.
Key Terms
• fertilization: the act of fecundating or impregnating animal or vegetable gametes
• zona pellucida: a glycoprotein membrane surrounding the plasma membrane of an oocyte
• acrosome: a structure forming the end of the head of a spermatozoon
• polyspermy: the penetration of an ovum by more than one sperm
Fertilization
Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm are haploid, which means they each contain one set of chromosomes; upon fertilization, they will combine their genetic material to form a zygote that is diploid, having two sets of chromosomes. A zygote that has more than two sets of chromosomes will not be viable; therefore, to ensure that the offspring has only two sets of chromosomes, only one sperm must fuse with one egg.
In mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. When a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reaction, take place. In placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane. The fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. Fusion between the oocyte plasma membrane and sperm follows and allows the sperm nucleus, centriole, and flagellum, but not the mitochondria, to enter the oocyte. The nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome. This process ultimately leads to the formation of a diploid cell called a zygote. The zygote divides to form a blastocyst and, upon entering the uterus, implants in the endometrium, beginning pregnancy.
Process of fertilization: (a) Fertilization is the process in which sperm and egg fuse to form a zygote. (b) Acrosomal reactions help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus.
To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg. If this mechanism fails, multiple sperm can fuse with the egg, resulting in polyspermy. The resulting embryo is not genetically viable and dies within a few days. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.05%3A_Fertilization_and_Early_Embryonic_Development/43.5A%3A__Fertilization.txt |
A zygote undergoes rapid cell divisions (cleavage) to form a spherical ball of cells: the blastula; this will further develop into a blastocyst.
Learning Objectives
• Describe the events that occur from the formation of a zygote to gastrulation
Key Points
• A single-celled zygote will undergo multiple rounds of cleavage, or cell division, in order to produced a ball of cells, called a blastula, with a fluid-filled cavity in its center, called a blastocoel.
• In animals with little yolk in the egg, the zygote undergoes holoblastic cleavage, in which the entire zygote is cleaved repeatedly; in animals with a lot of yolk in the egg, the zygote undergoes meroblastic cleavage, in which only part of the zygote is cleaved.
• The blastula eventually organizes itself into two layers: the inner cell mass (which will become the embryo) and the outer layer or trophoblast (which will become the placenta ); the structure is now called a blastocyst.
• During gastrulation, the blastula folds in on itself to form three germ layers, the ectoderm, the mesoderm, and the endoderm, that will give rise to the internal structures of the organism.
Key Terms
• blastula: a 6-32-celled hollow structure that is formed after a zygote undergoes cell division
• blastomere: any cell that results from division of a fertilized egg
• meroblastic: undergoing only partial cleavage
• holoblastic: cleaving, and separating into separate blastomeres
• inner cell mass: a mass of cells within a primordial embryo that will eventually develop into the distinct form of a fetus in most eutherian mammals
• gastrulation: the stage of embryo development at which a gastrula is formed from the blastula by the inward migration of cells
• trophoblast: the membrane of cells that forms the wall of a blastocyst during early pregnancy, providing nutrients to the embryo and later developing into part of the placenta
Cleavage and Blastula Stage
The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage. After the cleavage has produced over 100 cells, the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel). Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula. During cleavage, the cells divide without an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells. Each cell within the blastula is called a blastomere.
Cleavage can take place in two ways: holoblastic (total) cleavage or meroblastic (partial) cleavage. The type of cleavage depends on the amount of yolk in the eggs. In placental mammals (including humans) where nourishment is provided by the mother’s body, the eggs have a very small amount of yolk and undergo holoblastic cleavage. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage.
In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass and an outer layer called the trophoblast. The inner cell mass is also known as the embryoblast; this mass of cells will go on to form the embryo. At this stage of development, the inner cell mass consists of embryonic stem cells that will differentiate into the different cell types needed by the organism. The trophoblast will contribute to the placenta and nourish the embryo.
Gastrulation
The typical blastula is a ball of cells. The next stage in embryonic development is the formation of the body plan. The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems.
Differentiation of germ layers: The three germ layers give rise to different cell types in the animal body: the ectoderm forms the nervous system and the outer layer of skin, the mesoderm gives rise to muscles and connective tissues, and the endoderm gives rise to the lining of the digestive system and other internal organs.
The three germs layers are the endoderm, the ectoderm, and the mesoderm. The ectoderm gives rise to the nervous system and the epidermis; the mesoderm gives rise to the muscle cells and connective tissue in the body; and the endoderm gives rise to columnar cells found in the digestive system and many internal organs.
Contributions and Attributions
• zona pellucida. Provided by: Wiktionary. Located at: http://en.wiktionary.org/wiki/zona_pellucida. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44846/latest...ol11448/latest. License: CC BY: Attribution
• Fertilization. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Fertilization. License: CC BY-SA: Attribution-ShareAlike
• polyspermy. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/polyspermy. License: CC BY-SA: Attribution-ShareAlike
• acrosome. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/acrosome. License: CC BY-SA: Attribution-ShareAlike
• fertilization. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/fertilization. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Fertilization and Early Embryonic Development. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44846/latest...43_05_01ab.jpg. License: CC BY: Attribution
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44846/latest...ol11448/latest. License: CC BY: Attribution
• OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44846/latest...ol11448/latest. License: CC BY: Attribution
• blastula. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/blastula. License: CC BY-SA: Attribution-ShareAlike
• blastomere. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/blastomere. License: CC BY-SA: Attribution-ShareAlike
• gastrulation. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/gastrulation. License: CC BY-SA: Attribution-ShareAlike
• trophoblast. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/trophoblast. License: CC BY-SA: Attribution-ShareAlike
• meroblastic. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/meroblastic. License: CC BY-SA: Attribution-ShareAlike
• holoblastic. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/holoblastic. License: CC BY-SA: Attribution-ShareAlike
• inner cell mass. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/inner_cell_mass. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Fertilization and Early Embryonic Development. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44846/latest...43_05_01ab.jpg. License: CC BY: Attribution
• OpenStax College, Fertilization and Early Embryonic Development. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44846/latest...e_43_05_03.jpg. License: CC BY: Attribution
• OpenStax College, Fertilization and Early Embryonic Development. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44846/latest...e_43_05_04.jpg. License: CC BY: Attribution | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.05%3A_Fertilization_and_Early_Embryonic_Development/43.5B%3A_Cleavage_the_Blastula_Stage_and_Gastrulation.txt |
During organogenesis, the three germ layers of the embryo differentiate and further specialize to form the various organs of the body.
Learning Objectives
• Describe the process of organogenesis in vertebrates
Key Points
• Cells in the ectoderm are signaled by molecules called growth factors to form the neural plate, which rolls up to form a structure called the neural tube; the neural tube will eventually develop into the brain and spinal cord.
• The differing expression of various genes controls the differentiation of the mesoderm into connective tissue, as well as the ribs, spine, skeletal muscle, and lungs.
• The endoderm forms the lining of the digestive tract, as well as the linings of all the glands that will empty into the digestive tract; it also forms a wide variety of internal organs.
Key Terms
• organogenesis: the formation and development of the organs of an organism from embryonic cells
• ectoderm: outermost of the three tissue layers in the embryo of a metazoan animal, which will produce the epidermis (skin) and nervous system of the adult
• mesoderm: one of the three tissue layers in the embryo of a metazoan animal, which will produce many internal organs of the adult such as the muscles, spine and circulatory system
• endoderm: one of the three tissue layers in the embryo of a metazoan animal, which will produce the digestive system and other internal organs of the adult
• neural plate: a thick, flat bundle of ectoderm formed in vertebrate embryos after induction by the notochord
Organogenesis
Organogenesis is the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism. Organs form from the germ layers through the differentiation: the process by which a less-specialized cell becomes a more-specialized cell type. This must occur many times as a zygote becomes a fully-developed organism. During differentiation, the embryonic stem cells express specific sets of genes which will determine their ultimate cell type. For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal cells. Therefore, the process of differentiation is regulated by cellular signaling cascades.
In vertebrates, one of the primary steps during organogenesis is the formation of the neural system. The ectoderm forms epithelial cells and tissues, as well as neuronal tissues. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate. If the signaling by growth factors were disrupted, then the entire ectoderm would differentiate into neural tissue. The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube. In further development, the neural tube will give rise to the brain and the spinal cord.
The mesoderm that lies on either side of the vertebrate neural tube will develop into the various connective tissues of the animal body. A spatial pattern of gene expression reorganizes the mesoderm into groups of cells called somites, with spaces between them. The somites will further develop into the ribs, lungs, and segmental (spine) muscle. The mesoderm also forms a structure called the notochord, which is rod-shaped and forms the central axis of the animal body.
The endoderm consists, at first, of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube (except part of the mouth and pharynx) and the terminal part of the rectum (which is lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and air cells of the lungs; the urinary bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus. Additionally, the endoderm forms internal organs including the stomach, the colon, the liver, the pancreas, the urinary bladder, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.06%3A_Organogenesis_and_Vertebrate_Formation/43.6A%3A_Organogenesis.txt |
Through the expression patterns of different genes, the three axes of the body are established, aiding in tissue and organ development.
Learning Objectives
• Describe the formation of body axes in vertebrates
Key Points
• As an animal develops, it must organize its internal and external structures such that the anterior/posterior (forward/backward), dorsal / ventral (back/belly), and lateral/medial (side/middle) axes are correctly determined.
• Proteins that are part of the Wnt signaling pathway help determine the anterior/posterior axis by guiding the axons of the spinal cord in an anterior/posterior direction.
• Together with the sonic hedgehog (Shh) protein, Wnt determines the dorsal/ventral axis; Wnt levels are highest in the dorsal region and lessen toward the ventral region, while Shh levels are highest in the ventral region and lessen toward the dorsal region.
Key Terms
• dorsal: with respect to, or concerning the side in which the backbone is located, or the analogous side of an invertebrate
• ventral: on the front side of the human body, or the corresponding surface of an animal, usually the lower surface
• notochord: a flexible rodlike structure that forms the main support of the body in the lowest chordates; a primitive spine
• Wnt signaling pathway: a group of signal transduction pathways made of proteins that pass signals from outside of a cell through cell surface receptors to the inside of the cell
Vertebrate Axis Formation
Even as the germ layers form, the ball of cells still retains its spherical shape. However, animal bodies have lateral-medial (toward the side-toward the midline), dorsal-ventral (toward the back-toward the belly), and anterior-posterior (toward the front-toward the back) axes. As the body forms, it must develop in such a way that cells, tissues, and organs are organized correctly along these axes.
How are these established? In one of the most seminal experiments ever to be carried out in developmental biology, Spemann and Mangold took dorsal cells from one embryo and transplanted them into the belly region of another embryo. They found that the transplanted embryo now had two notochords: one at the dorsal site from the original cells and another at the transplanted site. This suggested that the dorsal cells were genetically programmed to form the notochord and define the dorsal-ventral axis. Since then, researchers have identified many genes that are responsible for axis formation. Mutations in these genes leads to the loss of symmetry required for organism development. Many of these genes are involved in the Wnt signaling pathway.
In early embryonic development, the formation of the primary body axes is a crucial step in establishing the overall body plan of each particular organism. Wnt signaling can be implicated in the formation of the anteroposterior and dorsoventral axes. Wnt signaling activity in anterior-posterior development can be seen in several organisms including mammals, fish, and frogs. Wnt signaling is also involved in the axis formation of specific body parts and organ systems that are a part of later development. In vertebrates, sonic hedgehog (Shh) and Wnt morphogenetic signaling gradients establish the dorsoventral axis of the central nervous system during neural tube axial patterning. High Wnt signaling establishes the dorsal region while high Shh signaling indicates in the ventral region. Wnt is also involved in the dorsal-ventral formation of the central nervous system through its involvement in axon guidance. Wnt proteins guide the axons of the spinal cord in an anterior-posterior direction. Wnt is also involved in the formation of the limb dorsal-ventral axis. Specifically, Wnt7a helps produce the dorsal patterning of the developing limb.
Contributions and Attributions
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44850/latest...ol11448/latest. License: CC BY: Attribution
• Organogenesis. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Organogenesis. License: CC BY-SA: Attribution-ShareAlike
• Germ layer. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Germ_layer%23Endoderm. License: CC BY-SA: Attribution-ShareAlike
• ectoderm. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ectoderm. License: CC BY-SA: Attribution-ShareAlike
• mesoderm. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/mesoderm. License: CC BY-SA: Attribution-ShareAlike
• organogenesis. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/organogenesis. License: CC BY-SA: Attribution-ShareAlike
• endoderm. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/endoderm. License: CC BY-SA: Attribution-ShareAlike
• neural plate. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/neural_plate. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Organogenesis and Vertebrate Formation. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44850/latest...e_43_06_01.jpg. License: CC BY: Attribution
• Mesoderm. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Mesoderm.png. License: Public Domain: No Known Copyright
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44850/latest...ol11448/latest. License: CC BY: Attribution
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• ventral. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ventral. License: CC BY-SA: Attribution-ShareAlike
• notochord. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/notochord. License: CC BY-SA: Attribution-ShareAlike
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• dorsal. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/dorsal. License: CC BY-SA: Attribution-ShareAlike
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• Mesoderm. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Mesoderm.png. License: Public Domain: No Known Copyright
• OpenStax College, Organogenesis and Vertebrate Formation. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44850/latest...e_43_06_03.jpg. License: CC BY: Attribution | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.06%3A_Organogenesis_and_Vertebrate_Formation/43.6B%3A_Vertebrate_Axis_Formation.txt |
Once the zygote implants in the uterine wall, embryonic and fetal development continue through three trimesters to birth.
Learning Objectives
• Describe the development of the human fetus from fertilization through the third trimester
Key Points
• After fertilization, the zygote implants itself in the uterine wall; its outer layer grows into the endometrium, where it begins to produce human chorionic gonadotropin.
• During the first trimester, the placenta forms along with the internal organs and structures; however, not all of the internal organs function at this point.
• During the second trimester, internal organs continue to develop and the fetus becomes active.
• The third trimester is one of rapid growth, in which the fetus reaches its full size; pregnancy often becomes uncomfortable for the mother.
Key Terms
• zygote: a diploid fertilized egg cell
• chorion: allows exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment
• human chorionic gonadotropin: a peptide hormone, produced during pregnancy, that prevents the breakdown of the corpus luteum and maintains progesterone production
• placenta: a vascular organ in mammals that supplies food and oxygen from the mother to the fetus, while passing back waste; it is implanted in the wall of the uterus
Human gestation
Twenty-four hours before fertilization, the egg has finished meiosis and become a mature oocyte. When fertilized (at conception), the egg, now known as a zygote, travels through the oviduct to the uterus. The developing embryo must implant into the wall of the uterus within seven days or it will deteriorate and die. The outer layers of the zygote ( blastocyst ) grow into the endometrium by digesting the endometrial cells. Wound healing of the endometrium closes up the blastocyst into the tissue. Another layer of the blastocyst, the chorion, begins releasing a hormone called human chorionic gonadotropin (hCG) which makes its way to the corpus luteum, keeping it active. This ensures adequate levels of progesterone that will maintain the endometrium of the uterus for the support of the developing embryo. Pregnancy tests determine the level of hCG in urine or serum: if the hormone is present, the test is positive.
First trimester
The gestation period is divided into three equal periods or trimesters. During the first two to four weeks of the first trimester, nutrition and waste are handled by the endometrial lining through diffusion. As the trimester progresses, the outer layer of the embryo begins to merge with the endometrium and the placenta forms. This organ takes over the nutrient and waste requirements of the embryo and fetus, with the mother’s blood passing nutrients to the placenta and removing waste from it. Chemicals from the fetus, such as bilirubin, are processed by the mother’s liver for elimination. Some of the mother’s immunoglobulins will pass through the placenta, providing passive immunity against some potential infections.
Internal organs and body structures begin to develop during the first trimester. By five weeks, limb buds, eyes, the heart, and liver have been basically formed. By eight weeks, the term fetus applies; the body is essentially formed. The individual is about five centimeters (two inches) in length and many of the organs, such as the lungs and liver, are not yet functioning. Exposure to any toxins is especially dangerous during the first trimester, as all of the body’s organs and structures are going through initial development. Anything that affects that development can have a severe effect on the fetus’ survival.
Second trimester
During the second trimester, the fetus grows to about 30 cm (12 inches). As it becomes active, the mother usually feels the first movements. All organs and structures continue to develop. The placenta has taken over the functions of nutrition and waste, along with the production of estrogen and progesterone from the corpus luteum, which has degenerated. The placenta will continue functioning up through the delivery of the fetus.
Third trimester
During the third trimester, the fetus grows to 3 to 4 kg (6 ½ -8 ½ lbs.) and about 50 cm (19-20 inches) long. This is the period of the most rapid growth during the pregnancy. Organ development continues to birth (and some systems, such as the nervous system and liver, continue to develop after birth). The mother will be at her most uncomfortable during this trimester. She may urinate frequently due to pressure on the bladder from the fetus. There may also be intestinal blockage and circulatory problems, especially in her legs. Clots may form in her legs due to pressure from the fetus on returning veins as they enter the abdominal cavity. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.07%3A_Human_Pregnancy_and_Birth/43.7A%3A_Human_Gestation.txt |
Labor and birth are divided into three stages: the dilation of the cervix, the delivery of the baby, and the expulsion of the placenta.
Learning Objectives
• Describe the process of labor and birth in humans
Key Points
• At the end of gestation, estrogen receptors on the uterine wall bind oxytocin, which causes the uterine muscles to contract; as the muscles contract, they signal for the release of more oxytocin in a positive feedback loop.
• During the first stage of labor, the cervix thins and dilates to allow passage of the baby into the birth canal; typically over the course of several hours, the cervix will dilate to its full width of 10 centimeters.
• During the second stage of labor, contractions become very strong and, aided by the pushing of the mother, the baby is expelled from the uterus.
• During the third stage of labor, the placenta, amniotic sac, and the remainder of the umbilical cord are expelled from the uterus, usually within a few minutes after birth.
• When the baby begins suckling at the breast after delivery, prolactin signals the release of milk from the mammary glands, providing nutrition and immunity against disease to the infant.
Key Terms
• prolactin: a peptide gonadotrophic hormone secreted by the pituitary gland; it stimulates growth of the mammary glands and lactation in females
• parturition: the act of giving birth; childbirth
• oxytocin: a hormone that stimulates contractions during labor, and then the production of milk
Labor and Birth
Labor is the physical effort of expulsion of the fetus and the placenta from the uterus during birth (parturition). The total gestation period from fertilization to birth is about 38 weeks (birth usually occurring 40 weeks after the last menstrual period). Toward the end of the third trimester, estrogen causes receptors on the uterine wall to develop and bind the hormone oxytocin. At this time, the baby reorients, facing forward and down with the back or crown of the head engaging the cervix (uterine opening). This causes the cervix to stretch, sending nerve impulses to the hypothalamus, which signals for the release of oxytocin from the posterior pituitary. The oxytocin causes the smooth muscle in the uterine wall to contract. At the same time, the placenta releases prostaglandins into the uterus, increasing the contractions. A positive feedback relay occurs between the uterus, hypothalamus, and the posterior pituitary to assure an adequate supply of oxytocin. As more smooth muscle cells are recruited, the contractions increase in intensity and force.
There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm, a process that may take many hours, especially in a woman bearing her first child. At some point, the amniotic sac bursts and the amniotic fluid escapes. During stage two, the baby is expelled from the uterus with the umbilical cord still attached. The uterus contracts and the mother pushes as she compresses her abdominal muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall, usually within a few minutes. If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.
The mother’s mammary glands go through changes during the third trimester to prepare for lactation and breastfeeding. When the baby begins suckling at the breast, signals are sent to the hypothalamus causing the release of prolactin from the anterior pituitary, which signals the mammary glands to produce milk. Oxytocin is also released, promoting the release of the milk. The milk contains nutrients for the baby’s development and growth as well as immunoglobulins to protect the child from bacterial and viral infections. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.07%3A_Human_Pregnancy_and_Birth/43.7B%3A_Labor_and_Birth.txt |
Contraception, also known as birth control, is methods used to prevent pregnancy; some of these methods are more successful than others.
Learning Objectives
• Describe the various methods of birth control and their associated failure rates
Key Points
• Barrier methods include those that prevent the sperm from reaching the egg ( condoms, diaphragms, cervical caps, sponges, and spermicides ); these methods have a failure rate of 15-24%.
• Hormonal methods include those that prevent a woman from ovulating (oral birth control pills, hormone injections, and hormone implants); these have a failure rate of 8%.
• Sterilization, such as a tubal ligation in a woman or a vasectomy in a man, is an extremely effective (although permanent) method of contraception, with a failure rate of 1%.
• More “natural” methods of birth control include avoiding intercourse when a woman is ovulating, or withdrawing the penis from the vagina before ejaculation; these methods are much less successful, with a failure rate of around 25%.
Key Terms
• condom: a flexible sleeve made of latex or other impermeable material such as sheepskin, worn over an erect penis during intercourse as a contraceptive or as a way to prevent the spread of STDs
• intrauterine device: a contraceptive device consisting of a spiral or similar shape of plastic or metal inserted through the vagina into the uterus in order to prevent the implantation of a fertilized egg
• vasectomy: the surgical removal of all or part of the vas deferens, usually as a means of male sterilization
• contraception: the use of a device or procedure to prevent conception as a result of sexual activity
• spermicide: a substance used for killing sperm
Contraception and Birth Control
Contraception, also known as birth control, is methods or devices used to prevent pregnancy. Strictly speaking, contraception aims to prevent the sperm and egg from joining, while birth control can refer to methods used to prevent a fertilized egg from developing into a fetus. Both terms are, however, frequently used interchangeably.
There are many methods of birth control, including barriers to sperm, hormones that prevent ovulation, sterilization procedures, and “natural” methods. Each method has an associated “failure rate”, which is the number of pregnancies resulting from the method’s use over a twelve-month period.
Barrier methods include those that prevent the sperm from reaching the egg. These include condoms, diaphragms, cervical caps, sponges, and spermicides; they have a failure rate of 15-24%. Barrier methods such as condoms, cervical caps, and diaphragms serve to block sperm from entering the uterus, thereby preventing fertilization. Chemicals such as spermicides, which are designed to kill sperm, are often used in conjunction; sponges, for example, are saturated with spermicides and are placed in the vagina at the cervical opening. Combinations of spermicidal chemicals and barrier methods achieve lower failure rates than do the methods when used separately.
Hormonal methods use synthetic progesterone (sometimes in combination with estrogen) to inhibit the hypothalamus from releasing FSH or LH, preventing an egg from being available for fertilization. The method of administering the hormone affects failure rate, although, in general, hormonal methods have a failure rate of 8%. The most reliable method, with a failure rate of less than 1 percent, is the implantation of the hormone under the skin. The same rate can be achieved through the sterilization procedures of vasectomy in the man or of tubal ligation in the woman, or by using an intrauterine device (IUD). IUDs are inserted into the uterus where they establish an inflammatory condition that prevents fertilized eggs from implanting into the uterine wall.
Sterilization is a one-time, permanent, surgical procedure. In a vasectomy, the vasa deferentia of a male are severed and then tied/sealed in a manner that prevents sperm from entering into the seminal stream (ejaculate). Tubal ligation or tubectomy is a surgical procedure for sterilization in which a woman’s fallopian tubes are clamped and blocked, or severed and sealed; either method prevents eggs from reaching the uterus for fertilization. Both of these procedures have a less than 1% failure rate.
