When is homeostasis most effective

Among those influenced by Bernard were such physiological luminaries as William M. Bayliss, Ernest H. Starling, Joseph Barcroft, J. Haldane, and C.

Sherrington in England, and L. Henderson and Walter B. Cannon in America Adolph, ; Cooper, ; Gross, Walter Cannon later popularized this phrase when he used it as the title for his book in which he introduced the concept of homeostasis. In , Charles R. Richet — , a student of Bernard who later won the Nobel Prize in Physiology and Medicine, stressed the dynamic stability of the internal environment. The following quote, we shall see, presaged the definition supplied by Walter Cannon.

By an apparent contradiction, it maintains its stability only if it is excitable and capable of modifying itself according to external stimuli and adjusting its response to the stimulation.

In a sense, it is stable because it is modifiable — the slight instability is the necessary condition for the true stability of the organism. Homeostasis is often mistakenly taken to mean unchanging or stagnant. Homeostasis, then, is the tendency of a system to maintain an internal stability as the result of the coordinated response of its parts to any situation or stimulus that disturbs normal conditions or function. Thus, the term homeostasis attempts to convey two ideas: 1 an internal stability within a range of values and 2 the coordinated dynamic response that maintains this internal stability self-regulatory goal-seeking behavior.

The word does not imply, something set and immobile, a stagnation. It means a condition — a condition which may vary, but is relatively constant. As emphasized by Cannon, homeostasis is not static; it is, rather, a dynamic self-adjusting system that maintains viability in the face of changing environmental demands. Echoing Bernard, homeostasis is a unique property of living organisms and, may be responsible for life itself.

More recently, Turner described homeostasis as a dynamic disequilibrium — dynamic, as a stable internal environment requires continuous monitoring and adjustments once again, a self-regulatory process in order to maintain a balance between opposing forces what he calls disequilibrium so that a free and independent life is possible. In short, homeostasis is life. The final piece of the homeostasis puzzle was supplied by the application of control theory from systems engineering to explain self-regulation in biological systems.

The interaction of these regulatory mechanisms not only increases the stability of the system but provides redundancy back-up such that failure of one component does not necessarily lead to catastrophe.

Thus, from its inception physiological investigations have been directed toward understanding the organism be it microbe, plant, animal, or human as a single functional entity.

Both feedback and feedforward are the mechanisms by which homeostasis is obtained. I shall begin this section with a discussion of the contribution of feedback to homeostatic regulation and then briefly discuss feedforward also known as central command mechanisms. A feedback system is a closed loop structure in which the results of past actions changes in the internal environment of the system are fed into the system via information, feedback to control future action; the system affects its own behavior modified from Forrester, There are two types of feedback systems: negative feedback that seeks a goal and responds as a consequence of failure to meet this goal maintains a stable range of values and positive feedback that produces growth processes wherein the actions build on the results that then generate still greater action a growth cycle.

These feedback systems are themselves subject to higher levels of control; that is, the operational range of the regulated variables can be adjusted to support the behavioral response to environmental stimuli. Homeostasis is the result of the complex interaction and competition between multiple negative and positive feedback systems and provides the basis for physiological regulation.

Once again we can trace the origin of self-regulatory systems to the ancient Greeks. A water clock depends upon a steady flow of water to measure an unvarying flow of time.

If the water level is not relatively constant, the water outflow will vary depending on the height of the water column supplying the clock faster with a full container and slower as the water level in the container falls.

The water clock designed by Ktesibios used a float valve similar to that used in the modern flush toilet to maintain a constant water level in the clock water reservoir. As water levels fall, the float also falls thereby opening a valve that allows water to flow into the clock reservoir and to replenish the water level. Then, as the water returns to the desired level, the float rises and closes the valve. Thus, the clock water reservoir could be regulated such that there is no net gain or loss in the water level and thereby it maintains a constant water outflow rate from which an accurate estimate of time can be obtained.

The accuracy of this type of water clock was not supplanted until the 17th century when a pendulum was employed to regulate the clock mechanism. A major limitation of early steam engines was that their speed was affected by both the steam pressure generated by the boiler and work load placed upon the engine.

