Give a brief account of the counter current mechanism.

Thecounter current mechanism operating inside the kidney is the mainadaptation for the conservation of water. There are two countercurrent mechanisms inside the kidneys. They are Henle’s loopand vasa rectae.

Henle’sloop is a U-shaped part of the nephron. Blood flows in the two limbsof the tube in opposite directions and this gives rise to countercurrents. The Vasa recta is an efferent arteriole, which forms acapillary network around the tubules inside the renal medulla. Itruns parallel to Henley’s loop and is U-shaped. Blood flows inopposite directions in the two limbs of vasa recta. As a result,blood entering the renal medulla in the descending limb comes inclose contact with the outgoing blood in the ascending limb.

Theosmolarity increases from 300 mOsmolL ^-1 in the cortex to1200 mOsmolL ^-1 in the inner medulla by counter currentmechanism. It helps in maintaining the concentration gradient, whichin turn helps in easy movement of water from collecting tubules. Thegradient is a result of the movement of NaCl and urea.