Natural methods include avoiding intercourse when ovulation is occurring (“natural family planning) or withdrawing the penis from the vagina before ejaculation. Nearly 25% of the couples using natural family planning or withdrawal can expect a failure of the method. Natural family planning is based on the monitoring of the menstrual cycle and having intercourse only during times when the egg is not available. A woman’s body temperature may rise a degree Celsius at ovulation and the cervical mucus may increase in volume, becoming more pliable. These changes give a general indication of when intercourse is more or less likely to result in fertilization. Withdrawal involves the removal of the penis from the vagina during intercourse, before ejaculation occurs; it has a failure rate of 27%. This is, therefore, a risky method. The high failure rate is due to the possible presence of sperm in the bulbourethral gland’s secretion, which may enter the vagina prior to removing the penis.
Termination of an existing pregnancy can be spontaneous or voluntary. Spontaneous termination in the first trimester is referred to as spontaneous abortion, while in the second and third trimesters is referred to as a miscarriage. Spontaneous abortion usually occurs very early in the pregnancy (typically within the first few weeks). Pregnancy loss occurs when the embryo/fetus cannot develop properly so the gestation is naturally terminated. Voluntary termination of a pregnancy is called an elective abortion. Laws regulating elective abortion vary between states; the laws tend to view fetal viability as the criteria for allowing or preventing the procedure.
43.7D: Infertility
Learning Objectives
• Define infertility and discuss ways in which it can be treated
Infertility is the inability to conceive or carry a fetus to birth. About 75% of causes of infertility can be identified. These include diseases (such as sexually-transmitted diseases that can cause scarring of the reproductive tubes in either men or women) or developmental problems frequently related to abnormal hormone levels in one of the individuals. Inadequate nutrition, especially starvation, can delay menstruation. Stress can also lead to infertility. Short-term stress can affect hormone levels, while long-term stress can delay puberty, causing less-frequent menstrual cycles. Other factors that affect fertility include toxins (such as cadmium), tobacco smoking, marijuana use, gonadal injuries, and aging.
If the cause of infertility is identified, several assisted reproductive technologies (ART) are available to aid conception. A common type of assisted reproductive technology is in vitro fertilization (IVF) where an egg and sperm are combined outside the body and then placed in the uterus. Eggs are obtained from the woman after extensive hormonal treatments that prepare mature eggs for fertilization and prepare the uterus for implantation of the fertilized egg. Sperm are obtained from the male and combined with the eggs, which are then supported through several cell divisions to ensure viability of the zygotes. When the embryos have reached the eight-cell stage, one or more is implanted into the woman’s uterus. If fertilization is not accomplished by simple IVF, a procedure known as intracytoplasmic sperm injection (ICSI) can be used to inject the sperm into an egg. IVF procedures produce a surplus of fertilized eggs and embryos that can be frozen and stored for future use; the procedures can also result in multiple births.
Key Points
• Infertility can have many causes, such as sexually transmitted diseases, toxins, malnutrition, drug use, stress, and age.
• Infertility can be treated by assisted reproductive technologies in which science intervenes, attempting to aid conception through artificial means.
• In vitro fertilization is a type of assisted reproductive technology in which an egg is fertilized by a sperm outside the body, grows to the eight- cell stage, and is then implanted in the uterus.
Key Terms
• infertility: the inability to conceive children
• assisted reproductive technology: the technological means of inducing pregnancy (by means of artificial insemination or other techniques) | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/43%3A_Animal_Reproduction_and_Development/43.07%3A_Human_Pregnancy_and_Birth/43.7C%3A_Contraception_and_Birth_Control.txt |
Ecology is the study of organisms, populations, and communities as they relate to one another and interact in the ecosystems they comprise.
Learning Objectives
• Describe the study of ecology
Key Points
• In ecology, ecosystems are composed of organisms, the communities they comprise, and the non-living aspects of their environment.
• The four main levels of study in ecology are the organism, population, community, and ecosystem.
• Ecosystem processes are those that sustain and regulate the environment.
• Ecological areas of study include topics ranging from the interactions and adaptations of organisms within an ecosystem to the abiotic processes that drive the development of those ecosystems.
Key Terms
• ecology: the branch of biology dealing with the relationships of organisms with their environment and with each other
• ecosystem: a system formed by an ecological community and its environment that functions as a unit
• ecophysiology: the study of the relationships between, and adaptation of, the physiology of an organism and its environment
An Introduction to Ecology
Ecology is the study of the interactions of living organisms with their environment. Within the discipline of ecology, researchers work at four specific levels, sometimes discretely and sometimes with overlap. These levels are organism, population, community, and ecosystem. In ecology, ecosystems are composed of dynamically-interacting parts, which include organisms, the communities they comprise, and the non-living (abiotic) components of their environment. Ecosystem processes, such as primary production, pedogenesis (the formation of soil), nutrient cycling, and various niche construction activities, regulate the flux of energy and matter through an environment. These processes are sustained by organisms with specific life-history traits. The variety of organisms, called biodiversity, which refer to the differing species, genes, and ecosystems, enhances certain ecosystem services.
In essence, ecologists seek to explain:
• life processes
• interactions, interrelationships, behaviors, and adaptations of organisms
• the movement of materials and energy through living communities
• the successional development of ecosystems
• the abundance and distribution of organisms and biodiversity in the context of the environment
There are many practical applications of ecology in conservation biology, wetland management, natural resource management (agroecology, agriculture, forestry, agroforestry, fisheries), city planning (urban ecology), community health, economics, basic and applied science, and human social interaction (human ecology). Organisms and resources comprise ecosystems which, in turn, maintain biophysical feedback mechanisms that moderate processes acting on living (biotic) and nonliving (abiotic) components of the planet. Ecosystems sustain life-supporting functions and produce natural capital, such as biomass production (food, fuel, fiber and medicine), the regulation of climate, global biogeochemical cycles, water filtration, soil formation, erosion control, flood protection, and many other natural features of scientific, historical, economic, or intrinsic value.
There are also many subcategories of ecology, such as ecosystem ecology, animal ecology, and plant ecology, which look at the differences and similarities of various plants in various climates and habitats. In addition, physiological ecology, or ecophysiology, studies the responses of the individual organism to the environment, while population ecology looks at the similarities and dissimilarities of populations and how they replace each other over time.
Finally, it is important to note that ecology is not synonymous with environment, environmentalism, natural history, or environmental science. It is also different from, though closely related to, the studies of evolutionary biology, genetics, and ethology. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.01%3A_The_Scope_of_Ecology/44.1A%3A_Introduction_to_Ecology.txt |
Organismal and population ecology study the adaptations that allow organisms to live in a habitat and organisms’ relationships to one another.
Learning Objectives
• Describe populations as studied in population ecology and organisms as studied in organismal ecology
Key Points
• Organismal ecology focuses on the morphological, physiological, and behavioral adaptations that let an organism survive in a specific habitat.
• Population ecology studies the number of individuals in an area, as well as how and why their population size changes over time.
• The Karner blue butterfly, an endangered species, makes a good model for both organismal and population ecology since it is dependent, as a population, on a specific plant that grows within specific areas, which, thus, influences butterfly distribution and numbers.
Key Terms
• conspecific: an organism belonging to the same species as another
• population: a collection of organisms of a particular species, sharing a particular characteristic of interest, most often that of living in a given area
• oviposit: to lay eggs
Organismal Ecology
Researchers studying ecology at the organismal level are interested in the adaptations that enable individuals to live in specific habitats. These adaptations can be morphological (pertaining to the study of form or structure), physiological, and behavioral. For instance, the Karner blue butterfly (Lycaeides melissa samuelis) is considered a specialist because the females preferentially oviposit (that is, lay eggs) on wild lupine. This preferential adaptation means that the Karner blue butterfly is highly dependent on the presence of wild lupine plants for its continued survival.
After hatching, the larval caterpillars emerge to spend four to six weeks feeding solely on wild lupine. The caterpillars pupate (undergo metamorphosis), emerging as butterflies after about four weeks. The adult butterflies feed on the nectar of flowers of wild lupine and other plant species. A researcher interested in studying Karner blue butterflies at the organismal level might, in addition to asking questions about egg laying, ask questions about the butterflies’ preferred temperature (a physiological question) or the behavior of the caterpillars when they are at different larval stages (a behavioral question).
Population Ecology
A population is a group of interbreeding organisms that are members of the same species living in the same area at the same time. Organisms that are all members of the same species, a population, are called conspecifics. A population is identified, in part, by where it lives; its area of population may have natural or artificial boundaries. Natural boundaries might be rivers, mountains, or deserts, while examples of artificial boundaries include mowed grass or manmade structures such as roads. The study of population ecology focuses on the number of individuals in an area and how and why population size changes over time. Population ecologists are particularly interested in counting the Karner blue butterfly, for example, because it is classified as federally endangered. However, the distribution and density of this species is highly influenced by the distribution and abundance of wild lupine. Researchers might ask questions about the factors leading to the decline of wild lupine and how these affect Karner blue butterflies. For example, ecologists know that wild lupine thrives in open areas where trees and shrubs are largely absent. In natural settings, intermittent wildfires regularly remove trees and shrubs, helping to maintain the open areas that wild lupine requires. Mathematical models can be used to understand how wildfire suppression by humans has led to the decline of this important plant for the Karner blue butterfly. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.01%3A_The_Scope_of_Ecology/44.1B%3A_Organismal_Ecology_and_Population_Ecology.txt |
Community ecology studies interactions between different species; abiotic and biotic factors affect these on an ecosystem level.
Learning Objectives
• Distinguish between community ecology and ecosystem ecology
Key Points
• Community ecology focuses on the processes driving interactions between differing species and their overall consequences.
• Ecosystem ecology studies all organismal, population, and community components of an area, as well as the non-living counterparts.
• The mutualistic relationship between the Karner blue butterfly and ants are of interest to community ecology studies since both species interact within an area and affect each other’s survival rate; in turn, they are both affected by nutrient -poor soils, which are part of the ecosystem ecology.
Key Terms
• community: a group of interdependent organisms inhabiting the same region and interacting with each other
• conspecific: an organism belonging to the same species as another
• heterospecific: an organism belonging to a different species to another
Community Ecology
A biological community consists of the different species within an area, typically a three-dimensional space, and the interactions within and among these species. Community ecologists are interested in the processes driving these interactions and their consequences. Questions about conspecific interactions often focus on competition among members of the same species for a limited resource. Ecologists also study interactions among various species; members of different species are called heterospecifics. Examples of heterospecific interactions include predation, parasitism, herbivory, competition, and pollination. These interactions can have regulating effects on population sizes and can impact ecological and evolutionary processes affecting diversity.
For example, the larvae of the Karner blue butterfly form mutualistic relationships with ants. Mutualism is a form of a long-term relationship that has coevolved between two species and from which each species benefits. For mutualism to exist between individual organisms, each species must receive some benefit from the other as a consequence of the relationship. Researchers have shown that there is an increase in the probability of survival when Karner blue butterfly larvae (caterpillars) are tended by ants. This might be because the larvae spend less time in each life stage when tended by ants, which provides an advantage for the larvae. Meanwhile, the Karner blue butterfly larvae secrete a carbohydrate-rich substance that is an important energy source for the ants. Both the Karner blue larvae and the ants benefit from their interaction.
Ecosystem Ecology
Ecosystem ecology is an extension of organismal, population, and community ecology. The ecosystem is composed of all the biotic components (living things) in an area along with that area’s abiotic components (non-living things). Some of the abiotic components include air, water, and soil. Ecosystem biologists ask questions about how nutrients and energy are stored, along with how they move among organisms and the surrounding atmosphere, soil, and water.
The Karner blue butterflies and the wild lupine live in an oak-pine barren habitat. This habitat is characterized by natural disturbance and nutrient-poor soils that are low in nitrogen. The availability of nutrients is an important factor in the distribution of the plants that live in this habitat. Researchers interested in ecosystem ecology could ask questions about the importance of limited resources and the movement of resources, such as nutrients, though the biotic and abiotic portions of the ecosystem. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.01%3A_The_Scope_of_Ecology/44.1C%3A_Community_Ecology_and_Ecosystem_Ecology.txt |
Biogeography is an ecological field of interest that focuses on the distribution of organisms and the abiotic factors that affect them.
Learning Objectives
• Explain the role of biogeography in the analysis of species distribution
Key Points
• The composition of plant and animal communities change as abiotic factors, which include temperature and altitude, start to vary.
• Some species exist only in specific geographical areas while others can thrive in a variety of areas; however, no single species can be found everywhere in the world.
• Studying an area where a species is not found is also of importance to ecologists in determining unique patterns of species distribution.
• As with animals, plant species can also be either endemic, usually found in isolated land masses, or generalists, found in many regions.
Key Terms
• biogeography: the study of the geographical distribution of living things
• generalist: species which can thrive in a wide variety of environmental conditions
• endemic: unique to a particular area or region; not found in other places
Biogeography
Biogeography is the study of the geographic distribution of living things and the abiotic (non-living) factors that affect their distribution. Abiotic factors can include temperature, moisture, nutrients, oxygen, and energy availability, as well as disturbances from events such as wind and fire. Differences in temperature and rainfall are primarily based on latitude and elevation. As these abiotic factors change, the composition of plant and animal communities also changes. For example, if you were to begin a journey at the equator and walk north, you would notice gradual changes in plant communities. At the beginning of your journey, you would see tropical wet forests with broad-leaved evergreen trees, which are characteristic of plant communities found near the equator. As you continued to travel north, you would see these broad-leaved evergreen plants eventually give rise to seasonally-dry forests with scattered trees. You would also begin to notice changes in temperature and moisture. At about 30 degrees north, these forests would give way to deserts, which are characterized by low precipitation.
Moving farther north, you would see that deserts are replaced by grasslands or prairies. Eventually, grasslands are replaced by deciduous temperate forests. These deciduous forests give way to the boreal forests found in the subarctic, the area south of the Arctic Circle. Finally, you would reach the Arctic tundra, which is found at the most northern latitudes. This trek north reveals gradual changes in both climate and the types of organisms that have adapted to environmental factors associated with ecosystems found at different latitudes. However, different ecosystems exist at the same latitude due in part to abiotic factors such as jet streams, the Gulf Stream, and ocean currents. If you were to hike up a mountain, the changes you would see in the vegetation would parallel those as you move to higher latitudes.
Ecologists who study biogeography examine patterns of species distribution. No species exists everywhere. For example, the Venus flytrap is endemic to a small area in North and South Carolina. An endemic species is one which is naturally found only in a specific geographic area that is usually restricted in size. Other species are generalists, living in a wide variety of geographic areas. The raccoon, for example, is native to most of North and Central America.
Species distribution patterns are based on biotic and abiotic factors and the influences these factors have had during the very long periods of time required for species evolution. Therefore, early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century. Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers. Biologists estimate that Australia, for example, has between 600,000 and 700,000 species of plants and animals. Approximately 3/4 of living plant and mammal species are endemic species found solely in Australia.
Sometimes ecologists discover unique patterns of species distribution by determining where species are not found. Hawaii, for example, has no native land species of reptiles or amphibians and has only one native terrestrial mammal, the hoary bat. Most of New Guinea, as another example, lacks placental mammals.
Plants can be endemic or generalists. Endemic plants are found only in specific regions of the earth, while generalists are found in many regions. Isolated land masses, such as Australia, Hawaii, and Madagascar, often have large numbers of endemic plant species. Some of these plants are endangered due to human activity. The forest gardenia (Gardenia brighamii), for instance, is endemic to Hawaii; only an estimated 15–20 trees are thought to exist. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.02%3A_Biogeography/44.2A%3A_Biogeography.txt |
The availability of energy and nutrient sources affects species distribution and their adaptation to land or aquatic habitats.
Learning Objectives
• Assess how energy availability affects species distribution within an ecosystem
Key Points
• In land habitats, plant adaptations include life cycles that are dependent on the availability of light; for example, species will flower or grow at varying times to ensure they capture enough available light suitable to their needs.
• In aquatic ecosystems, species growth and distribution are adapted to deal with the sometimes-limited availability of light due to its absorption by water, plants, suspended particles, microorganisms, and water depth.
• Ocean upwelling and spring and fall turnovers are important processes regulating the distribution of nutrients in an aquatic ecosystems.
• Nutrient availability is connected to the energy needs of organisms in aquatic ecosystems since sequestered energy is reused by living organisms from dead ones.
Key Terms
• ephemeral: lasting for a short period of time
• upwelling: the oceanographic phenomenon that occurs when strong, usually seasonal, winds push water away from the coast, bringing cold, nutrient-rich deep waters up to the surface
• thermocline: a layer within a body of water or air where the temperature changes rapidly with depth
Energy Sources
Energy from the sun is captured by green plants, algae, cyanobacteria, and photosynthetic protists. These organisms convert solar energy into the chemical energy needed by all living things. Light availability can be an important abiotic force directly affecting the evolution of adaptations in photosynthesizers. For instance, plants in the understory of a temperate forest are shaded when the trees above them in the canopy completely leaf out in the late spring. Not surprisingly, understory plants have adaptations to successfully capture available light. One such adaptation is the rapid growth of spring ephemeral plants, such as the spring beauty. These spring flowers achieve much of their growth and finish their life cycle (reproduce) early in the season before the trees in the canopy develop leaves.
In aquatic ecosystems, the availability of light may be limited because sunlight is absorbed by water, plants, suspended particles, and resident microorganisms. Toward the bottom of a lake, pond, or ocean, there is a zone that light cannot reach. Photosynthesis cannot take place there and, as a result, a number of adaptations have evolved that enable living things to survive without light. For instance, aquatic plants have photosynthetic tissue near the surface of the water. The broad, floating leaves of a water lily cannot survive without light. In environments such as hydrothermal vents, some bacteria extract energy from inorganic chemicals because there is no light for photosynthesis.
Nutrient Cycling
The availability of nutrients in aquatic systems is also an important aspect of energy or photosynthesis. Many organisms sink to the bottom of the ocean when they die in the open water. When this occurs, the energy found in that organism is sequestered for some time unless ocean upwelling occurs. Ocean upwelling is the rising of deep ocean waters that occurs when prevailing winds blow along surface waters near a coastline. As the wind pushes ocean waters offshore, water from the bottom of the ocean moves up to replace this water. As a result, the nutrients once contained in dead organisms become available for reuse by other living organisms.
In freshwater systems, the recycling of nutrients occurs in response to air temperature changes. The nutrients at the bottom of lakes are recycled twice each year: in the spring and fall turnover, which recycles nutrients and oxygen from the bottom of a freshwater ecosystem to the top of a body of water. These turnovers are caused by the formation of a thermocline: a layer of water with a temperature that is significantly different from that of the surrounding layers. In wintertime, the surface of lakes found in many northern regions is frozen. However, the water under the ice is slightly warmer, while the water at the bottom of the lake is warmer yet at 4 °C to 5 °C (39.2 °F to 41 °F). Water is densest at 4 °C; therefore, the deepest water is also the densest. The deepest water is oxygen poor because the decomposition of organic material at the bottom of the lake uses up available oxygen that cannot be replaced by means of oxygen diffusion into the water due to the surface ice layer.
In springtime, air temperatures increase and surface ice melts. When the temperature of the surface water begins to reach 4 °C, the water becomes heavier and sinks to the bottom. The water at the bottom of the lake, displaced by the heavier surface water, rises to the top. As it rises, the sediments and nutrients from the lake bottom are brought along with it. During the summer months, the lake water stratifies, or forms layers, with the warmest water at the lake surface.
As air temperatures drop in the fall, the temperature of the lake water cools to 4 °C; this causes fall turnover as the heavy cold water sinks and displaces the water at the bottom. The oxygen-rich water at the surface of the lake then moves to the bottom of the lake, while the nutrients at the bottom of the lake rise to the surface (). During the winter, the oxygen at the bottom of the lake is used by decomposers and other organisms requiring oxygen, such as fish. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.02%3A_Biogeography/44.2B%3A_Energy_Sources.txt |
Temperature and water are important abiotic factors that affect species distribution.
Learning Objectives
• Describe species adaptations to temperature fluctuations and the availability of water
Key Points
• Temperature is a factor that influences species distribution because organisms must either maintain a specific internal temperature or inhabit an environment that will keep the body within a temperature range that supports their metabolism.
• Many species have developed adaptations, such as migration, hibernation, and estivation, to deal with temperature fluctuations in the environments in which they live.
• Water retention is vital to all living beings; adaptations have evolved within both terrestrial and aquatic species to minimize water loss.
Key Terms
• osmosis: The net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane
• estivate: to go into stasis or torpor in the summer months
• extremophile: an organism that lives under extreme conditions of temperature, salinity, etc; commercially important as a source of enzymes that operate under similar conditions
• torpor: a state of being inactive or stuporous
• hibernation: a state of inactivity and metabolic depression in animals during winter
Temperature
Temperature affects the physiology of living things as well as the density and state of water. It exerts an important influence on living organisms because few can survive at temperatures below 0 °C (32 °F) due to metabolic constraints. It is also rare for them to survive at temperatures exceeding 45 °C (113 °F). This is a reflection of evolutionary response to typical temperatures. Enzymes are most efficient within a narrow and specific range of temperatures; enzyme degradation can occur at higher temperatures. Therefore, organisms must either maintain an internal temperature or inhabit an environment that will keep the body within a temperature range that supports metabolism. Some animals have adapted to enable their bodies to survive significant temperature fluctuations, as seen in hibernation or reptilian torpor. Similarly, some bacteria have adapted to survive in extremely-hot temperatures found in places such as geysers. Such bacteria are examples of extremophiles: organisms that thrive in extreme environments.
Temperature can limit the distribution of living things. Animals faced with temperature fluctuations may respond with adaptations, such as migration, in order to survive. Migration, the movement from one place to another, is common in animals, including many that inhabit seasonally-cold climates. Migration solves problems related to temperature, locating food, and finding a mate. In migration, for instance, the arctic tern (Sterna paradisaea) makes a 40,000 km (24,000 mi) round trip flight each year between its feeding grounds in the southern hemisphere and its breeding grounds in the Arctic Ocean. Monarch butterflies (Danaus plexippus) live in the eastern United States in the warmer months, but migrate to Mexico and the southern United States in the wintertime. Some species of mammals also make migratory forays: reindeer (Rangifer tarandus) travel about 5,000 km (3,100 mi) each year to find food. Amphibians and reptiles are more limited in their distribution because they lack migratory ability. Not all animals that can migrate do so as migration carries risk and comes at a high energy cost.
Some animals hibernate or estivate to survive hostile temperatures. Hibernation enables animals to survive cold conditions, while estivation allows animals to survive the hostile conditions of a hot, dry climate. Animals that hibernate or estivate enter a state known as torpor, a condition in which their metabolic rate is significantly lowered. This enables the animal to wait until its environment better supports its survival. Some amphibians, such as the wood frog (Rana sylvatica), have an antifreeze-like chemical in their cells, which retains the cells’ integrity and prevents them from bursting.
Water
Water is required by all living things because it is critical for cellular processes. Since terrestrial organisms lose water to the environment by simple diffusion, they have evolved many adaptations to retain water.
Examples of adaptations used by terrestrial and aquatic species include the following:
• Plants have a number of interesting features on their leaves, such as leaf hairs and a waxy cuticle, that serve to decrease the rate of water loss via transpiration.
