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Kidney Q

Discussion in 'MCAT Study Question Q&A' started by futuredoctor10, Jan 5, 2009.

  1. futuredoctor10

    Classifieds Approved 10+ Year Member

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    What is the "counter-current multiplier mechanism" of the kidney?

    AAMC lists this as a topic under BS > Excretory System!
     
  2. swamprat

    Physician Classifieds Approved 10+ Year Member

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    It has to do with the kidneys ability to concentrate urine. It involves the recycling of urea(600 mOsm) and Na+(300 mOsm) and Cl-(300 mOsm) and basically you have an osmotic gradient on the outside of the loop as you go deeper into the medulla of the kidney. At the bottom of the loop its 1200 mOsm. I don't rlly have any more details off hand but I don't think they would really get that much more in depth into it either(someone correct me if im wrong).
     
  3. thatscorrect7

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    i think thats what the multiplier means.

    what is the counter current exchanger?
     
  4. swamprat

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    The fluid is going in opposite directions. Down the descending limb deeper into the medulla and then up toward the cortex through the ascending limb. The same deal goes with the peritubular capillaries that wrap around the limbs to absorb/secrete stuff. It's like that so you keep even distributions of everything(from what i remember).
     
  5. not so calm now

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    I'm going to tack on another kidney question.

    When solutes are getting reabsorbed, does this mean going back into plasma? or is it interstitial fluid?
     
    #5 not so calm now, Feb 4, 2009
    Last edited: Feb 27, 2009
  6. aevea

    aevea New Member
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    Counter current: the blood flow of the capillaries of the vasa recta is opposite in direction of the flow of filtrate in the loop of Henle. That is, as the filtrate flows towards the medulla through the descending limb, the neighboring capillaries carry plasma towards the cortex and as the filtrate flows towards the cortex in the ascending loop, the capillaries carry the blood towards the medulla.

    Counter current exchange: The descending loop of Henle is permeable to water but not ions and the medulla has a higher solute concentration than the cortex. So, as the filtrate descends, water flows out and is absorbed into the capillaries and carried towards the cortex.

    The ascending loop o' Henle is permeable to ions but not water (!), so as the filtrate ascends, ions diffuse out and are carried by the neighboring capillaries towards the medulla. Along the ascending loop ions are actively pumped out of the filtrate and also carried towards the medulla, ultimately establishing the osmotic gradient. The gradient that's established through active transport is "multiplied" by the counter current exchange.

    The pay off: The immediate pay off is reducing filtrate volume by reclaiming both water and ions from the filtrate into the plasma. The secondary pay off is establishing the concentration gradient so that ADH can control urine concentration by altering the permeability of the collecting ducts which pass through the same gradient.
     

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