How does the lungs know which is O2 and CO2? We breathe a ton of different gasses daily but what happens to the other gases other than O2 and CO2 when inhaled?

Question

How does the lungs know which is O2 and CO2? We breathe a ton of different gasses daily but what happens to the other gases other than O2 and CO2 when inhaled?

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Answer 1

They don’t know _per se_, they’re just designed to absorb oxygen and release CO2. The way our lungs have evolved, they’re simply presenting an easily crossed, large area membrane across which oxygen and CO2 molecules will flow according to a concentration gradient. Its simply a passive mechanism that only works if the concentration of oxygen and CO2 molecules are in a certain proportion to each other in the atmosphere and in our blood.

While there is still oxygen at high altitudes for example, the lower air pressure means that gradient is bork, so oxygen doesn’t want to flow into our blood across the membrane anymore, the concentration of oxygen molecules on the outside is the same or lower than the concentration of oxygen molecules in our blood. Same reason why its possible to do mouth to mouth: even exhaled breath has enough oxygen in it to provide the necessary concentration gradient, thus oxygen flows into our blood.

For CO2, well, if we’re breathing in an environment with heavy CO2 concentration, again high enough to eliminate that gradient between our blood and the atmosphere, the CO2 stops leaving our blood. OUr lungs simply can’t get rid of the CO2 in our blood. This is why in enclosed spaces like space capsules or submarines it isn’t enough to replenish the oxygen we breathe in its also important to scrub the CO2 we exhale (go watch Apollo 13.)

On the upside, our lungs don’t care about nitrogen at all, it simply doesn’t have much bearing on our breathing other than it affects the overall air pressure. You can breathe in a pure oxygen environment with no nitrogen if you adjust the air pressure of the just-oxygen to match the same concentration of oxygen molecules that would be present in a regular air mixture (I think its like 10 or 15% of regular air pressure, if you’re using a pure oxygen setting). They used to do this in space capsules until the Apollo 1 accident. No point in lugging a few hundred pounds of useless nitrogen into orbit when we don’t even need it, just use low-pressure pure oxygen.