• Freshwater organisms, surrounded by water, are constantly in danger of having water rush into their cells because of osmosis. Many adaptations of organisms living in freshwater environments have evolved to ensure that solute concentrations in their bodies remain within appropriate levels. One such adaptation is the excretion of dilute urine.
• Marine organisms are surrounded by water with a higher solute concentration than the organism and, thus, are in danger of losing water to the environment because of osmosis. These organisms have morphological and physiological adaptations to retain water and release solutes into the environment. For example, marine iguanas (Amblyrhynchus cristatus) sneeze out water vapor that is high in salt in order to maintain solute concentrations within an acceptable range while swimming in the ocean and eating marine plants. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.02%3A_Biogeography/44.2C%3A_Temperature_and_Water.txt |
Soil structure, oxygen availability, wind, and fire are abiotic factors that have influences on species distribution and quantity.
Learning Objectives
• Identify further abiotic factors that affect species distribution and abundance
Key Points
• Soil structure, pH, and its nutrient content affect the distribution of plants, which in turn influences the distribution of the animals that feed on them.
• Oxygen availability is an important abiotic factor affecting species in both aquatic and terrestrial environments.
• Wind and fire impose physical disturbances that species must be adapted to in order to live in affected areas.
Key Terms
• inorganic: relating to a compound that does not contain carbon
• transpiration: the loss of water by evaporation in terrestrial plants, especially through the stomata; accompanied by a corresponding uptake from the roots
Other Important Abiotic Factors
Inorganic nutrients, soil structure, and aquatic oxygen availability are further abiotic factors that affect species distribution in an ecosystem. The same is true for terrestrial factors, such as wind and fire, which can impact the types of species that inhabit regions exposed to these types of disturbances.
Inorganic Nutrients and Soil
Inorganic nutrients, such as nitrogen and phosphorus, are important in the distribution and the abundance of living things. Plants obtain these inorganic nutrients from the soil when water moves into the plant through the roots. Therefore, soil structure (the particle size of soil components), soil pH, and soil nutrient content play an important role in the distribution of plants. Animals obtain inorganic nutrients from the food they consume. Therefore, animal distributions are related to the distribution of what they eat. In some cases, animals will follow their food resource as it moves through the environment.
Oxygen Availability
Some abiotic factors, such as oxygen, are important in aquatic ecosystems as well as terrestrial environments. Terrestrial animals obtain oxygen from the air they breathe. Oxygen availability can be an issue for organisms living at very high elevations, where there are fewer molecules of oxygen in the air. In aquatic systems, the concentration of dissolved oxygen is related to water temperature and the speed at which the water moves. Cold water has more dissolved oxygen than warmer water. In addition, salinity, water current, and tide can be important abiotic factors in aquatic ecosystems.
Other Terrestrial Factors
Wind can be an important abiotic factor because it influences the rate of evaporation and transpiration. The physical force of wind is also important because it can move soil, water, or other abiotic factors, as well as an ecosystem’s organisms.
Fire is another terrestrial factor that can be an important agent of disturbance in terrestrial ecosystems. Some organisms are adapted to fire and, thus, require the high heat associated with fire to complete a part of their life cycle. For example, the jack pine, a coniferous tree, requires heat from fire for its seed cones to open. Through the burning of pine needles, fire adds nitrogen to the soil and limits competition by destroying undergrowth.
44.2E: Abiotic Factors Influencing Plant Growth
The two most important abiotic factors affecting plant primary productivity in an ecosystem are temperature and moisture.
Learning Objectives
• Identify the abiotic factors that affect plant growth
Key Points
• Primary production, on which almost all of life on earth is dependent, occurs through either photosynthesis or chemosynthesis.
• Annual biomass production, used to estimate net primary productivity by plants in an area, is directly influenced by an environment’s abiotic factors, which include temperature and moisture.
• Warm and wet climates have the greatest amount of plant biomass because they offer conditions in which photosynthesis, plant growth, and the resulting net primary productivity are highest.
Key Terms
• biomass: the total mass of all living things within a specific area, habitat, etc.
• eco-region: a region, smaller than an ecozone, that contains a distinct biodiversity of flora and fauna
• chemosynthesis: the production of carbohydrates and other compounds using the oxidation of chemical nutrients as a source of energy rather than sunlight; it is limited to certain bacteria and fungi
Abiotic Factors Influencing Plant Growth
Temperature and moisture are important influences on plant production (primary productivity) and the amount of organic matter available as food (net primary productivity). Primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through chemosynthesis, which uses the oxidation or reduction of chemical compounds as its source of energy. Almost all life on earth is directly or indirectly reliant on primary production. The organisms responsible for primary production, known as primary producers or autotrophs, form the base of the food chain. In terrestrial eco-regions, these are mainly plants, while in aquatic eco-regions, they are mainly algae.
Net primary productivity is an estimation of all of the organic matter available as food. It is calculated as the total amount of carbon fixed per year minus the amount that is oxidized during cellular respiration. In terrestrial environments, net primary productivity is estimated by measuring the aboveground biomass per unit area, which is the total mass of living plants, excluding roots. This means that a large percentage of plant biomass which exists underground is not included in this measurement. Net primary productivity is an important variable when considering differences in biomes. Very productive biomes have a high level of aboveground biomass.
Annual biomass production is directly related to the abiotic components of the environment. Environments with the greatest amount of biomass have conditions in which photosynthesis, plant growth, and the resulting net primary productivity are optimized. The climate of these areas is warm and wet. Photosynthesis can proceed at a high rate, enzymes can work most efficiently, and stomata can remain open without the risk of excessive transpiration. Together, these factors lead to the maximal amount of carbon dioxide (CO2) moving into the plant, resulting in high biomass production. The aboveground biomass produces several important resources for other living things, including habitat and food. Conversely, dry and cold environments have lower photosynthetic rates and, therefore, less biomass. The animal communities living there will also be affected by the decrease in available food. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.02%3A_Biogeography/44.2D%3A_Inorganic_Nutrients_and_Other_Factors.txt |
A biome consists of all the habitats of a community that make up similar ecosystems in a particular region.
Learning Objectives
• Differentiate biomes from other levels of ecological classification, including habitat
Key Points
• The climate, including precipitation and temperature, and the geography control the type of biome found in a region.
• There are two major classifications of biomes, which are terrestial and aquatic, and these include the types of biomes known as deserts, forests, grasslands, savannas, tundra, and freshwater environments.
• A habitat is the location where a group of one type of organism (a population ) lives, while a biome is a community made of all the habitats in a given region and climate.
• Different organisms inhabit different types of biomes.
• Each type of biome can be found in multiple locations on Earth depending on its climate, geography, and organisms.
Key Terms
• biome: any major regional biological community such as that of forest or desert
• ecotone: a transition area between two adjacent ecosystems
• habitat: a specific place or natural conditions in which a plant or animal lives
• population: a collection of organisms of a particular species, sharing a particular characteristic of interest, most often that of living in a given area
• ecosystem: a system formed by an ecological community and its environment that functions as a unit
What Constitutes a Biome?
A group of living organisms of the same kind that live in the same place simultaneously is known as a population. Populations live together in habitats, which together make up a community. An ecosystem is a community of living organisms interacting with the non-living components of that environment.
A biome is a community on a global scale, where habitats flank each other, and is usually defined by the temperature, precipitation, and types of plants and animals that inhabit it. The Earth’s biomes are categorized into two major groups: terrestrial and aquatic. Terrestrial biomes are based on land, while aquatic biomes include both ocean and freshwater biomes. The major types of biomes include: aquatic, desert, forest, grassland, savannas, and tundra.
Biome Attributes
Generally, biome classification is determined by the climate and geography of an area. Each biome consists of communities that have adapted to the different climate and environment inside the biome. Specifically, there are special vegetation adaptations as well as physical and behavioral adaptions made by animals in order to accommodate the environment. The eight major terrestrial biomes on Earth are each distinguished by characteristic temperatures and amount of precipitation. Comparing the annual totals of precipitation and fluctuations in precipitation from one biome to another provides clues as to the importance of abiotic factors in the distribution of biomes. Temperature variation on a daily and seasonal basis is also important for predicting the geographic distribution of the biome and the vegetation type in the biome.
The distribution of these biomes shows that the same biome can occur in geographically distinct areas with similar climates. Biomes have no distinct boundaries. Instead, there is a transition zone called an ecotone, which contains a variety of plants and animals. For example, an ecotone might be a transition region between a grassland and a desert, with species from both. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.03%3A_Terrestrial_Biomes/44.3A%3A_What_constitutes_a_biome.txt |
Tropical wet forests are characterized by high precipitation and humidity, while savannas have scattered trees and an extensive dry season.
Learning Objectives
• Recognize the distinguishing characteristics of tropical wet forests and savannas
Key Points
• Tropical wet forests, located near the equator, have temperatures that range from 20°C – 34°C (68°F – 93°F), with little variation in seasonal temperatures.
• The lack of seasonality, constant daily sunlight, ideal temperatures, and high rate of precipitation in tropical wet forests lead to increased plant growth and high species diversity.
• The annual rainfall in tropical wet forests ranges from 125 to 660 cm (50–200 in); there is a high rate of precipitation even in the dry months.
• Savannas, grasslands with scattered trees, are located in Africa, South America, and northern Australia.
• Temperatures in savannas range from 24°C – 29°C (75°F – 84°F), with annual rainfall ranges from 10–40 cm (3.9–15.7 in).
• Because savannas are very dry, trees do not grow as well as they do in other forest biomes and diversity is minimal.
Key Terms
• deciduous: of or pertaining to trees which lose their leaves in winter or the dry season
• biome: any major regional biological community such as that of forest or desert
• understory: the layer of plants that grow in the shade of the canopy of a forest
Tropical Wet Forest
Tropical wet forests, also referred to as tropical rainforests, are found in equatorial regions. The vegetation is characterized by plants with broad leaves that fall off throughout the year. Unlike the trees of deciduous forests, the trees in this biome do not have a seasonal loss of leaves associated with variations in temperature and sunlight; these forests are “evergreen” year-round.
The temperature and sunlight profiles of tropical wet forests are very stable in comparison to that of other terrestrial biomes, with the temperatures ranging from 20°C – 34°C (68°F – 93°F). Compared to other forest biomes, tropical wet forests have little variation in seasonal temperatures. This lack of seasonality leads to year-round plant growth, rather than the seasonal (spring, summer, and fall) growth seen in other biomes. In contrast to other ecosystems, tropical ecosystems do not have long days and short days during the yearly cycle. Instead, a constant daily amount of sunlight (11–12 hrs per day) provides more solar radiation and, thereby, a longer period of time for plant growth.
The annual rainfall in tropical wet forests ranges from 125-660 cm (50–200 in), with some monthly variation. While sunlight and temperature remain fairly consistent, annual rainfall is highly variable. Tropical wet forests have wet months in which there can be more than 30 cm (11–12 in) of precipitation, as well as dry months in which there are fewer than 10 cm (3.5 in) of rainfall. However, the driest month of a tropical wet forest still exceeds the annual rainfall of some other biomes, such as deserts.
Tropical wet forests have high net primary productivity because the annual temperatures and precipitation values in these areas are ideal for plant growth. Therefore, the extensive biomass present in the tropical wet forest leads to plant communities with very high species diversity. Tropical wet forests have more species of trees than any other biome. On average, between 100 and 300 species of trees are present in a single hectare (2.5 acres) of South America. One way to visualize this is to compare the distinctive horizontal layers within the tropical wet forest biome. On the forest floor is a sparse layer of plants and decaying plant matter. Above that is an understory of short shrubby foliage. A layer of trees rising above this understory is topped by a closed upper canopy: the uppermost overhead layer of branches and leaves. Some additional trees emerge through this closed upper canopy. These layers provide diverse and complex habitats for the variety of plants, fungi, animals, and other organisms within the tropical wet forests. For instance, epiphytes are plants that grow on other plants. Host plants are typically unharmed. Epiphytes are found throughout tropical wet forest biomes. Many species of animals use the variety of plants and the complex structure of the tropical wet forests for food and shelter. Some organisms live several meters above ground, having adapted to this arboreal lifestyle.
Savannas
Savannas are grasslands with scattered trees located in Africa, South America, and northern Australia. Savannas are hot, tropical areas with temperatures averaging from 24°C – 29°C (75°F – 84°F) and an annual rainfall of 10–40 cm (3.9–15.7 in). They have an extensive dry season. For this reason, forest trees do not grow as well as they do in the tropical wet forest or other forest biomes. As a result, there are relatively few trees within the grasses and forbs (herbaceous flowering plants) that dominate the savanna. Since fire is an important source of disturbance in this biome, plants have evolved well-developed root systems that allow them to quickly re-sprout after a fire.
Animals commonly found in savannas in Africa include the African elephant, lions, gazelles, giraffes, ostriches, and many other mammals, birds, plants and invertebrates. The northern Australian savannas also have many types of plants, animals, insects, and reptiles, including marsupials (kangaroos and wallabies), bats, and rodents. In addition to the native animals such as foxes and Patagonian maras (rabbit-like rodents), savannas in South America are commonly used for grazing domestic livestock, such as sheep, goats, and cattle because of their open grasslands and herbaceous layer of plants. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.03%3A_Terrestrial_Biomes/44.3B%3A_Tropical_Wet_Forest_and_Savannas.txt |
Subtropical deserts are characterized by their dry environments, while chaparrals are characterized by the presence of shrubs.
Learning Objectives
• Recognize the distinguishing characteristics of subtropical deserts and chaparrals
Key Points
• Subtropical deserts are centered on the Tropics of Cancer and Capricorn.
• Subtropical deserts can be hot or cold, but they are all very dry,having very low annual precipitation.
• Because precipitation is so low in subtropical deserts, most plants are annuals which utilize adaptations to conserve water.
• Chaparrals (scrub forests) are found in California, along the Mediterranean Sea, and along the southern coast of Australia.
• Chaparrals are very wet in the winter, but very dry in the summer months; most chaparral plants stay dormant during the summer.
• Most chaparral plants are shrubs adapted to fires; some seeds only germinate after a fire.
Key Terms
• chaparral: a region of shrubs, typically dry in the summer and rainy in the winter
• subtropical desert: dry region centered on the Tropics of Cancer and Capricorn where evaporation exceeds precipitation
Subtropical Deserts
Subtropical deserts, which exist between 15° and 30° north and south latitude, are centered on the Tropics of Cancer and Capricorn. In some years, evaporation exceeds precipitation in this very dry biome. Subtropical hot deserts may have daytime soil surface temperatures above 60°C (140°F) and nighttime temperatures approaching 0°C (32°F). In cold deserts, temperatures may be as high as 25°C (77°F) and may drop below -30°C (-22°F). Subtropical deserts are characterized by low annual precipitation of fewer than 30 cm (12 in), with little monthly variation and lack of predictability in rainfall. In some cases, the annual rainfall can be as low as 2 cm (0.8 in), such as in central Australia (“the Outback”) and northern Africa.
Types of Deserts
There are several types of deserts including high-pressure deserts, mid-continent deserts, rain-shadow deserts, and upwelling deserts. In high-pressure deserts, the high atmospheric pressure enables the air to retain more moisture and there is little rainfall. High-pressure deserts include the Sahara, Arabian, Thar, and Kalahari deserts, and the desert regions within the Arctic and Antarctic circles. Areas in the middle of a continent can receive little rainfall because moisture tends to condense before it reaches the middle of a large continent. Modern examples of mid-continent deserts are the Turkmenistan, Gobi, and Great Australian deserts. Third, rain-shadow deserts are created when moisture from the ocean condenses on one side of a mountain range. These mountain ranges usually have a rainforest on one side and a desert on the other. Examples of rain-shadow deserts include the Mojave desert in the rain-shadow of the Sierra Nevada, the Patagonian desert in the rain-shadow of the Andes, and the Iranian desert in the rain-shadow of the Zagros mountains. Finally, upwelling deserts exist adjacent to areas where cold currents rise to the ocean surface, reducing evaporation. Examples include the Atacama desert, the Western Sahara, and the Namib desert.
Adaptations for Deserts
The type of vegetation and limited animal diversity of this biome are closely related to the low and unpredictable precipitation. Very dry deserts lack perennial vegetation that lives from one year to the next. Instead, many plants are annuals that grow quickly, reproduce when rainfall does occur, and then die. Many other plants in these areas are characterized by having a number of adaptations that conserve water, such as deep roots, reduced foliage, and water-storing stems. Seed plants in the desert produce seeds that can remain in dormancy for extended periods between rains. To reduce water loss and conserve energy, many desert animals like the fennec fox are nocturnal and burrow during the day.
Chaparral
The chaparral, also called the scrub forest, is found in California, along the Mediterranean Sea, and along the southern coast of Australia. The annual rainfall in this biome ranges from 65 cm to 75 cm (25.6–29.5 in), with the majority of rain falling in the winter. Due to the very dry summers, many chaparral plants are dormant during that season. The chaparral vegetation is dominated by shrubs and is adapted to periodic fires, with some plants producing seeds that only germinate after a hot fire. The ashes left behind after a fire are rich in nutrients, such as nitrogen, that fertilize the soil and promote plant regrowth. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.03%3A_Terrestrial_Biomes/44.3C%3A_Subtropical_Deserts_and_Chaparral.txt |
Temperate grasslands are areas with low annual precipitation, fluctuating seasonal temperatures, and few trees.
Learning Objectives
• Recognize the distinguishing characteristics of temperate grasslands
Key Points
• Temperate grasslands are found throughout central North America, where they are also known as prairies; they are also found in Eurasia, where they are known as steppes.
• Temperate grasslands have hot summers and cold winters; the growing season occurs during the spring, summer, and fall.
• Because of the low annual precipitation, temperate grasslands have very few trees.
• Grasses are the dominant vegetation; their roots and rhizomes provide increased fertility to the soil.
• Fires caused by lightening occur often in grasslands; without fires grasslands are converted to scrub forests.
Key Terms
• steppe: the grasslands of Eastern Europe and Asia
• prairie: an extensive area of relatively flat grassland with few, if any, trees, especially in North America
Temperate Grasslands
Temperate grasslands are found throughout central North America, where they are also known as prairies, and within Eurasia, where they are known as steppes. Temperate grasslands have pronounced annual fluctuations in temperature, with hot summers and cold winters. The annual temperature variation produces specific growing seasons for plants. Plant growth is possible when temperatures are warm enough and when ample water is available to sustain it, which typically occurs in the spring, summer, and fall. During much of the winter, temperatures are low and water, which is stored in the form of ice, is not available for plant growth.
Annual precipitation ranges from 25 cm to 75 cm (9.8–29.5 in). Because of relatively-lower annual precipitation in temperate grasslands, there are few trees, except for those found growing along rivers or streams. The dominant vegetation tends to consist of grasses; some prairies sustain populations of grazing animals. The vegetation is very dense and the soils are fertile because the subsurface of the soil is packed with the roots and rhizomes (underground stems) of these grasses, which anchor plants into the ground and replenish the organic material (humus) in the soil when they die and decay.
Fires, mainly caused by lightning, are a natural disturbance in temperate grasslands. When fire is suppressed, the vegetation eventually converts to scrub and dense forests. Often, the restoration or management of temperate grasslands requires the use of controlled burns to suppress the growth of trees and maintain the grasses. Burning causes new grass to grow, which brings back the grazing animals.
Organisms Found in Temperate Grasslands
Mites, insect larvae, nematodes and earthworms inhabit deep soil, which can reach 6 meters (20 ft) underground in undisturbed grasslands on the richest soils of the world. These invertebrates, along with symbiotic fungi, extend the root systems, break apart hard soil, enrich it with urea and other natural fertilizers, trap minerals and water, and promote growth. Some types of fungi make the plants more resistant to insect and microbial attacks. Grasslands also are home to a vast variety of mammals, reptiles, birds, and insects. Typical large mammals include the Giant Anteater and populations of grazing animals, such as the Blue Wildebeest, Przewalski’s Horse, and the American Bison. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.03%3A_Terrestrial_Biomes/44.3D%3A_Temperate_Grasslands.txt |
Temperate forests are characterized by fluctuating seasonal temperatures and constant-but-moderate rainfall.
Learning Objectives
• Recognize the distingushing chracteristics of temperate forests
Key Points
• Temperate forests are the most common biome in eastern North America, Western Europe, Eastern Asia, Chile, and New Zealand.
• Temperatures in temperate forests fluctuate; there are defined growing seasons during spring, summer, and early fall.
• Because temperate forests have moderate annual precipitation, the dominant plants are the deciduous trees.
• Deciduous trees experience a dormant period in the winter, which is why temperate forests have less net productivity than tropical forests.
• Temperate forests are more open than tropical wet forests since their trees do not grow as tall.
• The soils of the temperate forests are rich in inorganic and organic nutrients; this is due to the thick layer of leaf litter on forest floors, which returns nutrients to the soil.
Key Terms
• deciduous: of or pertaining to trees which lose their leaves in winter or the dry season
• temperate forest: forest concentrations formed in the northern and southern hemisphere, or in temperate regions; main characteristics include wide leaves, large and tall trees, and seasonal vegetation
Temperate Forests
Temperate forests are the most common biome in eastern North America, Western Europe, Eastern Asia, Chile, and New Zealand. This biome is found throughout mid-latitude regions. Temperatures ranging between -30°C – 30°C (-22°F – 86°F) drop below freezing on an annual basis, resulting in defined growing seasons during the spring, summer, and early fall. Precipitation is relatively constant throughout the year, ranging between 75 cm and 150 cm (29.5–59 in).
Because of the moderate, annual rainfall and temperatures, deciduous trees are the dominant plant in this biome. Deciduous trees lose their leaves each fall, remaining leafless in the winter; thus, no photosynthesis occurs during the dormant winter period. Each spring, new leaves appear as the temperature increases. Because of the dormant period, the net primary productivity of temperate forests is less than that of tropical wet forests. In addition, temperate forests show less diversity of tree species than do tropical wet forest biomes.
The trees of the temperate forests leaf out and shade much of the ground; however, this biome is more open than tropical wet forests because trees in the temperate forests do not grow as tall as the trees in tropical wet forests. The soils of the temperate forests are rich in inorganic and organic nutrients due to the thick layer of leaf litter on forest floors. As this leaf litter decays, nutrients are returned to the soil. The leaf litter also protects soil from erosion, insulates the ground, and provides habitats for invertebrates (such as the pill bug or roly-poly, Armadillidium vulgare) and their predators, such as the red-backed salamander (Plethodon cinereus).
Organisms Found in Temperate Deciduous Forests
The leaf litter is home to invertebrates (such as the pill bug or roly-poly, Armadillidium vulgare) and their predators, including the red-backed salamander (Plethodon cinereus). Pileated woodpeckers depend upon dead or dying trees as a source of food and for constructing their nests, and many migratory birds, such as the spring warblers, time their arrival to coincide with the opening of the tree canopy, which provides the insects that are their principal food sources for raising young. Many well-known animals are found in temperate deciduous forests including squirrels, deer, and bears. The top predators in deciduous forest were once wolves and cougars, but their populations have been in decline. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.03%3A_Terrestrial_Biomes/44.3E%3A_Temperate_Forests.txt |
The boreal forest is characterized by coniferous trees, while the arctic tundra is characterized by permanently frozen soils.
Learning Objectives
• Recognize the distinguishing characteristics of boreal forests and arctic tundra
Key Points
• The boreal forest is found across most of Canada, Alaska, Russia, and northern Europe; the arctic tundra lies north of the boreal forest.
