tween ICF and ECF are maintained by active transport-mediated ion pumps within cell membranes.
Water exchange between the intravascular and interstitial spaces occurs in the capillaries. Capillaries of different tissues have varied anatomic structures and therefore different permeability to water and solutes. The transcapillary forces that determine if net filtration (i.e., water leaving the vascular space) or net absorption (i.e., water entering the vascular space) will occur are hydrostatic and oncotic pressures. Oncotic pressure is the osmotic pressure attributed to serum protein concentration (e.g., serum albumin levels) differences across the capillary membrane. Generally, filtration occurs at the arterial end of the capillary, while absorption occurs at the venous end.
Incomplete fluid replacement resulting in decreased total body water affects each fluid space as a consequence of free fluid exchange (Costill and Fink, 1974; Durkot et al., 1986; Nose et al., 1983). The distribution of body water loss among the fluid spaces, as well as among different body organs during water deficit (dehydration or hypohydration), was determined in an animal model (Nose et al., 1983). The fluid deficit in rats thermally dehydrated by 10 percent of body weight was apportioned between the intracellular (41 percent) and extracellular (59 percent) spaces. Organ fluid loss was 40 percent coming from muscle, 30 percent from skin, 14 percent from viscera, and 14 percent from bone. Neither the brain nor liver lost significant water content. Various dehydration methods influence the partitioning of water loss from the fluid spaces (Mack and Nadel, 1996).
Body water balance depends on the net difference between water gain and water loss. Water gain occurs from consumption (liquids and food) and production (metabolic water), while water losses occur from respiratory, skin, renal, and gastrointestinal tract losses. Water is normally consumed by mouth via liquid and food, and this mixture is digested and absorbed within the gastrointestinal tract. Therefore, water intake can be estimated from measured liquid volumes and tables of food composition. Water losses can be estimated from a variety of physiological and biophysical measurements and calculations (Adolph, 1933; Consolazio et al., 1963; Johnson, 1964). Depending upon a person’s age, health, diet, activity level, and environmental exposure, different physiological and biophysical methods can be used to quantify the water balance components. Table