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#1
02-05-2012
 USMLE Forums Addict Steps History: Not yet Posts: 101 Threads: 21 Thanked 11 Times in 10 Posts Reputation: 21
Isotonic solution and the ECF volume!

If an isotonic solution is added to the body, then because the solutes also come along it causes an increase in volume in the ECF (expansion). But there is no change in osmolarity in the ICF. Doesn't water normally follow solutes? So if there is an increase in solute then how come ICF doesn't decrease in volume? How does ECF increase in volume because of the solutes? For it to increase doesn't it mean that ICF should decrease?

#2
02-05-2012
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 85 Threads: 13 Thanked 22 Times in 15 Posts Reputation: 32
Isotonic solution doubt

Osmolarity depends on the total number of solute particles
Tonicity depends onosmotic pressure or the effective osmolarity =reflection quotientXosmolarity
The reflection quotient is the measure of permiability and 0 means fully permiable and 1 means no permiabilty.
A hyperosmotic solution may be isotonic if one of the solutes is fully permiable as then it will diffuse across the memb and carry water along with it.
eg.Solutions A and B are separated by a semipermiable memb .A contains 1mMol sucrose and 1mM urea.B contains 1mM sucrose.The reflection coefficient of sucrose is one and that for ures is zero.Which of the following statements is correct?
1.Solution A has a higher effective pressure than B
2.Solution Ahas a lower effective pressure than B
3.Solutions A and B are isoosmotic
4.Solution A is hyperosmotic and solutions are isotonic
5Solution A is hyposmotic and solutions are isotonic

#3
02-05-2012
 USMLE Forums Addict Steps History: Not yet Posts: 101 Threads: 21 Thanked 11 Times in 10 Posts Reputation: 21

Thanks so much for the reply. Why does the volume increase when an isotonic solution is added to the ECF?
#4
02-06-2012
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 85 Threads: 13 Thanked 22 Times in 15 Posts Reputation: 32
Isotonic solution

Quote:
 Originally Posted by usmle_taker Thanks so much for the reply. Why does the volume increase when an isotonic solution is added to the ECF?
ECF is a bag of fluids if some more solution is added to the bag the volume has to increase as fluids are incompressible, if it was a gas the volume could remain the same but the pressure would increase.
 The above post was thanked by: usmle_taker (02-06-2012)

 Tags Electrolytes-, Physiology-

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