Explain the Counter Current Mechanism?
What is counter current mechanism? Counter current mechanism is the mechanism by which the kidneys produce osmotically concentrated urine. Let us see how. In the proximal convoluted tubule (PCT), the substances like glucose and amino acids are reabsorbed mainly as isotonic solution. Most of the reabsorption occurs here and ' this is the obligatory absorption. If more solute is to be absorbed, more water is reabsorbed and vice versa. This process mainly occurs in the PCT. When the filtrate reaches the loop of Henle, it is still isotonic (having the same or equal osmotic pressure) with the blood plasma. According to Wirz, the filtrate becomes concentrated as it passes down the descending limb of the loop. This portion of the loop lies in the renal medulla and is hyperosmotic with respect to the plasma. Hence water is lost from the descending limb to make the tubular fluid also hyperosmotic.
In the ascending limb, a reverse set of actions occurs as you have already studied earlier under the water balance function of kidneys. The tubule passes from a hyperoslnotic zone to a hypoosmotic zone. To maintain the balance there is an active secretion of sodium chloride unaccompanied by water in the ascending limb, so that the tubular fluid becomes hypoosmotic with the plasma. This mechanism is known as the hair pin counter current multiplier mechanism. The filtrate as it passes down the descending limb of the loop of Henle, gets multiplied in its content of sodium which is through the interstitium and to descending limb through the secretion in the ascending limb. The vasa recta (arterial capillary) act as a counter current exchanger system, faithfully exchanging ions with the interstitium. Through these countercurrent mechanisms, kidney can maintain osmolarity between 300 mOsm/L to 1200 mOsm/L. So far we studied about the contribution of the kidneys in urine formation and its role in maintaining the pH and performing other regulatory functions. Besides these, did you know that the kidneys also have non-excretory functions?