exercise duration, environmental conditions (dry heat versus humid heat), state of training and heat acclimatization, sex, and age (see Chapter 5). The increased heat production of exercise, an increased sweat rate, and inadequate hydration predispose soldiers in hot environments to dehydration.
Along with exercise intensity, sweat rate is related to environmental conditions, clothing, and acclimatization state (Shapiro et al., 1982). In hot, dry conditions, water loss from the skin and respiratory surfaces can be as much as 2 to 3 liters per hour (Wenger, 1988). In hot, moist (humid) conditions, sweat losses are measurably less than in hot, dry conditions. In a study that measured physiologic changes and sweat losses in healthy young men during hyperthermia induced by humid heat in an environmental chamber, total sweat losses averaged 7 liters per 24 hours (Beisel et al., 1968). However, humidity per se does not appear to affect core (rectal) temperature (Morimoto, 1967). In terms of military apparel, the nuclear-biological-chemical (NBC) protective clothing worn by many military personnel prevents the normal dissipation of body heat because of the cloth's lack of moisture permeability and its insulating properties. As a result, body temperature may rise excessively, producing high levels of sweat (1 to 2 liters per hour) that cannot evaporate effectively because air turnover is reduced, and caution must be taken (Muza et al., 1988; Pimental et al., 1987).
If the fluid involved in excessive sweat loss is not replaced, total body water, along with the total blood volume, will be decreased. A water loss as small as 1 percent of body weight will induce changes such as increased heart rate during rest and exercise, and decreased performance. However, a 1 percent loss is difficult to discern relative to what might be regarded as initial water balance. Thus, it is hard to attribute physiological changes to a 1 percent loss, but such changes can be readily observed at losses of 2.0 to 2.5 percent. A 10 percent loss of body weight through dehydration3 is life-threatening (Adolph, 1947). Water loss from the blood leads to a decrease in sweat rates and skin blood flow (Sawka and Pandolph, 1990; Wyndham, 1977), which results in less evaporative cooling and a risk of heat stroke (Wyndham, 1977). The normal compensatory response to exercise and heat stress is increased peripheral blood flow to maximize heat dissipation and prevent hyperthermia. However, in dehydrated individuals with greatly diminished blood volume, skin blood flow is reduced to maintain cardiac output and blood pressure.
Reductions in blood volume can result in a reduced flow of blood to organs during exercise and reduced venous flow in return. This reduced venous return to the heart decreases stroke volume and causes a compensatory increase in the heart rate to maintain cardiac output and blood pressure.