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loop of henle and urine concentration

Discussion in 'MCAT Study Question Q&A' started by dextor2003, Jul 3, 2010.

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  1. dextor2003

    dextor2003

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    144. Long loops of Henle on juxtamedullary nephrons allow
    for greater concentration of urine. For an individual with
    highly concentrated urine, filtrate entering the loop of
    Henle is likely to be:

    A. more concentrated than filtrate exiting the loop
    Henle.
    B. less concentrated than filtrate exiting the loop of
    Henle.
    C. more voluminous than filtrate exiting the loop of
    Henle.
    D. less voluminous than filtrate exiting the loop of
    Henle.

    A is correct. The loop of Henle concentrates the medulla via a net loss of solute to the medulla. This process is
    critical to the function of other parts of the nephron; a medulla with a high concentration of solute allows for
    the passive absorption of water from the filtrate in other areas of the nephron.

    i dont think i get the explanation. tell me if my thinking is correct. so a concentrated urine means either too much solutes in the filtrate, or not enough water in the filtrate (ie, too much water absorption). and water is passively reabsorbed into the medulla as you go down the descending loop of henle...and that happens because water was initially absorbed right before that in the proximal tubule because of the overall net secretion of solutes into the filtrate (so the filtrate had a higher osmotic pressure than the surroundings)...and i dont know where to go from here...i get all the facts, but im not sure how to connect them to get to the answer
  2. rls303

    rls303

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    I think what it means to say is that the filtrate is already so concentrated. Once it goes up through the ascending loop of henle, it looses some solutes to the surrounding medulla. However, since his urine is so concnetrated, he would probably have had a very concentrated filtrate coming in already.

    I think that this is what the passage was trying to convey, someone please correct me if I'm wrong.
  3. MCAT guy

    MCAT guy ...

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    Lets try to clear up the thinking here...

    Concentrated urine I would say you are correct (little water or lots of solute in filtrate). The reason the water is reabsorbed upon descending the loop is not because of any overall net secretion, it is because the medulla is super concentrated and if there's much of any water in that loop, the osmotic pressure will pull out the water into the medulla. Your next sentence is 100% correct though.

    This is an ok question I guess. What they are trying to test you on is this:

    Lets say a guy has really high concentrated urine, really high. Well why do we have the urine go down this loop losing water but then lose a bunch of solute on the way up the loop?

    The purpose of this whole procedure is to ensure that the medulla is HIGHLY concentrated, which means that it can pull water out of urine when necessary at the collecting duct (Why? So before urinating we can pull water out if our body needs more water, using ADH). So we need the medulla really concentrated. How do we do this? first we go down the loop, pulling out water (concentrating filtrate), then we go up the loop and pull out a bunch of Na+ actively (concentrating the medulla), this is really a set up phase, like I said before, so that we can pull out extra water when we want.

    So all the question really wants you to understand is that the point of the loop of henle is to concentrate the medulla, which allows up to pull out water later on (collecting duct). So if a guy has concentrated urine, it will tend to lose solute to concentrate the medulla.
  4. dextor2003

    dextor2003

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    i think i understand your thinking...i think (maybe im wrong on this though) that i was reading the question incorrectly. i read it as asking what caused the concentrated urine in the first place..and since there could be many reasons for that, i wasnt able to connect any of the answer choices to the question...if that makes any sense..but i understand it now much better

    thank you !
  5. Kaushik

    Kaushik I'm on a horse. Moderator Emeritus

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    You're looking at this wrong. The urine isn't really concentrated/diluted before entering the Loop of Henle. Concentration/dilution primarily occurs at the late distal tubule and collecting duct (based on ADH levels). So you won't ever really see a question asking about concentration/dilution of the filtrate before the distal tubule/collecting duct.
  6. rls303

    rls303

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    So basically what you are saying is that we get little ion reabsorption in the ascending loop of henle as well as the distal tubule, because there is already such a high concenreation of the ions in the plasma?

    Thanks in advance!
  7. MCAT guy

    MCAT guy ...

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    Kaus is right, I interchangeable used the word "urine" and "filtrate", while I should have just used to word "filtrate". The concepts are all correct, but the terminology was off a bit.

    If you understand how it works (conceptually), you shouldn't miss any MCAT questions on it.

    Good luck.

    In the end the idea is, the filtrate came into the loop highly concentrated, and left in a lower concentration... then made its way to the collecting duct at the end.

    Another way of looking at this could be,

    if the urine is really concentrated at the end, then I probably had to pull out a lot of water, correct? Well if I had to pull out a lot of water then I had to have a highly concentrated medulla, correct? Well if I had a highly concentrated medulla then I had to add lots of solute to it when it went UP the loop of henle... which means I had a HIGH concentration of filtrate entering the loop and then was able to take some of that concentration and pull it out when the loop ascended.
  8. fingerscrossedd

    fingerscrossedd

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    what a great, helpful thread - a few years later, and this thread just helped elucidate a very important concept!
  9. brood910

    brood910

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    BUMP. I understand that highly concentrated urine means the filtrate entering the loop is highly concentrated too, but how do you know if it is "more" concentrated than filtrate exiting the loop?
    Doesnt ADH makes urine even more concentrated by helping water absorption? According to EK's diagram, urine [] is the highest in the duct with ADH present.

    EDIT: -_-........ I read the choice as exiting the duct, not the loop. I am stupid. Never mind guys.
    Last edited: Jan 29, 2014
  10. Iatro

    Iatro

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    Its too long a description to write out, but I think it is easier to think of loops of henle on a more macroscopic level. The deeper you go into the medulla, the more concentrated the salt in the surrounding tissues, thus the greater the osmotic pressure driving the H2O out. Water reabsorption just so happens to occur on the descending and salt on the ascending. If it was reverse it (unfortunately for biology it is not) it would be much easier to understand!

    Khan academy has a good video on this very topic.

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