• The boreal forest has cold, dry winters and short, cool, wet summers with precipitation that takes the form of snow; due to this environment, evergreen coniferous trees are the dominant plants.
• The soil in boreal forest regions is usually acidic and contains little available nitrogen.
• Boreal forests have lower productivity than tropical or temperate forests; they also have less diversity, with only a tree layer and ground layer.
• Temperatures in the arctic tundra are cold year-round and precipitation is very low.
• Plants in the arctic tundra have a very short growing season of approximately 10–12 weeks, but during this time, growth is rapid; plants are low to the ground and the soil is permanently frozen.
Key Terms
• permafrost: permanently frozen ground
• arctic tundra: a biome found in the far Northern Hemisphere, north of the boreal forests, where the subsoil is permanently frozen
• boreal forest: a biome found in the Northern Hemisphere and characterized by coniferous forests consisting mostly of pines, spruces, and larches
Boreal Forests
The boreal forest, also known as taiga or coniferous forest, the world’s largest terrestrial biome, is found south of the Arctic Circle and across most of Canada, Alaska, Russia, and northern Europe. This biome has cold, dry winters and short, cool, wet summers. Temperatures vary from −54°C – 30°C (-65°F – 86°F) throughout the whole year. The summers, while short, are generally warm and humid. In much of the taiga, -20°C (-4°F) would be a typical winter day temperature, while 18°C (64°F) would be an average summer day.The annual precipitation, from 40 cm -100 cm (15.7–39 in), usually takes the form of snow. Little evaporation occurs because of the cold temperatures.
The long and cold winters in the boreal forest have led to the predominance of cold-tolerant, cone-bearing plants. These are evergreen, coniferous trees such as pines, spruces, and firs, which retain their needle-shaped leaves year-round. Evergreen trees can photosynthesize earlier in the spring than can deciduous trees because less energy from the sun is required to warm a needle-like leaf than a broad leaf. This benefits evergreen trees, which grow faster than deciduous trees in the boreal forest. In addition, soils in boreal forest regions tend to be acidic, with little available nitrogen. Leaves are a nitrogen-rich structure that deciduous trees must produce yearly. Therefore, coniferous trees that retain nitrogen-rich needles may have a competitive advantage over the broad-leafed deciduous trees.
The net primary productivity of boreal forests is lower than that of temperate forests and tropical wet forests. The aboveground biomass of boreal forests is high because these slow-growing tree species are long-lived, accumulating standing biomass over time. Plant species diversity is less than that seen in temperate forests and tropical wet forests. Boreal forests lack the pronounced elements of the layered forest structure seen in tropical wet forests. The structure of a boreal forest is often only a tree layer and a ground layer. When conifer needles are dropped, they decompose more slowly than do broad leaves; therefore, fewer nutrients are returned to the soil to fuel plant growth.
Arctic Tundra
The Arctic tundra, lying north of the subarctic boreal forest, is located throughout the Arctic regions of the northern hemisphere. The average winter temperature is -34°C (-29.2°F), while the average summer temperature is from 3°C – 12°C (37°F – 52°F). Plants in the arctic tundra have a very short growing season of approximately 10–12 weeks. However, during this time, there are almost 24 hours of daylight, so plant growth is rapid. The annual precipitation of the Arctic tundra is very low (about 15-25 cm), with little annual variation in precipitation. As in the boreal forests, there is little evaporation due to the cold temperatures.
Plants in the Arctic tundra are generally low to the ground. There is little species diversity, low net primary productivity, and low aboveground biomass. The soils of the Arctic tundra may remain in a perennially frozen state referred to as permafrost. The permafrost makes it impossible for roots to penetrate deep into the soil and slows the decay of organic matter, which inhibits the release of nutrients from organic matter. During the growing season, the ground of the Arctic tundra can be completely covered with plants or lichens.
The biodiversity of the tundras is low: there are 1,700 species of vascular plants and only 48 species of land mammals. Notable animals in the Arctic tundra include caribou (reindeer ), musk ox, arctic hare, arctic fox, snowy owl, lemmings, and polar bears. Due to the harsh climate, tundra regions have seen little human activity, even though they are sometimes rich in natural resources such as oil and uranium. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.03%3A_Terrestrial_Biomes/44.3F%3A_Boreal_Forests_and_Arctic_Tundra.txt |
Abiotic factors that influence aquatic biomes include light availability, depth, stratification, temperature, currents, and tides.
Learning Objectives
• Differentiate among the abiotic factors that affect aquatic biomes
Key Points
• In aquatic biomes, light is an important factor that influences the communities of organisms found in both freshwater and marine ecosystems.
• In freshwater biomes, stratification, a major abiotic factor, is related to the energy aspects of light.
• Marine systems are influenced by the physical water movements, such as currents and tides, along with the thermal properties of water.
• Oceans zones can be categorized into photic or aphotic zones, depending on the presence or absence of light and photosynthesis.
Key Terms
• stratification: the process leading to the formation or deposition of layers
• photic: of, related to, or irradiated by light; especially describing that part of the near-surface ocean is which photosynthesis is possible
• aphotic: describing that part of the deep oceans and lakes where photosynthesis is not possible
Abiotic Factors Influencing Aquatic Biomes
As with terrestrial biomes, aquatic biomes are influenced by a series of abiotic factors. However, these factors differ since water has different physical and chemical properties than does air. Even if the water in a pond or other body of water is perfectly clear (there are no suspended particles), water, on its own, absorbs light. As one descends into a deep body of water, there will eventually be a depth which the sunlight cannot reach. While there are some abiotic and biotic factors in a terrestrial ecosystem that might obscure light (such as fog, dust, or insect swarms), usually these are not permanent features of the environment. The importance of light in aquatic biomes is central to the communities of organisms found in both freshwater and marine ecosystems. In freshwater systems, stratification due to differences in density is perhaps the most critical abiotic factor and is related to the energy aspects of light. The thermal properties of water (rates of heating and cooling) are significant to the function of marine systems and have major impacts on global climate and weather patterns. Marine systems are also influenced by large-scale physical water movements, such as currents; these are less important in most freshwater lakes.
The ocean is categorized by several areas or zones. All of the ocean’s open water is referred to as the pelagic realm (or zone). The benthic realm (or zone) extends along the ocean bottom from the shoreline to the deepest parts of the ocean floor. Within the pelagic realm is the photic zone, which is the portion of the ocean that light can penetrate (approximately 200 m or 650 ft). At depths greater than 200 m, light cannot penetrate; thus, this is referred to as the aphotic zone. The majority of the ocean is aphotic, lacking sufficient light for photosynthesis. The deepest part of the ocean, the Challenger Deep (in the Mariana Trench, located in the western Pacific Ocean), is about 11,000 m (about 6.8 mi) deep. To give some perspective on the depth of this trench, the ocean is, on average, 4267 m or 14,000 ft deep. These realms and zones are relevant to freshwater lakes as well, as they determine the types of organisms that will inhabit each region. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.04%3A__Aquatic_Biomes/44.4A%3A_Abiotic_Factors_Influencing_Aquatic_Biomes.txt |
The ocean and coral reefs make up two types of marine biomes where organisms are influenced by depth and light availability.
Learning Objectives
• Describe coral reefs and the various zones in the ocean and the types of organisms living in each
Key Points
• The ocean is divided into different zones with groups of species adapted to deal with the differences in light level, as well as other biotic and abiotic conditions particular to the zones.
• The intertidal zone is characterized by its high and low tides, as well as wave action; as the zone closest to land, it can be a sandy, rocky, or muddy beach.
• The neritic zone is silted, well-oxygenated, low in pressure, and stable in temperature; it is adjacent to the oceanic zone, in which warm and cold waters mix due to currents in the open ocean.
• The benthic zone is nutrient rich because of the sand, silt, and dead organisms that comprise the bottom of the region; as water depth increases, temperature within this deepwater region decreases.
• The abyssal zone is the deepest part of the ocean and, as such, it is cold, has very high pressure, high oxygen content, and low nutrient content.
• Coral reefs are ocean ridges formed by a mutualistic relationship between cnidarians and photosythetic algae; climate change and run-off are just two reasons why these important organisms are now in decline.
Key Terms
• desiccation: the state or process of drying-out
• zooxanthellae: animals of the genus Symbiodinium, a yellow dinoflagellate, notably found in coral reefs
• photic: of, related to, or irradiated by light; especially describing that part of the near-surface ocean is which photosynthesis is possible
Marine Biomes
The ocean is the largest marine biome. It is a continuous body of salt water that is relatively uniform in chemical composition; it is a weak solution of mineral salts and decayed biological matter. Within the ocean, coral reefs are a second kind of marine biome.
Ocean
Physical diversity has a significant influence on the ocean, which is categorized into different zones based on how far light reaches into the water. Each zone has a distinct group of species adapted to the biotic and abiotic conditions particular to that zone.
The intertidal zone (between high and low tide) is the oceanic region that is closest to land. It includes sandy beaches, but can also be rocky or muddy. This zone is an extremely variable environment because of tides. Organisms, exposed to air and sunlight at low tide, are underwater most of the time, especially during high tide. Therefore, living things that thrive in the intertidal zone are adapted to being dry for long periods of time. Since the shore of the intertidal zone is repeatedly struck by waves, the organisms found there are adapted to withstand damage from the pounding action of the waves. The exoskeletons of shoreline crustaceans are tough, protecting them from desiccation and wave damage.
The neritic zone extends from the intertidal zone to depths of about 200 m (or 650 ft) at the edge of the continental shelf. Since light can penetrate this depth, photosynthesis can occur in the neritic zone. The water here contains silt, is well-oxygenated, low in pressure, and stable in temperature. Phytoplankton and floating Sargassum, a marine seaweed, provide a habitat for some sea life found in the neritic zone, including zooplankton, protists, small fishes, and shrimp, which are the base of the food chain for most of the world’s fisheries.
Beyond the neritic zone is the open ocean area known as the oceanic zone. Within the oceanic zone there is thermal stratification where warm and cold waters mix because of ocean currents. Abundant plankton serve as the base of the food chain for larger animals. Nutrients are scarce in this less-productive part of the marine biome. When photosynthetic organisms and the protists and animals that feed on them die, their bodies fall to the bottom of the ocean where they remain. In contrast to freshwater lakes, the open ocean lacks a process for bringing the organic nutrients back up to the surface. The majority of organisms in the aphotic zone include sea cucumbers and other organisms that survive on the nutrients contained in the dead bodies of organisms in the photic zone.
A lower layer is the benthic realm, the deepwater region beyond the continental shelf. The bottom of the benthic realm comprises sand, silt, and dead organisms. Temperature decreases, remaining above freezing, as water depth increases. Due to the dead organisms that fall from the upper layers of the ocean, this nutrient-rich portion of the ocean allows a diversity of life to exist, including fungi, sponges, sea anemones, marine worms, sea stars, fishes, and bacteria.
The deepest part of the ocean, the abyssal zone, at depths of 4000 m or greater, is very cold and has very high pressure, high oxygen content, and low nutrient content. There are a variety of invertebrates and fishes found in this zone, but the abyssal zone does not have plants due to the lack of light.
Coral Reefs
Coral reefs are ocean ridges formed by marine invertebrates living in warm, shallow waters of the ocean. They are found in the photic zone and are important in shore protection. Other coral reef systems are fringing islands, which are directly adjacent to land, or atolls, which are circular reef systems surrounding a former landmass that is now underwater. The coral organisms are colonies of cnidarian polyps that secrete a calcium carbonate skeleton, which slowly accumulates, forming the underwater reef. Corals found in shallower waters have a mutually symbiotic relationship with photosynthetic unicellular algae, which provides corals with the majority of the nutrition and the energy they require. The waters are nutritionally poor; therefore, without this mutualism, it would not be possible for large corals to grow. In contrast, corals living in deeper and colder water attain energy and nutrients by capturing prey using stinging cells on their tentacles.
Global Decline of Coral Reefs
Climate change and human activity pose dual threats to the long-term survival of the world’s coral reefs. Corals evolved to survive at the upper limit of ocean water temperature. Excessive warmth induced by climate change causes reefs to expel their symbiotic, food-producing algae, resulting in a phenomenon known as bleaching. When bleaching occurs, the reefs lose much of their characteristic color as the algae and the coral animals die if loss of the symbiotic zooxanthellae is prolonged. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.04%3A__Aquatic_Biomes/44.4B%3A_Marine_Biomes.txt |
Estuaries, composed of a mix of fresh and salt water and their living communities, are influenced by salinity and the changing tides.
Learning Objectives
• Explain the ecology of estuaries
Key Points
• Estuaries acts as nursery grounds for crustaceans, mollusks, and fish.
• Salinity, regulated by the influx of seawater and outflow of freshwater once or twice each day, is a determining factor in the types of organisms that can live there.
• To deal with the short-term and rapid variation in salinity, estuary species have developed specialized adaptations that enable them to live with the salty conditions; as a result, most plant species found in estuaries are halophytes.
Key Terms
• brackish: salty or slightly salty, as a mixture of fresh and sea water, such as that found in estuaries
• halophyte: any plant that tolerates an environment having a high salt content
• estuary: coastal water body where ocean tides and river water merge
Estuaries: Where the Ocean Meets Fresh Water
Estuaries form a unique marine biome that occurs where a source of fresh water, such as a river, meets the ocean. Therefore, both fresh water and salt water are found in the same vicinity. Mixing results in a diluted (brackish) saltwater. Estuaries form protected areas where many of the young offspring of crustaceans, mollusks, and fish begin their lives. Salinity of estuaries is a very important factor that influences the organisms found there and their adaptations. The salinity, which varies, is based on the rate of flow of its freshwater sources. Once or twice a day, high tides bring salt water into the estuary. Low tides, occurring at the same frequency, reverse the current of salt water.
The short-term and rapid variation in salinity due to the mixing of fresh water and salt water is a difficult physiological challenge for the plants and animals that inhabit estuaries. Many estuarine plant species are halophytes: plants that can tolerate salty water on their roots or sea spray. In some halophytes, filters in the roots remove the salt from the water that the plant absorbs. Other plants are able to pump oxygen into their roots. Animals, such as mussels and clams, have developed behavioral adaptations that expend a lot of energy to function in this rapidly-changing environment. When these animals are exposed to low salinity, they stop feeding, close their shells, and switch from aerobic respiration (in which they use gills) to anaerobic respiration (a process that does not require oxygen). When high tide returns to the estuary, the salinity and oxygen content of the water increases, causing these animals to open their shells, begin feeding, and to return to aerobic respiration.
44.4D: Freshwater Biomes
Lakes, ponds, rivers, streams, and wetlands are all freshwater biomes, which differ in depth, water movement, and other abiotic factors.
Learning Objectives
• Differentiate among the freshwater biomes of lakes and ponds, rivers and streams, and wetlands
Key Points
• Temperature, as well as the availability of nitrogen and phosphorus, are factors that affect living things in lakes and ponds.
• When available in large amounts, nitrogen and phosphorus cause potentially-detrimental algal blooms in lakes; nitrogen is also a limiting factor for plant growth in bogs.
• The continuous movement of rivers and streams are their defining characteristic; these bodies of water carry large amounts of water from the source to a lake or ocean.
• Wetlands are shallow bodies of water with soil that is either permanently or periodically saturated with water; every type of wetland has three shared characteristics: their hydrology, hydrophytic vegetation, and hydric soils.
Key Terms
• algal bloom: a dense spread of algae on the surface of water
• percolation: the seepage or filtration of a liquid through a porous substance
• hydrology: the science of the properties, distribution, and effects of water on a planet’s surface, in the soil and underlying rocks, and in the atmosphere
Freshwater Biomes
Freshwater biomes occur throughout the world’s terrestrial biomes. They include lakes and ponds, rivers and streams, and wetlands.
Lakes and Ponds
Lakes and ponds can range in area from a few square meters to thousands of square kilometers. Temperature is an important abiotic factor affecting organisms found there. In the summer, thermal stratification of lakes and ponds occurs when the upper layer of water is warmed by the sun, but does not mix with deeper, cooler water. Light can penetrate within the photic zone of the lake or pond. Phytoplankton found here carry out photosynthesis, providing the base of the food web. At the bottom of lakes and ponds, bacteria in the aphotic zone break down dead organisms that sink to the bottom.
Nitrogen and phosphorus are important limiting nutrients. Because of this, they are determining factors in the amount of phytoplankton growth that occurs in lakes and ponds. When there is a large input of nitrogen and phosphorus (from sewage and run-off from fertilized lawns and farms, for example), the growth of algae skyrockets, resulting in a large accumulation called an algal bloom. These blooms can become so extensive that they reduce light penetration in water. As a result, the lake or pond becomes aphotic: photosynthetic plants cannot survive. When the algae die and decompose, severe oxygen depletion of the water occurs. Fish and other organisms that require oxygen are more likely to die. The resulting dead zones are found across the globe.
Rivers and Streams
Rivers and streams are continuously moving bodies of water that carry large amounts of water from the source, or headwater, to a lake or ocean. Abiotic features of a river or stream vary along its length. The origin point of streams (source water) is usually cold, low in nutrients, and clear.
Because the source channel is narrow, the current is often faster here than at any other point of the river or stream. This fast-moving water results in minimal silt accumulation at the bottom of the river or stream, resulting in clear water. Photosynthesis occurs primarily in algae growing on rocks since the swift current in channels inhibits the growth of phytoplankton. An additional input of energy can come from leaves or other organic material that falls into the river or stream from trees and other plants that border the water. When the leaves decompose, the organic material and nutrients in the leaves are returned to the water. Plants and animals have adapted to this fast-moving water. For instance, leeches have elongated bodies and suckers on both ends that attach to the substrate, keeping the leech anchored in place.
As the river or stream flows away from the source, the width of the channel gradually widens and the current slows. This slow-moving water, caused by the gradient decrease and the volume increase as tributaries unite, has more sedimentation. The water is as clear as it is near the source since phytoplankton can be suspended in slow-moving water. The water is also warmer. Worms and insects can be found burrowing into the mud. The higher order predator vertebrates, which include waterfowl, frogs, and fishes, often depend on taste or chemical cues to find prey due to the murkiness of the water.
Wetlands
Wetlands are environments in which the soil is either permanently or periodically saturated with water. They differ from lakes in that they are shallow bodies of water. Emergent vegetation consists of wetland plants that are rooted in the soil, but have portions of leaves, stems, and flowers extending above the water’s surface. Types of wetlands include marshes, swamps, bogs, mudflats, and salt marshes. The three shared characteristics among these types are their hydrology, hydrophytic vegetation, and hydric soils.
Freshwater marshes and swamps are characterized by slow and steady water flow. Bogs develop in depressions where water flow is low or non-existent. Bogs usually occur in areas where there is a clay bottom with poor percolation: the movement of water through the pores in the soil or rocks. The water found in a bog is stagnant and oxygen-depleted because the oxygen that is used during the decomposition of organic matter is not replaced, resulting in a slowing of decomposition. This leads to organic acids and other acids building up, which lower the pH of the water. At a lower pH, nitrogen becomes unavailable to plants, creating a challenge for them. Some types of bog plants capture insects and extract the nitrogen from their bodies. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.04%3A__Aquatic_Biomes/44.4C%3A_Estuaries-_Where_the_Ocean_Meets_Fresh_Water.txt |
Climate refers to long-term, predictable atmospheric conditions, while weather refers to atmospheric conditions during a short period of time.
Learning Objectives
• Distinguish between climate and weather
Key Points
• Climate refers to the long-term, predictable atmospheric conditions of a specific area; it does not address the amount of rain that fell on one particular day or the colder-than-average temperatures on a given day in a biome.
• Weather refers to the conditions of the atmosphere during a short period of time; weather forecasts are usually made for 48-hour cycles.
• Specific, one-off weather occurrences are not necessarily indicators of climate change.
Key Terms
• biome: any major regional biological community such as that of forest or desert
• climate: long-term manifestations of weather and other atmospheric conditions in a given area or country in a period long enough to ensure that representative values are obtained (generally 30 years)
• weather: the short term state of the atmosphere at a specific time and place, including the temperature, humidity, cloud cover, precipitation, wind, etc.
Climate and Weather
A common misconception about global climate change is that a specific weather event occurring in a particular region (for example, a very cool week in June in central Indiana) is evidence of global climate change. However, a cold week in June is a weather-related event and not a climate-related one. These misconceptions often arise because of confusion over the terms climate and weather.
Climate refers to the long-term, predictable atmospheric conditions of a specific area. The climate of a biome is characterized by having consistent temperature and annual rainfall ranges. Climate does not address the amount of rain that fell on one particular day in a biome or the colder-than-average temperatures that occurred on one day. In contrast, weather refers to the conditions of the atmosphere during a short period of time. Weather forecasts are usually made for 48-hour cycles; while long-range weather forecasts are available, they can be unreliable.
To better understand the difference between climate and weather, imagine that you are planning an outdoor event in northern Wisconsin. You would be thinking about climate when you plan the event in the summer rather than the winter because you have long-term knowledge that any given Saturday in the months of May to August would be a better choice for an outdoor event in Wisconsin than any given Saturday in January. However, you cannot determine the specific day that the event should be held because it is difficult to accurately predict the weather on a specific day. Climate can be considered “average” weather.
44.5B: Causes of Global Climate Change
Global climate change is cyclical and happens naturally; however, modern human society’s impact has had unprecedented negative effects.
Learning Objectives
• Explain the drivers of climate change, past and present
Key Points
• Data show a correlation between the timing of temperature changes and drivers of climate change.
• Prior to the Industrial Era (pre-1780), there were three drivers of climate change that were not related to human activity: Milankovitch cycles, sun intensity, and volcanic eruptions.
• Greenhouse gases, probably the most significant drivers of the climate, include carbon dioxide, methane, water vapor, nitrous oxide, and ozone.
• Human activity, such as the burning of fossil fuels, releases carbon dioxide and methane, two of the most important greenhouse gases, into the atmosphere.
• Deforestation, cement manufacture, animal agriculture, the clearing of land, and the burning of forests are other human activities that release carbon dioxide.
Key Terms
• Milankovitch cycle: any of the three cyclic variations in the earth’s orbit around the sun, respectively the obliquity of its axis, the precession of the equinoxes, and the eccentricity of its orbit
• greenhouse gas: any gas, such as carbon dioxide, that contributes to the greenhouse effect (continued warming) when released into the atmosphere
• greenhouse effect: the process by which a planet is warmed by its atmosphere
Current and Past Drivers of Global Climate Change
Since it is not possible to go back in time to directly observe and measure climate, scientists use indirect evidence to determine the drivers, or factors, that may be responsible for climate change. The indirect evidence includes data collected using ice cores, boreholes (narrow shafts bored into the ground), tree rings, glacier lengths, pollen remains, and ocean sediments. The data shows a correlation between the timing of temperature changes and drivers of climate change. Before the Industrial Era (pre-1780), there were three drivers of climate change that were not related to human activity or atmospheric gases: the Milankovitch cycles, solar intensity, and volcanic eruptions.
Natural Causes of Climate Change
The Milankovitch cycles describe how slight changes in the earth’s orbit affect the earth’s climate. The length of the Milankovitch cycles ranges between 19,000 and 100,000 years. In other words, one could expect to see some predictable changes in the earth’s climate associated with changes in the earth’s orbit at a minimum of every 19,000 years.
