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Homeostasis

Homeostasis

BIBLIOGRAPHY

The concept of homeostasis is widely used, in physiology and psychology, to identify what seems to be a general attribute of living organisms: the tendency to maintain and restore certain steady states or conditions of the organism. An obvious example is that of body temperature, which in the human tends to fluctuate only in a narrow range about the value 98.6° F. When the temperature rises above the normal range, corrective reflexes (perspiration, reduced metabolism, etc.) go into action to restore the steady state. Persistent deviation may initiate other actions (moving into the shade, plunging into water, etc.). If body temperature drops, other corrective actions are observed.

Many bodily steady states follow this pattern. Blood glucose level, blood pH, and osmotic pressure are examples. The key concepts are: an observable steady state that persists over time with minor changes; thresholds above and below this normal range; a sensory input that reports changes in the steady state; and effector mechanisms for restoring the steady state.

When a deviation goes beyond either the upper or the lower threshold, energy is mobilized to restore the steady state to its optimal value. Physiologists have been concerned mainly with the reflexes triggered by such deviations, but psychologists have emphasized those homeostatic actions that are seen in learned behavior. Man will exert considerable energy to protect optimal states. He may take restorative action (building a fire when cold) or forestalling action (moving south before winter arrives). The simple reflex level and the complex learned response to homeostatic disturbance are often labeled differently: Stagner and Karwoski (1952) called the former “static homeo-stasis” and the latter “dynamic homeostasis”; Cofer and Appley (1964) used the terms “physiological homeostasis” and “behavioral homeostasis.”

The biochemical and reflex defenses function adequately to protect some constancies; if blood osmotic pressure drops too low or vitamin concentration is too high, kidney mechanisms correct the situation. In other cases, the reflex machinery may fail, and learned behavior is activated. There are probably two thresholds in the system, one for reflexive response and another (further from optimum) that initiates voluntary action.

Damaging the biochemical mechanism forces increased reliance on the learned systems of defense. Thyroidectomized rats build nests to protect against heat loss; parathyroidectomy or adren-alectomy leads to increased drinking of solutions containing calcium or sodium. It seems reasonable, therefore, to consider the cerebral cortex as the highest level of a homeostatic protective mechanism.

Heterostasis. The concept of heterostasis must be introduced into any systematic discussion of homeostasis. Action to restore one steady state may disrupt another steady state (perspiration to cool the body upsets the osmotic equilibrium in the blood, triggering thirst). Heterostasis points to a hierarchy of steady states; deviation of an “impor tant” value will initiate action that disrupts one lower in the scale, but the reverse is not true. Thirst dominates over hunger, and the need for oxygen is prepotent over either. Deviations from optimum dominate behavior as a function of their relevance to organismic survival. Oxygen lack can be tolerated for only a few minutes, water lack for a day or two, food lack for many days.

Sensory controls. Homeostasis depends upon a negative feedback loop; disturbance of a steady state initiates sensory cues that trigger responses tending to restore the previous state; and as the sensory input indicates that the optimal value is being approached, the intensity of corrective action diminishes. Each steady state must have built into its control system, in neural or biochemical form, some representation of the tolerable range of values and upper and lower thresholds. However, all such monitoring systems probably report to a central mechanism, giving rise to something like Mor gan’s (1957) “central motive state,” a state of tension that elicits dynamic homeostatic responses. Tension is thus an important intervening variable in homeostatic theory.

Some types of stimuli have inherent equilibrium-disturbing or equilibrium-restoring properties. Sweet tastes not only signal reduction of hunger for infants; they also seem to have general tensionreducing value. Pain disturbs equilibrium and raises tension. Soft, warm contact may be a universal tension reducer for mammals (cf. Harlow 1958). This need not imply a one-to-one correspondence between pleasure or “subjective utility” and homeostatic value in a goal object. For example, the organism may make mistakes (saccharin is sought by the organism because it is misperceived as equilibrium restoring); and learning may cause the organism to value objects that are purely symbolic and not in themselves effective either to disturb or to restore steady states [seeLearning, article onreinforcement].

Imprinting. Imprinting may be a determiner of cue value. When a gosling first hatches, it seems to fixate on the first moving object and endow this with positive valence; the gosling will follow this object and behave as if a steady state were disturbed when the object disappears [seeImprinting].

