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Thread: Why don't all RBCs burst

  1. #1 Why don't all RBCs burst 
    Forum Freshman
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    Sep 2012
    Hello all.
    Just wondering why red blood cells don't all osmotically burst.

    Here's what I am thinking:
    RBCs are filled with lots of haemoglobin
    Haemoglobin is a molecule so should contribute to osmolarity
    The osmolarity in RBCs should therefore be really high
    Water flows from low to high osmolarity
    Shouldn't water flow into the RBCs from the blood plasma then, and cause the RBCs to burst

    Obviously this doesn't happen in real life. So why don't RBCs osmotically burst? Any thoughts?

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  3. #2  
    Administrator KALSTER's Avatar
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    Sep 2007
    South Africa
    Well, the plasma is not pure water. I'd guess it is probably in equilibrium.

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    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
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  4. #3  
    Forum Freshman
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    Sep 2012
    Yeah, I think you are right, it made me think about it again differently.

    My main difficulty was that I though each particle of haemoglobin exerted 1 osmol of oncotic pressure, and there are a lot of haemoglobin particles in a RBC. But plasma is only around 300 milli-osmols. (Plasma osmolality - Wikipedia, the free encyclopedia)
    But it turns out that apparently the concentration of haemoglobin in RBCs is only around 5 mmol (Mean corpuscular hemoglobin concentration - Wikipedia, the free encyclopedia).
    And then it hit me, you'd need a whole mole (a really big number) of haemoglobin to exert 1 osmol of oncotic pressure! haha
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  5. #4  
    Forum Professor Zwirko's Avatar
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    Sep 2008
    55 N, 3 W
    Haemoglobin is not diffusing or being transported across the membrane. Remember, osmosis is about small soulte molecules diffusing (usually) across semi-permeable membranes.

    I think perhaps the concept of tonicity is what you are looking for rather than osmolarity, since haemoglobin is unable to diffuse across the membrane. For example, a hypertonic solution will cause water to flow out from a RBC, while a hypotonic one will cause water to flow in to the cell( possibly bursting it in the process).

    As I think Kalster implied, concentrations of water and other stuff should be in equilibrium more or less on both sides of the meembrane.
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