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https://www.msdmanuals.com/home/lung-and-airway-disorders/biology-of-the-lungs-and-airways/exchanging-oxygen-and-carbon-dioxide

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?

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  • $\begingroup$ The answer to your question on carbon anhydrase (which is now deleted?) is that yes, CA (surprisingly) can catalyze the hydrolysis of esters such as para-nitrophenylacetate AND it can catalyse the hydration of aldehydes such as acetaldehyde and ketones such as pyruvate. The PNPA reaction is nice as the product, p-nitrophenolate, is colored and the reaction may be monitored in a spectrophotometer At one time it was though that the enz was absolutely specific for carbon dioxide. See this great ref for more details. $\endgroup$ – user1136 Nov 7 '18 at 11:29
  • $\begingroup$ In addition, from an evolutionary point of view, CA activity appears to have independently arisen on at least three occasions, see Structure and mechanism of carbonic anhydrase. (If you are interested in carbonyl group hydration, and its importance, see this articlel by R.P Bell) $\endgroup$ – user1136 Nov 7 '18 at 11:42
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    $\begingroup$ @user1136 Are there any other compounds that can specifically catalyze the hydration of CO2 ? And thanks for the answer $\endgroup$ – Dhruva Nov 7 '18 at 12:47
  • $\begingroup$ I don't know, except to say that hydration of carbon dioxide occurs at a very significant rate in the absence of a catalyst, but attainment of equilibrium is greatly accelerated by CA, and that the degree of hydration is greatly influenced by pH See Kern, especially table 1 $\endgroup$ – user1136 Nov 7 '18 at 13:09
  • $\begingroup$ One other interesting fact: although CA activity has arisen independently in three (evolutionary unrelated) protein superfamilies, all of them contain zinc (ref cited in previous comment). $\endgroup$ – user1136 Nov 7 '18 at 13:13
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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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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 0.5% 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 ten times to around 5%.

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.

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  • $\begingroup$ 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? $\endgroup$ – Dhruva Nov 5 '18 at 3:16
  • $\begingroup$ 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 '18 at 13:54
  • $\begingroup$ But we don't suffocate frequently, right? But still CO2 is accumulating in the blood. $\endgroup$ – Dhruva Nov 7 '18 at 13:37
  • $\begingroup$ 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 '18 at 14:47

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