Adaptation level. Adaptation level modifies the sensory controls in homeostatic cycles. As Cannon (1932) pointed out, the “baseline” (herein referred to as optimal value) may shift as a result of experience. Infants learn to adapt to a two-hour, three-hour, or four-hour feeding schedule and become “hungry” at the proper time. If subjected to radio and television noises, they show distress at first but later may seem to enjoy the sound level (Chodorkoff 1960). Europeans find room temperatures comfortable that Americans experience as chilly and disturbing.

Effector processes. The effector mechanisms set off by homeostatic disturbances vary from simple biochemical buffering of the blood stream, through hormonal modifications and autonomic nervous system reflex changes, to complex and elaborate learned responses. As Bernard (1859) noted, homeostasis in its simplest form involves protection of constancies of body fluids. Life escaped from the sea when techniques evolved for carrying the sea (as cell environment) within the organism. He wrote that internal constancies are basic to the free life—free from the limitations of an oceanic environment, free to live on land and in the air.

The more complex activities described above expand this conception markedly. First, the organism may remove itself from disturbing stimuli (avoid heat, seek shade). Second, the organism may restore the constancy of the external environment (air conditioning, stores of food). There seems to be general agreement that these two kinds of responses, while remote from biochemical secretion or mechanical reflexes, qualify as homeostatic in nature. Third, the organism may seek to protect (or restore) symbolic conditions associated with homeostatic success—bank accounts, institutional systems, and the like. There is less agreement that these phenomena belong under the homeostatic rubric, yet it is difficult to find a logical rule by which to exclude them.

Evaluation of homeostatic theory. Critics have objected to homeostatic theory as being too conservative, as implying that motivation is conceived solely as operating to restore pre-existing conditions. In a very narrow sense this criticism is true: unless the essential steady states are restored to their normal range, the organism dies. (It is also true that most people are conservative unless de prived.) In a broader sense, homeostatic theory says that energy is mobilized to take action that will restore and protect these steady states, but that the action may be novel and inventive. Fire, clothing, and other inventions serve homeostatic uses. The individual, frustrated by inadequate habits, may acquire new ones which will reduce tensions.

The assertion that all energy mobilizations derive from disturbances of steady states does encounter problems. The phenomena of curiosity, manipulative motivation, and sensory deprivation, as well as the sex drive, offer difficulties. Some psychologists have preferred to treat homeostasis as one segment of the topic of motivation. Those who prefer to press for an inclusive view, in which all motivation obeys homeostatic principles, resort to assumptions such as efficient organismic functioning “demanding” a certain level of sensory stimulation, just as it “demands” a certain level of glucose or vitamin B. “Sensory deprivation” may be motivating because the central nervous system has a normal activity level that is disrupted by either an excess or a deficit of external stimulation [seeperception, article onperceptual deprivation; Stimulation drives; see also Hebb 1955; Miller I960].

Aspiration level studies can also be incorporated into this schema. Motivation, in the sense of effort directed toward a specific goal, obeys aspiration principles. An incentive below present attainment levels is ignored; one that is too far above present achievement is perceived as impossible. Effective incentives, therefore, must fall within the “normal range” as a function of adaptation level. Repeated successes lead to a shift upward, failures to a shift downward, in aspiration level. Prior experience determines what will be anticipated [seeAchievement Motivation].

Homeostatic conception of personality. Person alities can be described by the hierarchy of steady states that are defended, by the cues that disturb these states, and by the techniques utilized to protect and restore equilibriums (Stagner 1961).

Preferred steady states. Some individuals value physical comfort; others, a quiet family life; others, a buzz of activity. Variations in adaptation or aspiration level may be involved. Objects that symbolize internal equilibrium, such as the mother, a house, or a bank account, become values protected in themselves. The self-image, the individual’s percept of himself, becomes an object of value. Threats to the self-image (failure, social ridicule, loss of status) provoke blood pressure and glandular changes as well as vigorous action to restore the integrity of the ego.

Sources of disturbance. Some persons manifest phobias and other disturbances that are obviously not realistic. Neurotic aggressions and anxieties fit into this category. Others are disturbed only by realistic threats to physical constancies or to ac quiredvalues.

Methods of restoring equilibriums. The indi vidual may rely chiefly upon overt muscular action to protect and restore valued constancies, or he may utilize symbols and fantasy. The whole range of coping and defensive reactions indicates indi vidual differences in ways of maintaining steady states.

