Drainage of plasma proteins from the renal medullary interstitium in rats

1. Lymph vessels are scarce or lacking in the renal inner medulla, raising the question of whether plasma proteins entering the medullary interstitium are removed by diffusion through the interstitium to lymphatics in the outer medulla or cortex, or by convection into the vasa recta. 2. Using micropipettes, we infused 125I-albumin into the papilla of anaesthetized rats and watched its disappearance from the injection site as well as the uptake in the thoracic duct and plasma. 3. Tracer infused into the renal cortex appeared almost immediately in the thoracic duct lymph, and rose to a sevenfold higher concentration than in plasma, whereas tracer infused into the papilla appeared first and increased more sharply in plasma than in the lymph. No spread from the papillary injection site was observed. Tracer injected in renal hilar lymphatics was quantitatively recovered in the thoracic duct. 4. The plasma concentration pattern following papillary infusion was similar to that obtained by intravenous injection, indicating uptake in blood and subsequent distribution to extracellular fluid and lymph from all organs. 5. We conclude that plasma proteins normally diffusing out from the vasa recta are brought back through water flux (1) from the collecting ducts due to the high sodium chloride concentration in the papillary interstitium and (2) from the interstitium into the vasa recta driven by plasma protein osmotic pressure. Accordingly, there is no need for lymph vessels in the inner medulla.