Biology Asked on June 21, 2021
I am a high school student and I am a little confused in 2 things related to our urinary system:
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.
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.
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.
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.
Answered by rotaredom on June 21, 2021
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