Relation of plasma to interstitial fluid volume in essential hypertension.

Extracellular water (radiobromine), plasma volume (radioiodinated human serum albumin), and peripheral plasma renin activity were measured simultaneously in men with uncomplicated, untreated essential hypertension and in normal subjects. The ratio of plasma volume to interstitial fluid volume (PV/IF) was significantly lower in hypertensive patients (0.194 vs. 0.223, P < 0.001). This could not be explained only on the basis of the diminished plasma volume found in hypertensive patients (17.3 ml/ cm of body height vs. 18.7 ml/cm, P < 0.05), as three normal subjects with marked plasma volume contraction had normal PV/IF ratios (0.224). This disturbed ratio in hypertensives suggests, therefore, an abnormality in the mechanism regulating the distribution of extracellular water in essential hypertension. Peripheral plasma renin activity correlated inversely with plasma volume, but not with extracellular water volume, suggesting that these two indices of body fluid volume are not always functionally interchangeable.