Counter current system : (a) The counter current system is formed by two limbs of Henle's loop. Another is formed by vasa recta.
(b) In this the glomerular filtrate flows in opposite directions in two limbs.
(c) The filtrate entering descending limbs flows close to that which is leaving the ascending limb.
(d) Sodium and chloride ions are reabsorbed from the ascending limb into the surrounding medullary tissue where it is retained by vasa rectae. .
(e) Water cannot flow out of this limb as it is impermeable to water. So, its contents become progressively dilute.