liters per day or more in a patient with diarrhea. Obligatory urine volume is limited by the concentrating power of the kidneys, but it can vary from 250 to 1400 ml per day depending on diet. Urine volume is usually 700 ml per day, but a high-protein diet demands more obligatory water to excrete the osmotically active products of protein metabolism.
Water requirements during exercise in the heat primarily depend on evaporative cooling. Metabolism and environmental heat exchange determine the required evaporative cooling (Ereq) to achieve thermal balance. Because respiratory water loss contributes little to evaporative cooling in warm or hot environments, cooling must come primarily from cutaneous sweat secretion. The rate of sweating and its regulation are determined by core and skin temperatures, skin wettedness, heat storage, metabolism, and the set point.
The U.S. military deploys troops to tropical and desert climates, and therefore military men and women are exposed to both wet and dry heat. Figure 5-1 shows the sweat responses as well as the mean changes in rectal temperature, heart rate, and metabolic rate of four distance runners walking 5.6 km per hour in dry heat, in wet heat, and in a cool environment. Experiments were performed 4 to 5 weeks apart and consisted of 4 hours of continuous walking, lunch (30 minutes), followed by another 2 to 3 hours of walking. Water was ingested ad libitum, but the subjects were constantly informed of their weight loss and were successful in maintaining fluid balance. All men walked 6 hours in the neutral and hot-dry environments except one subject who stopped walking after 5.5 hours in dry heat with a rectal temperature of 39°C and a heart rate of 136 beats per minute (bpm). Another subject walked for 7 hours in dry heat and finished with a rectal temperature of 38.3°C and a heart rate of 132 bpm. Sweat rate in the desert environment averaged 1210 ± 56 (x ± SE) ml per hour (Table 5-2).
In the hot-wet environment, sweat rate averaged only 716 ± 56 (x ± SE) ml per hour, which resulted in higher rectal temperatures (39.3°C) and heart rates (132 bpm). The reduced rate of sweating in this environment was associated with sweat gland fatigue (Brown and Sargent, 1965; Hertig et al., 1961; Kerslake, 1972; Nadel and Stolwijk, 1973; Robinson and Gerking, 1947). The mechanism responsible for this phenomenon is not clear, but evidence suggests that it is related to excessive wetting of the skin (Brebner