2+ Year Member
Sep 20, 2014
Medical Student (Accepted)
Hi everyone,
I've been having a bit of confusion concerning the circulatory system and how different factors affect the arterial pO2. One of these questions was answered previously, but the explanation still made no sense to me.
Thanks for any help!

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.

The answer: A
An unchanged arterial pO2. Carbon monoxide has a very strong affinity for the iron on the heme of hemoglobin which is responsible for binding oxygen. For this reason, the number of hemoglobins able to transport oxygen is decreased. Does this decrease the arterial pO2? No. The reason is that only the oxygen dissolved in the plasma contributes to the partial pressure. The oxygen on hemoglobin is bound and is thus not soluble in the fluid. Thus, carbon monoxide will not affect the arterial pO2.

I chose C. Wouldn't a higher concentration of CO in the blood lead to more O2 displaced from the hemoglobin, and thus a higher amount of O2 dissolved in the blood which leads to a higher arterial pO2? I'm really confused by this :/

Which of the statements below is FALSE regarding pCO2 during exercise?
A. The alveolar pCO2 determines arterial pCO2
B. Alveloar ventilation increases
C. Venous pCO2 increases
D. Arterial pCO2 increases

D is correct, arterial pC02 increases. During exercise, it is clear that the cells will be using more oxygen and thus producing more carbon dioxide. For that reason, the levels of venous pC02 will increase. Will this lead to a rise in the arterial partial pressure of carbon dioxide? The answer is no, because the rate of ventilation will increase. We know that we breathe "harder" when we are exercising. In other words, the excess carbon dioxide :is released as a result of the higher rate of ventilation. The increase in the carbon dioxide production is equivalent to the increase in breathing rate. The result of this is that the arterial partial pressure of carbon dioxide does not change, making D a false statement. The correct choice is D.

The bigger question I have for this question is which arteries are they referring to when they state arterial pCO2? Because if there is more CO2 being picked up by the veins, wouldn't the pulmonary arteries' pCO2 increase because the deoxygenated blood has to go through the pulmonary pathway in order to rid itself of the excess CO2? Or are they only referring to the systemic arteries when they talk about arterial pCO2?

Also, how does the alveolar pCO2 determine arterial pCO2? Is it because the amount of CO2 absorbed by the alveoli in the lung's capillaries determines how much pCO2 is left in the blood when it reaches the systemic arteries? Or is it because the amount of CO2 absorbed by the alveoli is directly proportional to the amount of CO2 initially propelled to the lungs by the pulmonary arteries?

I guess the biggest question I have is which circulation's arteries are referred to in "arterial pCO2"?
Thanks for any help or feedback! I feel like there are gaps in my knowledge which are seriously bothering me..:(
May 30, 2015
Now that I look back at these, I feel like drawing out equilibria and using Le Chatelier's principle would be pretty useful.
E.g. for #95:
O2 (g) <----> O2 (aq)
O2 (aq) + Hb <----> Hb-O2
Hence, the strong CO binding to Hb effectively removes Hb from the 2nd equation, shifting that rxn to the left to produce more O2 (aq). You'd be right if we were only looking at the 2nd eqn, but increasing O2 (aq) should shift 1st rxn to the left and hence increase O2 (g). Hence, you should expect: decreased Hb-O2, relatively unchanged serum O2 (aq), and increased alveolar O2 (g).
A more intuitive reasoning would be: solubility of free gas in a solvent is relatively constant and sensitive primarily to temperature; so unless temperature changes, solubility shouldn't change.

For 98, B and C are easy to eliminate; I agree that arterial pCO2 sounds ambiguous as it could increase in pulmonary but decrease in systemic, so let's compare A and D and see the worse option.
For A:
CO2 (g) <----> CO2 (aq)
Equilibrium is a 2-way causal link. Hence, if higher gas, then higher aqueous. ALSO, if higher aqueous, then higher gas. The latter absolutely matches choice A.
For D:
Yeah you can argue that pCO2 can increase in pulmonary arteries, but you can also argue that pCO2 will be unchanged or may be even lower if the person is undergoing hyperventilation (choice B, which we already know is true).
Hence, D has more loopholes you can exploit than does A
In short, don't see questions like this as content gaps (you clearly know your stuff); just see it as an opportunity to practice POE skills. Also, the above equilibria are my way of simplifying the problem to its bare bones to more clearly see the answer - do this when reviewing the problem. When doing timed sets, use your intuition more since that's quicker.
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2+ Year Member
Sep 20, 2014
Medical Student (Accepted)
Thank you so much for this explanation! It makes a ton more sense now.