Human Kidney

[notsowisetooth]

In the Human Kidney:

a) water is actively transported from the loop of henle to the medulla

b)water is actively transported from the medulla to the loop of henle

c) salt is passively transported from the loop of henle to the medulla

d) salt is actively transported from the loop of henle to the medulla

e) salt is actively transported from the medulla to the loop of henle.


The answer is D. The explanation says: In the Kidney, salt is actively transported from the loop of Henle to the medulla. The water follws passively along its osmotic gradient.

But I'm looking at my Kaplan quicksheet, and in the inner medulla, salt is passively transported out of the loop of Henle. What's going on?

Is this question just too general?

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[dfymarine]

Kaplan is wrong.


salt is polar, it can not pass through the nonpolar membrane without any facilitated mechanism.

 

[Dannelid]

dfymarine is right. The way i remember it is that the concentration of solutes in the medulla is high, so it must be active transport to go from lower concentration in the loop of henle to higher concentration in the medulla.

 

[UCB05]

dfymarine is right. The way i remember it is that the concentration of solutes in the medulla is high, so it must be active transport to go from lower concentration in the loop of henle to higher concentration in the medulla.

Moreover, it's how the kidney maintains its concentration gradient Water passively moves into the medulla in the descending loop, salt actively transported in the ascending loop. Without the active transport, the gradient would even out and the kidneys would be poop.

 

[Dannelid]

Without the active transport, the gradient would even out and the kidneys would be poop.

Hah you're very right. Poop indeed; interesting juxtaposition.

 

[notsowisetooth]

Okay, so salts are never passively transported in the kidney?

 

[Shinpe]

Kaplan is wrong.


salt is polar, it can not pass through the nonpolar membrane without any facilitated mechanism.

Passive diffusion has nothing to do with being polar or not. It just means that the particle is going down its electrochemical gradient. If it's polar, transport channels are probably necessary, just like in water, but this would be called facilitated diffusion and is still passive since no ATP is used. Active diffusion is when energy (ATP or ion gradient) is used to pump a particle opposite to its electrochemical gradient.

The descending limb of the loop of Henle (LoH) is permeable to water and not ions so water passively diffuses out as it encounters more and more concentrated medulla interstitial fluid.
The ascending loop has two parts however: first a thin portion and then a thick portion. Thin portion is impermeable to water but permeable to solute (by facilitated diffusion) and so NaCl diffuses out here as you go up the medulla and see more dilute interstitial fluid. When you reach the thick segment, this part doesn't allow passive diffusion of NaCl but instead actively pumps it out in order to make the lumen hyperosmotic.

Some books ignore the role of the thin ascending loop, I think because the thick part plays the major role. But I still think that question is badly worded as both D and C can be right. I would have still chosen D though because that contributes more to the whole situation.

 

[notsowisetooth]

ahhhhhhh.

The role of the thin part of the ascending loop of Henle was throwing me off.

Thank you Shinpe.

 

[gatorPredent]

So water is mainly reabsorbed at the proximal convulated tubule. Also at the collecting duct. Can the descending loop of henle be included as well?

 

[nze82]

This should clarify how the nephron works:
http://www.youtube.com/watch?v=glu0dzK4dbU

 

[Shinpe]

So water is mainly reabsorbed at the proximal convulated tubule. Also at the collecting duct. Can the descending loop of henle be included as well?

About 70% of the filtrate water +salt is reabsorbed in the proximal tubules. Water reabsorption in the descending loop of Henle helps with the gradient in the medulla. Water reabsorption in the collecting ducts helps fine-tune the water content in response to ADH levels.

 

[soliqas]

Kaplan isn't wrong, this question is too general. Salts are passively transported from the lower part of the ascending loop into the inner medulla while salts are actively transported from the thicker part of the ascending loop into the outer medulla. This comes straight from Campbell's biology study guide.

 

[notsowisetooth]

I might be thinking about this too much, but what I don't understand is if water is being transported out of the descending LoH to the medulla, but then salts are being actively transported out of the ascending LoH to the medulla, how is this producing an osmotic gradient?

Wouldn't this be counterproductive as water is diffusing to the the medula (diluting interstitial fluid), but then salt is being transported into the medulla as well (counterproductively concentrating the interstitial fluid)?


Does that make any sense?

 

[nze82]

I might be thinking about this too much, but what I don't understand is if water is being transported out of the descending LoH to the medulla, but then salts are being actively transported out of the ascending LoH to the medulla, how is this producing an osmotic gradient?

Wouldn't this be counterproductive as water is diffusing to the the medula (diluting interstitial fluid), but then salt is being transported into the medulla as well (counterproductively concentrating the interstitial fluid)?


Does that make any sense?

This is exactly why they call it the "Counter Current Exchange System." As the filtrate travels down the descending loop, it loses water to its surrounding (Medulla) via osmosis.
When the filtrate reaches the descending arm, salt is actively transported into the medulla. Why?

