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 countercurrent mechanism, first, we have to understand how a countercurrent multiplier works.
Countercurrent mechanism & Countercurrent multiplier
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.
- Concurrent flow: 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.
- Countercurrent flow: 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.
How is concentrated urine formed?
The countercurrent multiplier or the countercurrent mechanism is used to concentrate urine in the kidneys by the nephrons of the human excretory system.
The nephrons involved in the formation of concentrated urine extend all the way from the cortex of the kidney to the medulla. And they are accompanied by vasa recta. The flow of filtrate in the two limbs of the Henle’s loop is in opposite directions, and so is the flow of blood cells in vasa recta. NaCl is transported from the ascending limb of the Henle’s loop to the descending limb of the vasa recta. The ascending limb of the vasa recta, in turn, transports NaCl to the interstitium (the tissue between the loop of Henle and the vasa recta). Thus, a concentration gradient of 300mm in the cortex to 1200mm in the medulla is created (mOsm or milliosmoles is a unit of osmolarity i.e. the concentration of osmotically active substances). Urea contributes to this process by being transported by the descending limb of the loop of Henle to the interstitium.
As urine flows downwards in the collecting tubule, it encounters higher and higher concentrations of solutes in the interstitium. Hence it goes on losing water due to osmosis. This is how urine is concentrated.
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