Personality defense mechanisms. Sigmund Freud has often been cited as a forerunner of homeostatic theory, although he never discussed negative feedback loops (Fletcher 1942; Walker 1956). His conception of repression, projection, dreams, rationalization, and other mechanisms as devices for protecting the individual against threatened disturbance, or devices for restoring equilibrium, places him in this category [seeDefense mechanisms].

When anxiety and tension are aroused, infantile ways of restoring equilibrium may be reinstated. Eating, drinking, and sexual contact are primitive devices for reducing tension. Unfortunately, if the steady state that has been disturbed involves a need for other substances, these defenses have only temporary effect; the tension returns promptly. Neurotic behavior, in homeostatic terms, is behavior that diminishes tension without correcting the basic imbalance (Menninger et al. 1963).

One must note that some psychologists take issue vigorously with attempts to conceptualize personality in homeostatic terms. Allport (1955) asserts that man can be understood more by what he is striving for in the future than by what he is seeking to restore of the past.

Homeostasis as a social phenomenon. Cannon (1932) noted that the concept of homeostasis seemed relevant also to social processes. One such implication derives from the process of mutual assistance (Dempsey 1951). There is some evidence (Chodorkoff 1960) that the mother-infant relationship is a true dyadic system, in which each organism can be a cue disturbing the other, and in which the behavior of one may be equilibrium restoring for the other. Most convincing is the fact that social conditions can trigger the same reflexive mechanisms involved in physiological homeostasis (cf. Denenberg et al. 1964; Hoagland 1964). Hoagland, for example, reported that rats reared in overcrowded cages develop glandular disorders and die at earlier ages than less crowded littermates. These animals show distinct evidences of tension and behavioral disturbance. Hoagland also commented that equilibrium develops at a given population density. Below this point, reproduction is usually accelerated; but above the equilibrium value, infertility and early death reduce the population. Thus he is disposed to favor the view that homeostatic functions are true group processes, not merely manifestations of individual disturb ance. Allee et al. (1949) have pointed out that ecological balance often shows all the properties ascribed to homeostatic equilibrium.

To apply the concept of homeostasis to even more complex social phenomena, such as the market economy, one must identify the steady state, the factors disrupting this state, the cues indicating disturbance, and the restorative mechanisms. Equilibrium theory in economics seems to meet these requirements. In a specific economic market, for example, clothing, one finds a tendency for the price to stabilize. An increase in consumer demand will upset the existing steady state; information spreads by way of efforts to purchase; producers in the clothing industry scent possibilities for profit and increase output accordingly. Thus there is a feedback loop analogous to that observed in individuals.

Failures of homeostasis. One criticism of homeostatic theory is that humans often do things that disrupt, rather than restore, equilibrium. To some extent this is merely a function of errors in informational input or in guiding action. A young lion who charges his prey from too great a distance is not thereby proved to be disobeying homeostatic principles; he simply has not learned effective responses.

In some cases homeostasis operates inefficiently at the individual level because of inadequate sensory controls. Man is not endowed with receptors sensitive to gamma radiation, so, without warning, he may die of fall-out. If the species survives, mutation may provide such sense organs.

Group phenomena also show catastrophic effects of poor information input. War between nations has frequently come about because the leaders of one nation did not accurately perceive the situation (for example, in 1939 Hitler thought England would not fight over Poland). Disputes between a union and management can be settled more amicably if information flows freely between the two (Muench 1960).

These illustrations do not prove that the homeostatic model explains group action; no such logical proof exists at this time. Nevertheless, the homeo static model has been a fruitful one in pointing to areas for research and in suggesting genotypic unities that cut across phenotypic discontinuities, just as the theory of gravitation cut across events from atoms to galaxies. Consequently, homeostasis has been a healthy influence in psychology and in the other social sciences.

Ross stagner

[Directly related are the entriesDrives; Motivation. Other relevant material may be found inCyber Netics; Stress; and in the biography ofCannon.]

BIBLIOGRAPHY

An extensive and scholarly discussion of homeostatic theories may he found in Cofer & Appley 1964. The homeostatic conception of personality is supported in Stagner 1961, and the opposing view is well stated in Allport 1955. Applications to social groups are proposed in Cannon 1932 and 1941 and to industry in Stagner 1956.

Allee, W. C. et al. 1949 Principles of Animal Ecology. Philadelphia: Saunders.

Allport, Gordon W. 1955 Becoming: Basic Considerations for a Psychology of Personality. New Haven: Yale Univ. Press.

