The medullary receptors are most sensitive to PaCO2. The initial ventilatory response to CO2 is large and occurs rapidly. The peripheral chemoreceptors in the carotid bodies are mainly sensitive to reduced arterial oxygen tension. Irritant receptors in the large airways protect the delicate alveolar surfaces from particles, chemical vapors, and physical factors. Many factors influence ventilation in exercise in a manner not completely understood, but there is no doubt that the controlled variable is PaCO2.
Sleep is a complex phenomenon consisting of several phases during which breathing control varies.
Both the sensitivity to CO2 and O2 are diminished, possibly because the reticular activating system is depressed.