James Watt — vastly improved the efficiency and safety of the steam engine by the development of a centrifugal feedback valve that controlled the speed of the engine Rosen, This, in turn, opened a valve to decrease the flow of steam into the engine and a slower speed was restored. Conversely, as the engine speed decreased, so also would the rotation of the flyweights, thereby decreasing the outward centrifugal force. The flyweights would drop pulled down by gravity closer together, closing the steam valve so more steam could enter into the engine and increase its speed.

As with the water clock and its water reservoir level, a constant engine speed could be maintained despite fluctuating steam pressure and changing work load without the constant supervision of a human monitor. Figure 4. See text for details. Source: public domain, as modified from, https:www. In , Harold S. Black — applied feedback regulation to electrical circuits to amplify transatlantic telephone signals Black, His negative feedback amplifier patented in can be considered to be one of the most important developments in the field of electronics.

Further advances in systems control theory were achieved during World War II with the development of servo-control negative feedback mechanisms for anti-aircraft weapons. In , two influential papers were published that established that the mathematical principles of control theory, as first described by Maxwell, could be applied to explain behavior in living organisms.

Interestingly, Rosenblueth worked closely with Cannon and undoubtedly was influenced by his ideas. In his book Cybernetics, Wiener developed the first formal mathematical analysis of feedback control in biological systems, concepts that have subsequently been extensively applied in modeling physiological systems as, for example, by Arthur Guyton — and his many students with regard to cardiovascular regulation.

Thus, the concept of feedback regulation in living organisms may be said to have co-evolved with the mathematical concepts of control theory in mechanical systems. Negative feedback regulation is a particularly important mechanism by which homeostasis is achieved, as will be described in the following paragraphs. The water clock and centrifugal steam governor described in the preceding paragraphs provide classic examples of negative feedback systems.

Thus, the float simultaneously affects the water levels and is affected by water level forming a circular causality or a cycle of causation. It is important to emphasize that this is an automatic self-regulatory system, meaning that it requires no external adjustment once the operating level around which the variable is regulated has been set.

A simplified general form of a closed loop feedback system is illustrated in Figure 5. Effector activity opposes and thereby buffers against changes in the variable. A solid line is used in this diagram to indicate a direct relationship increase leads to increase, decrease leads to decrease between the components, while a dashed line represents an inverse relationship increase leads to a decrease and vice versa.

Negative feedback regulation must contain an odd number of dashed lines in order to maintain the variable within a narrow range of the desired value.

Figure 5. A schematic representation of negative feedback regulation. A solid line indicates that the connected components are directly related an increase in one component leads to increase the connected component, while a decrease will lead to decrease in the connected components. A dashed line indicates the connected components are inversely related an increase in one component leads to a decrease in the connected component while a decrease will lead to an increase in the connected component.

An odd number of dashed lines are a necessary condition for any negative feedback cycle of causation. Negative feedback acts to maintain the controlled variable within a narrow range of values see text for a detailed description. A commonly used example of negative feedback is the regulation of room temperature by a thermostatically controlled heating and cooling system as displayed in Figure 6. Room temperature is the regulated variable, the sensor is a thermometer, the comparator is the thermostat—the device that compares the desired temperature operating point with the actual temperature error detection , and the effector is the heating or cooling system.

In this example, an increase in outside heat is detected by the sensor and the information is conveyed to the thermostat. The temperature information is compared to operating point and if there is sufficient difference between actual and desired temperature, the cooling system is activated and the heating system is inactivated reducing the error signal.

The converse would happen if environmental temperature should fall, the cooling system would be turned off and the heating units activated. Thus, stable room temperatures can be maintained despite a wide range of fluctuating external conditions.

Figure 6. A schematic representation of the regulation of room temperature to illustrate the concept of negative feedback regulation. A solid line indicates that the connected components are directly related an increase in one component leads to an increase the connected components, while a decrease will lead to a decrease in the connected components.

A dashed line indicates that the connected components are inversely related an increase in one component leads to a decrease in the connected component while a decrease will lead to an increase in the connected component.

