The osmolarity is about 1200 mOsmL^-1 in

There is 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. The proximity between the Henle’s loop and vasa recta, as well as the countercurrent in them help in maintaining this gradient.