Water and sodium exchange in renal tubule fluid

¹ Department of Physiology, Washington University School of Medicine, Saint Louis, Missouri

The entry and exit of water into and out of the renal tubular fluid of dogs was measured by infusing tritium-labeled water (THO) into the renal artery either during (entry) or before (exit) a period of stopped urine flow. The results show that THO moves more slowly between urine lying in the most distal nephron segments (presumably collecting ducts) than between urine lying in more proximal segments and blood plasma. From the reduced rate of labeling of renal medullary, as compared with cortical, tissue with THO and Na²² infused during stopped urine flow, it is concluded that the slow exchange of collecting duct with plasma water is partly due to a slow rate of labeling of the renal medullary extratubular water and Na by the blood stream. From the failure of collecting duct water and Na to come into isotopic equilibrium with medullary water and Na either during a 5-minute period of stopped urine flow or during the subsequent collection of a few stop-flow urine samples, it is concluded that the collecting duct wall offers significant resistance to diffusion of THO and Na²² and thus also contributes to the slow exchange of these substances between plasma and collecting fluid. The bearing of these results on present theories about the way in which the kidney handles water and Na is discussed.