BRS phys ch1 q9

[bigslick]

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From BRS Physiology (5th), chapter 1 quiz, #9:

In a hospital error, a 60 year old woman is infused with large volumes of a solution that causes lysis of her RBCs. The solution was most likely:

A) 150 mM NaCl
B) 300 mM mannitol
C) 350 mM mannitol
D) 300 mM urea
E) 150 mM CaCl2

The explanations say the correct answer is D b/c the reflection coefficient of urea is <1.0, and therefore 300 mM urea is hypotonic. But I was thinking urea is completely permeable to cell membranes (reflection coefficient of 0), and so it shouldn't cause water to undergo osmosis? The explanations also mention about how 150 mM NaCl and 300 mM mannitol are isotonic.. does anybody know how they found this out (like by calculation) or is it just memorized?

 

[Morsetlis]

It's correct: urea (small) and glycerol (uses the water channels) have a reflection coefficient of 0, or near it. Therefore, 300 mM urea solution ~= pure water! RBC's would be more likely to lyse in pure water.

The only other reasonable choice is E, but pure water would be a better choice.

 

[bigslick]

oh that makes sense.. thanks!

 

[ijn]

The explanations also mention about how 150 mM NaCl and 300 mM mannitol are isotonic.. does anybody know how they found this out (like by calculation) or is it just memorized?

For STEP 1 purposes definitely memorize that normal plasma osmolality is around 280-300 mOsm. The calculation they did to determine that mannitol and NaCl were isotonic was multiplying by the # of osmotically active particles that each dissociates into when in a solution (1 and 2 respectively.)