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blood circulation. The increased blood flow to the surface raises the temperature of the skin and allows sensible heat loss by radiative and convective mechanisms. Evaporative heat loss occurs through sweating (see Chapters 3, 4, and 5).

Heavy exercise at increased ambient temperatures decreases the skin-to-ambient-temperature gradient, thus substantially decreasing sensible heat loss. Under these conditions, most heat loss by the body will occur through evaporative cooling (i.e., sweating). As is well known, heat loss by this mechanism can be greatly decreased under conditions of high humidity. The resulting dehydration from excess sweating can reduce blood volume and cardiac filling. If compensatory circulatory and cardiac changes are insufficient, skin and muscle blood flow will be impaired, thus reducing sensible heat loss and physical performance. A state of adequate hydration is therefore important in maintaining the effectiveness of the physiological mechanisms involved in heat dissipation.

Heat Stress

Thermoregulation can be defined as the summation of the mechanisms by which the body adapts to a heat stress in order to maintain thermoneutrality. Body core and skin temperatures have been used as indices of the ability of the body to thermoregulate, along with cardiovascular changes in heart rate, blood volume (see Harrison, 1985, for a comprehensive review) and blood pressure, with sweat rate as a visible mechanism of adaptation. Acclimatization is the process of adapting to prolonged exposure to a new environment, so that the mechanisms that result in initial responses are modified to allow increased endurance with less strain on body functions.

The ability to defend one's body temperature against heat stress is influenced by level of activity, acclimatization state, aerobic fitness, and hydration level. In heat-acclimatized2 individuals, the thermoregulatory mechanisms involved in dissipating heat become fully operative. Although some investigators report that to perform a given submaximal exercise task the metabolic rate is greater in a hot compared to a temperate environment (Consolazio et al., 1961, 1963; Dimri et al., 1980, Fink et al., 1975), other investigators report lower metabolic rates in the heat (Brouha et al., 1960; Petersen and Vejby-Christensen, 1973; Williams et al., 1962; Young et al., 1985). A person's state of heat acclimatization does not account for whether individuals demonstrate an increased or decreased metabolic rate during submaximal exercise in the heat; other mechanisms explain this discrep-

2  

The term heat acclimatization is used here to refer to the adaptive changes that occur due to exposure to a hot natural environment; heat acclimation will be used to refer to adaptive changes to a hot environment under controlled conditions, such as in an environmental chamber.



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