Well, think about it....
If water was transported into the medulla without salt being secreted into the medulla, then eventually the osmotic pressure inside the descending tube would equal the osmotic pressure of the medulla. Under such circumstances, there would be no net movement of water molecules. Subsequently, water loss would come to a halt. However, as long as salt is being secreted into the medulla, the solute concentration of medulla remains higher than that of the solute inside the descending limb, allowing osmosis to happen and water loss to continue.

Hope this helps!

 

[Shinpe]

This should clarify how the nephron works:
http://www.youtube.com/watch?v=glu0dzK4dbU

That video ignored the passive diffusion of salt in the thin ascending limb. Me no likey lol

 

[notsowisetooth]

this is exactly why they call it the "counter current exchange system." as the filtrate travels down the descending loop, it loses water to its surrounding (medulla) via osmosis.
When the filtrate reaches the descending arm, salt is actively transported into the medulla. Why?

Well, think about it....
If water was transported into the medulla without salt being secreted into the medulla, then eventually the osmotic pressure inside the descending tube would equal the osmotic pressure of the medulla. Under such circumstances, there would be no net movement of water molecules. Subsequently, water loss would come to a halt. However, as long as salt is being secreted into the medulla, the solute concentration of medulla remains higher than that of the solute inside the descending limb, allowing osmosis to happen and water loss to continue.

Hope this helps!

oooooh!

 

[nze82]

That video ignored the passive diffusion of salt in the thin ascending limb. Me no likey lol

Passive diffusion of salt in the ascending limb?!
As long as I'm concerned salt is actively transported into the medulla, and that was the whole point of the video and this discussion!

 

[Shinpe]

Passive diffusion of salt in the ascending limb?!
As long as I'm concerned salt is actively transported into the medulla, and that was the whole point of the video and this discussion!

Read my earlier post, and your Campbell bio book or probably any physiology book. The thin portion of the ascending limb is permeable to salt but not water and so salt passively diffuses out. It's only in the thick segment where salt is actively diffuses out. Again, the thin portion is very often ignored because it is the thick portion that plays the main role in setting up the concentration gradient and not the thin portion.

 

[nze82]

Read my earlier post, and your Campbell bio book or probably any physiology book. The thin portion of the ascending limb is permeable to salt but not water and so salt passively diffuses out. It's only in the thick segment where salt is actively diffuses out. Again, the thin portion is very often ignored because it is the thick portion that plays the main role in setting up the concentration gradient and not the thin portion.

Which is probably why the video ignored it

 

[Shinpe]

Which is probably why the video ignored it

Well that doesn't make it right.

Mainly to answer the OP's question. It's asking what happens in the human kidney. Not "which of these is mainly responsible for creating the osmotic gradient in medulla?" If that had been the question, the answer would definitely be D. But the way the question is right now, I think it has two answers, C and D.

 

[nze82]

Well that doesn't make it right.

Mainly to answer the OP's question. It's asking what happens in the human kidney. Not "which of these is mainly responsible for creating the osmotic gradient in medulla?" If that had been the question, the answer would definitely be D. But the way the question is right now, I think it has two answers, C and D.

How about you never watch that video again if you don't like it so much?
That's the easiest solution that comes to my mind at this point.
I believe I answered the OP's question in my previous post.

 

[notsowisetooth]

yo...

you guys both answered my question.

Thank you. I appreciate it

 

[Shinpe]

How about you never watch that video again if you don't like it so much?
That's the easiest solution that comes to my mind at this point.
I believe I answered the OP's question in my previous post.

How about you stop being defensive and take some criticism when you're not entirely right? or at least take a chill pill

The OP's question was that he/she read in Kaplan that there is passive diffusion in inner meddulla (where the thin ascending limb is), which is correct, but that wouldn't be the right answer, because the question ignored that fact. The middle medulla has the thick portion and actively transports salt. Right now you are the telling the OP to forget about this whole thing, which I don't agree with, because the people who are writing the DAT are going to know better (or at least be more precise) than those who wrote that question or made that video.

And don't worry, I won't watch the video again. It has a good general view but all I was saying is that it's not quite to the point for this question.

 

[nze82]

How about you stop being defensive and take some criticism when you're not entirely right? or at least take a chill pill

The OP's question was that he/she read in Kaplan that there is passive diffusion in inner meddulla (where the thin ascending limb is), which is correct, but that wouldn't be the right answer, because the question ignored that fact. The middle medulla has the thick portion and actively transports salt. Right now you are the telling the OP to forget about this whole thing, which I don't agree with, because the people who are writing the DAT are going to know better (or at least be more precise) than those who wrote that question or made that video.

And don't worry, I won't watch the video again. It has a good general view but all I was saying is that it's not quite to the point for this question.

Great! We finally came to an agreement.
And I'm not the one who made that video, so I can care less whether people like it or not. I think it's a good review of the general function of the kidney, so I posted it.

 

[Shinpe]

Great! We finally came to an agreement.
And I'm not the one who made that video, so I can care less whether people like it or not. I think it's a good review of the general function of the kidney, so I posted it.

Cheers .........

 

[orangutan777]

should we know both the countercurrent and two solute models of the kidney (as presented in shaum's)? active/passive transport depends on the model (at least in the acending loop)