Loop Of Henle

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MedPR

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I'm not understanding the ascending loop of henle.

So the solute concentration in the medulla increases as you descend deeper into it. Therefore, it makes sense that water inside the loop of henle will passively diffuse out of the descending loop since the descending loop is highly permeable to water.

Then, once you hit the turn (transition between descending and ascending loop) the fluid in the loop of henle is isotonic to the medulla.

As you ascend the thin-walled ascending loop, sodium and calcium ions passively move out since the previously isotonic loop-of-henle-fluid is now hypertonic as you ascend in the medulla.

Here's where I am lost.

1. Why is the thick-walled ascending loop impermeable to ions? Is this one of those "it just is" types of things?

2. The reason why ions diffuse into the medulla in the thin-walled ascending loop is because of the concentration gradient. In the thick-walled ascending loop, the concentration gradient doesn't really matter because the loop is impermeable to ions. So why pump out the ions?

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1. Why is the thick-walled ascending loop impermeable to ions? Is this one of those "it just is" types of things?

2. The reason why ions diffuse into the medulla in the thin-walled ascending loop is because of the concentration gradient. In the thick-walled ascending loop, the concentration gradient doesn't really matter because the loop is impermeable to ions. So why pump out the ions?

1. How it's done? Tight junctions between cells.

2. From my notes: The active transport contributes to medullary high osmolarity. It also allows water retention from the DCT (distal convoluted tubule) if ADH is released.

Hmm. I'm not sure what I meant by this last sentence with DCT and ADH. I'll look in my book, but what I think it means is that active transport can be reduced to reduce osmolarity and increase water retention since less water moves out of the filtrate by osmosis.
 
1. Because at this point, you want to dilute the urine which can't be done if the ions are permeant. (more ions pumped out than water)
2. Again, you pump out ions in the TAL because you want to dilute the urine and further reabsorb ions.
 
The ascending loop of henle is impermeable to water but permeable to solutes. On its way up, concentration of solutes inside the loop is higher than outside, therefore ions diffuse out but water stays, keeping urine dilute.
 
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The ascending loop of henle is impermeable to water but permeable to solutes. On its way up, concentration of solutes inside the loop is higher than outside, therefore ions diffuse out but water stays, keeping urine dilute.

This is the right idea, but solutes don't passively diffuse into the medulla. Rather, there are active transport pumps that cotransport sodium, potassium and chloride out of the ascending limb and into the medulla. As the filtrate ascends the loop, it is actually less concentrated than the immediate surrounding medulla, so the active transporters are needed to pump the ions against their gradient. (Note, aldosterone stimulates the formation of Na/K pumps on the ascending limb, DCT and collecting duct. Chloride and water follow the NaCl, resulting in salt and water retention, higher blood pressure, and lower overall urine volume)

This process serves to a.) dilute the urine as the body needs, and b.) ensure the osmolarity of the medulla stays high, which thus pulls water out of the descending loop of Henle. Urea actually plays a larger role in part b., but that's getting into too much detail for the MCAT.
 
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The ascending loop of henle is impermeable to water but permeable to solutes. On its way up, concentration of solutes inside the loop is higher than outside, therefore ions diffuse out but water stays, keeping urine dilute.


I'm asking about the thick ascending loop, not the thin ascending loop.
 
This is the right idea, but solutes don't passively diffuse into the medulla. Rather, there are active transport pumps that cotransport sodium, potassium and chloride out of the ascending limb and into the medulla. As the filtrate ascends the loop, it is actually less concentrated than the immediate surrounding medulla, so the active transporters are needed to pump the ions against their gradient. (Note, aldosterone stimulates the formation of Na/K pumps on the ascending limb, DCT and collecting duct. Chloride and water follow the NaCl, resulting in salt and water retention, higher blood pressure, and lower overall urine volume)

This process serves to a.) dilute the urine as the body needs, and b.) ensure the osmolarity of the medulla stays high, which thus pulls water out of the descending loop of Henle. Urea actually plays a larger role in part b., but that's getting into too much detail for the MCAT.


So you're saying that while still in the thin-ascending loop, the filtrate is MORE concentrated than the medulla, but by the time it gets up into the thick-ascending loop, so many ions have already diffused from filtrate to medulla that the filtrate is now less concentrated? Hence why the ascending loop must be impermeable in order to maintain osmotic gradient with the medulla?

Please say yes, because that makes sense!:)
 
So you're saying that while still in the thin-ascending loop, the filtrate is MORE concentrated than the medulla, but by the time it gets up into the thick-ascending loop, so many ions have already diffused from filtrate to medulla that the filtrate is now less concentrated?

I think you're on the right track. At the bottom of the loop the filtrate is isotonic to the heavily concentrated medulla (~1200 mOsm). The ascending limb is filled with ATPase pumps that actively pump out ions like sodium, potassium, chloride and the like (I believe they start as soon as the tube ascends, but the pumps are definitely mostly concentrated on the thick segment). Normally water would simply follow the solute transport via osmosis, but because all or most of the ascending limb is impermeable to it, water remains in the tubule, making the urine increasingly dilute.

Hence why the ascending loop must be impermeable in order to maintain osmotic gradient with the medulla?

Exactly. Otherwise it'd expend a lot of energy pumping out ions only to have them diffuse back in, or water diffuse back out.
 
I think you're on the right track. At the bottom of the loop the filtrate is isotonic to the heavily concentrated medulla (~1200 mOsm). The ascending limb is filled with ATPase pumps that actively pump out ions like sodium, potassium, chloride and the like (I believe they start as soon as the tube ascends, but the pumps are definitely mostly concentrated on the thick segment). Normally water would simply follow the solute transport via osmosis, but because all or most of the ascending limb is impermeable to it, water remains in the tubule, making the urine increasingly dilute.



Exactly. Otherwise it'd expend a lot of energy pumping out ions only to have them diffuse back in, or water diffuse back out.


Ok I got it. Thank you!
 
Hey Guys,
I was confused on how the overall function of the loop of henle is to increase the solute concentration and therefore increase the osmolority of the medulla and when the filtrate enters the descending loop the water leaves so the solute concentration would increase but when it goes back up and the sodium ions leave it seems like you are countering the effect? So how exactly does the solute concentration increase?
Thank You!!
 
There are more solutes than just sodium. The purpose of the loop is to preserve things like sodium, potassium, glucose etc.

Everything that doesn't have an active transport mechanisms somewhere in the loop will contribute to the increased solute concentration of the urea.
You keep pumping ions out so that water diffuses into the medulla. That makes the water still in the collecting duct more concentrated with all the "garbage" you don't want in your body anyway.
 
If you have water leave the descending limb of the loop of henle, the osmolarity in the surrounding Medulla would go down, so on the way up the ascending loop of henle, NaCl can leave to re-establish the osmolarity in the surrounding Medulla, but water from here on OUT, to DCT, cannot leave, until you reach the collecting duct
 
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