The osmolarity of the inner medullary interstitium is maintained by

The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help in maintaining
an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL^–1 in the cortex to about 1200 mOsmolL^–1 in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule.