4. Give a brief account of the counter current mechanism.

Answers (1)
S Sonika

The function of the countercurrent mechanism that operates inside the kidney is to conserve the water and make the urine concentrated. The countercurrent mechanism depends upon the loops of Henle, vasa recta, collecting ducts and interstitial fluid. The blood flows in the two limbs of the tube in opposite directions giving rise to the counter-currents. The proximity between the loop of Henle and vasa recta, as well as the countercurrent in them,  help in maintaining an in increasing osmolarity towards the inner medullary interstitial fluid i.e.  300 mOsmol/L in the cortex to 1200 mOsmol/L in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of the loop of Henle which is exchanged with the descending capillary of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of the loop of Henle which is transported back to the medullary interstitial fluid by the collecting duct. 

The countercurrent mechanism helps to maintain a concentration gradient in the medullary interstitial fluid which helps in easy absorption of water from the filtrate present in the collecting duct so that the concentration of the filtrate is increased. The overall function of the counter current mechanism is to concentrate sodium chloride in the interstitial fluid and cause water to diffuse out of the collecting ducts and concentrate the urine. This leads to the production of hypertonic urine.