TBR: Expiring Air from Lungs Question

[justadream]

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TBR Bio Book I page 38 #39

“What is observed during normal expiration?”

One of the correct answers is “Moles of oxygen exceed moles of CO2”

TBR says “expiration occurs to displace the CO2, so it must have a greater mole fraction of CO2 than oxygen”


I have 2 problems with this

1) Shouldn’t CO2 movement only be affected by the CO2 concentration gradient (not the concentration gradient of any other gas like O2)? Using TBR’s logic, then let’s imagine a scenario in which you are trying to get rid of 2 substances from the blood (Substance X and CO2) that have the same exchange rate (as in, leave the blood just as easily to enter the alveoli):


Inspired Air: Partial pressure of both =0.0000.

In the Blood: PX = 1000mm Hg and PCO2 = 990mm Hg

Let’s assume 50% efficiency in transfer:

Expired air: PX = 500mm Hg and PCO2 had 445 mmHg

So according to TBR’s logic, this CO2 cannot be expired because it’s partial pressure is less than that of Substance X?



2) Is it really true that in the expired air, PO2 is < PCO2? I can’t find any information from textbooks supplying info on this. All they say is that the PCO2 is 40mm Hg in the expired air. The PO2 in the inspired air (in the alveoli) is 100mm Hg but it is unclear how much this decreases from exchanging with the capillaries. )

 

[DrknoSDN]

1) Shouldn’t CO2 movement only be affected by the CO2 concentration gradient (not the concentration gradient of any other gas like O2)? Using TBR’s logic, then let’s imagine a scenario in which you are trying to get rid of 2 substances from the blood (Substance X and CO2) that have the same exchange rate (as in, leave the blood just as easily to enter the alveoli):

No, Hemoglobin has cooperative binding with oxygen so oxygen being present along with 2,3-BPG, and the entire bicarb mechanism cause additional release at the blood/gas exchange. The example given is not really relevant considering the complexity of the respiratory system. There is simplification, and then there is oversimplification. Gotta avoid the latter.

 

[justadream]

@DrknoSDN

Okay, so even if you consider cooperative binding (and thus, that CO2 movement is affected by O2 movement and vice versa)

Does PO2 have to be greater than PCO2 in order for respiration to work?