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Homework Help: Test question: Respiring cells release CO2 into the bloodstream

  1. Feb 10, 2013 #1
    Test question: "Respiring cells release CO2 into the bloodstream..."

    1. testquestionrespiringcells_zps600ae6a9.jpg

    2. For this question, I was thinking about the nature of CO2.
    It's not an acid or base, so how could it drop or raise the pH level?!
    So I put that the pH level won't change (B), but the real answer is C (it will decrease slightly).

    How is this? How can a neutral substance make the blood more acidic?
    Or is it really NOT neutral...? Hm.:confused:

    Would you please explain why the pH value gets lowered?

    Thank you so much! :)
  2. jcsd
  3. Feb 10, 2013 #2


    Staff: Mentor

  4. Feb 10, 2013 #3
    Re: Test question: "Respiring cells release CO2 into the bloodstream..

    Okay, I read the article you posted.

    I hope I understood correctly.

    So is it that once cells use up oxygen, CO2 is released and then absorbed by red blood cells.
    But once the CO2 is absorbed, it combines with H2O to form H2CO3, which dissociates into
    H+ and HCO2-.

    The H+ plus attached to a hemaglobin, and the HCO2- gets pumped out of the red blood cell. The red blood cell takes in Cl- in its place.

    So now the HCO2- is in the blood plasma.

    Once the blood gets to the lungs, the HCO2- gets back into the red blood cell and the Cl- gets kicked out. The H+ detaches from the hemaglobin, and so H2CO3 is formed again.

    Now the H2CO3 gets broken down again into water and CO2.
    The CO2 diffuses into the lungs and is breathed out. ...I guess the water stays.

    Okay, makes sense. But weren't they asking about the pH right at the moment when the CO2 leaves the cell (before taken up by the red blood cells)?

    And they couldn't be talking about when the HCO2- gets kicked out of the red blood cell and into the plasma because then the blood would be more basic, right?
    HCO2- wants a H+, so it is a base, right?
    ...So I still am not sure as to how the blood gets more acidic when CO2 leaves cells! :/
    Hm. =_=
  5. Feb 10, 2013 #4


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    Staff: Mentor

    Re: Test question: "Respiring cells release CO2 into the bloodstream..

    Adding CO2 to the system is equivalent to adding H2CO3 to the system. Further reactions can lower the effect, but they can't change the direction - adding acid always lowers pH.
  6. Feb 10, 2013 #5
    Re: Test question: "Respiring cells release CO2 into the bloodstream..

    Oh, okay! Since the CO2 combines with water to form H2CO3 (carbonic acid), the blood becomes more acidic.

    I guess it was required for us to know the carbonic acid step in order to answer this question!
    ....(The chapters the test was over did not include information about how CO2 and O2 are expelled and taken up by cells!)

    Ugh. =_=# Oh well.

    Thank you so much, Borek and jedishrfu! Makes sense now! :D
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