Negative feedback acts to maintain the room temperature within a narrow range of values despite changes in ambient temperature see text for a detailed description. With this caveat firmly in mind, the concept of self-regulation in biological system can be illustrated by the regulation of blood pressure.

As early as the midth century, it became obvious that arterial blood pressure was maintained within a narrow range of values via the activation of neutrally mediated reflex adjustments Adolph, However, it was not until to s that the principles of negative feedback were applied to explain the homeostatic regulation of arterial blood pressure.

A detailed description of intricacies of blood pressure regulation is beyond the scope of the present essay for a recent review see Dampney, Nonetheless, a simplified feedback cycle, analogous to the one we used for room temperature, is seen in Figure 7. Figure 7. A simplified schematic representation of the regulation of arterial blood pressure as a physiological example of negative feedback regulation.

Negative feedback regulation acts to maintain the arterial blood pressure within a narrow range of values see text for a detailed description. Algebraically, blood pressure BP — analogous to voltage, E, in an electrical circuit is the product of the cardiac output CO — analogous to current, I, in an electrical circuit and systemic vascular resistance also known as total peripheral resistance TPR — analogous to electrical resistance, R.

Cardiac output is itself the product of the amount of blood ejected per beat [stroke volume SV ] multiplied by the number of beats per minute [heart rate HR ]. Returning to Figure 7 , the sensors are receptors baroreceptors located in arterial blood vessels aortic arch and carotid sinuses that respond to changes in arterial pressure increases in BP increase receptor activity.

The comparator function is performed by a cluster of nerve cells within the medulla of brain [nucleus tractus solitarius NTS ] where the signal is processed to affect the output of the effector system. The signal is processed at the NTS and then effects excitatory [rostral ventral lateral medulla RVLM via interneuron connections] and inhibitory [nucleus ambiguus NA , monosynaptically] areas within the medulla to elicit the motor response see Figure 8 for more details.

The motor output from the central nervous system to target organs is conducted by means of two sets of nerves to the heart: parasympathetic nerves originating in the NA that decrease HR and sympathetic nerves originating in the intermediolateral column, IML of the spinal cord, regulated by neurons from the RVLM that increase HR and SV.

The sympathetic nerves also go to blood vessels, the activation of which decreases vessel diameter and thereby increases TPR. Thus, if BP should increase, the so-called baroreceptor reflex is activated. An increase in parasympathetic activity coupled with a decrease in sympathetic activity would reduce cardiac output decreasing HR and SV and decrease TPR.

The opposite changes would occur if blood pressure should decrease. Thus, negative feedback regulation buffers against transitory changes and thereby helps maintain a stable blood pressure on a beat-by-beat basis throughout the day despite changing environmental or behavioral conditions.

Figure 8. A simplified schematic representation of the central neural structures involved in baroreceptor reflex regulation of arterial blood pressure. Arterial pressure receptors located in the carotid sinuses and aortic arch nerve firing increases as arterial pressure increases convey afferent information via the glossopharyngeal IXth and vagus Xth nerves to the brain, respectively. This information is first processed by neurons located in the nucleus tractus solitarius NTS.

This mechanism of molecules moving across a cell membrane from the side where they are more concentrated to the side where they are less concentrated is a form of passive transport called simple diffusion Figure 8. Simple Diffusion across the Cell Plasma Membrane. The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.

Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Very small polar molecules, such as water, can cross via simple diffusion due to their small size. Charged atoms or molecules of any size cannot cross the cell membrane via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer.

Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane. A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar.

To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.

There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell. Water also can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself. Osmosis is the diffusion of water through a semipermeable membrane Figure 8.

The movement of water molecules is not itself regulated by cells, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells in the extracellular fluid is equal to the concentration of solutes inside the cells in the cytoplasm.

T onicity is used to describe the variations of solute in a solution with the solute inside the cell. Three terms— hypotonic, isotonic, and hypertonic —are used to compare the relative solute concentration of a cell to that of the extracellular fluid surrounding the cells.

