Adaptation under Extreme Environmental Conditions

Living systems tend to maintain in equilibrium variables associated with their continued existence. Environ mental conditions which force these variables beyond their normal ranges produce stress within the system. Feedback mechanisms then seek to restore equilibrium. Stress arises from overloads or underloads of energy and information, as well as from inputs which have threatening implications. The process of adapting to stress depends partly on the nature of the stress-producing agent and partly on the mechanisms available to the system. Exposure to heat, for example, involves dilation of blood vessels, reduction of body activity, and sweating to reduce body heat. At the same time, the system undergoes neurophysiological and endocrine changes which are found in many other types of stress. When the system is subjected to extreme environmental conditions for an extended period of time, adaptive responses may become increasingly costly. A possible outcome is one of the "dis eases of adaptation" described by Selye. Eventually, adap tive mechanisms may fail and the system may collapse. The stress encountered by most individuals arises from the symbolic, rather than the physical, aspects of their environ ment. Among the exceptions are groups such as antarctic explorers, mountain-climbers, prisoners of war, and crews of space vehicles. Although problems arise under these condi tions, most studies of men under stress reveal a remarkable capacity for adaptation to extreme environments.