Urea enhancement of water reabsorption in the renal medulla

Abstract

In the dog, osmotic diuresis produced by the infusion of urea results in a higher rate of abstraction of solute-free water, at any given osmolal clearance, than does mannitol or sodium chloride diuresis. It is proposed that the ratio of urea to nonpermeating solute within the fluid entering the collecting duct system will influence the degree to which the urine can be concentrated and to which solute-free water can be abstracted from an isotonic precursor fluid. Urea, as a result of its ability to permeate the collecting duct epithelium, is relatively ineffective in opposing the osmotic force resulting from the presence of a hypertonic sodium chloride solution in the peritubular space. The osmotic movement of water out of the collecting duct is consequently limited primarily by the concentration of nonpermeating solute within the tubular fluid. Since during the infusion of urea the relative concentration of nonpermeating solute within the tubular lumen is reduced, the potential for water abstraction from this fluid is accordingly enhanced.