Bernard, Claude 1859 Leçons sur les propriétés physiologiques et les altérations pathologiques des liquides de Vorganisme. 2 vols. Paris: Bailliére.

Cannon, Walter B. (1932) 1963 The Wisdom of the Body. Rev. & enl. ed. New York: Norton.

Cannon, Walter B. 1941 The Body Physiologic and the Body Politic. Science 93:1–10.

Chodorkoff, Joan R. 1960 Infant Development as a Function of Mother-Child Interaction. Ph.D. dissertation, Wayne State Univ.

Cofer, C N.; and Appley, M. H. 1964 Homeostatic Concepts and Motivation. Pages 302-365 in C N. Cofer and M. H. Appley, Motivation: Theory and Research. New York: Wiley.

Dempsey, Edward W. 1951 Homeostasis. Pages 209-235 in S. S. Stevens (editor), Handbook of Experimental Psychology. New York: Wiley.

Denenberg, W. H.; Hudgens, G. A.; and Zarrow, M. X. 1964 Mice Reared With Rats: Modification of Be havior by Early Experience With Another Species. Science 143:380–381.

Fletcher, John M. 1942 Homeostasis as an Explana tory Principle in Psychology. Psychological Review 49:80–87.

Harlow, Harry F. 1958 The Nature of Love. American Psychologist 13:673–685.

Hebb, Donald O. 1955 Drives and the C.N.S. (Concep tual Nervous System). Psychological Review 62:243–254.

Hoagland, Hudson 1964 Cybernetics of Population Control. Bulletin of the Atomic Scientists 20, no. 2: 2–6.

Maslow, A. H. 1937 The Influence of Familiarization on Preference. Journal of Experimental Psychology 21:162–180.

Menninger, Karl; Mayman, Martin; and Pruyser, Paul 1963 The Vital Balance: The Life Processes in Men tal Health and Illness. New York: Viking.

Miller, James G. 1960 Information Input Overload and Psychopathology. American Journal of Psychiatry 116:695–704.

Morgan, Clifford T. 1957 Physiological Mechanisms of Motivation. Volume 5, pages 1-35 in Nebraska Symposium on Motivation. Edited by Marshall R. Jones. Lincoln: Univ. of Nebraska Press.

Muench, George A. 1960 A Clinical Psychologist’s Treatment of Labor-Management Conflicts. Personnel Psychology 13.165–172.

Southwick, Charles H. 1964 Peromyscus Leucopus: An Interesting Subject for Studies of Socially Induced Stress Responses. Science 143:55–56.

Stagner, Ross 1956 Psychology of Industrial Conflict. New York: Wiley. → See especially pages 89-116, “Motivation: Principles,” and pages 117-154, “Motivation: Some Applications.”

Stagner, Ross 1961 The Nature of Personality Structure. Pages 69-86 in Ross Stagner, Psychology of Personality. 3d ed. New York: McGraw-Hill.

Stagner, Ross; and Karwoski, T. F. 1952 Psychology. New York: McGraw-Hill.

Walker, Nigel 1956 Freud and Homeostasis. British Journal for the Philosophy of Science 7:61–72.

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homeostasis

homeostasis (Greek: staying the same) is a fundamental idea in our understanding of the workings of the body. The concept had its origin in the 1870s, when the French physiologist Claude Bernard showed that, although the concentration of sugar in the blood could be raised or lowered by a number of processes, the net effect of these processes was to keep the concentration of sugar within certain limits. Bernard extended the idea to other constituents of blood — for which he had less evidence — and in a timeless phrase referred to the constancy of the internal environment (‘le milieu intérieur’): ‘La fixité du milieu intérieur est la condition de la vie libre, independante.’

Bernard contrasted this constancy with that of the changeable world that surrounded the animal (‘le milieu extérieur’). He likened the protective function of the internal milieu to that of a greenhouse, though to us this may seem rather an odd analogy. The constitution of the internal milieu (extracellular fluids, including blood and lymph) has been suggested to represent some primal sea in which vertebrates have evolved. It is a likeable hypothesis, but one which is rather difficult to test.

Bernard's proposal attracted little contemporary attention, which was hardly surprising, for it was about 50 years ahead of its time. But during the period 1915–35 two American physiologists, W. B. Cannon (1871–1945) and L. J. Henderson (1878–1942), revived it. Cannon was particularly concerned with demonstrating the importance of the autonomic nervous system in maintaining the constancy of the milieu intérieur: he realized that the constancy of blood pressure was an essential part of the maintenance. It was Cannon who actually coined the word ‘homeostasis’, and in his Wisdom of the body (1932) he described how several of the body's systems were involved in homeostatic mechanisms.

