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 .
Effects of changes in dietary sodium (Na) intake on extracellular fluid (ECF) volume. The dietary intake of Na was increased from 2 to 5 g, and then returned to 2 g. The relationship between dietary Na intake (dashed line) and ECF volume (solid line) is derived from the model of Walser . In this model the rate of Na excretion is assumed to be proportional to the content of Na in the body (At) above a zero point (A0) at which Na excretion ceases. This relation can be expressed as dAt/dt = I - k(At - A0), where I is the dietary Na intake and t is time. The ECF volume is approximated as the total body Na content divided by the plasma Na concentration. (This assumption is strictly incorrect because approximately 25% of Na is tightly bound in bone; however, this amount is nearly invariant and can be ignored in the current analysis.) According to this construct, when dietary Na intake changes from level 1 to level 2, the ECF volume approaches a new steady state exponentially with a time constant of k according to the following equation:
Urinary sodium excretion, g/d 2 3 4 5
Sodium intake, g/d
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