system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Humans need oxygen to survive, but air isn’t 100 percent oxygen so what happens to all the other chemicals we breath in?

I’ve always known air isn’t pure oxygen, that there’s a lot of chemicals that make up the air around us but another post sparked this question in me when explaining there is nitrogen and argon and other stuff in the air around us.

What is happening for our bodies to breath in this soup of chemicals and only use oxygen, then exhale Carbon dioxide? Or monoxide? I’m not even sure what it is we exhale..  but are these other chemicals harmful in any way?

Do we use the nitrogen and other stuff we inhale? Or does or body filter out just oxygen and lets everything else go when we exhale?
gpt
The nitrogen does form an interesting function. It doesn’t get absorbed and basically just gets exhaled out again. However this means that the nitrogen serves to splint open the little air sacs in the lungs (alveoli) and prevent them from collapsing.

Very sick patients in intensive care requiring very high amounts of oxygen on a ventilator can experience what is known as “absorption atelectasis”. If someone is on very high amounts of oxygen (especially in the 90-100% range) the air they are breathing will therefore have little or no nitrogen. This means when they absorb oxygen from the air, the total volume of gas in the air sacs will reduce, meaning they shrink a little bit. This is problematic when people are difficult to ventilate and senior ICU drs will be extremely reluctant to leave patients on 100% oxygen, partially for that reason. Perversely, by the air sacs collapsing a bit the patients ability to absorb oxygen from the air will get worse.