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.
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.
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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.
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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.
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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.
In the field of demography, the term homeostasis refers to the way in which populations control their long-term growth. Drawing on concepts of self-regulating systems in biology, demographic homeostasis usually is regarded as a phenomenon in which interrelated social, cultural, and economic institutions ensure that the long-term rate of population growth is close to zero. During the demographic transition of the last 200 years, there has been a huge expansion in the human population. In contrast, the centuries that preceded the transition were characterized by much more modest population growth. Thus, the study of homeostasis focuses primarily on the pre-transitional era. However, with the end of the demographic transition in sight during the twenty-first century, it is likely that homeostasis will become a topic of research interest in the future.
For almost two centuries those trying to explain the nature of equilibrating mechanisms in populations have looked to the economist T. R. Malthus (1766–1834) for inspiration. Malthus proposed that competition between individuals is the basis of the equilibrating mechanism and that population balance can be arrived at through two possible means: the positive check of worsening mortality and the preventive check of reduced fertility.
The first edition of Malthus's Essay (1798), written in the style of a polemical pamphlet, with a limited exposition of theory and little supporting evidence, expressed this contrast with particular starkness and emphasized the positive check of mortality. Later, better-documented editions tended to stress the role of the preventive check through the mechanism of delayed marriage. In either case the model remained austere, with total population size being the key regulating element. Thus, the Malthusian model often is referred to as being based on density dependence. Because it includes mechanisms by which countervailing forces work to keep a population in balance with its resource base, it sometimes is characterized as showing negative feedback. Both density dependence and negative feedback are features of population control systems that can be described as homeostatic. In fact, these terms are used so interchangeably in the context of population dynamics that they can be regarded as near synonyms.
There are two forms of evidence that suggest the existence of homeostatic mechanisms in human populations: the fact that observed growth has been near zero over most of human history and detailed assessments of the observed strength of homeostatic forces.
Although the nineteenth and twentieth centuries saw a remarkable growth in human numbers (especially since around 1950), population growth over the greater part of human history must have been very close to zero. In light of the long duration of human prehistory, growth must have been close to zero during the Paleolithic era. Even the expansion of human numbers that took place with the development of agriculture, starting about 10,000 years ago, involved only marginally higher growth rates. As Joel E. Cohen puts it, "Before the several local inventions of agriculture, local human populations grew at long-term rates just above zero. Where agriculture was invented, local human populations grew ever so slightly faster" (Cohen, p. 32). However, events that occurred during this period cannot be quantitatively documented.
Although it most likely never will be possible to make reliable estimates of world population size before the last 200 years or so, there are some populations in which the ratio of facts to deductive reasoning is sufficiently favorable to make tolerably robust estimates of population growth for the last 2,000 years. The best documented of these cases are Europe and China, both of which show periods of growth, decline, and stagnation at various times during the last two millennia. Until the relatively recent past, however, even episodes of growth took place at modest rates compared with modern experience, and the growth rate over the very long run was extremely low. For example, the average annual growth rate in China from 2 to 1500 c.e. was in the neighborhood of 0.03 percent.
That long-term stability partly masks shorter-term fluctuations. In addition to explaining the factors leading to very slow growth, models of population change must consider the fluctuations. However, it is not easy to distinguish the cultural, economic, political, and environmental causes of such swings. Analyses of fluctuations in both European and Chinese history now generally emphasize nondemographic factors such as the degree of political stability and climatic change. In many cases periods of growth appear to be associated with eras of political stability, whereas times of generalized social unrest saw population declines. Possibly the most sophisticated analysis of this phenomenon is found in C. Y. Cyrus Chu and Ronald D. Lee (1994), who used data for China. Similarly, in Europe the demise of the Western Roman Empire appears to have been associated with a long-term decline in population size.
In contrast to the slow growth of settled populations, the potential for rapid growth clearly exists for all populations but seems to be realized only in some circumstances. In particular, rapid growth in pretransitional populations appears to be restricted to places and eras where a population was able to settle new territory that previously was either not settled or thinly settled. Such expansions often were associated with the spread of alternative forms of farming or other technological innovations that allowed more effective exploitation of natural resources. A variation on this situation occurred in areas where catastrophic mortality from diseases introduced by new settlers devastated the indigenous population, effectively rendering the land thinly settled (Crosby 1986).
