Breath regulation
The carbon dioxide partial pressure PaCO2 and the oxygen partial pressure PaO2 primarily control respiration. Added to this is the pH of the blood, which respiration uses to stay within vital limits.
Chemoreceptors, nervous stimuli from the lungs, and impulses from the brain help in this process. The blood-brain barrier is permeable to CO2, thus receptors can measure the partial pressure of carbon dioxide in the cerebrospinal fluid. From there, respiratory rate is controlled, with carbon dioxide having priority over oxygen.
That is, 'our breathing is basically controlled primarily by the CO2 and less by the O2 in the blood.
Only in severe lung diseases the oxygen partial pressure gets a regulating effect, when the PaCO2 is chronically increased and the respiration has adjusted to it. If then the PaO2 is decreased, this also triggers a stimulation of respiration.