I don't understand the Loop of Henle and what it does.

[Kemosabe]

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I get that the descending end moves water passively into extracellular tissue (interstitial fluid) via osmosis because on the ascending end Na+ is moved into extracellular tissues via the ATPase. This is the counter-current multiplier and what is responsible for keeping the mendulla hypertonic to blood. Since the bottom of the loop of the loop of henle is isosmotic to the mendulla would sodium be able to diffuse passively through it?
Is this the main purpose of the loop of henle and do I have this idea down correctly? From what I understand it also plays a role in concentrating urine, but I don't quite understand how it does. I'm thinking that based off of the process of transporting h2o and ions it causes urine to be dilute, which is why fluid in the distal tubule is generally hypotonic, but I also saw an exam krackers question that said something like "if filtrate is more concentrated along the descending loop of henle then the urine produced will be more concentrated", why is that?

Also, do I have the idea of the countercurrent multiplier down correctly?

 

[NextStepTutor_3]

Awesome questions - the loop of Henle is one of the most confusing concepts in all of biology! You have the right general idea, but I did want to explain the part about the concentration of urine. The wording of this statement tends to give people the wrong idea.

You're totally right that fluid is generally more dilute when it leaves the loop of Henle than when it enters. You may have seen diagrams with osmolality values that describe this very effect - fluid entering the loop is usually depicted at 300 mOsm/L while fluid that reaches the distal tubule is closer to 100. With this in mind, the loop of Henle still facilitates the concentration of urine when it is necessary. This occurs entirely due to the fact that the collecting duct runs parallel to the loop.

Like you said, sodium is actively transported out of the ascending loop, causing a net loss of solute from the filtrate. This process, as part of the countercurrent multiplier, helps establish and maintain the increasing solute gradient as we travel down the medulla. Now, let's return to the collecting duct, which also travels down the medulla, into increasingly more hypertonic surroundings. When the collecting duct is impermeable to water, this has little effect, and we can excrete fairly dilute urine. However, when ADH is present (as in cases of low blood pressure) the collecting duct becomes more permeable and water can diffuse out. What promotes the diffusion of water from collecting duct into extracellular fluid? The fact that the medulla is so concentrated, allowing water to keep leaving the duct even as the urine becomes increasingly concentrated. This is what people usually mean when they say "the loop of Henle helps concentrate urine." Filtrate doesn't become more concentrated IN the loop, but without the loop's countercurrent setup, the concentration process would be far less effective later in the nephron.

Any other questions? The kidney is such a vast topic that it's hard to mention everything!

 

[shefv]

Just to clarify:

- I know that water moves out of the descending loop of henle via osmosis and is carried away by the vasa recta which helps to maintain the concentration gradient (i.e. more solute in the surroundings, and less water) so that osmosis can continue to occur. So if there are already more solutes around the loop of henle, then why is Na+ reabsorbed from the ascending side?

- Is it the reabsorption of Na+ from the loop of henle that makes the medulla hypertonic, thereby helping to create a concentration gradient to assist with the movement of water from the collecting ducts?

- If no ADH is present, is there some water still reabsorbed back due to this concentration gradient? can we say that ADH works to increase that movement of water out of the collecting duct?

 

[NextStepTutor_3]

To answer your last question first: if no ADH is present, little to no water is reabsorbed from the collecting duct! That's because, even though the concentration gradient exists, the duct is usually impermeable to water. ADH functions to promote the insertion / activation of aquaporins (water channels) in the lining of the collecting duct; without these channels, water is mainly trapped in the duct.