Descending loop of Henle

[Lothric]

Hey,

FA says that the thin descending loop of Henle reabsorbs water via medullary hypertonicity. I don't understand - if the medulla is hypertonic, that means it has alot of solute concentrations. Shouldn't water then STAY in the medulla rather than getting reabsorbed? In fact, I think water should move TOWARDS the medulla so that the concentration lowers.

I thought water moved from low concentration to high...?

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[goodvibes27]

Hi,
See as you said the medulla is hypertonic, so basically the interstitial tissue around the descending loop of henle is hypertonic.
As the urine enters the descending loop of henle water will be reabsorbed, aka it moves from the renal tubular lumen into the medullary interstitium. This would result in the increased concentration of the urine.
And yeah, water will move from hypotonic / low concentration / low osmolality to hypertonic / high concentration / high osmolality.

And diffusion of particles occurs from high concentration to lower concentration. Example, diffusion of gases in the lungs.


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[iBS1972]

I think what OP is asking is if the medulla is hypertonic, or when solute is being actively pumped out of the the ascending limb of LoH, why doesn't water leave the peritubular vessels as well?
I will attempt to answer this. You are right, water stays in the medulla, not much of it gets back into the peritubular vessels (aka vasa recta). The osmolarity of the the afferent renal vessel (supplying the nephron) and the efferent vessel (leaving the nephron) is about the same. In fact, the blood leaving the nephron is actually slight more concentrated than the blood entering the nephron--so you are absolutely correct.

This is only to answer your question. But it also leaves a lot of questions unanswered, such as "if this is the case, how does the kidney give back water to our blood when we are dehydrated?" In short, ADH. In the collecting ducts there's aquaporins that allow free flow of fluid from the lumen out into the medulla. The fluid in the lumen is almost always going to be hypotonic to the interstitium of the medulla (because much of the solute has been actively pumped out of the lumen in ascending limb of henle). If hypothalamus detects high blood osmolarity, it will secrete ADH, which signals the collecting duct cells to insert more aquaporins into the membrane, allowing even more fluid from the collecting tubule to enter interstitium. What happens then depends on interstitial osmolarity and vasa recta osmolarity.

Hope this answered your question.