The gradient of increasing hyperosmolarity of medullary interstitium is maintained by a counter current mechanism and the proximity between the Henle’s loop and vasa recta.
This gradient is mainly caused by NaCl and urea. The transport of these substances is facilitated by the special arrangement of Henle’s loop and vasa recta.
There are two aspect of this mechanism
(i) Counter current multiplication (by the Henle’s loop).
(ii) Counter current exchange (by the vasa recta).