Heat Acclimatization

Prolonged or repeated exposure to stressful environmental conditions elicits significant physiological changes, called acclimatization, that reduce the resulting strain. (Such changes are often referred to as acclimation when produced in a controlled experimental setting.) Some degree of heat acclimatization occurs either by heat exposure alone or by regular strenuous exercise, which raises core temperature and provokes heat-loss responses. Indeed, the first summer heat wave produces enough heat acclimatization that most people notice an improvement in their level of energy and general feeling of well-being after a few days. However, the acclimatization response is greater if heat exposure and exercise are combined, causing a greater rise of internal temperature and more profuse sweating. Evidence of acclimatization appears in the first few days of combined exercise and heat exposure, and most of the improvement in heat tolerance occurs within 10 days. The effect of heat ac-

Heat Acclimatization

Cardiovascular strain and compensatory responses during heat stress. This figure first shows the effects of skin vasodilation on peripheral pooling of blood and the thoracic reservoirs from which the ventricles are filled; and second, the effects of compensatory vasomotor adjustments in the splanchnic circulation. The valves on the right represent the resistance vessels that control blood flow through the liver/splanchnic, muscle, and skin vascular beds. Arrows show the direction of the changes during heat stress. (Modified from Row-ell LB. Cardiovascular aspects of human thermoregulation. Circ Res 1983;52:367-379.)

climatization on performance can be dramatic, and acclimatized subjects can easily complete exercise in the heat that earlier was difficult or impossible.

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