Kidney - osmolarity/reabsorbtion

[IndianVercetti]

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alright, so I understand the mechanism of how the loop of henle and the collecting duct and convoluted tubule reabsorb solutes/H2O (and also which reabsorbs/secretes what)

However, I was wondering if someone could explain why certain areas of the nephron reabsorbtion/secretion system have active transport, while others have passive transport. I know it has to do with the osmolarity gradient, but I was wondering if someone could explain what would be required knowledge of this topic as for the MCAT.

thanks.

 

[StarryNights]

I kno, I got confused on this too since I used Kaplan and it talked about concentration gradient but didn't really go in depth. All I got out of it was that the ascending portion of the loop of Henle (permeable to salt but not water) is the only place where sodium is reabsorbed passively thanks to gradient. If no one replies to this thread, I'm going to have to look online.

 

[PiBond]

alright, so I understand the mechanism of how the loop of henle and the collecting duct and convoluted tubule reabsorb solutes/H2O (and also which reabsorbs/secretes what)

However, I was wondering if someone could explain why certain areas of the nephron reabsorbtion/secretion system have active transport, while others have passive transport. I know it has to do with the osmolarity gradient, but I was wondering if someone could explain what would be required knowledge of this topic as for the MCAT.

thanks.

the high osmolarity of the medulla is what causes water to move out by osmosis as the filtrate moves down the descending loop. and the bottom of the loop of henle the filtrate is wicked concentrated and as it enters the ascending loop it needs to become less concentrated (thus actively transporting out ions like Na+, K+, and Cl-). it all boils down to the fact that the body needs these ions to concentrate the interstitium so that water moves out of the filtrate (and into the vasa recta) so we dont piss it out and lose a ton of water. there's also a heat component to it that reduces the amount of heat lost

that's what i remember from back in the day but you might want to double check some of my facts

 

[IndianVercetti]

the bottom of the loop of henle the filtrate is wicked concentrated and as it enters the ascending loop it needs to become less concentrated (thus actively transporting out ions like Na+, K+, and Cl-).

yea, this is what would make sense, if the Na+, K+, Cl- were being ACTIVELY transported (reabsorbed), since the inner medulla has a high solute concentration. Maybe kaplan has an error in it, when it shows these ions being PASSIVELY reabsorbed??

 

[PiBond]

yea, this is what would make sense, if the Na+, K+, Cl- were being ACTIVELY transported (reabsorbed), since the inner medulla has a high solute concentration. Maybe kaplan has an error in it, when it shows these ions being PASSIVELY reabsorbed??

yeah, it's definitely active transport....Na+K+ ATPase and a K+ Cl- symporter i think

 

[StarryNights]

I read through my old bio notes, and it says that moving down deeper into the medulla and hitting the concentrated part is what causes water to move out of the descending limb (since it's not permeable to salt). Then as the ascending limb moves up and up, the outside gets less and less concentrated to the point where filtrate is hypertonic to the outside so Na+ leaves passively. Wth?? Is this what Kaplan meant? I'm seriously confused now.

 

[IndianVercetti]

I read through my old bio notes, and it says that moving down deeper into the medulla and hitting the concentrated part is what causes water to move out of the descending limb (since it's not permeable to salt). Then as the ascending limb moves up and up, the outside gets less and less concentrated to the point where filtrate is hypertonic to the outside so Na+ leaves passively. Wth?? Is this what Kaplan meant? I'm seriously confused now.

I checked it up, pretty much here's what I learned- Na+ is ALWAYS actively reabsorbed, especially in the inner medulla cause the interstitial solute concentration is just so friggin high.

I'm not sure which kaplan version book you're using, but I'm using the new colored version, and I think they screwed up and showed Na+ being passively reabsorbed in the inner medulla instead of actively reabsorbed.

I'm still confused why Cl- is suddenly ALWAYS being passively reabsorbed??

 

[IndianVercetti]

OHH it's obvious: Cl- just tags along with Na+ as it the Na+ is being actively reabsorbed. Nice! It feels good to figure something out!

 

[PiBond]

I checked it up, pretty much here's what I learned- Na+ is ALWAYS actively reabsorbed, especially in the inner medulla cause the interstitial solute concentration is just so friggin high.

I'm not sure which kaplan version book you're using, but I'm using the new colored version, and I think they screwed up and showed Na+ being passively reabsorbed in the inner medulla instead of actively reabsorbed.

I'm still confused why Cl- is suddenly ALWAYS being passively reabsorbed??

just a guess, but there could be an electrical gradient (having so many positive ions in medulla interstitium) that it's more significant than its concentration gradient. once again, just speculation. regardless i think it's beyond the scope of the MCAT

 

[BennieBlanco]

alright, so I understand the mechanism of how the loop of henle and the collecting duct and convoluted tubule reabsorb solutes/H2O (and also which reabsorbs/secretes what)

However, I was wondering if someone could explain why certain areas of the nephron reabsorbtion/secretion system have active transport, while others have passive transport. I know it has to do with the osmolarity gradient, but I was wondering if someone could explain what would be required knowledge of this topic as for the MCAT.

thanks.

This is probably beyond the MCAT.

The PCT can actively transport/diffuse Na+. It has a symporter to move glucose into the PCT cell but then diffuses out of the cells into the capillaries. HCO3- diffuses from the PCT cells to the capillaries. There is also passive diffusion of CL, K, Ca, Mg, Urea, H2O in the PCT. Sodium and H+ have an antiporter also here.


In the vasa recta there is a symporter to move in Na/K/Cl but then Na is actively transported into the vasa recta. Cl- can diffuse. Cations can just diffuse into the vasa recta also (including Na, K, Ca2+, Mg2+).

Last we see in the DCT and collecting duct that Na can diffuse into the principal cells only to be actively transported via a sodium/potassium pump, where it can diffuse into the capillaries (after going into the interstitial fluid).

As you can see, #1 it is complicated, and #2 the MCAT couldn't expect students to have this much detail down for reabsorption/secretion.

Often the reason people don't have clarity in these questions is: #1 it is overly detailed and someone tried to explain it in a prep book in a paragraph. Or #2 it is just to heavy for the MCAT.

There is no simple, diffusion happens here and active transport happens here. Everything is basically happening everywhere.