Explanation

Ascending limb of vasa recta. The gradient of medullary region is primarily by $NaCl$ and urea. $NaCl$ is transported by ascending limb of Henle’ loop which is exchanged with descending limb of vasa recta. Similarly small amount of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to interstitium by collecting tubule. The above described transport of substances facilitated by special arrangement of Henle’s loop and vasa recta is called counter current mechanism