Gases exert partial pressures in proportion to their relative numbers of molecules. The partial pressure of water vapor in a saturated gas at body temperature is 47 torr. The volume of CO 2 eliminated from the lungs each minute is the amount coming out with each expiration times the number of expirations per minute. This equation shows that alveolar PCO 2 is determined by the ratio of CO 2 production to alveolar ventilation. This is done by keeping minute ventilation high enough to maintain alveolar ventilation at a sufficient level.
Hyperventilation is the opposite.
Hypercapnia means high PCO 2 . In general, hypercapnia means that that there is alveolar hypoventilation. Hypocapnia means low PCO 2 . In general, hypocapnia means that there is alveolar hyperventilation. Since alveolar and arterial PCO 2 levels are closely coupled, hypoventilation causes arterial PCO 2 to become elevated.