Cannon's fellow professor at Harvard, L. J. Henderson, analysed the way in which the body maintained the hydrogen ion concentration of body fluids (usually expressed as pH) within narrow limits. There is a short-term pH homeostasis which is a property of blood itself: a bicarbonate-buffering system. If this is not adequate, the kidneys cope with any larger deviation. Henderson published his findings in a classic work, Blood: a study in general physiology (1928). The kidneys are, incidentally, the homeostatic organs par excellence: every renal activity is involved in maintaining the internal milieu, whether it is the concentration of ions in blood, blood volume, blood pressure itself, or the excretion of alien substances.

How do the body's systems actually maintain the constancy? The most conspicuous mechanism is generally known as ‘negative feedback’, illustrated below.

As an example, blood glucose concentration could be the ‘regulated variable’ in the diagram. The control system for the variable is the hormone insulin, whose main action is to accelerate the entry of glucose into many of the cells of the body, thereby lowering its plasma concentration. Insulin is released from cells in the Islets of Langerhans of the pancreas (the controller), the most important stimulus for its release being a rise in blood glucose concentration, as occurs after a meal (‘disturbance’ in the diagram). The reason for this being a ‘negative’ feedback system is that the action of insulin, by lowering the blood sugar, tends to remove the stimulus for its own release. Negative feedback is a ubiquitious principle in engineering and electronics.

It is clear from this example that the mechanism does not keep glucose concentration (the regulated variable) at a fixed level. The level oscillates, because there are delays in both arms of the system — it takes a finite time for insulin to lower blood glucose concentrations, and also for elevated glucose concentrations to increase the production of insulin from the pancreas.

Another regulated variable is carbon dioxide. The control of a constant partial pressure of carbon dioxide (PCO2) in blood is a very precise feedback loop, and its control system is the act of breathing. The body produces the gas constantly, adding it to blood. The CO2 sensor in this system consists of neurons in the medullary respiratory centre of the brain; the control system consists of motor nerves passing from the brain to the diaphragm and intercostal muscles. These nerves stimulate the act of breathing, which transfers carbon dioxide from blood into the lungs, lowers the blood PCO2, and temporarily removes the stimulus to the medullary respiratory centre. Because the body is still producing carbon dioxide, the blood PCO2 begins to rise again, the medullary receptors are stimulated, and the cycle repeats itself. A CO2-sensitive electrode inserted into an artery shows small, regular oscillations whose frequency corresponds precisely to the act of breathing.

The speed of response of the carbon dioxide loop is far greater than that of the glucose loop, a difference that derives from nervous compared with hormonal mechanisms: the PCO2 varies by only about 10% around its average level, whereas glucose varies by about 40%. The concentration ranges of some other constituents of blood provide us with clues about the nature of the relevant homeostatic mechanisms. Sodium ions (135–145 mmol/litre) and chloride ions (95–105 mmol/litre) have narrow ranges; this is the result of a mixture of nervous and hormonal mechanisms; the range is wider for potassium (3.5–5.0 mmol/litre) which is adjusted by hormonal action in the kidneys. By contrast, the hormones that provide the control systems regulating these variables show far wider concentration ranges in blood, according to the changes in secretion rates stimulated by disturbances in the variable they control. Thus ACTH (adrenocorticotrophic hormone) has a range of 3.3–15.4 pmol/litre, aldosterone 100–500 pmol/litre, and insulin 0–15 mUnits/ml (unfed) and 15–100 Units/ml (after food).

Homeostasis can itself be reset or entrained by higher nervous centres. The diurnal variations shown by ACTH and cortisol demonstrate high concentrations between midnight and midday (cortisol concentration 280–700 mmol/litre) and midday and midnight (cortisol 140–280 mmol/litre). Similarly, on a longer time-scale, the changes seen in the female reproductive cycle represent a 28-day cycle of entrainment. On a longer time-scale still, the growth and development of the child must represent the ultimate homeostatic entrainment by the brain. We might envisage old age as representing a genetically programmed deterioration of homeostasis.

