[immibis]

If there's twice as much CO2 in the air, twice as much CO2 goes from the air into your blood.

[teekert]

CO2 enters the blood in the body, it comes out of the lungs. If you breath too hard you loose too much CO2 and your blood pH will go up. That’s why you breathe into a bag when hyper ventilating, to keep that precious CO2 in.