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Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations
Several reflex adjustments compensate for peripheral pooling of blood and decreases in blood volume to help maintain cardiac filling, cardiac output, and arterial pressure during exercise-heat stress. Splanchnic and renal blood flows are reduced during exercise in proportion to relative exercise intensity (that is, as a percentage of (Rowell, 1986). These blood flows also undergo a graded and progressive reduction in subjects who are heated while resting; and in the splanchnic bed, at least, the vasoconstrictor effects of temperature and of exercise appear to be additive, so that at any exercise intensity, the reduction in splanchnic blood flow is greater at a higher skin temperature (Rowell, 1986). Reduction of renal and splanchnic blood flow allows a corresponding diversion of cardiac output to skin and exercising muscle. A substantial volume of blood can thus be mobilized from these beds to help maintain cardiac filling during exercise and heat stress.
During exercise in the heat, the primary cardiovascular challenge is simultaneously to provide sufficient blood flow to exercising skeletal muscle to support metabolism and to provide sufficient blood flow to the skin to dissipate heat. In hot environments, the core-to-skin temperature gradient is less than in cool environments, so that skin blood flow must be relatively high to achieve sufficient heat transfer to maintain thermal balance (Rowell, 1986; Sawka and Wenger, 1988). This high skin blood flow causes pooling of blood in the compliant skin veins, especially below heart level. In addition, as discussed, sweat secretion can result in a net loss of body water, and thereby a reduction in blood volume (Sawka and Pandolf, 1990). Heat stress can reduce cardiac filling through pooling of blood in the skin and through reduced blood volume. Compensatory responses include reductions in splanchnic and renal blood flow; increased cardiac contractility, which helps to defend stroke volume in the face of impaired cardiac filling; and increased heart rate to compensate for decreased stroke volume. If these compensatory responses are insufficient, skin and muscle blood flow will be impaired, possibly leading to dangerous hyperthermia and reduced exercise performance.
Acclimatization state, aerobic fitness and hydration level are important factors influencing a person's ability to dissipate body heat to the environment.
The higher the ambient temperature, the greater the dependence on evaporative heat loss to maintain body heat balance.
During exercise, the elevation in core temperature is dependent on the metabolic rate, when the environment has sufficient capacity for heat exchange.