Tests of Homeostasis
Malthusian ideas and terminology are so widely employed in describing both long-and short-term population movements that it is common to find them used routinely in both historical and contemporary discussions of economic-demographic relations. However, the strength of these simple density-dependent mechanisms rarely has been tested for human populations. With European historical data, researchers are in a position to do this. Using European data, Lee (1987) has concluded that only a very small proportion (0.5 percent) of year-to-year changes in settled agricultural populations can be attributed to simple density-dependent mechanisms.
On a short-term basis this means that homeostasis is overwhelmed by a multitude of other factors. However, as Lee goes on to note, "It is essential to realise that as long as there is any trace at all of density dependence, no matter how weak, this tug, by its systematic persistence, comes to dominate human population dynamics over the long run, if not the short" (Lee 1987, p. 452). Thus, homeostatic mechanisms can be seen to operate in a long-term perspective, but they do this through such complex and indirect pathways that their effects are difficult to detect.
The potential for population growth must have been kept in check by powerful forces at most times. One potential force is, of course, high mortality and there is certainly evidence of spectacular mortality crises in the historical record. The return of bubonic plague to Europe in the middle of the fourteenth century, for example, is thought to have killed around a third of that continent's population. Even more dramatically, the arrival of Europeans in the Americas, Australia, and the Pacific islands led to the demographic collapse of indigenous populations. Such disasters may be attributed largely to the introduction of new diseases and the occurrence of so-called virgin-soil epidemics (Crosby 1986). However, they also may indicate the effects of the collapse of one form of social order and its replacement with another and even deliberate genocide.
In spite of the dramatic nature of such crises and their undoubted importance in the historical demography of the Americas, Australia, and the Pacific islands, it is not clear how significant events of this kind have been in regard to long-term demographic change in general. Historical evidence from Europe and Asia mostly indicates less extreme mortality and much less extreme fluctuations in population size, suggesting that control over fertility and opportunities for migration were more significant than was catastrophic mortality (Liu et al. 2001, Livi-Bacci 1993). The most persuasive interpretation of the data on long-term population growth, therefore, would seem to be that human societies developed regulatory mechanisms that kept long-run population growth rates close to zero.
The best-documented examples of these mechanisms come from Western Europe and East Asia. Although there was considerable local and regional variation in the nature of pretransitional demo-graphic regimes, the overall range of experience seems broadly similar in the Asian and European populations that have been studied to date. Total fertility generally was in the range of four to six children (i.e., well below any theoretical maximum), whereas life expectancy at birth ranged between 20 and 40 years.
Although the overall ranges of mortality and fertility in Asian and European populations had much in common, that cannot be said for their social, economic, and cultural contexts. Different patterns of social behavior, as well as differences in ecological factors, disease environments, and political stability, shaped the demographic equilibriums, and both fertility and mortality were subject to different sets of determinants. For example, Asian marriage patterns generally bore no relation to the so-called West European model, in which both men and women married late and many people remained single. Similarly, deliberate infanticide, which was almost unknown in Europe, played a visible role in some parts of Asia. Patterns of sex-specific mortality also differed markedly between Western Europe and parts of Asia.
In all cases, however, it is clear that extensive systems of demographic and economic interaction and elaborate social conventions shaped the process of reproduction. Moreover, in addition to social norms, individuals and couples thought about and took action that affected their childbearing. This seems to be especially evident in China, where there is a long history of collective intervention in demographic matters of various kinds, including fertility. Thus, Zhongwei Zhao notes, "As early as the Tang (618 to 907 AD) and Song (960 to 1279) periods, ideas and practice of limiting family size were already discussed and recorded in the works of some scholars" (Zhao, p. 214).
