> I think we use them as a proxy for o2 levels (same as our bodies do).

Probably. ISTR that depriving a body of oxygen results in a different response than overloading the body on CO2. It's why if you completely displace all air in the room with CO2, people choke, panic, etc, but if you use Nitrogen, people just keel over dead without realising it.

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mathgeek 3 hours ago | parent [-]

The evolved response to CO2 is part of the human body’s ability to filter and remove CO2 via the respiratory system. AFAIK we don’t have similar capacity for Nitrogen because it’s not a primary waste product of that system.

	▲	wongarsu 2 hours ago \| parent [-]
		Dissolving CO2 in water creates some carbonic acid (H2CO3) that will decompose back to water and CO2 when the CO2 concentration drops. Blood has a fair bit of water, and carbonic acid is much easier to detect than oxygen or nitrogen gas We evolved to detect CO2 because that's by far the easiest thing to detect that's still a reasonable proxy for the performance of our respiratory system