Counter current exchange occurs in

Explanation

Mammals have the ability to produce concentrated urine. The Henle.s loop and vasa recta plays an important role in this. The flow of the renal filterate in the two limbs of Henle.s loop is in opposite directions and thus form counter current. The flow of blood through vasa recta is also in counter current pattern. The proximity between the Henle.s loop and vasa recta, as well as the counter currents of renal fluid and blood in them help in maintaining an increasing osmolarity towards the inner medullary interstitium. This gradient is mainly caused by NaCl and urea. NaCl passes out the ascending limb of Henle.s loop, and it enters the blood of the descending limb of vasa recta. NaCl is returned to the intestitium from the ascending portion of the vasa recta. Similarly small amounts of urea enter the thin segment of ascending limb of Henle.s loop which is transported back to the interstitium, from the collecting duct. Transport of these substances facilitated by the special arrangement of Henle.s loop and vasa recta is called the counter current mechanism.
`" "` This mechanism helps to maintain a concentration gradient in the medullary interstitium. Presence of such interstitium gradient help easy passage of water from the collecting duct, there by concentrating the urine.