Claude Bernard's intuition about ‘le milieu intérieur’ has come a very long way in a century. The mechanisms of homeostasis are so ubiquitous, their patterns so subtly intertwined, that we are tempted to produce a teleological question, and ask why. What is so useful to the organism about this precision? We do not have to look far, because the workings of every cell in the body depend on the maintenance of a negative potential inside the cell. In turn, this negative potential depends upon the relative concentrations of ions inside and outside the cell: a high sodium concentration in the extracellular fluid, and a high potassium concentration inside the cell, the gradients across the cell wall being maintained by ionic pumps within the cell membrane. But these pumps could not begin to control this gradient if the ionic concentrations in blood (extracellular fluid) were not kept within narrow limits in the first place. The subject comes into sharp focus when we consider the situation in the heart, which is very dependent on a constant plasma potassium level, within the range of 3.5–5.0 mmol/litre. The elevation of this value by 1–2 mmol/litre constitutes a medical emergency: the excitable components of the heart begin to conduct nervous impulses spontaneously and, without treatment, death soon follows from uncoordinated contraction of different parts of the ventricles (ventricular fibrillation).

It soon becomes clear that the body's function involves countless homeostatic mechanisms, both within and outside cells. Not only are the mechanisms ubiquitous, but careful analysis often shows two or more feedback loops apparently serving the same function; a good example is the elaborate relationship that exists between the control of blood pressure and plasma volume. Perhaps the apparent redundancy provides the organism with back-up systems that improve evolutionary survival value. Improvement or not, such duplication makes the understanding of disease processes very much more difficult to disentangle.

J. R. Henderson


See also acid–base homeostasis; body fluids; hormones.

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Homeostasis

Homeostasis

Living cells can function only within a narrow range of such conditions as temperature, pH , ion concentrations, and nutrient availability, yet living organisms must survive in an environment where these and other conditions vary from hour to hour, day to day, and season to season. Organisms therefore require mechanisms for maintaining internal stability in spite of environmental change. American physiologist Walter Cannon (18711945) named this ability homeostasis (homeo means "the same" and stasis means "standing or staying"). Homeostasis has become one of the most important concepts of physiology, physiological ecology, and medicine. Most bodily functions are aimed at maintaining homeostasis, and an inability to maintain it leads to disease and often death.

The human body, for example, maintains blood pH within the very narrow range of 7.35 to 7.45. A pH below this range is called acidosis and a pH above this range is alkalosis. Either condition can be life-threatening. One can live only a few hours with a blood pH below 7.0 or above 7.7, and a pH below 6.8 or above 8.0 is quickly fatal. Yet the body's metabolism constantly produces a variety of acidic waste products that challenge its ability to maintain pH in a safe range.

Body temperature also requires careful homeostatic control. On a spring or fall day in a temperate climate, the outdoor Fahrenheit temperature may range from the thirties or forties at night to the eighties in the afternoon (a range of perhaps 4 to 27 degrees Celsius). In spite of this environmental fluctuation, our core body temperature is normally 37.2 to 37.6 degrees Celsius (99.0 to 99.7 degrees Fahrenheit) and fluctuates by only 1 degree or so over the course of 24 hours. Indeed, if core body temperatures goes below 33 degrees Celsius (91 degrees Fahrenheit) a person is likely to die of hypothermia , and if it goes above 42 degrees Celsius (108 degrees Fahrenheit), death from hyperthermia is likely.

Internal conditions are not absolutely stable but fluctuate within a narrow range around an average called the set point. The set point for core body temperature, for example, is about 37.4 degrees Celsius, but the temperature fluctuates within about (±0.5 degrees Celsius. Thus, it is more accurate to say the body maintains an internal dynamic equilibrium than to say it maintains absolute stability.

Negative Feedback and Stability

The usual means of maintaining homeostasis is a general mechanism called a negative feedback loop. The body senses an internal change and activates mechanisms that reverse, or negate, that change.

An example of negative feedback is body temperature regulation. If blood temperature rises too high, this is sensed by specialized neurons in the hypothalamus of the brain. They signal other nerve centers, which in turn send signals to the blood vessels of the skin. As these blood vessels dilate, more blood flows close to the body surface and excess heat radiates from the body. If this is not enough to cool the body back to its set point, the brain activates sweating. Evaporation of sweat from the skin has a strong cooling effect, as we feel when we are sweaty and stand in front of a fan.

If the blood temperature falls too low, on the other hand, this is also sensed by the hypothalamus and signals are sent to the cutaneous arteries (those supplying the skin) to constrict them. Warm blood is then retained deeper in the body and less heat is lost from the surface. If this is inadequate, then the brain activates shivering. Each muscle tremor in shivering releases heat energy and helps warm the body back toward its 37 degrees Celsius set point.

