Explanation

The flow of blood through the two limps of vasa recta is in counter current pattern (opposite) The proximity between the Henle’s loop and vasa recta as well as the counter current in them help in maintaining an increasing osmolarity toward the inner medullary interstitium. i.e., from 300 $mos\ mol\ L^{- 1}$ in cortex to about 1200 $mos\ mol\ L^{- 1}$ in the inner medulla. This is also called medullary gradient