as I was told from early education that the air we currently breathe in contains 21% oxygen and 77-ish% of nitrogen and the rest are other particles.
So I have a dumb question on what the body uses nitrogen we breathe in for?
In: 387
You breathe air into your lungs.
Your lungs have millions of what are called air sacs, which are thin walled structures that get thin blood vessels (capillaries) very close to the surface.
The gasses in the air (oxygen, nitrogen, and others) are able to diffuse through the thin walls of the air sac and get into the blood.
Gasses can dissolve in liquids but not much. Just like if you open a bottle of soda water, the CO2 that’s dissolved in there doesn’t really want to be there. If you leave it to its own devices, most of it will leave. Oxygen and nitrogen are similar. They only want to dissolve into the blood a small amount.
The result is that they dissolve into your blood until they’re “saturated”, and the blood doesn’t want any more. At saturation, nitrogen and oxygen are entering and leaving your blood at the same rate so the concentration stays steady.
Nitrogen gas (N2) doesn’t get used by your body at all, so any dissolved nitrogen in your blood is just along for the ride. The small amount that’s able to dissolve is completely harmless. The dissolved nitrogen in your blood pretty much just stays at equilibrium with the atmosphere. There are some gas solubility equilibrium calculations that you may learn in grade 12 chemistry that can be used to estimate how much nitrogen is in your blood.
Dissolved oxygen isn’t very useful in itself, because not much can dissolve. That’s why your blood has Hemoglobin. Hemoglobin is a protein molecule that’s able to “pick up” oxygen. It essentially scoops it up. This lets your blood carry oxygen much more efficiently than if it were only using dissolved oxygen.
Oxygen gets used in what you might consider mini-combustion reactions. It’s reacted with energy dense molecules like glucose, releasing carbon dioxide. Hemoglobin does two jobs, it also picks up CO2 when it’s done carrying oxygen, and it happily releases it in the lungs, allowing CO2 to diffuse through the air sac membrane into the lungs in the air where it will be breathed out.
What really helps the knowledge “click” for me is remembering that its not like my stomach, where everything that gets taken in gets processed and used/broken down. Most of what you inhale, including nitrogen and most of the oxygen, gets pushed right back out. We breath in air that’s about 21% oxygen, and we breath out air that’s about 17% oxygen (depending on how long we hold our breath). So about 4/5ths of the oxygen we breathe in doesn’t get used either, because it doesn’t have a chance to diffuse into our blood before we breathe it back out again.
One reason we don’t breathe in and hold our breath to use every last scrap of oxygen is because it’s not efficient. The whole process is limited by how fast oxygen will diffuse through the air sac membrane and into our blood, which isn’t very fast (hence why we have millions of air sacs). Diffusion is a “mass transfer” phenomenon, and the rate of diffusion depends on the material and the difference in concentration on either side. Higher concentrations of oxygen in the air result in a higher rate of diffusion out of the air and into the blood.
When we hold our breath, the oxygen percentage might drop to 15%, and the lower concentration slows down the diffusion. We get diminishing returns, and it results in lower blood oxygen. Given how easy it is to just push that stale air out and get a fresh breath, we just do that.
While it’s slightly off-topic to talk about oxygen diffusion, it’s part of the whole explanation about how and why we breathe out most of what we breathe in, and the same phenomenon affects nitrogen as well. We breathe it in, it dissolves into and out of our blood, and we breathe it back out. It’s just hitching a ride, like my grandpa who like to take the bus around town for no reason.
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