system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the urge to breathe based around CO2 and not oxygen?

When you hold your breath, it is rising levels of CO2 in the bloodstream that makes you feel the need to breathe again; your body is not aware of its oxygen levels.  This makes it possible to breathe a gas like nitrogen and suffocate without ever noticing that you can't breathe.  Additionally, if you lower your CO2 levels by hyperventilating, then hold your breath, it's possible to pass out without feeling the need to breathe, because your oxygen levels deplete before your CO2 levels rise to alarming levels.

Since oxygen levels are what we need to stay alive, why aren't our bodies' reflexes based around that?
gpt
> Since oxygen levels are what we need to stay alive, why aren't our bodies' reflexes based around that?

Because evolution simply finds a solution that works well enough to let you reproduce. Not always the best one.