1. Plasma and interstitial fluid volumes have been measured simultaneously in men with uncomplicated and untreated essential hypertension.
2. Plasma volume (Evans blue) was reduced in essential hypertension and correlated inversely with blood pressure.
3. Interstitial fluid volume, derived from bromine space and plasma volume, was also reduced in essential hypertension and correlated inversely with blood pressure. The mean reduction in both plasma and interstitial fluid volumes was 6–7%.
4. There is no evidence for volume expansion in essential hypertension and the relationship between plasma and interstitial fluid volumes is preserved. The contraction of plasma and interstitial fluid volumes is most likely to reflect a natriuresis and diuresis secondary to the raised renal perfusion pressure, and sodium retention is unlikely to be a primary event in essential hypertension.
