STUDIES ON THE RENAL CONCENTRATING MECHANISM. II. EFFECT OF SMALL ACUTE CHANGES IN SOLUTE EXCRETION*

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Abstract
The effect of small changes in osmolal clearance on the osmotic urine/plasma ratio achieved after 18 hours of water deprivation was studied in healthy men. Changes in osmolal clearance over the range of 0.9-5.1 ml/minute and in urine flow over the range of 0.24-1.71 ml/minute were observed in the resting state, after venesection, after light exercise and during mannitol and urea loading. The osmotic urine/plasma ratio was not constant. There was a significant inverse relationship between osmotic urine/plasma ratio and osmolal clearance at low rates of solute excretion and urine flow.