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Does carboxyhemoglobin cause a right or left shift in oxygen dissociation curve?

Discussion in 'Step I' started by NeedToStudy, Sep 28, 2014.

  1. NeedToStudy

    NeedToStudy 2+ Year Member

    Jan 16, 2014
    From what I understand when hemoglobin has increased affinity for oxygen the curve shifts left. When it has reduced affinity for oxygen it shifts right. Carboxyhemoglobin reduces the affinity of Hemoglobin for oxygen, so the curve should move right and yet in First Aid it says that Carboxyhemoglobin cause the curve to shift left and reduces the amount of oxygen unloaded into the tissues. What's going on here?
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  3. Seph

    Seph 5+ Year Member

    Jul 28, 2012
    CO strongly increases the affinity of hemoglobin for O2, which forces the curve left. But while doing so, it occupies O2 binding sites, so despite carboxyHb having high O2 affinity, each tetramer will carry less O2. Plus, even if it could carry the same number of O2 molecules, the increased affinity would make oxygen delivery to tissues impossible.
  4. NeedToStudy

    NeedToStudy 2+ Year Member

    Jan 16, 2014
    because there would be less unloading of O2 in the tissues?
  5. SBR249

    SBR249 7+ Year Member

    Mar 22, 2009
    I think CO basically locks Hb into the high affinity relaxed conformation. Normally, loading and unloading of one O2 molecule causes a conformation change that loads and unloads the other 3 binding sites (cooperativity). But since CO locks Hb in the loaded state, the other O2 in other sites will be stuck on the Hb resulting in less unloading.
  6. WhereistheRRT


    Nov 14, 2013

    Sent from my iPhone using Tapatalk
    Last edited: Sep 29, 2014
  7. seminoma

    seminoma 2+ Year Member

    Aug 9, 2014
    Yes. CO competitively inhibits O2 from binding Hb, but by doing so it also "activates" the Hb into the high-O2-affinity form thereby preventing Hb from releasing any bound O2 it may have.
  8. Robotman

    Robotman Banned Banned

    Aug 22, 2014
    Downward and to the left. If you like to put curves into relation of pharmdynamics like I do, Carboxyhemo is like a partial agonist down and to the left. 2-3 BPG, High CO2, high temp etc, are competitive antagonists. Anemias are irreverisble or noncompetitive antagonists.
  9. NeedToStudy

    NeedToStudy 2+ Year Member

    Jan 16, 2014
    There's another problem unfortunately. On page 597 it says carboxyhemoglobin causes left shift. However on 598 it says that CO2 cause rightward shift. Anyone have an answer?

    It says that an increase in all factors causes a shift to the right. So lets say all factors are the same except for CO2, the shift will go to the left. But if all factors, including CO2, are increased then there will be a shift to the right? Am I correct?
  10. Mr. Mojo Risin

    Mr. Mojo Risin On a moonlight drive 2+ Year Member

    Nov 5, 2014
    You are confusing the effects of carbon monoxide vs carbon dioxide.

    Increased CO --> left shift
    Increased CO2 --> right shift

    To help you remember, in a number sequence 2 is to the right of 1, so CO2 causes right shift. The number 1 is to the left of 2, so CO causes left shift. Hope this helps.
    Last edited: Nov 14, 2014
  11. NeedToStudy

    NeedToStudy 2+ Year Member

    Jan 16, 2014
    Thanks. Such a little detail threw me off. Sorry about that.
  12. Psai

    Psai Account on Hold Physician 2+ Year Member

    Jan 2, 2014
    carbon monoxide basically acts like o2 when it's attached to hemoglobin
    but it's not o2 so it takes up spaces on the hemoglobin
  13. seminoma

    seminoma 2+ Year Member

    Aug 9, 2014
    This is a really key point to remember. In addition to the left shift, CO always causes decreased O2-binding capacity... a fact that I always forget.
  14. syma

    syma 5+ Year Member

    Apr 17, 2011
    Binding of CO to one chain on the Hb, inceases the affinity of oxygen to the other units.
    seminoma likes this.
  15. NeedToStudy

    NeedToStudy 2+ Year Member

    Jan 16, 2014
    First Aid says that Carboxyhemoglobin decreases O2 unloading onto tissues. So I'm not really sure I understand. If the O2 is not binding to hemoglobin but not being unloaded to tissues then what happens to it?
    Last edited: Nov 19, 2014
  16. seminoma

    seminoma 2+ Year Member

    Aug 9, 2014
    O2 binding capacity = the number of O2 molecules Hb can bind. Typically the number is 4. Carboxyhemoglobin by definition has one (or more) CO molecules bound, so the O2 binding capacity is only 3 O2.

    O2 unloading is determined by left or right shift. Left shift = decreased unloading. Right shift = increased unloading.

    Thus, CO poisoning = left shift with decreased O2 binding capacity. You can think of shifts/unloading as binding affinity and capacity as amount bound.

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