In addition to its use in the Henderson-Hasselbalch equation, PCO2 provides information on pulmonary alveolar gas exchange (ventilation). If PCO2 is high, there is not a sufficient degree of alveolar ventilation. This may be due to primary lung disease (inability of the lungs to ventilate properly) or to some other reason. If PCO2 is low, there is alveolar hyperventilation, again either from primary or secondary etiology.