Consideration of these matters inevitably raises the question of how the institutions that underpin homeostasis arise and are sustained. Within demography there are two main schools of thought. One, which is found in the work of E. A. Wrigley (1978), stresses the role of unconscious rationality in generating homeostatic patterns. In this way of thinking, just as the "invisible hand" of classical economics guides markets even when individuals are unaware of its presence, analogous forces steer social arrangements toward homeostatic equilibriums. The second school, most succinctly presented by Ron Lesthaeghe (1980), gives greater emphasis to conscious "short-term goal setting" on the part of the elite groups that set the prevailing moral guidelines and controlled the economic bases of society. According to this line of reasoning, these institutions are best seen as instruments of social control, and the equilibrating mechanisms also are viewed as methods of social differentiation.
An approach to the question of origins that links individuals and populations and has been influential in anthropology in recent years is the Darwinian evolutionary perspective, as seen in the work of Laura Betzig and her colleagues (1988). This approach draws on modern evolutionary ideas of "inclusive fitness" and "kin-selection" to investigate the role of evolutionary forces in determining human fertility patterns. A problem with a Darwinian approach is the manifestly Lamarckian nature of much cultural transmission, in which acquired characteristics are transmitted just as easily as inherited characteristics are. An unanswered question is how social institutions that favored low growth or homeostasis could have evolved in the absence of "group selection." As James W. Wood states, "It is very unlikely that special behavioral and institutional mechanisms have evolved in order to restrain population growth or regulate population size. But that does not mean that factors do not exist that have that effect, even if it is not the reason for their existence"(Wood, p. 101).
In contrast to Malthus's ideas of negative density dependence, Ester Boserup (1965, 1981) proposed that increasing population density stimulated technological progress in pre-transitional populations, leading to positive density dependence. Her ideas have been synthesized with those of Malthus by Lee (1986), who shows that both can be accommodated within a wider model of the interaction between population and economic growth. Lee's ideas have been developed by Wood (1998), who generalizes the "Malthus and Boserup model" to deal with a broad definition of well-being and explicitly considers the question of variance and population heterogeneity.
Boserup, Ester. 1965. The Conditions for Agricultural Growth: The Economics of Agrarian Change under Population Pressure. Chicago: Aldine.
——. 1981. Population and Technological Change: A Study of Long-Term Trends. Chicago: University of Chicago Press.
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Cohen, Joel E. 1995. How Many People Can the Earth Support? New York and London: Norton.
Crosby, Alfred W. 1986. Ecological Imperialism: The Biological Expansion of Europe, 900–1900. Cambridge, Eng., and New York: Cambridge University Press.
Galloway, Patrick R. 1986. "Long Term Fluctuations in Climate and Population in the Pre-Industrial Era." Population and Development Review 12:1–24.
Lee, Ronald D. 1986. "Malthus and Boserup: A Dynamic Synthesis." In The State of Population Theory: Forward from Malthus, ed. David Coleman and Roger S. Schofield. Oxford and New York: Blackwell.
——. 1987. "Population Dynamics of Humans and Other Animals." Demography 24: 443–465.
Lesthaeghe, Ron. 1980. "On the Social Control of Human Reproduction." Population and Development Review 6: 527–548.
Liu, Ts'ui-jung, et al., eds. 2001. Asian Population History. Oxford and New York: Oxford University Press.
Livi-Bacci, Massimo. 1993. A Concise History of World Population. Oxford and New York: Blackwell.
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Wood, James W. 1998. "A Theory of Preindustrial Population Dynamics: Demography, Economy and Well-Being in Malthusian Systems." Current Anthropology 39: 99–135.
Wrigley, E. A. 1978. "Fertility Strategy for the Individual and the Group." In Historical Studies of Changing Fertility, ed. Charles Tilly. Princeton, NJ: Princeton University Press.
Zhao, Zhongwei. 1997. "Demographic Systems in Historic China: Some New Findings from Recent Research." Journal of the Australian Population Association 14: 201–232.
Humans, all other organisms, and even ecological systems, live in an environment of constant change. The persistently shifting, modulating, and changing milieu would not permit survival, if it were not for the capacity of biological systems to respond to this constant flux by maintaining a relatively stable internal environment. An example taken from mammalian biology is temperature which appears to be "fixed " at approximately 98.6°F (37°C). While humans can be exposed to extreme summer heat, and arctic mammals survive intense cold, body temperature remains constant within vary narrow limits. Homeostasis is the sum total of all the biological responses that provide internal equilibrium and assure the maintenance of conditions for survival.
