system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the human body check the CO2 amount instead of O2 to determine if you got enough oxygen?
gpt
CO2 forms a buffer system in your blood , it actually exists as several different components depending on how much of it there is (CO2 +H20 => H2CO3 <-> HCO3^- <-> CO3^(2-)).  If CO2 builds up in your blood it causes a pH shift.   The nifty thing about this is that it's (normally) easy to regulate by controlling breathing patterns (if your blood is too acidic, you have too much CO2, you breath more, and it is expelled.  Too basic and you breathe less letting CO2 accumulate). Shifts in pH are pretty easy ways to trigger effects in the body. Oxygen on the other hand is (for the most part) bound to hemoglobin and not just floating around free. Also dissolved oxygen won't participate in the same type of buffering system as CO2 - so it is "more difficult" to develop a system around measuring it.