We don’t breathe out with much force, and even if we did, we immediately inhale. So, how do we not just inhale what we just exhaled?
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We generally do, except when breathing hard during heavy exercise. This isn’t a big deal as long as there’s a net loss of some of the CO2 content in the blood.
CO2 in the blood is generally stored bicarbonate ions. There are enzymes in the internal surface of the lungs called “Carbonate Anhydrase” that rapidly convert bicarbonate ions into CO2 gas , carbonate ions, and hydroxide ions.
The production of hydroxide ions raises the pH of the blood. The hydroxide ions then react with hemoglobin in blood cells causing them to change shape in such a way that their oxygen binding affinity goes up by a factor of several tens of thousands of times. This causes them to absorb oxygen.
In other words, the process of absorbing and releasing O2 is *catalyzed* by a change in pH, caused by the blood absorbing or releasing CO2. The process in not 1-1 meaning that it’s nit the case that for every CO2 released, 1O2 is absorbed.
The concentration of CO2 in the air actually needs to be fairly high, around 1.5-2% before this process starts having problems. This is more than 10× it’s normal level in the air. Specifically, the reverse reaction starts taking place, CO2 gas is back-converted into bicarbonate ions, consuming OH- ions and reducing the blood pH. This starts reducing the efficiency of the hemoglobin to absorb O2. levels of 5% CO2 are immediately dangerous and will cause rapid incapacitation and eventual death after prolonged exposure of more than a few minutes.
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