The human species has a greater variety of living conditions than any other organism. The ability of humans to live and reproduce in such diverse circumstances is due to a combination of homeostatic mechanisms coupled with cultural (behavioral) responses.
The scientific concept of homeostasis emerged from two scientists: Claude Bernard, a French physiologist, and Walter Bradford Cannon, an American physician. Bernard contrasted the external environment which surrounds an organism and the internal environment of that organism. He was, of course, aware that the external environment fluctuated considerably in contrast to the internal environment which remained remarkably constant. He is credited with the enunciation of the constancy of the internal environment ("La fixité du milieu intérieur...") in 1859. Bernard believed that the survival of an organism depended upon this constancy, and he observed it not only in temperature control but in the regulation of all of the systems that he studied. The concept of the stable "milieu intérieur " has been accepted and extended to the many organ systems of all higher vertebrates. This precise control of the internal environment is effected through hormones, the autonomic nervous system, endocrines, etc.
The term "homeostasis," derived from the Greek homoios meaning similar and stasis meaning to stand, suggests an internal environment which remains relatively similar or the same through time. The term was devised by Cannon in 1929 and used many times subsequently. Cannon noted that, in addition to temperature, there were complex controls involving many organ systems that maintained the internal stability within narrow limits. When those limited are exceeded, there is a reaction in the opposite direction that brings the condition back to normal, and the reactions returning the system to normal is referred to as negative feedback. Both Bernard and Cannon were concerned with human physiology. Nevertheless, the concept of homeostasis is applied to all levels of biological organization from the molecular level to ecological systems, including the entire biosphere . Engineers design self-controlling machines known as servomechanisms with feedback control by means of a sensing device, an amplifier which controls a servomotor which in turn runs the operation of the device. Examples of such devices are the thermostats which control furnace heat in a home or the more complicated automatic pilots of aircraft. While the human-made servomechanisms have similarities to biological homeostasis, they are not considered here.
As indicated above, temperature is closely regulated in humans and other homeotherms (birds and mammals). The human skin has thermal receptors sensitive to heat or cold. If cold is encountered, the receptors notify an area of the brain known as the hypothalamus via a nerve impulse. The hypothalamus has both a heat-promoting center and a heat-losing center, and, with cold, it is the former which is stimulated. Thyroid-releasing hormone, produced in the hypothalamus, causes the anterior pituitary to release thyroid stimulating hormone which, in turn, causes the thyroid gland to increase production of thyroxine which results in increased metabolism and therefore heat. Sympathetic nerves from the hypothalamus stimulate the adrenal medulla to secrete epinephrine and norepinephrine into the blood which also increases body metabolism and heat. Increased muscle activity will generate heat and that activity can be either voluntary (stamping the feet for instance) or involuntary (shivering). Since heat is dissipated via body surface blood vessels, the nervous system causes surface vasoconstriction to decrease that heat loss. Further, the small quantity of blood that does reach the surface of the body, where it is chilled, is reheated by countercurrent heat exchange resulting from blood vessels containing cold blood from the limbs running adjacent to blood vessels from the body core which contain warm blood. The chilled blood is prewarmed prior to returning to the body core. A little noted response to chilling is the voluntary reaching for a jacket or coat to minimize heat loss.
The body responds with opposite results when excessive heat is encountered. The individual tends to shed unnecessary clothing, and activity is reduced to minimize metabolism. Vasodilation of superficial blood vessels allows for radiation of heat. Sweat is produced, which by evaporation reduces body heat. It is clear that the maintenance of body temperature is closely controlled by a complex of homeostasis mechanisms.
Each step in temperature regulation is controlled by negative feedback. As indicated above, with exposure to cold the hypothalamus, through a series of steps, induces the synthesis and release of thyroxine by the thyroid gland. What was not indicated above was the fact that elevated levels of thyroxine control the level of activity of the thyroid by negative feedback inhibition of thyroid stimulating hormone. An appropriate level of thyroid hormone is thus maintained. In contrast, with inadequate thyroxine, more thyroid stimulating hormone is produced. Negative feedback controls assure that any particular step in homeostasis does not deviate too much from the normal.
