I am a high school student and I am a little confused in 2 things related to our urinary system:

  1. We know in our kidneys a countercurrent mechanism exists due to which there is a steep gradient of osmolarity is observed as we go deep inside the medulla and due to which water gets reabsorbed from the descending limb of Henle's loop into the ascending limb of vasa recta but due to the countercurrent of vasa recta osmolarity of blood doesn't changes but as collecting duct descends deep into the medulla it also loses water. Where does this water get drained? Does it get absorbed into a blood vessel? I don't think so because if the blood is also moving down with the collecting duct then it should also lose water as osmolarity increases as we go down.

  2. We know that ADH increases the water reabsorption from the distal parts of nephron via aquaporins but if more aquaporins are there the collecting duct should pass urine of 1200mosml osmolarity, shouldn't it? and in that case will the concentration gradient in the medulla be maintained? if yes then how? I am not able to imagine how our kidney would behave if blood of increased osmolarity get into it.

  1. The water doesn't get drained. As blood flows down a vas rectum, in any given section of the arteriole the tonicity is slightly higher outside the arteriole vs inside the arteriole. This results in a small amount of water flowing out of the arteriole to attempt to equilibrate the osmolarity. But before this has even finished happening, the blood has flowed down another quarter of a millimeter (or whatever) where the tonicity is even higher outside the arteriole. In other words, down the entire vas rectum the water is flowing out. But when the vas rectum comes back up, the situation is reversed, so the same quantity of water ends up diffusing back into the blood vessel. Thus the osmolarity at any given level of the medulla remains constant.

  2. Correct, the maximum osmolarity of urine is 1,200 mosm/L. The concentration gradient in the medulla will change very slightly (temporarily), but don't forget that even as the urine in the collecting tubule is being concentrated to that maximum concentration, the whole process of creating the concentration gradient is actively happening in loops of Henle all around the collecting duct. So even if the initial effect of having a bit more water in the medulla from the collecting tubule would be to slightly lower the extracellular osmolarity, in the grand scheme of things it changes nothing since the rest of the nephron is still actively creating an effective osmolarity gradient.


  • Any physiology textbook... for instance (among others), I have Human Physiology, from Cells to Systems - in the First Canadian Edition it's pp. 553-63.
  • $\begingroup$ I want to know that if urine is already getting maximum concentrated {1200mosml/lit} then how the hell ADH works? what's the point of adding more aquaporins into the collecting duct and DCT if urine is already getting maximum concentrated by absorbing water? I think urine will get 1200 mosml/lit concentration only if ADH acts on it , i.e collecting duct is not so permeable for water i.e it doesn't allow water to get maximally reabsorbed on its own, it only does it when ADH acts on it? but if that's the case then osmolarity inside medulla shouldn't be maintained because pumps are actively $\endgroup$ – Arun Bhardwaj Apr 15 at 6:54
  • $\begingroup$ continuing- pumping out Na+ ions even when the collecting duct is not allowing water to reabsorb so, the concentration should increase inside the medulla ? 1200mosml/lit is the maximum concentration in medulla only when water is getting reabsorbed from collecting duct , but when its not being done then it should increase? $\endgroup$ – Arun Bhardwaj Apr 15 at 6:57
  • $\begingroup$ @ArunBhardwaj ADH is what allows the urine to be concentrated to that extent. In the absence of ADH, the aquoporins are endocytosed, so that water is no longer able to flow down its osmolarity gradient, and the urine is consequently much more dilute. In other words, ADH is not a water-removing mechanism so much as a control mechanism - the nephron is designed such that the urine will become concentrated by default - if it can escape the tubule. The presence of ADH allows it to escape, the absence of ADH prevents its escape (via presence/absence of aquaporins). $\endgroup$ – rotaredom Apr 15 at 11:32
  • $\begingroup$ but in the absence of ADH , the Na+ pumps in the ascending limb of henle's loop should also decrease or work slowly .otherwise the concentration inside medulla would increase much higher than 1200mosml/lit ,isn't it? because now no water is coming out through collecting duct it is only coming out through descending limb of henle's loop. $\endgroup$ – Arun Bhardwaj Apr 25 at 8:17

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