Acclimatization

Acclimatization, also physiological adaptation, process by which an organism becomes better adapted to exist in an environment different from the one to which it was indigenous. If the environmental difference is extreme, changes take place in the structure and physiology of the organism. Each organism, however, has certain limits of temperature and other conditions within which it can survive, and some alleged instances of acclimatizing have merely been instances of unsuspected hardiness in the organism.

Humans, for example, can acclimate to extreme conditions through changes in normal physiological processes. Persons who move from a temperate to a hot, dry climate undergo changes in heart rate and body temperature such that in time they perspire less, and their perspiration contains less salt. Although at an altitude of 7600 m (25,000 ft) most persons need to breathe high-pressure oxygen for survival, through gradual acclimatization they may become able to breathe unaided. This is due to an increase in the number of red blood cells, which contain oxygen-carrying hemoglobin. The increase is mediated by the hormone erythropoietin, secreted by the kidneys. In addition, a change in chemical composition within the red blood cells promotes migration of hemoglobin into the body tissues, where oxygen is needed.

The human organism also exhibits a response to the absence of natural light. As with most life forms, humans normally function in what is called a circadian rhythm corresponding to the length of the day. Humans who have lived underground under experimental conditions continue to show cyclical changes in physiology, demonstrating the existence of a natural biological clock (Biological Clocks). The period dictated by this internal clock, however, is slightly longer than one day.

Acclimatization may also refer to psychological changes occasioned by a change in environments, as from a rural to an urban setting.

See also Adaptation.