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The relatively constant conditions within organisms, or the physiological processes by which such conditions are maintained in the face of external variation.
Similar homeostatic controls are used to keep factors such as temperature and blood pressure nearly constant despite changes in an organism's activity level or surroundings. Such systems operate by detecting changes in the variable that the system is designed to hold constant and initiating some action that offsets any change. All incorporate a sensor within the system that responds when the actual condition differs from the desired one, a device to ensure that any action taken will reduce the difference between actual and desired, and an effector to take the needed action as directed. The crucial aspect is that information is fed back from effector to sensor and action is taken to reduce any imbalance—hence the term negative feedback.
Blood pressure is, at least on a moment-to-moment basis, regulated by a system for which the sensors are stretch-sensitive cells located in the neck arteries that carry blood from heart to brain. An increase in blood pressure triggers sensor activity; their signal passes to the brain; and, in turn, the nerve supplying the heart (the vagus) is stimulated to release a chemical (acetylcholine) that causes the heart to beat more slowly—which decreases blood pressure.
The volume of the blood is subject to similar regulation. Fluid (mainly plasma) moves between the capillaries and the intercellular fluid in response to changes in pressure in the capillaries. A decrease in blood volume is detected by sensors at the base of the brain; the brain stimulates secretion of substances that cause contraction of tiny muscles surrounding the blood vessels that lead into the capillaries. The resulting arteriolar constriction reduces the flow of blood to, and the pressure within, the capillaries, so fluid moves from intercellular space into capillaries, thus restoring overall blood volume.
Body temperature in mammals is regulated by a sensor that consists of cells within the hypothalamus of the brain. Several effectors are involved, which vary among animals. These include increasing heat production through nonspecific muscle activity such as shivering; increasing heat loss through sweating, panting, and opening more blood vessels in the skin (vasodilation); and decreasing heat loss through thickening of fur (piloerection) and curling up. Humans sweat, but they retain only a vestige of piloerection (“goose flesh”). See Thermoregulation
While the homeostatic mechanisms described involve the neural and endocrine systems of mammals, it is clear that such arrangements pervade systems from genes to biological communities, and that they are used by the simplest and the most complex organisms.
Organisms of every kind develop, mature, and even shift physiological states periodically—between day and night, with seasons, or as internal rhythms. Thus organisms cannot be considered constant except over short periods. However, all such changes appear to involve the same basic sensing of the results of the past activity of the system and the adjusting of future activity in response to that information. Development of an organism from a fertilized egg is far from a direct implementation of a genetic program; probably no program could anticipate all the variation in the external context in which an organism must somehow successfully develop. See Biological clocks, Nervous system (vertebrate)
homeostasisany process which regulates or maintains a system in a stable state in relation to a changing external environment in which this system operates. The term may be applied to mechanical systems or servomechanisms, to living beings, or to SOCIAL SYSTEMS. Although a central postulate of some sociological theories (especially see FUNCTIONALISM and STRUCTURAL-FUNCTIONALISM), the suggestion that social systems act in self-maintaining or self-equilibriating ways remains controversial. see also SYSTEMS THEORY.
in physiology, a relatively dynamic constant state with respect to the composition and properties of the internal environment and constancy of the basic physiological functions in man, animals, and plants. The term “homeostasis” was proposed by the American physiologist W. Cannon in 1929. However, the concept of the constancy of the internal milieu had already been formulated in 1878 by the French scientist C. Bernard.
Homeostasis results from the complex, coordinated, and regulated interrelationships occurring at both the organismic level and the organic, cellular, and molecular levels. As a result of the adaptive mechanisms, the physical and chemical parameters that determine the vital processes occurring in the organism change within relatively narrow limits, despite wide fluctuation of the external environment. Higher animals differ from lower animals in the increased complexity of their homeostatic mechanisms. In man, mammals, and birds, homeostasis includes the maintenance of a constant hydrogen ion concentration (pH) and blood composition, osmotic pressure, body temperature, blood pressure, and many other functions. Homeostasis is controlled by neurohumoral, hormonal, barrier, and secretory mechanisms. Thus, for example, arterial pressure is maintained by regulatory mechanisms that are activated in the manner of a chain reaction with feedback loops. (Changes in blood pressure are detected by baroreceptors in the blood vessels and the signal is conveyed to the vascular centers. Changes in the vascular centers then result in alterations in vascular tonus and heart rate; vascular neurohumoral chemoreceptors are simultaneously stimulated and blood pressure is restored to normal.) An example of homeostasis in plants is the maintenance of leaf water content by the opening and closing of the stomata.
The concept of homeostasis is also applicable to biological communities. For example, the maintenance of constant species composition and constant numbers of individuals in biocenoses is regarded as homeostasis.
Genetic homeostasis is the capacity of a population to maintain its genetic composition in dynamic equilibrium, thereby assuring maximal viability.
The term “homeostasis” is also used in cybernetics to designate any autoregulatory mechanism.
REFERENCESGellhorn, E. Reguliatornye funktsii avtonomnoi nervnoi sistemy. Moscow, 1948. (Translated from English.)
Kassil’, G. N. Gematoentsefalicheskii bar’er. Moscow, 1963.
Winchester, A. Osnovy sovremennoi biologii. Moscow, 1967. (Translated from English.)
Adolph, E. Razvitie fiziologicheskikh reguliatsii. Moscow, 1971. (Translated from English.)
Cannon, W. B. The Wisdom of the Body. New York, 1932.
Lerner, I. M. Genetic Homeostasis. New York, 1954.
G. N. KASSIL’ and E. K. GINTER