Exchangeable Sodium and Blood Volume

Eight normotensive and eight essential hypertensive humans were studied when on high and low sodium intake. Total 24-hr exchangeable body sodium, blood volume, serum sodium concentration, hematocrit, body weight, and mean arterial pressures, when the subjects were supine and upright, were measured. Plasma and nonplasma sodium fractions, nonplasma weight, and plasma, cell, and whole blood volumes, as referred to body weight and height, were calculated. Changes in these parameters with diet were also calculated. Specific differences between normotensives and hypertensives were found, in absolute values and with changes with diet. The hypertensives fell into two groups with regard to exchangeable sodium: in four the values varied with diet and in the other four, the values remained low regardless of diet. The "variable-sodium" patients had high upright arterial pressures when on high salt intake and normal upright pressures when on low salt intake. "Low-sodium" patients had high arterial pressures on both diets. Changes in upright pressures of hypertensives correlated significantly (r = 0.73) with changes in nonplasma exchangeable sodium.