The variation in the sun’s intensity is the second natural factor responsible for climate change. Solar intensity is the amount of solar power or energy the sun emits in a given length of time. There is a direct relationship between solar intensity and temperature: as solar intensity increases (or decreases), the earth’s temperature correspondingly increases (or decreases). Changes in solar intensity have been proposed as one of several possible explanations for the Little Ice Age.
Finally, volcanic eruptions are a third natural driver of climate change. Volcanic eruptions can last a few days, but the solids and gases released during an eruption can influence the climate over a period of a few years, causing short-term climate changes. The gases and solids released by volcanic eruptions can include carbon dioxide, water vapor, sulfur dioxide, hydrogen sulfide, hydrogen, and carbon monoxide.
Human Activity-Related Causes of Climate Change
Greenhouse gases are probably the most significant drivers of the climate. When heat energy from the sun strikes the earth, gases known as greenhouse gases trap the heat in the atmosphere, similar to how the glass panes of a greenhouse keep heat from escaping. The greenhouse gases that affect earth include carbon dioxide, methane, water vapor, nitrous oxide, and ozone. Approximately half of the radiation from the sun passes through these gases in the atmosphere, striking the earth. This radiation is converted into thermal radiation on the earth’s surface; a portion of that energy is re-radiated into the atmosphere. Greenhouse gases, however, reflect much of the thermal energy back to the earth’s surface. The more greenhouse gases there are in the atmosphere, the more thermal energy is reflected back to the earth’s surface. Greenhouse gases, as they absorb and emit radiation, are an important factor in the greenhouse effect, or the warming of earth due to carbon dioxide and other greenhouse gases in the atmosphere.
Beginning recently, atmospheric carbon dioxide concentrations have increased beyond the historical maximum of 300 ppm. The current increases in atmospheric carbon dioxide have happened very quickly: in a matter of hundreds of years rather than thousands of years. A key factor that must be recognized when comparing the historical data and the current data is the presence of modern human society. No other driver of climate change has yielded changes in atmospheric carbon dioxide levels at this rate or to this magnitude.
Human activity releases carbon dioxide and methane, two of the most important greenhouse gases, into the atmosphere in several ways. The primary mechanism that releases carbon dioxide is the burning of fossil fuels, such as gasoline, coal, and natural gas. Deforestation, cement manufacture, animal agriculture, the clearing of land, and the burning of forests are other human activities that release carbon dioxide. Methane (CH4) is produced when bacteria break down organic matter under anaerobic conditions (i.e., without oxygen), which can happen when organic matter is trapped underwater, as in rice paddies, or in the intestines of herbivores. Methane can also be released from natural gas fields and the decomposition that occurs in landfills. Another source of methane is the melting of clathrates: frozen chunks of ice and methane found at the bottom of the ocean. When water warms, these chunks of ice melt, releasing methane. As the ocean’s water temperature increases, the rate at which clathrates melt is increasing, releasing even more methane. This leads to increased levels of methane in the atmosphere, which further accelerates the rate of global warming. This is an example of the positive feedback loop that is leading to the rapid rate of increase of global temperatures. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.05%3A_Climate_and_the_Effects_of_Global_Climate_Change/44.5A%3A_Climate_and_Weather.txt |
Global climate change can be understood by analyzing past historical climate data, such as atmospheric CO2 concentrations in ice cores.
Learning Objectives
• Evaluate the evidence for global climate change
Key Points
• Climate change can be understood by approaching three areas of study: (1) current and past global climate change, (2) causes of past and present-day global climate change, and (3) ancient and current results of climate change.
• Since we cannot go back in time to directly measure climatic variables, such as average temperature and precipitation, we must rely on historical evidence of earth’s past climate, such as Antarctic ice cores.
• Three significant temperature anomalies, or irregularities, have occurred in the last 2000 years: the Medieval Climate Anomaly (or the Medieval Warm Period), the Little Ice Age, and the Industrial Era.
• With the beginning of the Industrial Era, atmospheric carbon dioxide began to rise.
Key Terms
• fossil fuel: any fuel derived from hydrocarbon deposits such as coal, petroleum, natural gas, and, to some extent, peat; these fuels are irreplaceable; their burning generates the greenhouse gas carbon dioxide
Global Climate Change
Climate change can be understood by approaching three areas of study: (1) evidence of current and past global climate change, (2) causes of past and present-day global climate change, and (3) ancient and current results of climate change.
It is helpful to keep these three different aspects of climate change clearly separated when consuming media reports about global climate change. It is common for reports and discussions about global climate change to confuse the data showing that earth’s climate is changing with the factors that drive this climate change.
Evidence for Global Climate Change
Since scientists cannot go back in time to directly measure climatic variables, such as average temperature and precipitation, they must, instead, indirectly measure temperature. To do this, scientists rely on historical evidence of earth’s past climates.
Antarctic ice cores are a key example of such evidence. These ice cores are samples of polar ice obtained by means of drills that reach thousands of meters into ice sheets or high mountain glaciers. Viewing the ice cores is like traveling backwards through time; the deeper the sample, the earlier the time period. Trapped within the ice are bubbles of air and other biological evidence that can reveal temperature and carbon dioxide data. Antarctic ice cores have been collected and analyzed to indirectly estimate the temperature of the earth over the past 400,000 years.
Before the late 1800s, the earth had been as much as 9°C cooler and about 3°C warmer. Atmospheric concentration of carbon dioxide also rose and fell in periodic cycles; note the relationship between carbon dioxide concentration and temperature. Carbon dioxide levels in the atmosphere have historically cycled between 180 and 300 parts per million (ppm) by volume.
Two significant temperature anomalies, or irregularities, have occurred in the last 2000 years. These are the Medieval Climate Anomaly (or the Medieval Warm Period) and the Little Ice Age. A third temperature anomaly aligns with the Industrial Era. The Medieval Climate Anomaly occurred between 900 and 1300 AD. During this time period, many climate scientists think that slightly-warmer conditions prevailed in many parts of the world; the higher-than-average temperature changes varied between 0.10 °C and 0.20 °C above the norm. Although 0.10 °C does not seem large enough to produce any noticeable change, it did free seas of ice. Because of this warming, the Vikings were able to colonize Greenland.
The Little Ice Age was a cold period that occurred between 1550 AD and 1850 AD. During this time, a slight cooling of a little less than 1 °C was observed in North America, Europe, and possibly other areas of the world. This 1 °C change is a seemingly-small deviation in temperature (as was observed during the Medieval Climate Anomaly); however, it also resulted in noticeable changes. Historical accounts reveal a time of exceptionally-harsh winters with much snow and frost. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.05%3A_Climate_and_the_Effects_of_Global_Climate_Change/44.5C%3A_Evidence_of_Global_Climate_Change.txt |
Results of climate change, past and present, have been documented and include species extinction, rising sea levels, and effects on organisms.
Learning Objectives
• Describe the effects of current and geological climate change
Key Points
• Global warming has been associated with at least one planet-wide extinction event during the geological past; scientists estimate that during the Permian period, approximately 70 percent of the terrestrial plant and animal species along with 84 percent of marine species became extinct.
• Glacier recession and melting ice caps are direct effects of current global climate change, ultimately leading to higher global sea levels; as glaciers and polar ice caps melt, there is a significant contribution of liquid water that was previously frozen.
• Changes in temperature and precipitation are causing plants to flower earlier, before their insect pollinators have emerged; mismatched timing of plants and pollinators could result in injurious ecosystem effects.
• This mismatched timing of plants and pollinators could result in injurious ecosystem effects because, for continued survival, insect-pollinated plants must flower when their pollinators are present.
Key Terms
• phenology: the study of the effect of climate on periodic biological phenomena
• Permian: of a geologic period within the Paleozoic era; comprises the Cisuralian, Guadalupian, and Lopingian epochs from about 280 to 248 million years ago
Documented results of climate change: past and present
Scientists have geological evidence of the consequences of long-ago climate change. Modern-day phenomena, such as retreating glaciers and melting polar ice, cause a continual rise in sea level. Changes in climate can negatively affect organisms.
Geological climate change effects
Global warming has been associated with at least one planet-wide extinction event during the geological past. The Permian extinction event occurred about 251 million years ago toward the end of the roughly 50-million-year-long geological time span known as the Permian period. This geologic time period was one of the three warmest periods in earth’s geologic history. Scientists estimate that approximately 70 percent of the terrestrial plant and animal species and 84 percent of marine species became extinct, vanishing forever near the end of the Permian period. Organisms that had adapted to wet and warm climatic conditions, such as annual rainfall of 300–400 cm (118–157 in) and 20 °C–30 °C (68 °F–86 °F) in the tropical wet forest, may not have been able to survive the Permian climate change.
Present climate change effects
A number of global events have occurred that may be attributed to recent climate change during our lifetimes. Glacier National Park in Montana, among others, is undergoing the retreat of many of its glaciers, a phenomenon known as glacier recession. In 1850, the area contained approximately 150 glaciers. By 2010, however, the park contained only about 24 glaciers greater than 25 acres in size. One of these glaciers is the Grinnell Glacier at Mount Gould. Between 1966 and 2005, the size of Grinnell Glacier shrank by 40 percent. Similarly, the mass of the ice sheets in Greenland and the Antarctic is decreasing: Greenland lost 150–250 km3 of ice per year between 2002 and 2006. In addition, the size and thickness of the Arctic sea ice is decreasing.
This loss of ice is leading to rises in the global sea level. On average, the sea is rising at a rate of 1.8 mm per year. However, between 1993 and 2010, the rate of sea-level increase ranged between 2.9 and 3.4 mm per year. A variety of factors affect the volume of water in the ocean, including the temperature of the water (the density of water is related to its temperature) and the amount of water found in rivers, lakes, glaciers, polar ice caps, and sea ice. As glaciers and polar ice caps melt, there is a significant contribution of liquid water that was previously frozen.
In addition to some abiotic conditions changing in response to climate change, many organisms are also being affected by the changes in temperature. Temperature and precipitation play key roles in determining the geographic distribution and phenology of plants and animals. Phenology is the study of the effects of climatic conditions on the timing of periodic lifecycle events, such as flowering in plants or migration in birds. Researchers have shown that 385 plant species in Great Britain are flowering 4.5 days sooner than was recorded during the previous 40 years. In addition, insect-pollinated species were more likely to flower earlier than wind-pollinated species. The impact of changes in flowering date would be mitigated if the insect pollinators emerged earlier. This mismatched timing of plants and pollinators could result in injurious ecosystem effects because, for continued survival, insect-pollinated plants must flower when their pollinators are present. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/44%3A_Ecology_and_the_Biosphere/44.05%3A_Climate_and_the_Effects_of_Global_Climate_Change/44.5D%3A_Past_and_Present_Effects_of_Climate_Change.txt |
Learning Objectives
• Explain the importance and function of population demography
Introduction
Imagine someone sailing down a river in a small motorboat on a warm day. She is enjoying the warm sunshine and cool breeze when suddenly a 20-pound silver carp hits her in the head. The presence of Asian carp in U.S. waterways make this risk very real on rivers and canal systems, particularly in Illinois and Missouri.
The several types of Asian carp include the silver, black, grass, and big head carp. These have been farmed and eaten in China for over 1,000 years; it is among the top aquaculture foods worldwide. In the United States, however, Asian carp is considered to be an invasive species. It disrupts the structure and composition of native fish communities to the point of threatening native aquatic species.
Biologists are working to understand the biology and ecology of Asian carp. The aim is to develop methods of controlling the species without damaging native fish. Understanding the population dynamics of the carp will help biologists develop and implement measures that reduce its population, allowing scientists to model the statistics of carp populations.
Population demography
Populations are dynamic entities, consisting of all of the species living within a specific area. They fluctuate based on a number of factors: seasonal and yearly changes in the environment, natural disasters such as forest fires and volcanic eruptions, and competition for resources between and within species.
Demography, the statistical study of population dynamics, uses mathematical tools to investigate how populations respond to changes in their biotic and abiotic environments. Researchers originally designed demographic tools, such as life tables, to study human populations. To determine insurance rates, life insurance companies developed methods to analyze life expectancies of individuals in a population. The term “demographics” is often used in discussions of human populations, but demographic approaches can be applied to all living populations.
Key Points
• Demographic studies help scientists understand the population dynamics of species, such as invasive species like the Asian carp.
• Population fluctuations depend on the weather, food availability, natural disasters such as forest fires or volcanic eruptions, predation, and biological competition.
• Researchers originally designed demographic tools to study human populations, but demographic approaches can be applied to all living populations.
Key Terms
• demography: the study of populations and how they change
• population dynamics: the variation of populations due to birth and death rates, by immigration and emigration, and concerning topics such as aging populations or population decline
• statistics: a systematic collection of data on measurements or observations, often related to demographic information such as population counts, incomes, population counts at different ages, etc.
45.1B: Population Size and Density
Learning Objectives
• Choose the appropriate method to sample a population, given features of the organisms in that population
Population size and density
Population size and density are the two most important statistics scientists use to describe and understand populations. A population’s size refers to the number of individuals (N) it comprises. Its density is the number of individuals within a given area or volume. These data allow scientists to model the fluctuations of a population over time. For example, a larger population may be more stable than a smaller population. With less genetic variation, a smaller population will have reduced capacity to adapt to environmental changes. Individuals in a low-density population are thinly dispersed; hence, they may have more difficulty finding a mate compared to individuals in a higher-density population. On the other hand, high-density populations often result in increased competition for food. Many factors influence density, but, as a rule-of-thumb, smaller organisms have higher population densities than do larger organisms.
Population research methods
Counting all individuals in a population is the most accurate way to determine its size. However, this approach is not usually feasible, especially for large populations or extensive habitats. Instead, scientists study populations by sampling, which involves counting individuals within a certain area or volume that is part of the population’s habitat. Analyses of sample data enable scientists to infer population size and population density about the entire population.
A variety of methods can be used to sample populations. Scientists usually estimate the populations of sessile or slow-moving organisms with the quadrat method. A quadrat is a square that encloses an area within a habitat. The area may be defined by staking it out with sticks and string, or using a square made of wood, plastic, or metal placed on the ground.
A field study usually includes several quadrat samples at random locations or along a transect in representative habitat. After they place the quadrats, researchers count the number of individuals that lie within the quadrat boundaries. The researcher decides the quadrat size and number of samples from the type of organism, its spatial distribution, and other factors. For sampling daffodils, a 1 m2 quadrat could be appropriate. Giant redwoods are larger and live further apart from each other, so a larger quadrat, such as 100 m2, would be necessary. The correct quadrat size ensures counts of enough individuals to get a sample representative of the entire habitat.
Scientists typically use the mark and recapture technique for mobile organisms such as mammals, birds, or fish. With this method, researchers capture animals and mark them with tags, bands, paint, body markings, or some other sign. The marked animals are then released back into their environment where they mix with the rest of the population. Later, a new sample is collected, including some individuals that are marked (recaptures) and some individuals that are unmarked.
The ratio of marked to unmarked individuals allows scientists to calculate how many individuals are in the population as an estimate of total population size. This method assumes that the larger the population, the lower the percentage of tagged organisms that will be recaptured since they will have mixed with more untagged individuals. For example, if 80 deer are captured, tagged, and released into the forest, and later 100 deer are captured with 20 of them are already marked, we can determine the population size (N) using the following equation:
numbermarkedinfirstcatch×totalnumberofsecondcatchnumberofmarkedrecapturesinsecondcatchnumbermarkedinfirstcatch×totalnumberofsecondcatchnumberofmarkedrecapturesinsecondcatch
Plugging the example data into the equation, the calculation gives an estimated total population size of 400.
80×10020=40080×10020=400
Using the example data, if only 10 already-marked deer had been recaptured, the calculated total population size would be 800.
The mark and recapture method has limitations. Some animals from the first catch may learn to avoid capture in the second round. Such behavior would cause inflated population estimates. Alternatively, animals may preferentially be retrapped (especially if a food reward is offered), resulting in an underestimate of population size. Also, some species may be harmed by the marking technique, reducing their survival. A variety of other techniques have been developed, including the electronic tracking of animals tagged with radio transmitters and the use of data from commercial fishing and trapping operations to estimate the size and health of populations and communities.
Key Points
• A population ‘s size refers to the number of individuals (N) it comprises.
• Population density is the number of individuals within a given area or volume.
• Scientists usually study populations by sampling, which involves counting individuals within a certain area or volume that is part of the population’s habitat.
• The quadrat method is used to sample sessile organisms, using a square within which all individuals are counted; extrapolation of the data to the entire habitat results in a population size estimate.
• The mark and recapture technique is used for mobile organisms; it involves marking a sample of individuals and then estimating population size from the number of marked individuals in subsequent samples.
Key Terms
• population density: the average number of a population’s individuals that inhabit a unit area or volume
• quadrat: a square area, marked with boundaries for studying the population size and density of plants and sessile animals
• mark and recapture: a sample technique is used for study of the populations of mobile organisms, estimating population size from the number of marked individuals in samples
45.1C: Species Distribution
Learning Objectives
• Differentiate among the ways in which species distribute themselves in space
Density and size are useful measures for characterizing populations. Scientists gain additional insight into a species’ biology and ecology from studying how individuals are spatially distributed. Dispersion or distribution patterns show the spatial relationship between members of a population within a habitat. Patterns are often characteristic of a particular species; they depend on local environmental conditions and the species’ growth characteristics (as for plants) or behavior (as for animals).
Individuals of a population can be distributed in one of three basic patterns: they can be more or less equally spaced apart (uniform dispersion), dispersed randomly with no predictable pattern (random dispersion), or clustered in groups (clumped dispersion).
Uniform dispersion is observed in plant species that inhibit the growth of nearby individuals. For example, the sage plant, Salvia leucophylla, secretes toxins, a phenomenon called negative allelopathy. The chemicals kill off surrounding plants in a circle around the individual sage plants, leading to a uniform distance between each plant. Animals that maintain defined territories, such as nesting penguins, also exhibit uniform dispersion.
Random dispersion occurs with dandelion and other plants that have wind-dispersed seeds that germinate wherever they happen to fall in a favorable environment. Clumped dispersion is seen in plants that drop their seeds straight to the ground, such as oak trees, or animals that live in groups, such as schools of fish or herds of elephants. Clumped dispersions may also result from habitat heterogeneity. If favorable conditions are localized, organisms will tend to clump around those, such as lions around a watering hole.
In this way, the dispersion pattern of the individuals within a population provides more information about how they interact with each other and their environment than does a simple density measurement. Just as lower density species might have more difficulty finding a mate, solitary species with a random distribution might have a similar difficulty when compared to social species clumped together in groups.
Key Points
• Dispersion or distribution patterns show the spatial relationship between members of a population within a habitat.
• Individuals of a population can be distributed in one of three basic patterns: uniform, random, or clumped.
• In a uniform distribution, individuals are equally spaced apart, as seen in negative allelopathy where chemicals kill off plants surrounding sages.
• In a random distribution, individuals are spaced at unpredictable distances from each other, as seen among plants that have wind-dispersed seeds.
• In a clumped distribution, individuals are grouped together, as seen among elephants at a watering hole.
Key Terms
• dispersion pattern: the spatial relationship between members of a population within a habitat, often characteristic of a particular species
• allelopathy: the release by a plant of a toxin to suppress growth of nearby competing plants, often resulting in a uniform dispersion pattern
• habitat heterogeneity: variation in physical environmental features within an area, such as topography, soil chemistry, temperature, moisture, and biological factors | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.01%3A_Population_Demography/45.1A%3A_Population_Demography.txt |
Learning Objectives
• Distinguish between life tables and survivorship curves as used in demography
Demography
Population size, density, and distribution patterns describe a population at a fixed point in time. To study how a population changes over time, scientists must use the tools of demography: the statistical study of population changes over time. The key statistics demographers use are birth rates, death rates, and life expectancies; although, in practice, scientists also study immigration and emigration rates, which also affect populations.
These measures, especially birth rates, may be related to the population characteristics described in prior sections. For example, a large population would have a relatively-high birth rate if it has more reproductive individuals. Alternatively, a large population may also have a high death rate because of competition, disease, or waste accumulation. A high population density may lead to more reproductive encounters between individuals, as would a clumped distribution pattern. Such conditions would increase the birth rate.
Biological features of the population also affect population changes over time. Birth rates will be higher in a population with the ratio of males to females biased towards females, or in a population composed of relatively more individuals of reproductive age.
The demographic characteristics of a population are the basic determinants of how the population changes over time. If birth and death rates are equal, the population remains stable. The population will increase if birth rates exceed death rates, but will decrease if birth rates are lower than death rates. Life expectancy, another important factor, is the length of time individuals remain in the population. It is impacted by local resources, reproduction, and the overall health of the population. These demographic characteristics are often displayed in the form of a life table.
Life tables
Life tables, which provide important information about the life history of an organism, divide the population into age groups and often sexes; they show how long a member of that group will probably live. The tables are modeled after actuarial tables used by the insurance industry for estimating human life expectancy. Life tables may include:
• the probability of individuals dying before their next birthday (i.e., mortality rate )
• the percentage of surviving individuals at a particular age interval
• the life expectancy at each interval
The life table shown is from a study of Dall mountain sheep, a species native to northwestern North America. The population is divided into age intervals, as seen in the leftmost column. The mortality rate per 1,000 individuals is calculated by dividing the number of individuals dying during an age interval by the number of individuals surviving at the beginning of the interval, multiplied by 1,000.
For example, between ages three and four, 12 individuals die out of the 776 that were remaining from the original 1,000 sheep. This number is then multiplied by 1,000 to get the mortality rate per thousand.
As can be seen from the mortality rate data (column D), a high death rate occurred when the sheep were between 6 and 12 months old, which then increased even more from 8 to 12 years old, after which there were few survivors. The data indicate that if a sheep in this population were to survive to age one, it could be expected to live another 7.7 years on average, as shown by the life expectancy numbers in column E.
Survivorship curves
Another tool used by population ecologists is a survivorship curve, which is a graph of the number of individuals surviving at each age interval plotted versus time (usually with data compiled from a life table). These curves allow comparison of life histories of different populations.
Humans and most primates exhibit a Type I survivorship curve because a high percentage of offspring survive early and middle years; death occurs predominantly in older individuals. These species have few offspring as they invest in parental care to increase survival.
Birds show the Type II survivorship curve because equal numbers of birds tend to die at each age interval. These species may also have relatively-few offspring and provide significant parental care.
Trees, marine invertebrates, and most fishes exhibit a Type III survivorship curve. Very few individuals survive the younger years; however, those that live to old age are likely to survive for a relatively-long period. Organisms in this category usually have large numbers of offspring and provide little parental care. Such offspring are “on their own” and suffer high mortality due to predation or starvation; however, their abundance ensures that enough individuals survive to the next generation, perpetuating the population.
Key Points
• The key statistics used in demography are birth rates, death rates, and life expectancies which may be influenced by population characteristics and biological factors.
• Birth rates, death rates, and life expectancies are the basic determinants of how a population changes over time.
• Life tables are demographic tools which shows a population’s life expectancy and mortality within age groups.
• A survivorship curve is a graph of the number of individuals surviving at each age interval plotted versus time.
• The characteristics and behavior of a species, such as number of offspring produced, its percentage of surviving offspring, and degree of parental care, determine the shape of its survivorship curve.