Historically, biologists have been particularly impressed with mammalian and human homeostasis. Lower vertebrates have received less attention. However, while internal physiology may vary more in a frog than in a human, there are mechanisms which assure the survival of frogs . For instance, when the ambient temperature drops significantly in the autumn in northern latitudes, leopard frogs move into lakes or rivers which do not freeze. Moving into lakes and rivers is a behavioral response to a change in the external environment which results in internal temperature stability. The metabolism and structure of the frog is inadequate to protect the frog from freezing, but the specific heat of the water is such that freezing does not occur except at the surface of the overwintering lake or river. Even though life at the bottom of a lake with an ice cover moves at a slower pace than during the warm summer months, a functioning circulatory system is essential for survival. In general, frog blood (not unlike crankcase oil prior to the era of multiviscosity oil) increases in viscosity with as temperature decreases. Frog blood, however, decreases in viscosity with the prolonged autumnal and winter cold temperatures, thus assuring adequate circulation during the long nights under an ice cover. This is another control mechanism that assures the survival of frogs by maintaining a relatively stable internal environment during the harsh winter. With a return of a warm external environment, northern leopard frogs leave cold water to warm up under the spring sun. Warm temperature causes frog blood viscosity to increase to summer levels. It may be that the behavioral and physiological changes do not prevent oscillations that would be unsuitable for warm blooded animals but, in the frog, the fluctuations do not interfere with survival, and in biology, that is all that is essential.
There is homeostasis in ecological systems. Populations of animals in complex systems fluctuate in numbers, but the variations in numbers are generally between limits. For example, predators survive in adequate numbers as long as prey are available. If predators become too great in number, the population of prey will diminish. With fewer prey, the numbers of predators plummet through negative feedback thus permitting recovery of the preyed upon species. The situation becomes much more complex when other food sources are available to the predator.
Many organisms encounter a negative feedback on growth rate with crowding. This density dependent population control has been studied in larval frogs, as well as many other organisms, where excretory products seem to specifically inhibit the crowded species but not other organisms in the same environment. Even with adequate food, high density culture of laboratory mice results in negative feedback on reproductive potential with abnormal gonad development and delayed sexual maturity. Density independent factors affecting populations are important in population control but would not be considered homeostasis. Drought is such a factor, and its effects can be contrasted with crowding. Populations of tadpoles will drop catastrophically when breeding ponds dry. Instead of fluctuating between limits (with controls), all individuals are affected the same (i.e., they die). The area must be repopulated with immigrants at a subsequent time, and the migration can be considered a population homeostatic control. The inward migration results in maintenance of population within the geographic area and aids in the survival of the species.
[Robert G. McKinnell ]
Hardy, R. N. Homeostasis. London: Edward Arnold, Ltd., 1976.
Langley, L. L. Homeostasis. New York: Reinhold Publishing Co., 1965.
Tortora, G. J., and N. P. Anagnostakos. Principles of Anatomy and Physiology. 5th ed. New York: Harper and Row, 1987.
Homeostasis (a Greek term meaning same state), is the maintenance of constant conditions in the internal environment of the body, even in the presence of large swings in the external environment. Conditions such as body temperature and moisture content are maintained within a range of normal values around a set point despite constantly changing external conditions. For instance, when the external temperature drops, the body’s homeostatic mechanisms make adjustments that result in the generation of body heat, thereby maintaining the internal temperature at constant levels. In mammals, blood pressure, respiration rate, and blood glucose levels are all under homeostatic regulation.
In most organisms homeostasis is maintained by negative feedback mechanisms, sometimes called negative feedback loops. In negative feedback loops, a deviation from the normal range of a condition, called a set point, initiates a response that brings the back to the normal range. A thermostat that controls temperature in a house relies on negative feedback. If it is cold outside, the internal temperature of the house drops, as cold air seeps in through the walls. When the temperature drops below the set point, the thermostat turns on the furnace. When the temperature within the house rises above the set point, the thermostat turns off the furnace.
