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Victoria_5254
#1 Posted : Saturday, July 10, 2021 1:40:46 AM
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Hello!

I have a question about osmoregulation in the kidneys.

I understand why the filtrate in the proximal convoluted tubule is more concentrated than the primary filtrate (from the Bowman's capsule). But how does this not affect osmolarity? Our book says that it stays the same. If the filtrate is more concentrated, wouldn't that mean that osmolarity would increase?

Thanks for the help!
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INSTR_Molly_129
#2 Posted : Saturday, July 10, 2021 2:30:17 AM
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Hi Victoria,

I'm not finding the reference that the filtrate is becoming more concentrated, so please let me know if I'm not quite addressing your question properly and I'm happy to talk more about it with you :)

At the proximal convoluted tubule, the volume decreases, but the concentration does not increase to become more concentrated. At the proximal convoluted tubule, reabsorption of water and other ions are happening at the same time. As water is reabsorbed, the volume of the filtrate decreases. However, as we also reabsorb NaCl, bicarb, and potassium at this site, so the osmolarity will remain the same.

At the descending loop of Henle, the concentration does change and the filtrate becomes more concentrated. The osmolarity would therefore also change to become higher.

I hope that helps. Cheers and happy studying!

Molly
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Victoria_5254
#3 Posted : Saturday, July 10, 2021 3:32:49 AM
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Hi Molly,

Thanks for this!

I thought I heard it in the med reels video 2.05.17: Nephron Function (Bio/Biochem CC2 > Lesson 5: Digestion and Excretion). At about 1 minute and 50 seconds in... I may be completely misunderstanding what the instructor said, though! Especially because your explanation makes a whole lot of sense.

Thanks again.
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INSTR_Molly_129
#4 Posted : Saturday, July 10, 2021 4:55:11 PM
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Happy to help!

Cheers,

Molly
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