Either 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 servosystems, which are of wide technological use as well, 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. If that area is experimentally heated, the rest of the body cools off. 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 (but not pigs, cats, or dogs) sweat, but they retain only a vestige of piloerection (“goose flesh”). See also: Thermoregulation
Regulation using negative-feedback servosystems occurs in more localized neuromuscular systems as well. When stimulated by nerves, muscles pull harder—how much shorter the muscle becomes depends on the load it is pulling against. But one often makes a muscle shorten by a fixed and load-independent amount. It is done with sensors within the muscle that effectively monitor its length. Extra load (for example, a book laid on an outstretched arm) stretches muscles such as the biceps; that stretch is detected and stimulates the brain to send more frequent nerve impulses to the muscle, restoring its original length—but at the higher tension needed for its greater load. Similar machinery permits a person standing with slightly bent knees to lift one leg off the ground without collapsing—the tensions of the extensor muscles of the opposite leg are doubled to offset the doubling of their load. See also: Muscle
While the homeostatic mechanisms first described involved 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. If a bacterium such as Escherichia coli is grown in a culture medium that lacks the amino acid tryptophan, it reacts to that absence by turning on the appropriate genes and making the enzymes that it needs to synthesize tryptophan. If the same bacterium is transferred to a medium in which lactose rather than glucose is the sole energy source, it begins to make the enzymes necessary to live on lactose. In either case the bacterium does what is necessary to maintain the level of some substance within itself.
Servosystems of the kind described above are limited in their response speed by the necessity for the external change to affect the internal sensor. In practice, organisms commonly overlay such systems with anticipatory devices. The initial heat-conserving response to a sudden chill is triggered by information from cold-sensitive receptors in the skin. Since alteration in blood flow to the skin is part of the response to cold and since skin temperature is not closely regulated, these receptors cannot function as the primary source of the feedback signal—but they do initiate a rapid response. Similarly, when given access to water, a mammal deprived of water drinks rapidly. The animal stops drinking when sufficient water has been consumed to restore its normal content. But that happens well before the main detectors for water content (detectors that monitor blood composition) have been affected.
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 also: Biological clocks; Endocrine mechanisms; Nervous system (vertebrate); Servomechanism