In both cases, specialized neurons sense the abnormal body temperature and activate corrective negative feedback loops that return the temperature to normal. As a result, body temperature seldom goes more than 0.5 degrees Celsius above or below its set point. Other negative feedback loops regulate blood sugar concentration, water balance, pH, and countless other variables. Many such loops are regulated by the nervous system, and others by the hormones of the endocrine system.

Positive Feedback and Rapid Change

The counterpart to negative feedback is the positive feedback loop, a process in which the body senses a change and activates mechanisms that accelerate or increase that change. This can also aid homeostasis, but in many cases it produces the opposite effect and can be life-threatening.

An example of its beneficial effect is seen in blood clotting. Part of the complex biochemical pathway of clotting is the production of an enzyme that forms the matrix of the blood clot, but also speeds up the production of still more thrombin. That is, it has a self-catalytic , self-accelerating effect, so that once the clotting process begins, it runs faster and faster until, ideally, bleeding stops. Thus, this positive feedback loop is part of a larger negative feedback loop, one that is activated by bleeding and ultimately works to stop the bleeding.

Another example of beneficial positive feedback is seen in childbirth, where stretching of the uterus triggers the secretion of a hormone , oxytocin, which stimulates uterine contractions and speeds up labor. Yet another is seen in protein digestion, where the presence of partially digested protein in the stomach triggers the secretion of hydrochloric acid and pepsin, the enzyme that digests protein. Thus, once digestion begins, it becomes a self-accelerating process.

Often, however, positive feedback produces the very opposite of homeostasis: rapid loss of internal stability with potentially fatal consequences. For example, if the death of a small area of heart tissue triggers a heart attack (myocardial infarction), the heart pumps an inadequate amount of blood. Thus, the heart muscle itself is deprived of blood flow, and still more begins to die. This can lead to a rapid worsening of cardiac function until a person dies. Many diseases involve dangerous positive feedback loops.

Homeostasis, while described here with examples from human physiology, is a fundamental property of life and a necessity for survival of all living thingsnot just humans but all other animals as well as bacteria, plants, fungi, and protists. It enables all living organisms to maintain internal stability in spite of a ceaselessly changing and challenging environment.

see also Blood Clotting; Blood Sugar Regulation; Brain; Endocrine System; Hormones; Hypothalamus; Nervous Systems; Osmoregulation; Physiological Ecology; Temperature Regulation

Kenneth S. Saladin

Bibliography

Blessing, William W. The Lower Brainstem and Bodily Homeostasis. New York: Oxford University Press, 1997.

Cannon, Walter B. The Wisdom of the Body. New York: W. W. Norton, 1932.

Saladin, Kenneth S. Anatomy and PhysiologyThe Unity of Form and Function, 2nd ed. Dubuque, IA: McGraw-Hill Higher Education, 2001.

Willmer, P., G. Stone, and I. Johnson. Environmental Physiology of Animals. Oxford: Blackwell Science, 2000.

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Homeostasis

Homeostasis

In 1865 French physiologist Claude Bernard pointed out that in order for an organism to survive, a constant, or stable, internal environment was required. Based on this insight, and more than half a century later, in 1929, a physiologist from Harvard University, Walter B. Cannon, coined the term "homeostasis." Homeostasis means a stable state of the internal environment that is maintained by regulatory processes despite changes that may occur in the external environment.

Like the first animals to evolve, simple animals such as sponges and jellyfish have a body wall that is only a few cell layers thick, and each cell has direct access to the external environment. As a result, cells can take up nutrients and dispose of wastes by direct exchange with the external environment, but they are also directly exposed to fluctuations in that external environment. Such fluctuations (e.g., in ion concentrations or temperature), will affect the ability of the cells to perform the chemical processes necessary for survival, and therefore restrict the occurrence of these simple animals to more favorable environments.

When animals evolved that had a bulkier shape (like earthworms, fish, or humans today), only some of their cells were still in contact with the external environment. This required specializations such as a digestive system to bring food inside the body for digestion, and a circulatory system to disperse the nutrients to all the cells. The big advantage was that these internal cells were no longer directly exposed to the external environment but were surrounded by extracellular fluid creating an internal environment. Living cells can thrive in certain kinds of conditions and not in others. Various homeostatic control mechanisms regulating the internal environment can now cooperate to maintain the optimal conditions, independent from the external environment. This allows these animals to thrive in areas with less favorable external conditions. Homeostatic regulation of the extracellular fluid can include ion composition, pH levels, oxygen and carbon dioxide levels, nutrient and waste product levels, as well as temperature.

