Microvascular Fluid Exchange

Fluid comprises approximately 60% of total body volume with some variations with age and sex (1). The intracellular volume is about twice as large as the extracellular volume. About three-fourths of the extracellular volume comprises the interstitial space and the rest is plasma volume. The red blood cells belong to the intracellular compartment. Blood (intravascular volume) contributes to about 7% of body weight or 4 to 6 L in the adult man. At a normal hematocrit of about 40%, plasma volume in a 70 kg adult is about 3 L, but at lower hematocrit values, the plasma volume must be correspondingly larger to maintain normovolemia. In most organs of the body, the interstitial volume comprises 25% to 30% of total extravascular fluid volume of the tissue and the rest is intracellular volume. The relative interstitial volume is somewhat smaller in the brain (2).