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Also Read: Urine Formation and Osmoregulation

Unless one is drinking very large quantities of water, the urine has to be excreted in a concentrated form. Otherwise, the body will lose a lot of water and the person will suffer from the effects of dehydration and low blood pressure. “The mechanism that the kidneys use to concentrate urine is called the countercurrent mechanism .” In order to understand the countercurrent mechanism, first, we have to understand how a counter current multiplier works. Also Read:  Urine Formation and Osmoregulation

Let us say there are 2 tubes, through which a solution of the same substance is flowing. There is a free exchange of the solution between the two tubes. There can be two kinds of flow through these tubes.

Here, the solutions in the two tubes flow in the same direction. If at one end, one of them starts at 0% concentration and the other starts at 100% concentration. By the time they reach the other end of the tubes, the concentrations in each tube will be roughly 50%, as shown in the figure.

Here the solutions in the two tubes flow in opposite directions. In one tube 0% concentration of the solution starts to flow from one end, and in the other tube, 100% concentration of the solution starts to flow from the opposite end. Due to the free exchange of the substances between the two tubes, by the time the solutions reach the end of the tube, it will have acquired a concentration equal to the other tube at that end. This will become clear from the figure. Also read:  Renal Failure

Following are the important steps of the countercurrent mechanism:

Thick ascending Loop of Henle Transport

Equilibration of descending thin Loop of Henle

The counter-current multiplier or the countercurrent mechanism is used to concentrate urine in the kidneys by the nephrons of the  human excretory system .

Countercurrent mechanism in Henle’s loop

The  nephrons  involved in the formation of concentrated urine extend all the way from the cortex of the kidney to the medulla and are accompanied by vasa recta. The filtrate flows into the two limbs of the Henle’s loop in the opposite directions and therefore,  the flow of blood cells in vasa recta is also in the opposite directions. The concentrated urine is formed in the following ways:

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