When studying the physiology (structure and function) of animals, scientists are often concerned with the question of how the animals maintain homeostasis. Homeostatic control relies on negative feedback. This means that any deviation from the desired state of the internal environment will be reduced (hence negative) as a result of homeostatic control and bring the internal environment back to the desired state (e.g., regulation of body temperature).

This is in contrast to positive feedback that would enhance (hence positive) the difference leading to an escalation (e.g., child birth). Positive feedback is not useful for the homeostatic control of the internal environment.

There are three main structures that are part of all homeostatic control systems (see homeostatic feedback diagram): (1) sensors that sense the actual state of the condition that is being controlled. This information is passed on to the (2) control center, which compares the information from the sensors with the stored information, or set point, about what the condition should be. When the information from the sensors differs from the stored set point, the control center will send an error signal to (3) effectors to trigger an adequate response that will lead to a change in the controlled condition and bring it closer to the set point. When a difference between the controlled condition and the set point is no longer registered by the control center, the error signal ceases, and homeostasis is maintained until another disturbance causes a change in the controlled condition.

The nervous system and the endocrine system play a major role in homeostatic control by relaying the signals from the sensors to the control center and from the control center to the effectors.

An important example of homeostatic control is temperature regulation. Cellular processes are temperature dependent. For example, protein enzymes that catalyze the chemical reactions in cells have a preferred temperature range at which they perform optimally. Lower temperatures will slow them down, higher temperature may destroy them. Mammals and birds are two animal groups that have evolved the ability to maintain a stable body temperature independent of the environmental conditions (endothermy). This allows them to be active in environments over a wide temperature range. Humans normally regulate their body core temperature in a relatively narrow range (between 36 and 39°C [96.8° and 102.2°F]). To maintain this temperature, heat production has to balance heat loss. Homeostatic control of body temperature ensures that heat production and heat loss are approximately equal.

For example, when humans exercise, the muscle activity generates a lot of heat as a byproduct of muscle contraction. This additional heat (disturbance) will increase the body core temperature (controlled condition). The temperature is monitored by temperature sensors, and this information is passed on by neurons to a specific brain region, the hypothalamus (the temperature control center). If the core temperature exceeds the desired temperature (set point), an error signal is generated by the control center. This error signal in the form of action potentials (electric signals that travel along neurons) triggers sweat glands (effectors) to secrete sweat over the body surface (response). The evaporation of sweat leads to cooling off of the body surface. When heat loss through evaporation balances heat gain through exercise, a stable core temperature (and temperature homeostasis) will be maintained.

When humans experience severe heat loss (disturbance), the body temperature (controlled condition) monitored by the sensors drops below the desired temperature (set point). As a result, the hypothalamus (control center) sends action potentials (error signal) to skeletal muscles (effectors) to trigger muscle contractions and cause shivering (response). As a byproduct of this muscle activity heat is produced. When heat gain through shivering balances heat loss a stable core temperate (and temperature homeostasis) will be maintained.

see also Allometry; Cells; Functional Morphology.

Katrin F. Stanger-Hall

Bibliography

Campbell, Neil A. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman Inc.,1999.

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homeostasis

homeostasis The regulation by an organism of the chemical composition of its body fluids and other aspects of its internal environment so that physiological processes can proceed at optimum rates. It involves monitoring changes in the external and internal environment by means of receptors and adjusting the composition of the body fluids accordingly; excretion and osmoregulation are important in this process. Examples of homeostatic regulation are the maintenance of the acid–base balance and body temperature (see homoiothermy; poikilothermy).

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homeostasis

homeostasis In biology, processes that maintain constant conditions within a cell or organism in response to either internal or external changes. An example is the regulation of body temperature by the skin and liver.

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homeostasis

homeostasis (hoh-mi-oh-stay-sis) n. the physiological process by which the internal systems of the body are maintained at equilibrium, despite variations in the external conditions.
homeostatic adj.

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homeostasis

homeostasis See homoeostasis.

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homeostasis

homeostasis See HOMOEOSTASIS.

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homeostasis

homeostasis See HOMOEOSTASIS.

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