What is the role of Vasa recta in the concentration of urine ?

Explanation

1. The vasa recta, maintains the medullary osmotic gradient via counter current exchanger (the flow of blood through the ascending and descending vasa recta blood vessels) by passive transport.
2. The picture shows counter current exchanger where the vasa recta preserve the medullary gradient while removing reabsorbed water and solutes.

3. This system does not produce an osmotic gradient, but protects the medulla by removal of excess salts from the interstitial fluid and removing reabsorbed water.
4. The vasa recta leave the kidney at the junction between the cortex and medulla. The interstitial fluid at this point is isoosmotic to the blood.
5. When the blood leaves the efferent arteriole and enters vasa recta the osmolarity in the medulla increases (1200mOsm) and results in passive uptake of solutes and loss of water.
6. As the blood enters the cortex, the osmolarity in the blood decreases (300mOsm) and the blood loses solutes and gains water to form concentrated urine (hypertonic).
7. Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.