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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Air always wants to be at the same pressure everywhere, if it can. So, if you have two pockets of air at different pressures, some air from the high pressure area will move to the low pressure area until the pressure is balanced between them.
However, the same happens with the individual constituent parts of air, and this is called partial pressure. In other words, if you have two pockets of air at the same overall pressure, but one of them is 90% oxygen and 10% CO2, and the other is 10% oxygen and 90% CO2, then the oxygen and CO2 molecules will quickly move around until both pockets have the same concentration of oxygen and CO2.
Therefore, when you breath out, you’re breathing out air that has less oxygen and more CO2 than the surrounding air. Your breath has partial pressures that are different than the surrounding air, so there will be pressure forces that cause the CO2 and oxygen levels to normalize quickly, and most of that normalization will likely be done before you’ve even started taking your next breath.
Also, just the force of pushing the air out of your mouth is enough to push that air far enough away that your next breath won’t take in very much of the same air that you just breathed out. Imagine someone taking a drag off a cigarette, breathing out the smoke, and then taking their next breath. How much of the smoke that they blew out do you think they’re inhaling on the next breath?
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