Carbon Monoxide

[happyfellow]

The inspiration of carbon monoxide gas will result in:

A) an unchanged arterial pO2
B) a lowered arterial pO2
C) an increased arterial pO2
D) an increased percentage of hemoglobin saturated with O2

Answer is A. I was able to eliminate C and D with no problem. However, I don't understand why the answer is A. I know CO has an stronger affinity for hemoglobin than oxygen does...therefore there should be less oxygen in our blood because CO is displacing oxygen from its binding site at the RBC in the lungs, right?

Thanks for the help

-happy

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[docelh]

I'm with you on the overall concept. After breathing CO over a period of time, pO2 should fall as the body consumes whatever O2 is left in the blood. The issue with CO is not that it displaces O2, as that would be driven by partial pressure differences in the blood and alveoli air; but rather, once CO bonds to hemoglobin, it is extremely difficult to be displaced regardless of whether O2 partial pressure becomes high again in the lungs.

Not trying to rationalize the answer here (or maybe I am), but maybe they're referring to inhaling after one breath (if so, it seems like a silly question). If it's just one breath, the pO2 entering the lungs from the right ventricle is low and the pCO2 is high. pCO would exchange with pCO2 via diffusion and the blood coming back into the left atrium is high in pCO and has the same pO2.

 

[loveoforganic]

B seems right to me too.

 

[Charles_Carmichael]

My reasoning is that dissolved O2 is the only form of O2 that produces a partial pressure (O2 bound to hemoglobin does not contribute to the pO2 of blood). While CO would affect the amount of O2 bound to hemoglobin, it would have little to no effect on the dissolved O2 (since blood has a finite capacity to store dissolved O2).

A is the correct answer then.

 

[FutureInternist]

My reasoning is that dissolved O2 is the only form of O2 that produces a partial pressure (O2 bound to hemoglobin does not contribute to the pO2 of blood). While CO would affect the amount of O2 bound to hemoglobin, it would have little to no effect on the dissolved O2 (since blood has a finite capacity to store dissolved O2).

A is the correct answer then.