The effects of cortisone and ACTH on blood and extracellular fluid volumes have been studied in normotensive subjects without renal or cardiac disease, under conditions of (1) salt restriction and (2) controlled salt intake. When salt was restricted, blood volume did not change significantly but the volume of fluid available for dilution of radiosulfate increased slightly, the increment being derived from internal sources. When salt was supplied, extracellular fluid expanded considerably but venous pressure and plasma protein concentration did not change significantly. Blood and plasma volume remained constant or fell slightly as extracellular fluid volume expanded, and failed to reflect day-to-day fluctuations in the interstitial space. The stimulus which promotes renal adjustment to an excess of interstitial fluid apparently need not involve blood volume, venous pressure or plasma oncotic pressure.
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