The concentration gradient in the renal medulla is mainly caused by

There is an increasing osmolarity in the medullary interstitium i.e., from 300 mOsmolL^-1 in the outer medulla 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 the vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta. 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.