Bio Question about the Kidney Osmolarity

[magdi1989]

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why does TISSUE osmolarity increase from cortex the inner medulla? i just dont get it! lol

 

[Kahr]

Ugh, what a nasty question you ask, the kidney was one of my worst topics during biology class all those years ago.

Well, lets start with understanding the basic idea of osmolarity. This is the solute concentration (osmol/L), it is different than molarity, but you don't need to know the specifics right now.

Okay, so we have the cortex, the outer part of the kidney. Lets see why its osmolarity is relatively low. The blood is filtered into the renal system first right (glomerulus/bowman's capsule, etc), what goes in are a bunch of solutes and water (the big stuff stays in the blood). But as this filtrate comes up to our proximal tubule, a bunch actively leaves (amino acids, Na+/Cl-, glucose, etc), so logically water flows out as well due to osmosis. So this combined leaving of water and solutes (much of which goes back into the surrounding capillaries), equates to a relatively low osmolarity (if the water couldn't flow out, the osmolarity would be higher).

Now as the tubule descends into the medulla, water continues to leave, but it doesn't sit around in the tissue, it goes into the vasa recta (blood vessel situated around the tubules). As the tubule ascends through the medulla (yes we're going through the loop of henle right now), NaCl leaves passively at first, then it is pumped out as we go up. The catch here is that the ascending tubule is impermeable to water, so no water is flowing out into the tissue.

To summarize so far: water left going down into medulla, but taken up by blood vessels, NaCl left going back up, but water stayed in the tubule. Sounds like a recipe for a high osmolar environment in the medulla's tissues, right? It is! The reason for all this, is to keep the water flowing out osmotically in the descending tubule (back into the blood though, not the tissues!).

A bit more now. As we re-enter the cortex, water can flow out (passively) of the distal tubule into the cortex's tissues, further lowering the osmolarity in the cortex.

I just want to say that I don't think knowing the why of your question is totally necessary for the DAT. From what I keep hearing, the DAT is broad, not deep. So I think just knowing the facts will be enough. But anyway, hope this helps.

Cheers!

Edit: Sorry, I should've added links to these videos beforehand, as I think anyone needing some refreshment on the kidneys should watch them; I didn't give my above explanation based on them, but they're still fantastic.

Khan academy kidney videos:
http://www.youtube.com/watch?v=cc8sUv2SuaY&feature=related (kidney and nephron)
http://www.youtube.com/watch?v=czY5nyvZ7cU&feature=relmfu (secondary active transport in nephron)

 

[magdi1989]

ugh, what a nasty question you ask, the kidney was one of my worst topics during biology class all those years ago.

Well, lets start with understanding the basic idea of osmolarity. This is the solute concentration (osmol/l), it is different than molarity, but you don't need to know the specifics right now.

Okay, so we have the cortex, the outer part of the kidney. Lets see why its osmolarity is relatively low. The blood is filtered into the renal system first right (glomerulus/bowman's capsule, etc), what goes in are a bunch of solutes and water (the big stuff stays in the blood). But as this filtrate comes up to our proximal tubule, a bunch actively leaves (amino acids, na+/cl-, glucose, etc), so logically water flows out as well due to osmosis. So this combined leaving of water and solutes (much of which goes back into the surrounding capillaries), equates to a relatively low osmolarity (if the water couldn't flow out, the osmolarity would be higher).

Now as the tubule descends into the medulla, water continues to leave, but it doesn't sit around in the tissue, it goes into the vasa recta (blood vessel situated around the tubules). As the tubule ascends through the medulla (yes we're going through the loop of henle right now), nacl leaves passively at first, then it is pumped out as we go up. The catch here is that the ascending tubule is impermeable to water, so no water is flowing out into the tissue.

To summarize so far: Water left going down into medulla, but taken up by blood vessels, nacl left going back up, but water stayed in the tubule. Sounds like a recipe for a high osmolar environment in the medulla's tissues, right? It is! The reason for all this, is to keep the water flowing out osmotically in the descending tubule (back into the blood though, not the tissues!).

A bit more now. As we re-enter the cortex, water can flow out (passively) of the distal tubule into the cortex's tissues, further lowering the osmolarity in the cortex.

I just want to say that i don't think knowing the why of your question is totally necessary for the dat. From what i keep hearing, the dat is broad, not deep. So i think just knowing the facts will be enough. But anyway, hope this helps.

Cheers!

Edit: Sorry, i should've added links to these videos beforehand, as i think anyone needing some refreshment on the kidneys should watch them; i didn't give my above explanation based on them, but they're still fantastic.

Khan academy kidney videos:
http://www.youtube.com/watch?v=cc8suv2suay&feature=related (kidney and nephron)
http://www.youtube.com/watch?v=czy5nyvz7cu&feature=relmfu (secondary active transport in nephron)

woooooow!!! Bravo!!!

Thanks you so much! This helped a ton! 🙂