Vasa Recta - Vertebrate Kidney

The accumulation of water reabsorbed from the descending limb of Henle's loop or the collecting tubule would disturb the osmotic concentration gradient in the medulla. The capillary loops of vasa recta, which run parallel to the loop of Henle, remove the excess water and thereby maintain the concentration gradient in the medullay interstitium.

The vasa recta loops are freely permeable to water and solutes. As blood passes down the Vasa recta, water diffuses out into the interstitial fluid and salt diffuses in. Blood therefore becomes increasingly concentrated as it passes down in the vasa recta towards the renal papilla. When blood ascends towards the cortex, water moves in and salt moves out. The vasa recta also removes any excess salt that might accumulate in the medullae interstitium.