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Kidney Question

Discussion in 'MCAT Study Question Q&A' started by supertrooper66, May 31, 2008.

  1. supertrooper66

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    If you inject NaCl into the renal tubules, the filtrate osmolarity increases and the urine volume increases. I know the filtrate osmol. obviously goes up from the NaCl, but i don't necessarily understand why the urine volume increases. I just thought it would produce more concentrated urine. However, apparently, it increases urine volume because water flows into the tubules to make up for the increase in solute concentration. I don't get how and WHERE the water flows in from, though. In the setup of the nephron and whatnot, water is always leaving. Down the descending loop, the membrane is permeable to water and it flows out since generally the sodium outside the tubule is higher. Then we get to the actual loop and ascending part, which is only permeable to Na+. it leaves passively and through some active transport higher up the loop. Water also leaves the tubules at the proximal convoluted tubule and collecting duct. like i said, the water always flows out, tho, not in. so for this injecting NaCl question, that means the water can only possibly flow in from the proximal tubule, descending loop, or collecting duct. which one is it? also, if this is true, then it goes against the normal function of water leaving rather than entering. i'm confused on that.

    i understand we are increasing the Na+ levels in the tubules, but why do we need water to come in to compensate for that? why can't the ascending loop take care of it by removing the excess Na+ like it normally does?

    i hope this wasn't too confusing haha. thanks
     
  2. minhaj

    minhaj Awesomeness Incarnate
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    "I don't get how and WHERE the water flows in from, though."
    The water flows in from the renal medulla through osmosis.

    "In the setup of the nephron and whatnot, water is always leaving. Down the descending loop, the membrane is permeable to water and it flows out since generally the sodium outside the tubule is higher. Then we get to the actual loop and ascending part, which is only permeable to Na+. it leaves passively and through some active transport higher up the loop. Water also leaves the tubules at the proximal convoluted tubule and collecting duct. like i said, the water always flows out, tho, not in."

    While you recognize the movement of water under normal conditions, you must also remember that there are conditions that aren't ideal. The situation the question presents isn't ideal. The concentration of the solute in much greater in the tubules than in the renal medulla. Usually this is the other way round, which results in water flowing out to make the tubule isotonic with the medulla. However, in this question the since the tubule has a greater concentration of solute water flows in down the osmolarity gradient, in order to make the tubule and the renal medulla isotonic.

    "i understand we are increasing the Na+ levels in the tubules, but why do we need water to come in to compensate for that? why can't the ascending loop take care of it by removing the excess Na+ like it normally does?"

    Its not that we need to water to come in to compensate for increased Na+ levels, its the nature of osmosis. water flows in from a solution with low solute conc. to a solution with high solute conc.

    I hope that was helpful.
     
  3. supertrooper66

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    okay, so the descending loop is permeable to water both ways...in and out. well, duh, i guess it would have to be. the water is getting out through pores in the membrane, so if the concentrations are flipped there's no reason the water can't flow back in through those same pores. yeah, i get it. thanks!
     
  4. BloodySurgeon

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    If you inject enough NaCl into the renal tubules that the transportation max has been exceeded then the NaCl will be excreted and in the collecting ducts the osmolarity concentration increases and the ADH is inhibited and water is diluting the urine through the aquapores, increasing the urine volume.
     
  5. supertrooper66

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    oh yeah, i completely forgot about the aquaporins. so basically, you're still getting the standard Na+ leaving passively through the inner medulla and actively through the upper medulla (all this through the ascending loop) as well as through the cortex (distal convoluted tubule). the thing is you still have so much Na+ in there since you injected it in there manually. once you've reached the collecting duct, generally you have low Na+ inside compared to outside so the water flows out through the aquaporins. HOWEVER, in this case, you still have a higher concentration of Na+ inside than outside, so water flows IN through the aquaporins in the collecting duct. is that correct?

    i think the question would have been easier for me if they specifically said what part of the nephron they were injecting the NaCl in rather than just saying "renal tubule." haha
     
  6. Vihsadas

    Vihsadas No summer
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    A general rule that is helpful for kidney reabsorption and excretion problems is "Water follows Na+".

    GL!
     
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  7. Kaustikos

    Kaustikos Archerize It
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    Precisely what I thought. With an increase of Na+, you have a relatively higher amount of Na in the renal tubule that will go through the loops/etc. Now I know this is not the word of Vihsadas or anyone else, but I just follow this logic to help me out.

    An increased concentration of solute means less water will be followed out and more will be excreted through urine. I don't think it's that more water flows out, but less is secreted in the loops because of the higher [Na+] in the tubules in general. But I've basically turned off my advanced physiology training and dumbed myself down for the MCAT in order to help answer these questions and it's turned out extremely helpful.
    I'm sure bloodysurgeon is probably right with aquaphore channels, but I think what Vihsadas has the simpler answer you should be looking for.
    Who knows, though.:smuggrin:
     
  8. Doctor D

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    "Down the descending loop, the membrane is permeable to water and it flows out since generally the sodium outside the tubule is higher."

    Yes this is true however there will be less of a difference now since the solute is more concentrated. Therefore less water will leave than if the NaCl had never been added.
     
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