Key Terms
• life table: a demographic tool which shows a population’s life expectancy and mortality within age groups
• survivorship curve: a graph of the number of individuals surviving at each age interval plotted versus time
• mortality rate: the number of deaths per given unit of population over a given period of time | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.01%3A_Population_Demography/45.1D%3A_The_Study_of_Population_Dynamics.txt |
Learning Objectives
• Describe exponential growth of a population size
Exponential growth
In his theory of natural selection, Charles Darwin was greatly influenced by the English clergyman Thomas Malthus. Malthus published a book in 1798 stating that populations with unlimited natural resources grow very rapidly, after which population growth decreases as resources become depleted. This accelerating pattern of increasing population size is called exponential growth.
The best example of exponential growth is seen in bacteria. Bacteria are prokaryotes that reproduce by prokaryotic fission. This division takes about an hour for many bacterial species. If 1000 bacteria are placed in a large flask with an unlimited supply of nutrients (so the nutrients will not become depleted), after an hour there will be one round of division (with each organism dividing once), resulting in 2000 organisms. In another hour, each of the 2000 organisms will double, producing 4000; after the third hour, there should be 8000 bacteria in the flask; and so on. The important concept of exponential growth is that the population growth rate, the number of organisms added in each reproductive generation, is accelerating; that is, it is increasing at a greater and greater rate. After 1 day and 24 of these cycles, the population would have increased from 1000 to more than 16 billion. When the population size, N, is plotted over time, a J-shaped growth curve is produced.
The bacteria example is not representative of the real world where resources are limited. Furthermore, some bacteria will die during the experiment and, thus, not reproduce, lowering the growth rate. Therefore, when calculating the growth rate of a population, the death rate (D; the number organisms that die during a particular time interval) is subtracted from the birth rate (B; the number organisms that are born during that interval). This is shown in the following formula:
\[\dfrac{ΔN}{ΔT} =B− \dfrac{\Delta N}{\Delta T} =B−D\]
where \(ΔN\) = change in number, \(ΔT\) = change in time, \(B\) = birth rate, and \(D\) = death rate. The birth rate is usually expressed on a per capita (for each individual) basis. Thus, B (birth rate) = bN (the per capita birth rate “b” multiplied by the number of individuals “N”) and D (death rate) = dN (the per capita death rate “d” multiplied by the number of individuals “N”). Additionally, ecologists are interested in the population at a particular point in time: an infinitely small time interval. For this reason, the terminology of differential calculus is used to obtain the “instantaneous” growth rate, replacing the change in number and time with an instant-specific measurement of number and time.
\[\dfrac{dN}{dT}=BN DN=(BD)NdN/dT=BN DN=(BD)N\]
Notice that the “d” associated with the first term refers to the derivative (as the term is used in calculus) and is different from the death rate, also called “d.” The difference between birth and death rates is further simplified by substituting the term “r” (intrinsic rate of increase) for the relationship between birth and death rates:
\[\dfrac{dN}{dT}=rN \dfrac{dN}{dT}=rN\]
The value “r” can be positive, meaning the population is increasing in size; negative, meaning the population is decreasing in size; or zero, where the population’s size is unchanging, a condition known as zero population growth. A further refinement of the formula recognizes that different species have inherent differences in their intrinsic rate of increase (often thought of as the potential for reproduction), even under ideal conditions. Obviously, a bacterium can reproduce more rapidly and have a higher intrinsic rate of growth than a human. The maximal growth rate for a species is its biotic potential, or rmax, thus changing the equation to:
\[\dfrac{dN}{dT}=r_{max}N\]
Key Points
• To get an accurate growth rate of a population, the number that died in the time period (death rate) must be removed from the number born during the same time period (birth rate).
• When the birth rate and death rate are expressed in a per capita manner, they must be multiplied by the population to determine the number of births and deaths.
• Ecologists are usually interested in the changes in a population at either a particular point in time or over a small time interval.
• The intrinsic rate of increase is the difference between birth and death rates; it can be positive, indicating a growing population; negative, indicating a shrinking population; or zero, indicting no change in the population.
• Different species have a different intrinsic rate of increase which, when under ideal conditions, represents the biotic potential or maximal growth rate for a species.
Key Terms
• fission: the process by which a bacterium splits to form two daughter cells
• per capita: per person or individual | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.02%3A_Environmental_Limits_to_Population_Growth/45.2A%3A_Exponential_Population_Growth.txt |
Learning Objectives
• Describe logistic growth of a population size
Exponential growth is possible only when infinite natural resources are available; this is not the case in the real world. Charles Darwin recognized this fact in his description of the “struggle for existence,” which states that individuals will compete (with members of their own or other species ) for limited resources. The successful ones will survive to pass on their own characteristics and traits (which we know now are transferred by genes) to the next generation at a greater rate: a process known as natural selection. To model the reality of limited resources, population ecologists developed the logistic growth model.
Carrying Capacity and the Logistic Model
In the real world, with its limited resources, exponential growth cannot continue indefinitely. Exponential growth may occur in environments where there are few individuals and plentiful resources, but when the number of individuals becomes large enough, resources will be depleted, slowing the growth rate. Eventually, the growth rate will plateau or level off. This population size, which represents the maximum population size that a particular environment can support, is called the carrying capacity, or $K$.
The formula we use to calculate logistic growth adds the carrying capacity as a moderating force in the growth rate. The expression “K – N” is indicative of how many individuals may be added to a population at a given stage, and “K – N” divided by “K” is the fraction of the carrying capacity available for further growth. Thus, the exponential growth model is restricted by this factor to generate the logistic growth equation:
\begin{align*} \dfrac{dN}{dT} &=r_{max} \dfrac{dN}{dT} \[4pt] &=r_{max} \times N \times (\dfrac{K- N}{K}) \dfrac{dN}{dT} \[4pt] &=rmax∗(dN/dT)=rmax∗N∗((K N)/K) \end{align*}
Notice that when $N$ is very small, (K-N)/K becomes close to $K/K$ or 1; the right side of the equation reduces to $r_{max}N$, which means the population is growing exponentially and is not influenced by carrying capacity. On the other hand, when $N$ is large, $(K-N)/K$ come close to zero, which means that population growth will be slowed greatly or even stopped. Thus, population growth is greatly slowed in large populations by the carrying capacity $K$. This model also allows for negative population growth or a population decline. This occurs when the number of individuals in the population exceeds the carrying capacity (because the value of (K-N)/K is negative).
A graph of this equation yields an S-shaped curve; it is a more-realistic model of population growth than exponential growth. There are three different sections to an S-shaped curve. Initially, growth is exponential because there are few individuals and ample resources available. Then, as resources begin to become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment, with little change in population size over time.
Role of Intraspecific Competition
The logistic model assumes that every individual within a population will have equal access to resources and, thus, an equal chance for survival. For plants, the amount of water, sunlight, nutrients, and the space to grow are the important resources, whereas in animals, important resources include food, water, shelter, nesting space, and mates.
In the real world, the variation of phenotypes among individuals within a population means that some individuals will be better adapted to their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition (intra- = “within”; -specific = “species”). Intraspecific competition for resources may not affect populations that are well below their carrying capacity as resources are plentiful and all individuals can obtain what they need. However, as population size increases, this competition intensifies. In addition, the accumulation of waste products can reduce an environment’s carrying capacity.
Examples of Logistic Growth
Yeast, a microscopic fungus used to make bread and alcoholic beverages, exhibits the classical S-shaped curve when grown in a test tube ( a). Its growth levels off as the population depletes the nutrients that are necessary for its growth. In the real world, however, there are variations to this idealized curve. Examples in wild populations include sheep and harbor seals ( b). In both examples, the population size exceeds the carrying capacity for short periods of time and then falls below the carrying capacity afterwards. This fluctuation in population size continues to occur as the population oscillates around its carrying capacity. Still, even with this oscillation, the logistic model is confirmed.
Key Points
• The carrying capacity of a particular environment is the maximum population size that it can support.
• The carrying capacity acts as a moderating force in the growth rate by slowing it when resources become limited and stopping growth once it has been reached.
• As population size increases and resources become more limited, intraspecific competition occurs: individuals within a population who are more or less better adapted for the environment compete for survival.
Key Terms
• phenotype: the appearance of an organism based on a multifactorial combination of genetic traits and environmental factors, especially used in pedigrees
• carrying capacity: the number of individuals of a particular species that an environment can support; indicated by the letter “K”
45.2C: Density-Dependent and Density-Independent Population Re
Learning Objectives
• Differentiate between density-dependent and density-independent population regulation.
Density-dependent regulation
In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population. Most density-dependent factors, which are biological in nature (biotic), include predation, inter- and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites. Usually, the denser a population is, the greater its mortality rate. For example, during intra- and interspecific competition, the reproductive rates of the individuals will usually be lower, reducing their population’s rate of growth. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source.
An example of density-dependent regulation is shown with results from a study focusing on the giant intestinal roundworm (Ascaris lumbricoides), a parasite of humans and other mammals. The data shows that denser populations of the parasite exhibit lower fecundity: they contained fewer eggs. One possible explanation for this phenomenon was that females would be smaller in more dense populations due to limited resources so they would have fewer eggs. This hypothesis was tested and disproved in a 2009 study which showed that female weight had no influence. The actual cause of the density-dependence of fecundity in this organism is still unclear and awaiting further investigation.
Density-independent regulation and interaction with density-dependent factors
Many factors, typically physical or chemical in nature (abiotic), influence the mortality of a population regardless of its density. They include weather, natural disasters, and pollution. An individual deer may be killed in a forest fire regardless of how many deer happen to be in that area. Its chances of survival are the same whether the population density is high or low.
In real-life situations, population regulation is very complicated and density-dependent and independent factors can interact. A dense population that is reduced in a density-independent manner by some environmental factor(s) will be able to recover differently than would a sparse population. For example, a population of deer affected by a harsh winter will recover faster if there are more deer remaining to reproduce.
Key Points
• The density of a population can be regulated by various factors, including biotic and abiotic factors and population size.
• Density-dependent regulation can be affected by factors that affect birth and death rates such as competition and predation.
• Density-independent regulation can be affected by factors that affect birth and death rates such as abiotic factors and environmental factors, i.e. severe weather and conditions such as fire.
• New models of life history incorporate ecological concepts that are typically included in r- and K-selection theory in combination with population age structures and mortality factors.
Key Terms
• interspecific: existing or occurring between different species
• intraspecific: occurring among members of the same species
• fecundity: number, rate, or capacity of offspring production | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.02%3A_Environmental_Limits_to_Population_Growth/45.2B%3A_Logistic_Population_Growth.txt |
Learning Objectives
• Describe the energy budgets of, and the life history strategies used in, reproduction
Life history patterns and energy budgets
Energy is required by all living organisms for their growth, maintenance, and reproduction. At the same time, energy is often a major limiting factor in determining an organism’s survival. Plants, for example, acquire energy from the sun via photosynthesis, but must expend this energy to grow, maintain health, and produce energy-rich seeds to produce the next generation. Animals also have the additional burden of using some of their energy reserves to acquire food. In addition, some animals must expend energy caring for their offspring. Thus, all species have an energy budget in which they must balance energy intake with their use of energy for metabolism, reproduction, parental care, and energy storage, as when bears build up body fat for winter hibernation.
Parental care and fecundity
Fecundity is the potential reproductive capacity of an individual within a population. In other words, it describes how many offspring could ideally be produced if an individual has as many offspring as possible, repeating the reproductive cycle as soon as possible after the birth of the offspring. In animals, fecundity is inversely related to the amount of parental care given to an individual offspring. Species that produce a large number of offspring, such as many marine invertebrates, usually provide little if any care for those offspring, as they would not have the energy or the ability to do so. Most of their energy budget is used to produce many tiny offspring. Animals with this strategy are often self-sufficient at a very early age. This is because of the energy trade-off these organisms have made to maximize their evolutionary fitness. Since their energy is used for producing offspring instead of parental care, it makes sense that these offspring have some ability to be able to move within their environment to find food and perhaps shelter. Even with these abilities, their small size makes them extremely vulnerable to predation, so the production of many offspring allows enough of them to survive to maintain the species.
Animal species that have few offspring during a reproductive event usually give extensive parental care, devoting much of their energy budget to these activities, sometimes at the expense of their own health. This is the case with many mammals, such as humans, kangaroos, and pandas. The offspring of these species are relatively helpless at birth, needing to develop before they achieve self-sufficiency.
Plants with low fecundity produce few energy-rich seeds (such as coconuts and chestnuts) that have a good chance to germinate into a new organism. Plants with high fecundity usually have many small, energy-poor seeds (as do orchids) that have a relatively-poor chance of surviving. Although it may seem that coconuts and chestnuts have a better chance of surviving, the energy trade-off of the orchid is also very effective. It is a matter of where the energy is used: for large numbers of seeds or for fewer seeds with more energy.
Early versus late reproduction
The timing of reproduction in a life history also affects species survival. Organisms that reproduce at an early age have a greater chance of producing offspring, but this is usually at the expense of their growth and the maintenance of their health. Conversely, organisms that start reproducing later in life often have greater fecundity or are better able to provide parental care, but they risk not surviving to reproductive age. Examples of this can be seen in fish. Small fish, such as guppies, use their energy to reproduce rapidly, but never attain the size that would give them defense against some predators. Larger fish, such as bluefin tuna and mako sharks, use their energy to attain a large size, but do so with the risk that they will die before they can reproduce or reproduce to their maximum. These different energy strategies and trade-offs are key to understanding the evolution of each species as it maximizes its fitness and fills its niche.
Single versus multiple reproductive events
Some life history traits, such as fecundity, timing of reproduction, and parental care, can be grouped together into general strategies that are used by multiple species. Semelparous species are those that only reproduce once during their lifetime and then die. Such species use most of their resource budget during a single reproductive event, sacrificing their health to the point that they do not survive. Examples of semelparity are bamboo, which flowers once and then dies, and the Chinook salmon, which uses most of its energy reserves to migrate from the ocean to its freshwater nesting area, where it reproduces and then dies. In contrast, iteroparous species reproduce repeatedly during their lives. Some animals are able to mate only once per year, but survive multiple mating seasons. Primates, including humans and chimpanzees, are examples of animals that display iteroparity.
Key Points
• The amount of parental care given to an individual offspring is inversely related to the reproductive capacity of an animal species.
• Animal species that produce many small, vulnerable offspring tend to provide little or no care for them due to their energy budget constraints; just enough offspring survive to maintain the species.
• Animal species that have few offspring expend large amounts of their energy budgets on caring for helpless offspring that need to develop before being on their own.
• Plants with low fecundity produce few energy-rich seeds with high germination rates, while plants with high fecundity usually have many small, energy-poor seeds with poor survival rates.
• Species that reproduce early ensure a greater chance of having surviving offspring than do those that must survive to a later reproductive age.
• Semelparous species use all of their reproductive budgets on one single reproductive event, while iteroparous species spend it on multiple mating seasons.
Key Terms
• iteroparous: reproducing more than once in a lifetime
• semelparous: reproducing only once in a lifetime
• fecundity: number, rate, or capacity of offspring production | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.03%3A_Life_History_Patterns/45.3A%3A_Life_History_Patterns_and_Energy_Budgets.txt |
Learning Objectives
• Distinguish between K- and r-selected species
Early theories about life history: K-selected and r-selected species
While reproductive strategies play a key role in life histories, they do not account for important factors such as limited resources and competition. The regulation of population growth by these factors can be used to introduce a classical concept in population biology: that of K-selected versus r-selected species.
By the second half of the twentieth century, the concept of K- and r-selected species was used extensively and successfully to study populations. The concept relates not only to reproductive strategies, but also to a species’ habitat and behavior. This includes the way they obtain resources and care for their young, as well as length of life and survivorship factors. For this analysis, population biologists have grouped species into the two large categories, K-selected and r-selected, although they are really two ends of a continuum. The first variable is K (the carrying capacity of a population; density dependent), and the second variable is r (the intrinsic rate of natural increase in population size, density independent).
K-selected species
K-selected species are those in stable, predictable environments. Populations of K-selected species tend to exist close to their carrying capacity (hence the term K-selected) where intraspecific competition is high. These species produce few offspring, have a long gestation period, and often give long-term care to their offspring. While larger in size when born, the offspring are relatively helpless and immature at birth. By the time they reach adulthood, they must develop skills to compete for natural resources. Examples of K-selected species are primates including humans, other mammals such as elephants, and plants such as oak trees.
In plants, scientists think of parental care more broadly: how long fruit takes to develop or how long it remains on the plant are determining factors in the time to the next reproductive event. Oak trees grow very slowly and take, on average, 20 years to produce their first seeds, known as acorns. As many as 50,000 acorns can be produced by an individual tree, but the germination rate is low as many of these rot or are eaten by animals such as squirrels. In some years, oaks may produce an exceptionally large number of acorns; these years may be on a two- or three-year cycle depending on the species of oak (r-selection).
As oak trees grow to a large size (and for many years before they begin to produce acorns) they devote a large percentage of their energy budget to growth and maintenance. The tree’s height and size allow it to dominate other plants in the competition for sunlight, the oak’s primary energy resource. Furthermore, when it does reproduce, the oak produces large, energy-rich seeds that use their energy reserve to become quickly established (K-selection).
r-selected species
In contrast to K-selected species, r-selected species have a large number of small offspring (hence their r designation). This strategy is often employed in unpredictable or changing environments. Animals that are r-selected do not give long-term parental care and the offspring are relatively mature and self-sufficient at birth. Examples of r-selected species are marine invertebrates, such as jellyfish, and plants, such as the dandelion. Dandelions have small seeds that are dispersed long distances by wind; many seeds are produced simultaneously to ensure that at least some of them reach a hospitable environment. Seeds that land in inhospitable environments have little chance for survival since the seeds are low in energy content. Note that survival is not necessarily a function of energy stored in the seed itself.
Modern theories of life history
The r- and K-selection theory, although accepted for decades and used for much groundbreaking research, has now been reconsidered. Many population biologists have abandoned or modified it. Over the years, several studies attempted to confirm the theory, but these attempts have largely failed. Many species were identified that did not follow the theory’s predictions. Furthermore, the theory ignored the age-specific mortality of the populations which scientists now know is very important. New demographic-based models of life history evolution have been developed which incorporate many ecological concepts included in r- and K-selection theory, as well as population age structure and mortality factors.
Key Points
• K-selection species are defined as those present in stable and predictable environments that produce fewer offspring, have longer gestation periods, and provide long-term care after birth.
• r-selected species are defined as those present in fluctuating environments that have large numbers of offspring and do not provide long-term care after birth.
• Based on evidence that shows not all species follow solely the K- or r-selection theories, new models of life history are being developed which incorporate K- and r-selection theories with additional factors that affect life and survivorship such as population age structure and mortality factors.
Key Terms
• carrying capacity: the number of individuals of a particular species that an environment can support; indicated by the letter “K”
• intraspecific: occurring among members of the same species
Contributions and Attributions
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44881/latest...ol11448/latest. License: CC BY: Attribution
• semelparous. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/semelparous. License: CC BY-SA: Attribution-ShareAlike
• fecundity. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/fecundity. License: CC BY-SA: Attribution-ShareAlike
• iteroparous. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/iteroparous. License: CC BY-SA: Attribution-ShareAlike
• Chinook salmon fish. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...almon_fish.jpg. License: Public Domain: No Known Copyright
• OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44882/latest...ol11448/latest. License: CC BY: Attribution
• carrying capacity. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/carrying_capacity. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44882/latest...ol11448/latest. License: CC BY: Attribution
• intraspecific. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/intraspecific. License: CC BY-SA: Attribution-ShareAlike
• Chinook salmon fish. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...almon_fish.jpg. License: Public Domain: No Known Copyright
• OpenStax College, Biology. December 6, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44882/latest...ol11448/latest. License: CC BY: Attribution | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.03%3A_Life_History_Patterns/45.3B%3A_Theories_of_Life_History.txt |
Learning Objectives
• Predict the long-term consequences of exponential human population growth
Human Population Growth
Global human population growth is around 75 million annually, or 1.1% per year. The global population has grown from 1 billion in 1800 to 7 billion in 2012. It is expected to keep growing, though predictions differ as to when and if this growth will plateau.
The “population growth rate” is the rate at which the number of individuals in a population increases in a given time period as a fraction of the initial population. Specifically, population growth rate refers to the change in population over a time period, often expressed as a percentage of the number of individuals in the population at the beginning of that period. This can be written as the formula:
population growth rate=P(t2)−P(t1)P(t1)population growth rate=P(t2)−P(t1)P(t1)
Globally, the growth rate of the human population has been declining since 1962 and 1963, when it was 2.20% per annum. In 2009, the estimated annual growth rate was 1.1%. The CIA World Factbook gives the world annual birthrate, mortality rate, and growth rate as 1.89%, 0.79%, and 1.096% respectively. The last 100 years have seen a rapid increase in population due to medical advances and massive increase in agricultural productivity.
Each region of the globe has seen reductions in growth rate in recent decades, though growth rates remain above 2% in some countries of the Middle East and Sub-Saharan Africa, and also in South Asia, Southeast Asia, and Latin America. This does not mean that the population is declining;
rather, it means the population is growing more slowly. However, some countries do experience negative population growth, mainly due to low fertility rates, high death rates and emigration.
According to the UN’s 2010 revision to its population projections, world population will peak at 10.1 billion in 2100 compared to 7 billion in 2011. However, some experts dispute the UN’s forecast and have argued that birthrates will fall below replacement rates (the number of births needed to maintain a stable population) in the 2020s. According to these forecasters, population growth will be only sustained until the 2040s by rising longevity, but will peak below 9 billion by 2050, followed by a long decline.
Long-term Consequences of Population Growth
The “population explosion” seen in the last century has led to dire predictions. In 1968, biologist Paul Ehrlich wrote, “The battle to feed all of humanity is over. In the 1970s, hundreds of millions of people will starve to death in spite of any crash programs embarked upon now. At this late date, nothing can prevent a substantial increase in the world death rate. ” Although many critics view Ehrlich’s view as an exaggeration, the human population continues to grow exponentially. The laws of nature dictate that exponential growth cannot continue indefinitely.
Despite efforts to curb population growth, such as the “one-child policy” in China (introduced in 1979 but relaxed in the early 2000s), the human population continues to grow. A primary concern regarding this growth is that the demand for ever-more food will lead to widespread shortages, as forecast by Ehrlich.
In addition to the threat of food shortages, human population growth is damaging to the environment in potentially permanent ways. Most scientists agree that climate change caused by the emission of the greenhouse gas carbon dioxide (CO2) is a significant consequence of human activities. In a series of treaties in the late 20th century, many countries committed to reducing their CO2emissions to prevent continuous global warming; however these treaties have not been ratified by every country, largely due to economic and political concerns. The role of human activity in climate change is hotly debated in some circles. The future holds considerable uncertainty for curbing human population growth and protecting the environment.
Key Points
• Global human population growth is around 75 million annually, or 1.1% per year. The global population has grown from 1 billion in 1800 to 7 billion in 2012.
• Although the direst consequences of human population growth have not yet been realized, exponential growth cannot continue indefinitely.
• In the late 1970s, China’s “one-child” policy tried to control population growth, but restrictions were relaxed in the early 2000s.
• One of the major consequences of population growth is the potential for widespread food shortages.
• Most scientists agree that humans and human population growth are causing climate change by emission of the greenhouse gas carbon dioxide (CO2).
• International treaties to limit greenhouse gas emissions have not been ratified by every country due to economic and political concerns.
Key Terms
• greenhouse gas: Any gas, such as carbon dioxide, that contributes to the greenhouse effect (continued warming) when released into the atmosphere.
