Fluid volumes in typical adult men and women, given as percentages of body weight. In men (A), total body water typically is 60% of body weight (Total body water = Extracellular fluid [ECF] volume + Intracellular fluid [ICF] volume). The ECF volume comprises the plasma volume and the extravascular volume. The ICF volume comprises the water inside erythrocytes (RBCs) and inside other cells. The blood volume comprises the plasma volume plus the RBC volume. Thus, the RBC volume is a unique component of ICF volume that contributes directly to cardiac output and blood pressure. Typically, water comprises a smaller percentage of the body weight in a woman (B) than in a man; thus, when expressed as a percentage of body weight, fluid volumes are smaller. Note, however, that the percentage of total body water that is intracellular is approximately 70% in both men and women [5].

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