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Countercurrent mechanism in the kidney
Make it clearer
The interstitium of the renal medulla is the tissue that surrounds the tubules deep inside the kidney, and it is very salty. Making it that way is exactly what the countercurrent mechanism does.
The trick lies in the shape of the loop of Henle, the stretch of tubule that dips down into the medulla and comes back up, like a hairpin. The two limbs run side by side but in opposite directions, and each one behaves differently. The limb that goes down lets water out into that salty interstitium. The limb that goes up pulls out solutes, the dissolved substances such as salt, but does not let water through, so the fluid that moves on is more dilute.
The result is two things at once: deep tissue that gets saltier and saltier, and fluid that reaches the next stretch of tubule diluted.
The water that was reabsorbed does not stay there: it returns to the circulation through the vasa recta, the long, straight blood vessels of the kidney.
In brief
The countercurrent mechanism is how the loop of Henle makes the interstitium of the renal medulla, the tissue surrounding the tubules, very salty.
The loop is a hairpin: the descending limb lets water out into that hypertonic interstitium, and the ascending limb pulls out solutes but not water, a split that dilutes the urine.
Two tubes side by side running in opposite directions, each releasing something different into the tissue around them.
The reabsorbed water returns to the circulation through the vasa recta.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingHistology, Nephron, StatPearlsPublished
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