In a hypotonic solution , such as tap water, the extracellular fluid has a lower concentration of solutes than the fluid inside the cell, and water enters the cell. Note that water is moving down its concentration gradient If this occurs in an animal cell, the cell may burst, or lyse. Because the cell has a lower concentration of solutes, the water will leave the cell.

In effect, the solute is drawing the water out of the cell. This may cause an animal cell to shrivel, or crenate. In an isotonic solution , the extracellular fluid has the same solute concentration as the cell.

If the concentration of solutes of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell. Blood cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances as shown in Figure 8. Various organ systems, particularly the kidneys, work to maintain this homeostasis. Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis.

The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, the cytoplasm in plants is always slightly hypertonic compared to the cellular environment, and water will always enter a cell if water is available.

This influx of water produces turgor pressure, which stiffens the cell walls of the plant Figure 8. In nonwoody plants, turgor pressure supports the plant. If the plant cells become hypertonic, as occurs in drought or if a plant is not watered adequately, water will leave the cell. Plants lose turgor pressure in this condition and wilt. Another mechanism besides diffusion to passively transport materials between compartments is filtration. Unlike diffusion of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that pushes the fluid—and the solutes within it—from a higher pressure area to a lower pressure area.

Filtration is an extremely important process in the body. For example, the circulatory system uses filtration to move plasma and substances across the endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients.

Furthermore, filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream. For all of the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP.

During active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers, and usually against its concentration gradient. One of the most common types of active transport involves proteins that serve as pumps. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients from an area of low concentration to an area of high concentration.

These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes.

An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged at around mV relative to the outside. This process is so important for nerve cells that it accounts for the majority of their ATP usage. Active transport pumps can also work together with other active or passive transport systems to move substances across the membrane.

For example, the sodium-potassium pump maintains a high concentration of sodium ions outside of the cell. Therefore, if the cell needs sodium ions, all it has to do is open a passive sodium channel, as the concentration gradient of the sodium ions will drive them to diffuse into the cell.

In this way, the action of an active transport pump the sodium-potassium pump powers the passive transport of sodium ions by creating a concentration gradient. When active transport powers the transport of another substance in this way, it is called secondary active transport.

Symporters are secondary active transporters that move two substances in the same direction. Because cells store glucose for energy, glucose is typically at a higher concentration inside of the cell than outside. However, due to the action of the sodium-potassium pump, sodium ions will easily diffuse into the cell when the symporter is opened.

The flood of sodium ions through the symporter provides the energy that allows glucose to move through the symporter and into the cell, against its concentration gradient. Conversely, antiporters are secondary active transport systems that transport substances in opposite directions.

Other forms of active transport do not involve membrane carriers. Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested.

Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in.

Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance.

Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes. Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell.

When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis Figure 8.

Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses. To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.

A solution is a homogeneous mixture. The major component is the solvent , while the minor component is the solute. Solutions can have any phase; for example, an alloy is a solid solution. Solutes are soluble or insoluble , meaning they dissolve or do not dissolve in a particular solvent. The terms miscible and immiscible , instead of soluble and insoluble, are used for liquid solutes and solvents.

The statement like dissolves like is a useful guide to predicting whether a solute will dissolve in a given solvent. Dissolving occurs by solvation , the process in which particles of a solvent surround the individual particles of a solute, separating them to make a solution. For water solutions, the word hydration is used. If the solute is molecular, it dissolves into individual molecules.

If the solute is ionic, the individual ions separate from each other, forming a solution that conducts electricity. Such solutions are called electrolytes. If the dissociation of ions is complete, the solution is a strong electrolyte. If the dissociation is only partial, the solution is a weak electrolyte. Solutions of molecules do not conduct electricity and are called nonelectrolytes.

The amount of solute in a solution is represented by the concentration of the solution. The maximum amount of solute that will dissolve in a given amount of solvent is called the solubility of the solute.

Such solutions are saturated. Solutions that have less than the maximum amount are unsaturated. Most solutions are unsaturated, and there are various ways of stating their concentrations. Parts per million ppm and parts per billion ppb are used to describe very small concentrations of a solute.

