The image in the above link shows that CO2 and O2 both pass through the same membrane but still CO2 is exhaled and O2 is taken in the body.

Why doesn't the CO2 mix with O2 and stay in the blood ?

Why is only CO2 passed out through the membrane?


2 Answers 2


You actually inhale and exhale both CO2 and O2, what changes is the relative amounts of each.

Besides tiny percentages of various gases and non-gas particles, air contains primarily nitrogen (78%) and oxygen (21%). Carbon dioxide makes up roughly 0.04% of the air you inhale.

By the time you exhale, that air now contains about the same amount of nitrogen, but the amount of oxygen has decreased to around 15% and the amount of carbon dixoide has increased by about a hundred times to around 4%.

On the level of individual cells, both gases pass through the membranes in both directions, what matters is how much passes in each direction. As @porkchop explains, that depends on the relative concentrations. When it reaches the lungs, blood has relatively high CO2 and relatively low O2, compared to the outside air that you inhale. As long as the air stays inside, the two will equilibrate. There will be some molecules going each direction - some CO2 molecules will go into the blood, but many more will leave it and go into the air, and some O2 molecules will go into the air, but many more will come from the air into the blood. As a result, the net flow is CO2 going out of the blood, and O2 into the blood. Diagrams usually only illustrate the net flow.

  • $\begingroup$ But if the CO2 attains equilibrium shouldn't there be 5 % of CO2 also stay in the blood and shouldn't the concentration increase with time? $\endgroup$
    – Poin
    Nov 5, 2018 at 3:16
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    $\begingroup$ 5% isn't an appropriate way of describing it because blood isn't a gas, it's a liquid with CO2 solved in it. But yes blood leaving the lungs should have about the same "CO2 content" as exhaled air, and as it runs through the body it accumulates more CO2. There can be conditions under which more CO2 accumulates throughout the body than the blood can get rid of in the lungs (i.e. suffocation), and we also have a programmed responses to that condition (gasping). $\endgroup$
    – Armatus
    Nov 5, 2018 at 13:54
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    $\begingroup$ Blood is never completely devoid of CO2, but that doesn't mean the baseline increases over time. How much CO2 is left in the blood when it leaves the lungs depends on the concentration of CO2 in the inhaled air. As long as that's constant, the blood will always end up at the same baseline CO2 content after undergoing gas exchange. $\endgroup$
    – Armatus
    Nov 7, 2018 at 14:47
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    $\begingroup$ @poin yes. But CO2 in blood is also being converted to carbonic acid and eventually bicarbonate, so the chemistry is a bit more complicated $\endgroup$ Dec 8, 2022 at 11:54
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    $\begingroup$ @Poin Don't forget about the breathing part: blood and lung gas match in the lungs, then you breathe out the lung gas and bring in new gas that matches the outside air, then diffusion happens again until they match and so on. If you stop breathing yes, CO2 continues to accumulate in the blood and lungs and you asphyxiate. $\endgroup$
    – Bryan Krause
    Jan 10 at 14:23

It all comes down to concentration gradients. In deoxygenated blood, which is what enters the lungs, the O2 concentration is relatively low and the CO2 concentration is higher than that of ambient air. On the flip side, the air that we breathe in has a higher concentration of O2 than deoxygenated blood. The alveolar membrane is thin enough that the gases can diffuse across this gradient almost instantly. And just like that, you have oxygenation of blood, and removal of CO2.


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