Your hypothalamus tracks CO2 levels for your brain’s breathing control centers. Since CO2 is dissolved in the plasma, it’s easier to detect than oxygen, which is linked to proteins in the red blood cells.
Carbon dioxide is carried through your plasma as Hydrogen Carbonate ions. As Hydrogen carbonate is an acid, this means that the pH can be detected. If there is a low pH detected by the hypothalmus then it tells you to respire more.
I think :p