Distribution of Body Water
Total body water constitutes about 70 percent of lean body mass and is most simply divided into two major compartments: (a) intracellular water, which represents 50 percent of body weight or 35 liters in a 70-kg man, and (b) extracellular water, which represents 20 percent of body weight or 14 liters. The latter compartment is subdivided into plasma volume (5 percent body weight) and interstitial fluid volume (15 percent body weight). Intracellular water is not readily measured. It is calculated from measurements of total body water and extracellular fluid volume.
Respiratory water loss can range from 200 ml per day when breathing humidified air to 1500 ml per day when exercising at high altitude.
Water loss from cutaneous evaporation could range from 500 ml per day at rest in a cool environment to 10 liters per day during exercise in the heat.
Fecal losses could range from 100 ml per day when on a mixed diet to 32 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.
Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations.
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