volume change question

[StringBean]

OK, I've only gone over this a billion times & it still won't stick so i'm hoping someone can explain & help me out. (why is it that there are some things you read once & you remember... then there's those specific things you literally read 20 times & still forget? )

So why does hematocrit NOT change w/ hyperosmotic volume contraction & hyposmotic volume expansion... but it DOES change w/ hyperosmotic volume expansion & hyposmotic volume contraction?

I just keep thinking that it should change in all of them, i know there's a good reason why it doesn't change in the 2 but I can't find it... any help?

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[Idiopathic]

It can change in the first two cases, but it usually doesnt due to water shifting out of the RBC's or into the RBC's, which decreases/increases their volume respectively. Since hematocrit is the ration of RBC volume to plasma volume, in both these cases it can pretty much be normal, since the change in RBC volume will nearly parallel the change in plasma volume.

In hyperosmotic volume expansion, you are increasing plasma volume and causing water to shift out of RBC's, therefore lowering hematocrit. The reverse happens in hypoosmotic volume contraction.

 

[StringBean]

TOOOO funny... as soon as I read your reply I was like... "DUUUHHHH" that makes so much sense, what's the problem

Crazy how just having it said in a different way helps it make total sense. So simple I'm almost embarassed I asked.

Thanks!!!!!!!!!

 

[Idiopathic]

No problem...we have all been there. Good luck.