Ultimately, effective ventilation is measured by the partial pressure of CO2 in arterial blood (Paco2). All expired CO2 comes from alveolar gas. During normal breathing, the breathing rate or depth adjusts to maintain a steady Paco2 between 35 and 45 mm Hg. Hyperventilation manifests as a low Paco2; hypoventilation, as a high Paco2. During exercise or certain disease states, increasing breathing depth is far more effective than increasing the RR in improving alveolar ventilation.