Negative feedback loops require a receptor, a control center, and an effector. A receptor is the structure that monitors internal conditions. In the example above, the thermostat acts as a receptor. The human body has receptors in the blood vessels that monitor the pH of the blood. The blood vessels contain receptors that measure the resistance of blood flow against the vessel walls, thus monitoring blood pressure. Receptors sense changes in a condition and initiate the body’s homeostatic response.
These receptors are connected to a control center that integrates the information fed to it by the receptors. In the thermostat example, the control center is simply the switch that turns on or off the furnace. In most humans, the control center is most often the brain. When the brain receives information about a change or deviation in the body’s internal conditions, it sends out signals along nerves. These signals prompt changes that correct the deviation and bring the internal conditions back to the normal range.
Effectors are muscles, organs, or other structures that receive signals from the brain or control center. The effector in the thermostat example above is the furnace. In humans, when an effector receives a signal from the brain, it changes its function in order to correct the deviation. For example, if blood pressure is too high, the heart will pump more slowly and the blood vessels will expand.
An example of a negative feedback loop is the regulation of blood pressure (Figure 1). an increase in blood pressure is detected by receptors in the blood vessels that sense the resistance of blood flow against the vessel walls. The receptors relay a message to the brain, which in turn sends a message to the effectors, the heart and blood vessels. The heart rate decreases and blood vessels increase in diameter, which cause the blood pressure to fall back within the normal range. Conversely, if blood pressure decreases, the receptors
relay a message to the brain, which in turn causes the heart rate to increase, and the blood vessels to decrease in diameter. Some set points become “reset” under certain conditions. For instance, during exercise, the blood pressure normally increases. This increase is not abnormal; it is the body’s response to the increased demand of oxygen by muscle tissues. When the muscles require more oxygen, the body responds by increasing the blood flow to muscle tissues, thereby increasing blood pressure. This resetting of the normal homeostatic set point is required to meet the increased demand of oxygen by muscles.
Similarly, when the body is deprived of food, the set point of the metabolic rate can become reset to a lower-than-normal value. This lowering of the metabolic rate is the body’s attempt to stave off starvation and keep the body functioning at a slower rate. Many people who periodically deprive themselves of food in attempts to lose weight find that after the initial weight loss it becomes increasingly difficult to lose more pounds. This difficulty stems from the lowering of the metabolic set point. Exercise may counteract some of these effects by the increasing metabolic demands.
See also Physiology.
Schulkin, Jay Ed. Allostasis, Homeostasis, and the Costs of Physiological Adaptation. Cambridge, UK: Cambridge University Press, 2004.
Control center —The center that receives messages from receptors about a change in the body’s internal conditions and relays messages to effectors to change their function to correct the deviation; in most human homeostatic systems, the control center is the brain.
Effector —A muscle or organ that receives messages from the control center to change its function in order to correct a deviation in the body’s internal conditions.
Hormone —Chemical regulator of physiology, growth, or development which is typically synthesized in one region of the body and active in another and is typically active in low concentrations.
Negative feedback loop —A homeostatic mechanism that opposes or resists a change in the body’s internal conditions.
Positive feedback loop— A mechanism that increases or enlarges a change in the body’s internal conditions.
Receptor —A structure that monitors the body’s internal functions and conditions; detects changes in the body’s internal environment.
Set point —The range of normal functional values of an organ or structure.
Silverthorn, Dee Unglaub. Human Physiology: An Integrated Approach. San Francisco, CA: Benjamin Cummings, 2006.
Wong, May, et al. “Homeostasis.” 2001. <http://www3.fhs.usyd.edu.au/bio/homeostasis/Introduction.htm.> (accessed October 20, 2006).
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 (1871–1945) 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 things—not 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
Cannon, Walter B. The Wisdom of the Body. New York: W. W. Norton, 1932.
Saladin, Kenneth S. Anatomy and Physiology—The 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.
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.
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
Campbell, Neil A. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman Inc.,1999.
