Ambient air temperature and humidity modify respiratory water losses. Breathing hot, dry air during intense physical exercise can increase respiratory water losses by 120 to 300 mL/day (Mitchell et al., 1972). Breathing cold, dry air during rest and stressful physical exercise (Table 4-3) can increase respiratory water losses by approximately 5 mL/hour and approximately 15 to 45 mL/hour, respectively (Freund and Young, 1996). Freund and Young (1996) have calculated that for a 24-hour military scenario (8 hours of rest, 12 hours of moderate activity, and 4 hours of moderate-heavy activity), the respiratory water losses increase by approximately 340 mL/day when breathing −20°C versus +25°C air.

The kidneys are responsible for regulating the volume and composition of the ECF via a series of intricate neuroendocrine pathways (Andreoli et al., 2000). Renal fluid output can vary depending upon the specific macronutrient, salt, and water load. However, for persons consuming an average North American diet, some of these effects may not be discernable (Luft et al., 1983). Since there is a limit to how much the kidneys can concentrate urine, the minimal amount of water needed is determined by the quantity of end products that need to be excreted (e.g., creatinine, urea). On typical Western diets, an average of 650 mOsmol of electrolytes and other