• climate change: Changes in the earth’s climate, especially those said to be produced by global warming. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.04%3A_Human_Population_Growth/45.4A%3A_Human_Population_Growth.txt |
Learning Objectives
• Describe ways in which humans overcome density-dependent regulation of population size
Humans are uniquely able to consciously alter their environment to increase its carrying capacity. This capability is an underlying reason for human population growth as humans are able to overcome density-dependent limits on population growth, in contrast with all other organisms.
Human intelligence, society, and communication have enabled this capacity. For instance, people can construct shelters to protect them from the elements; food supply has increased because of agriculture and domestication of animals; and humans use language to pass on technology to new generations, allowing continual improvement upon previous accomplishments. Migration has also contributed to human population growth. Originating from Africa, humans have migrated to nearly every inhabitable area on the planet.
Public health, sanitation, and the use of antibiotics and vaccines have lessened the impact of infectious disease on human populations. In the fourteenth century, the bubonic plague killed as many as 100 million people: between 30 to 60 percent of Europe’s population. Today, however, the plague and other infectious diseases have much less of an impact. Through vaccination programs, better nutrition, and vector control (carriers of disease), international agencies have significantly reduced the global infectious disease burden. For example, reported cases of measles in the United States dropped from around 700,000 a year in the 1950s to practically zero by the late 1990s. Globally, measles fell 60 percent from an estimated 873,000 deaths in 1999 to 164,000 in 2008. This advance is attributed entirely to a comprehensive vaccination program.
Developing countries have also made advances in curbing mortality from infectious disease. For example, deaths from infectious and parasitic diseases in Brazil fell from second place as the most important causes of death in 1977 to fifth place in 1984. The improvement is attributed in part to increased access to essential goods and services, reflecting the country’s rising prosperity. Through changes in economic status, as in Brazil, as well as global disease control efforts, human population growth today is less limited by infectious disease than has been the case historically.
Key Points
• Humans’ ability to alter their environment is an underlying reason for human population growth, enabling people to overcome density-dependent limits on growth, in contrast with all other organisms.
• Abilities, such as construction of shelter, food cultivation, and the sharing of technology, have helped humans overcome factors that would have otherwise limited their population growth.
• Originating from Africa, human migration to nearly every inhabitable area of the globe has enabled colonization of areas where people were previously absent.
• Advances in medicine, notably vaccines and antibiotics, as well as improvements in nutrition and vector control, have significantly curbed mortality from disease.
Key Terms
• density-dependent: Processes that occur when population growth rates are regulated by the size of a population in a given amount of resources such as food or habitat area.
• vaccine: A substance given to stimulate the body’s production of antibodies and provide immunity against a disease, prepared from the agent that causes the disease, or a synthetic substitute.
• infectious disease: Illness caused by introduction of a pathogen or parasite into the body via contact with a transmitting agent such as vector organism or an infected person. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.04%3A_Human_Population_Growth/45.4B%3A_Overcoming_Density-Dependent_Regulation.txt |
Learning Objectives
• Explain how age structure in a population is associated with population growth and economic development
The variation of populations over time, also known as population dynamics, depends on biological and environmental processes that determine population changes. A population’s growth rate is strongly influenced by the proportions of individuals of particular ages. With knowledge of this age structure, population growth can be more accurately predicted. Age structure data allow the rate of growth (or decline) to be associated with a population’s level of economic development.
For example, the population of a country with rapid growth has a triangle-shaped age structure with a greater proportion of younger individuals who are at or close to reproductive age. This pattern typically occurs where fewer people live to old age because of sub-optimal living standards, such as occurs in underdeveloped countries.
Changing Population Age Structure: This 3:28 minute movie discusses age structures and gives examples.
Some developed countries, including the United States, have a slowly-growing population. This results in a column-shaped age structure diagram with steeper sides. In these cases, the population has fewer young and reproductive-aged individuals, with a greater proportion of older individuals. Some developed countries, such as Italy, have zero population growth. Countries with declining populations, such as Japan, have a bulge in the middle of their age structure diagram. The bulge indicates relatively-few young individuals, and a higher proportion of middle-aged and older individuals.
Globally, less-economically developed countries in Africa and Asia have the highest growth rates, leading to populations consisting mostly of younger people. Improved health care, beginning in the 1960s, is one of the leading causes of the increased growth rates that created the population explosion. For example, in the Middle East and North Africa, around 65 percent of the population is under the age of 30. These high growth rates lead to the so-called “youth bulge,” which some experts believe is a cause of social unrest and economic problems such as high unemployment.
All of the factors above also have an impact on the average life expectancy. As economic development and quality of health care increase, the life expectancy also increases.
Key Points
• Population dynamics are influenced by age structure, which is characteristic for populations growing at different rates.
• Age structure varies according to the age distribution of individuals within a population.
• Fast-growing populations with a high proportion of young people have a triangle-shaped age structure, representing younger ages at the bottom and older ages at the top.
• Slow-growing populations with a smaller proportion of young people have a column-shaped age structure, representing a relatively even distribution of ages.
• Improvements in health care have led to the population explosion in underdeveloped countries, causing a “youth bulge” which is associated with social unrest.
Key Terms
• population dynamics: Variation among populations due to birth and death rates, by immigration and emigration, and concerning topics such as aging populations or population decline.
• youth bulge: Age structure typical of fast-growing populations in which a majority of the population are relatively young.
• age structure: The composition of a population in terms of the proportions of individuals of different ages; represented as a bar graph with younger ages at the bottom and males and females on either side.
Contributions and Attributions
• greenhouse gas. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/greenhouse_gas. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44875/latest...ol11448/latest. License: CC BY: Attribution
• One-child policy. Provided by: Wikipedia. Located at: https://en.Wikipedia.org/wiki/One-child_policy. License: CC BY-SA: Attribution-ShareAlike
• Population Growth. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Human_population_growth. License: CC BY-SA: Attribution-ShareAlike
• climate change. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/climate_change. License: CC BY-SA: Attribution-ShareAlike
• 1280px-Los_Angeles_Aerial_view_2013.jpg. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/World_..._view_2013.jpg. License: CC BY-SA: Attribution-ShareAlike
• World-Population-1800-2100. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:W...-1800-2100.svg. License: CC BY-SA: Attribution-ShareAlike
• population curve. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Po...tion_curve.svg. License: Public Domain: No Known Copyright
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44875/latest...ol11448/latest. License: CC BY: Attribution
• Measles. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Measles%23Epidemiology. License: CC BY-SA: Attribution-ShareAlike
• density-dependent. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/density-dependent. License: CC BY-SA: Attribution-ShareAlike
• Boundless. Provided by: Boundless Learning. Located at: www.boundless.com//biology/de...ctious-disease. License: CC BY-SA: Attribution-ShareAlike
• vaccine. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/vaccine. License: CC BY-SA: Attribution-ShareAlike
• 1280px-Los_Angeles_Aerial_view_2013.jpg. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/World_..._view_2013.jpg. License: CC BY-SA: Attribution-ShareAlike
• World-Population-1800-2100. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:W...-1800-2100.svg. License: CC BY-SA: Attribution-ShareAlike
• population curve. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Po...tion_curve.svg. License: Public Domain: No Known Copyright
• Measles US 1944-2007 inset. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:M...2007_inset.png. License: CC BY: Attribution
• Countriesbyfertilityrate.svg.png. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Total_...tilityrate.svg. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44875/latest...ol11448/latest. License: CC BY: Attribution
• Population decline. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Population_decline. License: CC BY-SA: Attribution-ShareAlike
• Population pyramid. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Population_pyramid%23The_Middle_East_and_North_Africa. License: CC BY-SA: Attribution-ShareAlike
• Population pyramid. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Population_pyramid%23Youth_bulge. License: CC BY-SA: Attribution-ShareAlike
• Population dynamics. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Population_dynamics. License: CC BY-SA: Attribution-ShareAlike
• Boundless. Provided by: Boundless Learning. Located at: www.boundless.com//biology/de.../age-structure. License: CC BY-SA: Attribution-ShareAlike
• population dynamics. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/population%20dynamics. License: CC BY-SA: Attribution-ShareAlike
• Boundless. Provided by: Boundless Learning. Located at: www.boundless.com//biology/de...on/youth-bulge. License: CC BY-SA: Attribution-ShareAlike
• 1280px-Los_Angeles_Aerial_view_2013.jpg. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/World_human_population%23/media/File:Los_Angeles_Aerial_view_2013.jpg. License: CC BY-SA: Attribution-ShareAlike
• World-Population-1800-2100. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:World-Population-1800-2100.svg. License: CC BY-SA: Attribution-ShareAlike
• population curve. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Po...tion_curve.svg. License: Public Domain: No Known Copyright
• Measles US 1944-2007 inset. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Measles_US_1944-2007_inset.png. License: CC BY: Attribution
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• Changing Population Age Structure. Located at: http://www.youtube.com/watch?v=OpOEHjndywk. License: Public Domain: No Known Copyright. License Terms: Standard YouTube license
• Worldwide Life Expectency. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/World_...za_de_vida.PNG. License: CC BY-SA: Attribution-ShareAlike
• OpenStax College, Human Population Growth. October 17, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44875/latest...e_45_05_03.png. License: CC BY: Attribution | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.04%3A_Human_Population_Growth/45.4C%3A_Age_Structure_Population_Growth_and_Economic_Development.txt |
Learning Objectives
• Distinguish between foundation, keystone, and invasive species
Characteristics of Communities
Communities are complex entities that can be characterized by their structure (the types and numbers of species present) and dynamics (how communities change over time). Understanding community structure and dynamics enables community ecologists to manage ecosystems more effectively. There are three main types of species that serve as the basis for a community. These include the foundation species, keystone species, and invasive species. Each of these has a specific role in how communities are formed.
Foundation Species
Foundation species are considered the “base” or “bedrock” of a community, having the greatest influence on its overall structure. They are usually the primary producers: organisms that bring most of the energy into the community. Kelp, a brown algae, is a foundation species that forms the basis of the kelp forests off the coast of California.
Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them. An example is the photosynthetic corals of the coral reef. Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called zooxanthellae) that perform photosynthesis; this is another example of a mutualism. The exoskeletons of living and dead coral make up most of the reef structure, which protects many other species from waves and ocean currents.
Keystone Species
A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure. The intertidal sea star, Pisaster ochraceus, of the northwestern United States is a keystone species. Studies have shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. If these fish were to become extinct, the community would be greatly affected.
Invasive Species
Invasive species are foreign species whose introduction can cause harm to the economy and the environment. These species have many ways of entering foreign environments, including through ship’s ballast water: when planes take off, organisms can sometimes become stuck in the cargo area. When the plane arrives in its destination, the organisms are now in a foreign environment. Travelers sometimes illegally smuggle items, such as fruits, plants, or even animals as pets, from one state or country to another..
Invasive species are often better competitors than native species, resulting in population explosions. These new species usually overtake the native populations, driving them to localized extinctions.
One of the many recent proliferations of an invasive species concerns the growth of Asian carp populations. Asian carp were introduced to the United States in the 1970s by fisheries and sewage treatment facilities that used the fish’s excellent filter feeding capabilities to clean their ponds of excess plankton. Some of the fish escaped, however, and by the 1980s, they had colonized many waterways of the Mississippi River basin, including the Illinois and Missouri Rivers.
Voracious eaters and rapid reproducers, Asian carp may outcompete native species for food, potentially leading to native species extinctions. For example, black carp are voracious eaters of native mussels and snails, limiting this food source for native fish species. Silver carp eat plankton that native mussels and snails feed upon, reducing this food source by a different alteration of the food web. In some areas of the Mississippi River, Asian carp species have become predominant, effectively outcompeting native fish for habitat. In some parts of the Illinois River, Asian carp constitute 95 percent of the community’s biomass. Although edible, the fish is bony and not a desirable food in the United States. Moreover, their presence threatens the native fish and fisheries of the Great Lakes, which are important to local economies and recreational anglers. Asian carp have even injured humans. The fish, frightened by the sound of approaching motorboats, thrust themselves into the air, often landing in the boat or directly hitting the boaters.
One infested waterway of particular importance is the Chicago Sanitary and Ship Channel, the major supply waterway linking the Great Lakes to the Mississippi River. To prevent the Asian carp from leaving the canal, a series of electric barriers have been successfully used to discourage their migration; however, the threat is significant enough that several states and Canada have sued to have the Chicago channel permanently cut off from Lake Michigan. Local and national politicians have weighed in on how to solve the problem, but no one knows whether the Asian carp will ultimately be considered a nuisance, like other invasive species, such as the water hyacinth and zebra mussel, or whether it will be the destroyer of the largest freshwater fishery of the world.
Key Points
• A community is defined by the structure of different species that occupy it and how those structures change over time.
• Foundation species change the environment where other species live, modifying it to benefit the organisms that live there.
• Keystone species maintain biodiversity; their removal can greatly alter the dynamics within the community.
• Invasive species are non-native organisms introduced into an area that may be better competitors and reproduce faster than native species; they tend to upset the natural balance.
Key Terms
• invasive species: any species that has been introduced to an environment where it is not native and has since become a nuisance through rapid spread and increase in numbers, often to the detriment of native species
• community: a group of interdependent organisms inhabiting the same region and interacting with each other
• keystone species: a species that exerts a large, stabilizing influence throughout an ecological community, despite its relatively small numerical abundance | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.05%3A_Community_Ecology/45.5A%3A_The_Role_of_Species_within_Communities.txt |
Learning Objectives
• Distinguish between predation and herbivory and describe defense mechanisms against each
Predation and Herbivory
Most animals fall into one of two major categories when it comes to obtaining the energy they need to survive in the environment: predation or herbivory. An animal that hunts, kills, and eats other animals is called a predator. Examples of predators include tigers, snakes, and hawks. Herbivory, on the other hand, refers to animals that eat plant matter. Deer, mice, and most song birds are examples. To protect themselves against these feeding mechanisms, many organisms have developed methods that keep them from being eaten.
Predation is the hunting of prey by its predator. Populations of predators and prey in a community are not constant over time; in most cases, they vary in cycles that appear to be related. The most-often-cited example of predator-prey dynamics is seen in the cycling of the lynx (predator) and the snowshoe hare (prey), which is based on nearly 200-year-old trapping data from North American forests. This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, they kill so many hares that the hare population begins to decline. This is followed by a decline in the lynx population because of scarcity of food. When the lynx population is low, the hare population size begins to increase due, at least in part, to low predation pressure, starting the cycle anew.
Herbivory describes the consumption of plants by insects and other animals. Unlike animals, plants cannot outrun predators or use mimicry to hide from hungry animals. Some plants have developed mechanisms to defend against herbivory. Other species have developed mutualistic relationships; for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.
Defense Mechanisms against Predation and Herbivory
The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and herbivory. These defenses may be mechanical, chemical, physical, or behavioral.
Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey. Chemical defenses are produced by many animals as well as plants, such as the foxglove which is extremely toxic when eaten.
Many species use their body shape and coloration to avoid being detected by predators. The tropical walking stick is an insect with the coloration and body shape of a twig, which makes it very hard to see when stationary against a background of real twigs. In another example, the chameleon can change its color to match its surroundings. Both of these are examples of camouflage: avoiding detection by blending in with the background.
Some species use coloration as a way of warning predators they are not good to eat. For example, the cinnabar moth caterpillar, the fire-bellied toad, and many species of beetle have bright colors that warn of a foul taste, the presence of toxic chemical, and/or the ability to sting or bite, respectively. Predators that ignore this coloration and eat the organisms will experience their unpleasant taste or presence of toxic chemicals and learn not to eat them in the future. This type of defensive mechanism is called aposematic coloration, or warning coloration.
While some predators learn to avoid eating certain potential prey because of their coloration, other species have evolved mechanisms to mimic this coloration to avoid being eaten, even though they themselves may not be unpleasant to eat or contain toxic chemicals. In Batesian mimicry, a harmless species imitates the warning coloration of a harmful one. Assuming they share the same predators, this coloration then protects the harmless ones, even though they do not have the same level of physical or chemical defenses against predation as the organism they mimic. Many insect species mimic the coloration of wasps or bees, which are stinging, venomous insects, thereby discouraging predation.
Key Points
• Predation, the hunting and consuming of animals by other animals, often shows cyclical patterns of predator/prey population sizes; predators increase in numbers when prey species are plentiful.
• Herbivory is the eating of plant material for energy and can assist the plants with seed distribution.
• Plants have evolved spines and toxins to defend against being eaten by herbivores.
• Animals use bright colors to advertise that they are toxic; mimicry to hide from predators; or have spines, shells, and scales to protect themselves.
• Batesian mimicry is when a non-toxic species looks similar to a poisonous one, which deters predator attacks.
Key Terms
• camouflage: resemblance of an organism to its surroundings for avoiding detection
• herbivory: the consumption of living plant tissue by animals
• Batesian mimicry: the resemblance of one or more non-poisonous species to a poisonous species, for example, the scarlet king snake and the coral snake | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.05%3A_Community_Ecology/45.5B%3A_Predation_Herbivory_and_the_Competitive_Exclusion_Principle.txt |
Learning Objectives
• Differentiate among the types of symbiosis: commensalism, mutualism, and parasitism
Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time which impact the abundance and distribution of the associating populations. Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction.
Commensalism
A commensalistic relationship occurs when one species benefits from the close, prolonged interaction, while the other neither benefits nor is harmed. Birds nesting in trees provide an example of a commensal relationship. The tree is not harmed by the presence of the nest among its branches. The nests are light and produce little strain on the structural integrity of the branch. Most of the leaves, which the tree uses to obtain energy by photosynthesis, are above the nest, so they are unaffected. The bird, on the other hand, benefits greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators.
Mutualism
A second type of symbiotic relationship, mutualism, is where two species both benefit from their interaction. Some scientists believe that these are the only true examples of symbiosis. For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut. The termite benefits from the ability of bacterial symbionts within the protozoa to digest cellulose. The termite itself cannot do this; without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbionts benefit by having a protective environment and a constant supply of food from the wood-chewing actions of the termite.
Parasitism
A parasite is an organism that lives in or on another living organism, deriving nutrients from it. In this relationship the parasite benefits, but the organism being fed upon, the host, is harmed. The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the host. This is because the parasite needs the host to complete its reproductive cycle by spreading to another host.
The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species. A tapeworm is a parasite that causes disease in humans when contaminated, undercooked meat such as pork, fish, or beef is consumed. The tapeworm can live inside the intestine of the host for several years, benefiting from the food the host is bringing into its gut by eating; it may grow to be over 50 ft long by adding segments. The parasite moves from species to species as it requires two hosts to complete its life cycle.
Key Points
• Commensalism is when two organisms share the same environment, where one benefits and the other is unharmed.
• Trees and birds have a commensalistic relationship; the birds benefit from having a place to build their nests, while the trees are unharmed and not impacted by the bird’s presence.
• Mutualism is when two species sharing the same environment both benefit from their interactions.
• The protozoans living within the intestines of termites create a mutualistic relationship with them; the protozoans get a safe place to live while the termites get help digesting the cellulose in their diet.
• Parasitism occurs when two organisms interact, but while one benefits, the other experiences harm.
• Parasites harm their hosts, as with the tapeworm attaching itself to the intestine of a cow; the tapeworm absorbs the nutrients from the cow’s diet, preventing them from being absorbed by the cow.
Key Terms
• mutualism: Any interaction between two species that benefits both.
• commensalism: A sharing of the same environment by two organisms where one species benefits and the other is unaffected; e.g., barnacles on whales.
• parasitism: Interaction between two organisms, in which one organism (the parasite) benefits and the other (the host) is harmed. | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.05%3A_Community_Ecology/45.5C%3A_Symbiosis.txt |
Learning Objectives
• Distinguish between primary and secondary succession following disturbances to communities
Community dynamics
Community dynamics are the changes in community structure and composition over time. Sometimes these changes are induced by environmental disturbances such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state.
Succession describes the sequential appearance and disappearance of species in a community over time. In primary succession, newly-exposed or newly-formed land is colonized by living things. In secondary succession, part of an ecosystem is disturbed, but remnants of the previous community remain.
Primary succession and pioneer species
Primary succession occurs when new land is formed or rock is exposed; for example, following the eruption of volcanoes, such as those on the Big Island of Hawaii. As lava flows into the ocean, new land is continually being formed. On the Big Island, approximately 32 acres of land are added each year. First, weathering and other natural forces break down the substrate enough for the establishment of certain hearty plants and lichens with few soil requirements, known as pioneer species. These species help to further break down the mineral-rich lava into soil where other, less-hardy species will grow, eventually replacing the pioneer species. In addition, as these early species grow and die, they add to an ever-growing layer of decomposing organic material, contributing to soil formation. Over time, the area will reach an equilibrium state with a set of organisms quite different from the pioneer species.
Secondary succession
A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire. Wildfires will burn most vegetation and kill those animals unable to flee the area. Their nutrients, however, are returned to the ground in the form of ash. Thus, even when areas are devoid of life due to severe fires, they will soon be ready for new life to take hold.
Before a wildfire, vegetation is often dominated by tall trees with access to the major plant energy resource: sunlight. Their height gives them access to sunlight while also shading the ground and other low-lying species. After the fire, however, these trees are no longer dominant. Thus, the first plants to grow back are usually annual plants followed, within a few years, by quickly-growing and spreading grasses along with other pioneer species. Due to changes in the environment brought on by the growth of the grasses and other species, over many years, shrubs will emerge along with small pine, oak, and hickory trees. These organisms are called intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point where species composition is no longer changing and resembles the community before the fire. This equilibrium state is referred to as the climax community, which will remain stable until the next disturbance.
Key Points
• After an environmental disturbance such as a volcanic eruption or forest fire, communities are able to replace lost species through the process of succession.
• Primary succession occurs after a volcanic eruption or earthquake; it involves the breakdown of rocks by lichens to create new, nutrient -rich soils.
• The first species to colonize an area after a major disturbance are called pioneer species; they help to form the new environment.
• Secondary succession occurs after a disturbance such as a forest fire, where there is still some organic matter to allow new plants to grow.
• Both types of succession take place over long periods of time and result in the communities reaching a state of equilibrium.
Key Terms
• succession: an act of following in sequence
• equilibrium: the condition of a system in which competing influences are balanced, resulting in no net change | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.05%3A_Community_Ecology/45.5D%3A_Ecological_Succession.txt |
Learning Objectives
• Distinguish between innate and learned behaviors
Behavior is the change in activity of an organism in response to a stimulus. Behavioral biology is the study of the biological and evolutionary bases for such changes. The idea that behaviors evolved as a result of the pressures of natural selection is not new.
Animal behavior has been studied for decades, by biologists in the science of ethology, by psychologists in the science of comparative psychology, and by scientists of many disciplines in the study of neurobiology. Although there is overlap between these disciplines, scientists in these behavioral fields take different approaches. Comparative psychology is an extension of work done in human and behavioral psychology. Ethology is an extension of genetics, evolution, anatomy, physiology, and other biological disciplines. One cannot study behavioral biology without touching on both comparative psychology and ethology.
One goal of behavioral biology is to distinguish the innate behaviors, which have a strong genetic component and are largely independent of environmental influences, from the learned behaviors, which result from environmental conditioning.
