system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the atmosphere is 78% nitrogen, that must mean that the air we breathe is mostly nitrogen, right? So do our lungs take in nitrogen with no side effects or do they "filter out" the nitrogen to take in the oxygen?
gpt
Our lungs take in whatever is in the atmosphere, a special compound in our blood called hemoglobin (the one responsible for making it bright red) reacts with the oxygen.  Nitrogen (N2) is an inert gas, we breath it in, a little bit of it is absorbed in our blood but for the most part doesn't enter our bodies.