Although one can temporarily hold one's breath or breath in and out quickly (hyperventilating), automatic mechanisms control and coordinate most respiratory and cardovasular activity. The medulla oblongata, assisted by the pons (what a cool word...), serves as the control center of the respiratory and cardiovascular systems. The medulla, located near the brain's stem, sets the breathing rythm and moniters rises in the CO2 level. (because CO2 is a byproduct of oxygen consumption, O2 leves are not closely monitered).
If you got one interesting fact from this, it should be this: Difference in the atmosphere's amount of oxygen affects breathing levels only slightly, while even a minute change in CO2 levels in the atmosphere will drastically change it.