Innate behavior, or instinct, is important because there is no risk of an incorrect behavior being learned. These behaviors are “hard wired” into the system. In contrast, learned behaviors are flexible, dynamic, and can be altered relative to changes in the environment. Learned behaviors, even though they may have instinctive components, allow an organism to adapt to changes in the environment and are modified by previous experiences. Simple learned behaviors include habituation and imprinting—both are important to the maturation process of young animals.
Key Points
• Behavioral biology is the study of the biological and evolutionary bases for changes in activity in response to a stimulus.
• Comparative psychology is an extension of work done in human and behavioral psychology. Ethology is an extension of genetics, evolution, anatomy, physiology, and other biological disciplines.
• Innate behaviors have a strong genetic component and are largely independent of environmental influences; they are “hard wired.”
• Learned behaviors result from environmental conditioning; they allow an organism to adapt to changes in the environment and are modified by previous experiences..
Key Terms
• behavioral biology: A systematic approach to the understanding of human and animal behavior assuming that the behavior of a human or animal is a consequence of that individual’s history.
• comparative psychology: The scientific study of the behavior and mental processes of non-human animals, especially as these relate to the phylogenetic history, adaptive significance, and development of behavior.
45.6B: Movement and Migration
Learning Objectives
• Distinguish between kinesis, taxis, and migration in response to stimuli
Innate behaviors: movement and migration
Innate or instinctual behaviors rely on response to stimuli. The simplest example of this is a reflex action: an involuntary and rapid response to stimulus. To test the “knee-jerk” reflex, a doctor taps the patellar tendon below the kneecap with a rubber hammer. The stimulation of the nerves there leads to the reflex of extending the leg at the knee. This is similar to the reaction of someone who touches a hot stove and instinctually pulls his or her hand away. Even humans, with our great capacity to learn, still exhibit a variety of innate behaviors.
Kinesis and taxis
Another activity or movement of innate behavior is kinesis: undirected movement in response to a stimulus. Orthokinesis is the increased or decreased speed of movement of an organism in response to a stimulus. Woodlice, for example, increase their speed of movement when exposed to high or low temperatures. This movement, although random, increases the probability that the insect spends less time in the unfavorable environment. Another example is klinokinesis, an increase in turning behaviors. It is exhibited by bacteria such as E. coli which, in association with orthokinesis, helps the organisms randomly find a more hospitable environment.
A similar, but more-directed version of kinesis is taxis: the directed movement towards or away from a stimulus. This movement can be in response to light (phototaxis), chemical signals (chemotaxis), or gravity (geotaxis). It can be directed toward (positive) or away (negative) from the source of the stimulus. An example of a positive chemotaxis is exhibited by the unicellular protozoan Tetrahymena thermophila. This organism swims using its cilia, at times moving in a straight line and at other times making turns. The attracting chemotactic agent alters the frequency of turning as the organism moves directly toward the source, following the increasing concentration gradient.
Migration as innate behavior
Migration is the long-range seasonal movement of animals. An evolved, adapted response to variation in resource availability, it is a common phenomenon found in all major groups of animals. Birds fly south for the winter to get to warmer climates with sufficient food, while salmon migrate to their spawning grounds. The popular 2005 documentary March of the Penguins followed the 62-mile migration of emperor penguins through Antarctica to bring food back to their breeding site and to their young. Wildebeests migrate over 1800 miles each year in search of new grasslands.
Although migration is thought of as an innate behavior, only some migrating species always migrate (obligate migration). Animals that exhibit facultative migration can choose to migrate or not. Additionally, in some animals, only a portion of the population migrates, whereas the rest does not migrate (incomplete migration). For example, owls that live in the tundra may migrate in years when their food source, small rodents, is relatively scarce, but not migrate during the years when rodents are plentiful.
Key Points
• Innate behaviors are instinctual, relying on responses to stimuli.
• Kinesis is the undirected movement in response to a stimulus, which can include orthokinesis (related to speed) or klinokinesis (related to turning).
• Taxis is the directed movement towards or away from a stimulus, which can be in response to light (phototaxis), chemical signals ( chemotaxis ), or gravity (geotaxis).
• Migration is an innate behavior characterized as the long-range seasonal movement of animals; it is an evolved, adapted response to variation in resource availability.
• Migration is a variable innate behavior as some migrating species always migrate (obligate migration) while in other animals, only a portion of the population migrates (incomplete migration).
Key Terms
• orthokinesis: the speed of movement of the individual is dependent upon the intensity of the stimulus
• taxis: the movement of an organism in response to a stimulus; similar to kinesis, but more direct
• kinesis: the undirected movement of an organism in response to an external stimulus | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.06%3A_Innate_Animal_Behavior/45.6A%3A_Introduction_to_Animal_Behavior.txt |
Learning Objectives
• Differentiate among the ways in which animals communicate
Innate behaviors: living in groups
Not all animals live in groups, but even those that live relatively-solitary lives (with the exception of those that can reproduce asexually) must mate. Mating usually involves one animal signaling another so as to communicate the desire to mate. There are several types of energy-intensive behaviors or displays associated with mating called mating rituals. Other behaviors found in populations that live in groups are described in terms of which animal benefits from the behavior. In selfish behavior, only the animal in question benefits; in altruistic behavior, one animal’s actions benefit another animal; cooperative behavior occurs when both animals benefit. All of these behaviors involve some sort of communication between population members.
Communication within a species
Animals communicate with each other using stimuli known as signals. These signals are chemical ( pheromones ), aural (sound), visual (courtship and aggressive displays), or tactile (touch). These types of communication may be instinctual, learned, or a combination of both. These are not the same as the communication we associate with language, which has been observed only in humans and, perhaps, in some species of primates and cetaceans.
A pheromone is a secreted, chemical signal used to obtain a response from another individual of the same species. The purpose of pheromones is to elicit a specific behavior from the receiving individual. Pheromones are especially common among social insects, but they are used by many animal species to attract the opposite sex, to sound alarms, to mark food trails, and to elicit other, more-complex behaviors. Even humans are thought to respond to certain pheromones called axillary steroids. These chemicals influence human perception of other people. In one study, they were responsible for a group of women synchronizing their menstrual cycles. The role of pheromones in human-to-human communication is still somewhat controversial and continues to be researched.
Songs are an example of an aural signal: one that needs to be heard by the recipient. Perhaps the best known of these are songs of birds, which identify the species and are used to attract mates. Other well-known songs are those of whales, which are of such low frequency that they can travel long distances underwater. Dolphins communicate with each other using a wide variety of vocalizations. Male crickets make chirping sounds using a specialized organ to attract a mate, repel other males, and to announce a successful mating.
Courtship displays are a series of ritualized visual behaviors (signals) designed to attract and convince a member of the opposite sex to mate. These displays are ubiquitous in the animal kingdom. They often involve a series of steps, including an initial display by one member followed by a response from the other. If at any point the display is performed incorrectly or a proper response is not given, the mating ritual is abandoned and the mating attempt will be unsuccessful.
Aggressive displays are also common in the animal kingdom. As, for example, when a dog bares its teeth to get another dog to back down. Presumably, these displays communicate not only the willingness of the animal to fight, but also its fighting ability. Although these displays do signal aggression on the part of the sender, it is thought that they are actually a mechanism to reduce the amount of fighting that occurs between members of the same species: they allow individuals to assess the fighting ability of their opponent and thus decide whether it is “worth the fight.”
Distraction displays are seen in birds and some fish. They are designed to attract a predator away from the nest that contains their young. This is an example of an altruistic behavior: it benefits the young more than the individual performing the display, which is putting itself at risk by doing so.
Many animals, especially primates, communicate with other members in the group through touch. Activities such as grooming, touching the shoulder or root of the tail, embracing, lip contact, and greeting ceremonies have all been observed in the Indian langur, an Old World monkey. Similar behaviors are found in other primates, especially in the great apes.
Key Points
• Animals need to communicate with one another in order to successfully mate, which usually involves one animal signaling another; the energy-intensive behaviors or displays associated with mating are called mating rituals.
• Animal signaling is not the same as the communication we associate with language, which has been observed only in humans, but may also occur in some non-human primates and cetaceans.
• Animal communication by stimuli known as signals may be instinctual, learned, or a combination of both.
Key Terms
• pheromone: a chemical secreted by an animal that affects the development or behavior of other members of the same species, functioning often as a means of attracting a member of the opposite sex | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.06%3A_Innate_Animal_Behavior/45.6C%3A_Animal_Communication_and_Living_in_Groups.txt |
Learning Objectives
• Explain how altruistic behaviors can benefit populations
Altruistic Behaviors
Behaviors that lower the fitness of the individual engaging in the behavior, but increase the fitness of another individual, are termed altruistic. Examples of such behaviors are seen widely across the animal kingdom. Social insects, such as worker bees, have no ability to reproduce, yet they maintain the queen so she can populate the hive with her offspring. Meerkats keep a sentry standing guard to warn the rest of the colony about intruders, even though the sentry is putting itself at risk. Wolves and wild dogs bring meat to pack members not present during a hunt. Lemurs take care of infants unrelated to them. Although on the surface these behaviors appear to be altruistic, it may not be so simple.
Why Does Altruism Exist?
There has been much discussion over why altruistic behaviors exist. Do these behaviors lead to overall evolutionary advantages for their species ? Do they help the altruistic individual pass on its own genes? And what about such activities between unrelated individuals? One explanation for altruistic-type behaviors is found in the genetics of natural selection. In the 1976 book, The Selfish Gene, scientist Richard Dawkins attempted to explain many seemingly-altruistic behaviors from the viewpoint of the gene itself. Although a gene obviously cannot be selfish in the human sense, it may appear that way if the sacrifice of an individual benefits related individuals that share genes that are identical by descent (present in relatives because of common lineage). Mammal parents make this sacrifice to take care of their offspring. Emperor penguins migrate miles in harsh conditions to bring food back for their young. Selfish gene theory has been controversial over the years and is still discussed among scientists in related fields.
Even less-related individuals (those with less genetic identity than that shared by parent and offspring) benefit from seemingly altruistic behavior. The activities of social insects such as bees, wasps, ants, and termites are good examples. Sterile workers in these societies take care of the queen because they are closely related to it; as the queen has offspring, she is passing on genes from the workers indirectly. Thus, it is of fitness benefit for the worker to maintain the queen without having any direct chance of passing on its genes due to its sterility. The lowering of individual fitness to enhance the reproductive fitness of a relative and, thus, one’s inclusive fitness evolves through kin selection. This phenomenon can explain many superficially-altruistic behaviors seen in animals. However, these behaviors may not be truly defined as altruism in these cases because the actor is actually increasing its own fitness either directly (through its own offspring) or indirectly (through the inclusive fitness it gains through relatives that share genes with it).
Unrelated individuals may also act altruistically to each other; this seems to defy the “selfish gene” explanation. An example of this is observed in many monkey species where a monkey will present its back to an unrelated monkey to have that individual pick the parasites from its fur. After a certain amount of time, the roles are reversed and the first monkey now grooms the second monkey. Thus, there is reciprocity in the behavior. Both benefit from the interaction and their fitness is raised more than if neither cooperated or if one cooperated and the other did not. This behavior is still not necessarily altruism, as the “giving” behavior of the actor is based on the expectation that it will be the “receiver” of the behavior in the future; a concept termed reciprocal altruism. Reciprocal altruism requires that individuals repeatedly encounter each other, often the result of living in the same social group, and that cheaters (those that never “give back”) are punished.
Evolutionary Game Theory and Altruism
According to evolutionary game theory, a modification of classical game theory in mathematics, many of these so-called “altruistic behaviors” are not altruistic at all. The definition of “pure” altruism, based on human behavior, is an action that benefits another without any direct benefit to oneself. Most of the behaviors previously described do not seem to satisfy this definition; game theorists are good at finding “selfish” components in them. Others have argued that the terms “selfish” and “altruistic” should be dropped completely when discussing animal behavior, as they describe human behavior and may not be directly applicable to instinctual animal activity. What is clear, though, is that heritable behaviors that improve the chances of passing on one’s genes or a portion of one’s genes are favored by natural selection and will be retained in future generations as long as those behaviors convey a fitness advantage.
Key Points
• Behaviors that lower the fitness of the individual, but increase the fitness of another individual are termed altruistic; why altruistic behaviors exist has been the topic of some debate.
• One explanation for altruistic-type behaviors is found in the genetics of natural selection and the “selfish gene ” theory: although a gene cannot be selfish in the human sense, it may appear that way if the sacrifice of an individual benefits related individuals that share genes that are identical.
• Even less-related individuals, those with less genetic identity than that shared by parent and offspring, benefit from seemingly-altruistic behavior, such as sterile worker bees protecting the queen.
• Unrelated individuals may also act altruistically to each other, which seems to defy the “selfish gene” explanation; however, this altruism is typically reciprocal, in that both benefit from the interaction.
• Most of the behaviors described when speaking of altruism do not seem to satisfy the definition of “pure” altruism; some evolutionary game theorists suggest that we get rid of the terms “altruistic” and “selfish” altogether since they describe human behavior.
Key Terms
• kin selection: an evolutionary mechanism by which an organism’s behavior benefits the reproductive success of its relatives, including at the expense of its own survival or reproduction
• altruism: devotion to the interests of others; brotherly kindness; opposed to egoism or selfishness
• game theory: a branch of applied mathematics that studies strategic situations in which individuals or organizations choose various actions in an attempt to maximize their returns | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.06%3A_Innate_Animal_Behavior/45.6D%3A_Altruism_and_Populations.txt |
Learning Objectives
• Differentiate among monogamous, polygynous, and polyandrous mating systems, and distinguish between intersexual and intrasexual mate selection
Finding Sexual Partners
Not all animals reproduce sexually, but many that do have the same challenge: they need to find a suitable mate and often have to compete with other individuals to obtain one. Significant energy is spent in the process of locating, attracting, and mating with a sex partner.
Types of Mate Selection
Two types of selection that occur during the process of choosing a mate may be involved in the evolution of reproductive traits called secondary sexual characteristics. These types are: intersexual selection (the choice of a mate where individuals of one sex choose mates of the other sex) and intrasexual selection (the competition for mates between species members of the same sex). Intersexual selection is often complex because choosing a mate may be based on a variety of visual, aural, tactile, and chemical cues. An example of intersexual selection is when female peacocks choose to mate with the male with the brightest plumage. This type of selection often leads to traits in the chosen sex that do not enhance survival, but are those traits most attractive to the opposite sex (often at the expense of survival). Intrasexual selection involves mating displays and aggressive mating rituals such as rams butting heads; the winner of these battles is the one that is able to mate. Many of these rituals use up considerable energy, but result in the selection of the healthiest, strongest, and/or most dominant individuals for mating.
Mating Systems
Three general mating systems, all involving innate as opposed to learned behaviors, are seen in animal populations: monogamous (monogamy), polygynous (polygyny), and polyandrous (polyandry).
In monogamous systems, one male and one female are paired for at least one breeding season. In some animals, such as the gray wolf, these associations can last much longer, even a lifetime. Several explanations have been proposed for this type of mating system. The “mate-guarding hypothesis” states that males stay with the female to prevent other males from mating with her. This behavior is advantageous in such situations where mates are scarce and difficult to find. Another explanation is the “male-assistance hypothesis,” where males that remain with a female to help guard and rear their young will have more and healthier offspring. Monogamy is observed in many bird populations where, in addition to the parental care from the female, the male is also a major provider of parental care for the chicks. A third explanation for the evolutionary advantages of monogamy is the “female-enforcement hypothesis.” In this scenario, the female ensures that the male does not have other offspring that might compete with her own, so she actively interferes with the male’s signaling to attract other mates.
Polygynous mating refers to one male mating with multiple females. In these situations, the female must be responsible for most of the parental care as the single male is not capable of providing care to that many offspring. In resourced-based polygyny, males compete for territories with the best resources. They then mate with females that enter the territory, drawn to its resource richness. The female benefits by mating with a dominant, genetically-fit male; however, it is at the cost of having no male help in caring for the offspring. An example is seen in the yellow-rumped honeyguide, a bird whose males defend beehives because the females feed on the wax. As the females approach, the male defending the nest will mate with them. Harem mating structures are a type of polygynous system where certain males dominate mating while controlling a territory with resources. Elephant seals, where the alpha male dominates the mating within the group, are an example. A third type of polygyny is a lek system. Here there is a communal courting area where several males perform elaborate displays for females; the females choose their mate from this group. This behavior is observed in several bird species.
In polyandrous mating systems, one female mates with many males. These types of systems are much rarer than monogamous and polygynous mating systems. In pipefishes and seahorses, males receive the eggs from the female, fertilize them, protect them within a pouch, and give birth to the offspring. Therefore, the female is able to provide eggs to several males without the burden of carrying the fertilized eggs.
Key Points
• Two types of mate selection occur: intersexual selection (the choice of a mate where individuals of one sex choose mates of the other sex) and intrasexual selection (the competition for mates between species members of the same sex).
• Three general mating systems, all involving innate as opposed to learned behaviors, are seen in animal populations: monogamous ( monogamy ), polygynous ( polygyny ), and polyandrous (polyandry).
• In monogamous systems, one male and one female are paired for at least one breeding season; although in some animals, these partnerships can last even longer, sometimes an entire lifetime; males provide substantial parental care.
• Polygynous mating refers to one male mating with multiple females; in these situations, the female must be responsible for most of the parental care as the single male is not capable of providing care to that many offspring.
• In polyandrous mating systems, one female mates with many males; these types of systems are much rarer than monogamous and polygynous mating systems.
Key Terms
• polyandry: the mating pattern whereby a female copulates with several males
• polygyny: the mating patterns whereby a male copulates with several females
• monogamy: a form of sexual bonding involving an exclusive pair bond between two individuals
Contributions and Attributions
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• Hippocampus hystrix (Spiny seahorse). Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi..._seahorse).jpg. License: CC BY-SA: Attribution-ShareAlike | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.06%3A_Innate_Animal_Behavior/45.6E%3A_Mating_Systems_and_Sexual_Selection.txt |
Learning Objectives
• Distinguish between the simple learned behaviors of habituation and imprinting
Simple Learned Behaviors
The majority of the behaviors discussed in previous sections are innate or at least have an innate component. In other words, variations on the innate behaviors may be learned. Innate behaviors are inherited and do not change in response to signals from the environment. Conversely, learned behaviors, even though they may have instinctive components, allow an organism to adapt to changes in the environment and are modified by previous experiences. Simple learned behaviors include habituation and imprinting, both of which are important to the maturation process of young animals.
Habituation
Habituation is a simple form of learning in which an animal stops responding to a stimulus after a period of repeated exposure. This is a form of non-associative learning as the stimulus is not associated with any punishment or reward. Prairie dogs typically sound an alarm call when threatened by a predator, but they become habituated to the sound of human footsteps when no harm is associated with this sound; therefore, they no longer respond to them with an alarm call. In this example, habituation is specific to the sound of human footsteps, as the animals still respond to the sounds of potential predators.
Imprinting
Imprinting is a type of learning that occurs at a particular age or a life stage that is rapid and independent of the species involved. Hatchling ducks recognize the first adult they see, their mother, and make a bond with her. A familiar sight is ducklings walking or swimming after their mothers. This type of non-associative learning is very important in the maturation process of these animals as it encourages them to stay near their mother in order to be be protected, greatly increasing their chances of survival. However, if newborn ducks see a human before they see their mother, they will imprint on the human and follow it in just the same manner as they would follow their real mother.
Key Points
• Learned behaviors stand in opposition to innate behaviors: while learned behaviors may have an innate component, they allow the organism to modify its behavior according to environmental factors or previous experiences.
• Habituation is a simple form of learning in which an animal stops responding to a stimulus after a period of repeated exposure; it is a form of non-associative learning, as the stimulus is not associated with any punishment or reward.
• Imprinting is a type of learning that occurs at a particular age or a life stage that is rapid and independent of the species involved.
Key Terms
• imprinting: any kind of phase-sensitive learning (learning occurring at a particular age or a particular life stage) that is rapid and apparently independent of the consequences of behavior
• habituation: a learned behavior involving modifying behavior according to the environment or previous expriences
• innate: inborn; native; natural
45.7B: Conditioned Behavior
Learning Objectives
• Distinguish between classical and operant conditioning techniques
Conditioned behaviors are types of associative learning where a stimulus becomes associated with a consequence. Two types of conditioning techniques include classical and operant conditioning.
Classical Conditioning
In classical conditioning, a response called the conditioned response is associated with a stimulus that it had previously not been associated with, the conditioned stimulus. The response to the original, unconditioned stimulus is called the unconditioned response. The most cited example of classical conditioning is Ivan Pavlov’s experiments with dogs. In Pavlov’s experiments, the unconditioned response was the salivation of dogs in response to the unconditioned stimulus of seeing or smelling their food. The conditioning stimulus that researchers associated with the unconditioned response was the ringing of a bell. During conditioning, every time the animal was given food, the bell was rung. This was repeated during several trials. After some time, the dog learned to associate the ringing of the bell with food and to respond by salivating. After the conditioning period was finished, the dog would respond by salivating when the bell was rung, even when the unconditioned stimulus (the food) was absent. Thus, the ringing of the bell became the conditioned stimulus and the salivation became the conditioned response. Although it is thought by some scientists that the unconditioned and conditioned responses are identical, Pavlov discovered that the saliva in the conditioned dogs had characteristic differences when compared to the unconditioned dog.
Some believe that this type of conditioning requires multiple exposures to the paired stimulus and response, but it is now known that this is not necessary in all cases; some conditioning can be learned in a single pairing experiment. Classical conditioning is a major tenet of behaviorism, a branch of psychological philosophy that proposes that all actions, thoughts, and emotions of living things are behaviors that can be treated by behavior modification and changes in the environment.
Operant Conditioning
In operant conditioning, the conditioned behavior is gradually modified by its consequences as the animal responds to the stimulus. A major proponent of such conditioning was psychologist B.F. Skinner, the inventor of the Skinner box. Skinner put rats in his boxes that contained a lever that would dispense food to the rat when depressed. While initially the rat would push the lever a few times by accident, it eventually associated pushing the lever with getting the food. This type of learning is an example of operant conditioning. Operant learning is the basis of most animal training: the conditioned behavior is continually modified by positive or negative reinforcement (such as being given a reward or having a negative stimulus removed) or by positive or negative punishment (such as being given a punishment or having a pleasing stimulus removed). In this way, the animal is conditioned to associate a type of behavior with the punishment or reward. Over time, the animal can be induced to perform behaviors that they would not have done in the wild, such as the “tricks” dolphins perform at marine amusement park shows.
Key Points
• In classical conditioning, a response called the conditioned response is associated with a stimulus that it had previously not been associated with, the conditioned stimulus; the response to the original, unconditioned stimulus is called the unconditioned response.
• Classical conditioning is a major tenet of behaviorism, a branch of psychological philosophy that proposes that all actions, thoughts, and emotions of living things are behaviors that can be treated by behavior modification and changes in the environment.
• In operant conditioning, the conditioned behavior is gradually modified by its consequences as the animal responds to the stimulus.
• Operant conditioning relies on the use of reinforcement (i.e. a reward) and/or punishment to modify a conditioned behavior; in this way, the animal is conditioned to associate a type of behavior with the punishment or reward.
Key Terms
• classical conditioning: the use of a neutral stimulus, originally paired with one that invokes a response, to generate a conditioned response
• operant conditioning: a technique of behavior modification through positive and negative reinforcement and positive and negative punishment | textbooks/bio/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.07%3A_Learned_Animal_Behavior/45.7A%3A_Simple_Learned_Behaviors.txt |
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