Vasa recta

 

Vasa Recta Function

The vasa recta, the capillary networks that supply blood to the medulla, are highly permeable to solute and water. As with the loop of Henle, the vasa recta form a parallel set of hairpin loops within the medulla.

 Not only do the vasa recta bring nutrients and oxygen to the medullary nephron segments but, more importantly, they also remove the water and solute that is continuously added to the medullary interstitium by these nephron segments. The ability of the vasa recta to maintain the medullary interstitial gradient is flow dependent. A substantial increase in vasa recta blood flow dissipates the medullary gradient. Alternatively, decreased blood flow reduces oxygen delivery to the nephron segments within the medulla. Because transport of salt and other solutes requires oxygen and adenosine triphosphate, reduced medullary blood flow decreases salt and solute transport by nephron segments in the medulla. 

As a result, the medullary interstitial osmotic gradient cannot be maintained, which also reduces the ability to concentrate the urine.