Homeostasis (a Greek term meaning same state), is the maintenance of constant conditions in the internal environment of the body despite large swings in the external environment. Functions such as blood pressure , body temperature , respiration rate , and blood glucose levels are maintained within a range of normal values around a set point despite constantly changing external conditions. For instance, when the external temperature drops, the body's homeostatic mechanisms make adjustments that result in the generation of body heat , thereby maintaining the internal temperature at constant levels.
The body's homeostatically cultivated systems are maintained by negative feedback mechanisms, sometimes called negative feedback loops. In negative feedback, any change or deviation from the normal range of function is opposed, or resisted. The change or deviation in the controlled value initiates responses that bring the function of the organ or structure back to within the normal range.
Negative feedback loops have been compared to a thermostatically controlled temperature in a house, where the internal temperature is monitored by a temperature-sensitive gauge in the thermostat . If it is cold outside, eventually the internal temperature of the house drops, as cold air seeps in through the walls. When the temperature drops below the point at which the thermostat is set, the thermostat turns on the furnace. As the temperature within the house rises, the thermostat again senses this change and turns off the furnace when the internal temperature reaches the pre-set point.
Negative feedback loops require a receptor, a control center, and an effector. A receptor is the structure that monitors internal conditions. For instance, the human body has receptors in the blood vessels that monitor the pH of the blood. The blood vessels contain receptors that measure the resistance of blood flow against the vessel walls, thus monitoring blood pressure. Receptors sense changes in function and initiate the body's homeostatic response.
These receptors are connected to a control center that integrates the information fed to it by the receptors. In most homeostatic mechanisms, the control center is the brain . When the brain receives information about a change or deviation in the body's internal conditions, it sends out signals along nerves. These signals prompt the changes in function that correct the deviation and bring the internal conditions back to the normal range.
Effectors are muscles, organs, or other structures that receive signals from the brain or control center. When an effector receives a signal from the brain, it changes its function in order to correct the deviation.
An example of a negative feedback loop is the regulation of blood pressure (Figure 1). An increase in blood pressure is detected by receptors in the blood vessels that sense the resistance of blood flow against the vessel walls. The receptors relay a message to the brain, which in turn sends a message to the effectors, the heart and blood vessels. The heart rate decreases and blood vessels increase in diameter, which cause the blood pressure to fall back within the normal range or set point. Conversely, if blood pressure decreases, the receptors relay a message to the brain, which in turn causes the heart rate to increase, and the blood vessels to decrease in diameter. Some set points become "reset" under certain conditions. For instance, during exercise , the blood pressure normally increases. This increase is not abnormal; it is the body's response to the increased demand of oxygen by muscle tissues. When the muscles require more oxygen, the body responds by increasing the blood flow to muscle tissues, thereby increasing blood pressure. This resetting of the normal homeostatic set point is required to meet the increased demand of oxygen by muscles.
Similarly, when the body is deprived of food, the set point of the metabolic rate can become reset to a lower-than-normal value. This lowering of the metabolic rate is the body's attempt to stave off starvation and keep the body functioning at a slower rate. Many people who periodically deprive themselves of food in attempts to lose weight find that after the initial weight loss it becomes increasingly difficult to lose more pounds. This difficulty stems from the lowering of the metabolic set point. Exercise may counteract some of these effects by the increasing metabolic demands.
See also Physiology.
Marieb, Elaine Nicpon. Human Anatomy & Physiology. 5th ed. San Francisco: Benjamin/Cummings, 2000.
Reinhardt, H. Wolfgang, Paul P. Leyssac, and Peter Bie, eds. Mechanisms of Sodium Homeostasis: Sodium and Water Excretion in Mammals; Haemodynamic, Endocrine, and Neural Mechanisms. Boston: Blackwell Scientific Publishers, 1990.
Kozak, Wieslaw. "Fever: A Possible Strategy for Membrane Homeostasis During Infection." Perspectives in Biology and Medicine 37 (Autumn 1993): 1.
Skorupski, Peter, et al. "Integration of Positive and Negative Feedback Loops in a Crayfish Muscle." Journal of Experimental Biology 187 (February 1994): 305.
