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  Homeostasis (Greek, ‘equilibrium’), in the life sciences, is the maintenance of a constant internal environment in which conditions are the optimum for life. It is the self-regulating mechanism by which the dynamic equilibrium of all biological systems is maintained. In order for an organism to function, it must continually undergo physical and chemical changes; these must be controlled within fairly narrow limits by a co-ordinated process. The concept of an internal environment was proposed by the French physiologist Claude Bernard in the 19th century, and the actual term homeostasis was coined in 1939 by Walter Cannon. The principle of homeostasis permeates all levels of biology and extends beyond the internal environment to the external in the control of population interactions, and the control of the biosphere as a whole. It is also applicable to mechanized systems such as automatic pilots and thermostats. Two forms of control exist: feedback, where the output of a system modifies its activity; and simple on or off control.

All living cells control their internal environment by pumping chemical requirements and waste into and out of the cell by behavioural responses. In multicellular organisms the process is extended to the environment within the organism but outside the constituent cells, which co-operate to control their environment. Birds and mammals have developed some of the most complex homeostatic systems, controlling body temperature, for example, within narrow limits by physiological and behavioural responses. Populations are controlled by homeostatic processes, such as the feedback response in which the size of the predator population limits that of the prey and vice versa. The control of conditions throughout the biosphere, such as the level of atmospheric carbon dioxide, is homeostatic in principle. RB

See also endocrinology; Gaia hypothesis; humours; metabolism.Further reading W.B. Cannon, The Wisdom of the Body (1932); , Margaret Stanier, Physiological Processes.



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