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?


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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Your lungs don’t know anything. They’re just designed to take advantage of chemistry/physics.

When your CO2-laden, oxygen poor blood arrives at your lungs full of relatively oxygen-rich air the concentration of gasses starts to balance out. That is, CO2 flows out of your blood into your lungs and oxygen flows from your lungs into your blood. This is just basic physics. If I have a tank full of CO2 and a tank full of oxygen and connect them, the concentration of each gas in the connected system will approach an equilibrium.

However, your body also tips the scales of this process to go further than it otherwise would. Hemoglobin, the protein that bonds to CO2 and O2 to ferry it around your body, has a greater affinity for bonding with O2. If CO2 and O2 are around some hemoglobin in similar concentrations, the hemoglobin will bond with the oxygen. This means that, as oxygen flows into your blood it gets sucked up and carried away by red blood cells. This means the concentration of oxygen in blood at the lung-bloodstream interface remains low. Oxygen keeps flowing from your lungs into your blood.

Our cells are very complex chemical reactors. The cells in our lungs engage the O2 molecules in chemical reactions in order to take them, but don’t engage the molecules of other gases.

Well, we really _don’t_ breathe that many different gasses. Air is composed of 78% nitrogen, 21% oxygen and 1% trace gasses (everything else). When you take a breath, it is pretty much entirely nitrogen and oxygen.

Our bodies don’t “know” which gas is which. Instead, our red blood cells have chemical receptors which carry both oxygen and carbon dioxide. One of the laws of chemistry is that high-concentration solutions will diffuse into lower-concentration solutions until equilibrium is reached. When our high CO2 concentration blood is exposed to the low CO2 concentration air, the oxygen and CO2 will naturally exchange, trying to reach equilibrium. This is how our lungs exchange gasses.

The other gasses are dissolved in our blood using the same mechanism, but since concentrations (aside from nitrogen) are so low, they have very little impact. Nitrogen is dissolved in our blood in higher quantities, but since it is a very inert gas it doesn’t really affect anything either.

Note that these rules only apply at normal air pressures – I’m a SCUBA diver and one of the things we are _very_ concerned about is the build-up of dissolved nitrogen from spending time at depth.

The body is like a series of buckets connected together by pipes. If you fill one bucket up too high, it’ll flow into another bucket until both are the same. The lung exchanges gases with deoxygenated (high CO2 and low O2) blood. Because there’s more O2 in the air than blood, the O2 will go into the blood, and because there’s more CO2 in the blood than air, CO2 will flow out. Just like the buckets. And then the O2 and CO2 will go to cells as oxygenated blood (high O2 and low CO2) and the cells want O2 and want to get rid of CO2.

Gasses can dissolve in fluids like water, but because we want to carry around more O2 and CO2 than what the water in our blood can dissolve, we use hemoglobin to carry it like a boat in red blood cells. But we don’t have that for other gases. Most of the air is nitrogen, and because we can’t really use the nitrogen in the air, the amount in your blood and in the air is the same so it doesn’t really go up or down.

This is true for most gases but with some exceptions. Carbon monoxide poisoning is dangerous because hemoglobin likes to carry carbon monoxide more than it likes to carry oxygen so it kicks oxygen off the boat and then refuses to drop it off at the lungs. This means our cells don’t get oxygen because the hemoglobin is carrying the carbon monoxide which ends up starving the cells of oxygen and they die.

Our lungs operate on the principle of equilibrium. Through various membranes, your blood is “exposed” to the air and the gasses in your blood and in the air equalize. So if there is more gas in one than the other, that gas moves from the fluid that has it in excess to the fluid that is lacking it.

The blood that is passing through your lungs is blood that has expended all of its oxygen (and therefore lacks it) and has collected a lot of CO2 (and therefore has it in excess). When it is exposed to the air in your lungs, the excess O2 in the air gets absorbed by the blood and the excess CO2 in your blood gets absorbed by the air.

The other gasses in the air (primarily nitrogen) are essentially inert. So while your blood can absorb these gasses based on the same principles, it doesn’t do anything with them in a biochemical sense so those gases just reach an equilibrium point and stay there.