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- Control center
—The center that receives messages from receptors about a change in the body's internal conditions and relays messages to effectors to change their function to correct the deviation; in most homeostatic mechanisms, the control center is the brain.
—A muscle or organ that receives messages from the control center to change its function in order to correct a deviation in the body's internal conditions.
—Chemical regulator of physiology, growth, or development which is typically synthesized in one region of the body and active in another and is typically active in low concentrations.
- Negative feedback loop
—A homeostatic mechanism that opposes or resists a change in the body's internal conditions.
- Positive feedback loop
—A mechanism that increases or enlarges a change in the body's internal conditions.
—A structure that monitors the body's internal functions and conditions; detects changes in the body's internal environment.
- Set point
—The range of normal functional values of an organ or structure.
Homeostasis is the maintenance of stable internal conditions in a living thing. Organisms use a variety of systems and processes that help regulate and maintain a constant environment within their bodies. All organisms use a self-adjusting balance to make sure that what is going on inside their bodies is kept within certain boundaries.
One of the main characteristics of living things, or organisms, is that they have the ability to adjust to their environment. In the highly competitive struggle for survival, an organism would be at a great disadvantage if it could not adjust and be ready to cope with a changed situation. Such change can occur inside or outside an organism. Since living things are extremely complex organisms with constant energy demands, there are countless cellular reactions going on all the time. For example, chemicals are combining and breaking apart, fluids are passing in and out of membranes, and substances are being converted from one form to another. All of this constant activity means that the environment inside an organism's body is dynamic, and that it is always in a state of movement and change.
CLAUDE BERNARD DEVELOPS THE CONCEPT OF HOMEOSTASIS
The concept of homeostasis was developed by the French physiologist (a person specializing in the study of life processes, activities, and functions) Claude Bernard (1813–1878). Bernard investigated how the body keeps itself in a stable, or steady state. It was Bernard who first recognized the idea behind homeostasis—that an organism is designed and operates on the principle that it will always attempt to maintain a balance in its systems.
After Bernard, science eventually discovered that living things use two simple self-adjusting elements, input and output, as regulators. Although these elements are uncomplicated, an organism has many mechanisms and structures that it uses to maintain homeostasis. Some of these mechanisms work automatically and are under the control of the autonomic nervous system. It is this system that regulates the body's involuntary processes like internal body temperature, blood pressure, and food digestion, among other function.
Certain changes in the external environment may automatically trigger an organ to take a certain action. Under certain conditions, our bodies will perspire whether we want them to or not. Other mechanisms are controlled by the body's endocrine system. This system uses chemical messages, known as hormones, inside the body to regulate functions. A hormone is secreted by the organ that produces it when something happens to the organism that warrants regulation. Hormones then travel through the bloodstream to their target cells, which take the appropriate action.
THE FEEDBACK SYSTEM
For this system to really work, however, it must have some way of getting updated information as to what is going on inside and outside the organism. This is achieved by a feedback system that operates something like the thermostat in a house. Every thermostat has a sensor that, when set at a certain temperature, automatically turns the furnace on when the temperature gets lower than its setting, and turns it off when it reaches it. In this way, a built-in feedback "loop" regularly monitors its environment and is able to maintain a constant temperature by turning the furnace on or off.
The feedback system in our bodies works mostly with what is called negative feedback. Negative feedback operates by detecting an unwanted change and countering it in order to balance it. A typical example would be the body's use of feedback mechanisms to raise or lower its internal temperature during extremely cold or hot weather.
LEVELS OF HOMEOSTASIS
In the human body, homeostasis takes place at many different levels. These include the molecular level (in which atoms are linked together by chemical bonds), the cellular level, the organism level, and the population level. An example of homeostasis at the molecular level would be the body limiting how much of something is produced by a certain chemical reaction. At the cellular level, an example would be how certain cells
will stop dividing if they become so numerous that they touch each other. Sensations of hunger and thirst are good examples of a homeostasis mechanism at the organism level. Finally, at the population level, an increase in the number of prey animals usually results in an increase in the number of predators that eat the prey. In this way, the population size of both animals is kept in balance.