Molarity , defined as the number of moles of solute per liter of solution, is a common concentration unit in the chemistry laboratory. Equivalents express concentrations in terms of moles of charge on ions. When a solution is diluted, we use the fact that the amount of solute remains constant to be able to determine the volume or concentration of the final diluted solution.

Solutions of known concentration can be prepared either by dissolving a known mass of solute in a solvent and diluting to a desired final volume or by diluting the appropriate volume of a more concentrated solution a stock solution to the desired final volume.

The cell membrane provides a barrier around the cell, separating its internal components from the extracellular environment. The cell membrane is selectively permeable, allowing only a limited number of materials to diffuse through its lipid bilayer. All materials that cross the membrane do so using passive non energy-requiring or active energy-requiring transport processes.

During passive transport, materials move by simple diffusion or by facilitated diffusion through the membrane, down their concentration gradient.

Water passes through the membrane in a diffusion process called osmosis. During active transport, energy is expended to assist material movement across the membrane in a direction against their concentration gradient.

Active transport may take place with the help of protein pumps or through the use of vesicles. What materials can easily diffuse through the lipid bilayer, and why?

Why is receptor-mediated endocytosis said to be more selective than phagocytosis or pinocytosis? What do osmosis, diffusion, filtration, and the movement of ions away from like charge all have in common?

In what way do they differ? Which of the representations best corresponds to a 1 M aqueous solution of each compound? Justify your answers. Which of the representations shown in Problem 1 best corresponds to a 1 M aqueous solution of each compound?

Would you expect a 1. Why or why not? An alternative way to define the concentration of a solution is molality , abbreviated m. Molality is defined as the number of moles of solute in 1 kg of solvent. How is this different from molarity? Would you expect a 1 M solution of sucrose to be more or less concentrated than a 1 m solution of sucrose? Explain your answer. What are the advantages of using solutions for quantitative calculations?

If the amount of a substance required for a reaction is too small to be weighed accurately, the use of a solution of the substance, in which the solute is dispersed in a much larger mass of solvent, allows chemists to measure the quantity of the substance more accurately.

Calculate the number of grams of solute in 1. If all solutions contain the same solute, which solution contains the greater mass of solute? Complete the following table for mL of solution.

What is the concentration of each species present in the following aqueous solutions? What is the molar concentration of each solution? Give the concentration of each reactant in the following equations, assuming An experiment required A stock solution of Na 2 CrO 4 containing Describe how to prepare Calcium hypochlorite [Ca OCl 2 ] is an effective disinfectant for clothing and bedding. If a solution has a Ca OCl 2 concentration of 3.

In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather.

Mammals use layers of fat to achieve the same end. Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body.

Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, warming blood returning to the heart.

This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaption can be shut down in some animals to prevent overheating the internal organs.

The countercurrent adaption is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds.

In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears. Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm.

The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter.

Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat.

Severe cold elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.

The nervous system is important to thermoregulation , as illustrated in Figure The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain.

When bacteria are destroyed by leuckocytes, pyrogens are released into the blood. How might pyrogens cause the body temperature to rise? The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body.

When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood.

These pyrogens circulate to the hypothalamus and reset the thermostat. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. Finally, heat itself may also kill the pathogen.

A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism. Skip to content Chapter Learning Objectives By the end of this section, you will be able to: Define homeostasis Describe the factors affecting homeostasis Discuss positive and negative feedback mechanisms used in homeostasis Describe thermoregulation of endothermic and ectothermic animals. Homeostatic Process. Control of Homeostasis. Negative Feedback Mechanisms.

Positive Feedback Loop. Figure The birth of a human infant is the result of positive feedback. A person feels satiated after eating a large meal. The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the production of new red blood cells, is no longer released from the kidney.

Set Point. Concept in Action. Homeostasis: Thermoregulation. Endotherms and Ectotherms. Heat Conservation and Dissipation. Neural Control of Thermoregulation. The body is able to regulate temperature in response to signals from the nervous system. Exercises Which of the following statements about types of epithelial cells is false?