Hypothalamus sends signals to the Pituitary gland.
In a normal person the respiratory drive is controlled by the central chemoreceptors in the hypothalmous. These receptors are sensitive to changes in pH. CO2 effects pH by the hasselbach equation: H+HCO3 = CO2+H2O So basically the more CO2 your body has on bord the lower the pH is and the more stimmulation there is to the central chemoreceptors causing a desire to breath and blow off that CO2. In emphasima patients their Co2 is so chronically high their respiration is driven by their peripheral chemoreceptors that are sensitve to O2 levels in the blood not pH. This is why one needs to be very careful when giving oxygen to emphasima patients. If the O2 level of their blood is too high there would be no stimulation to breath and they could stop breathing.