Pulmonary gas pressures

The factors that determine the values for alveolar pO2 and pCO2 are:
Partial pressures.
The partial pressures (in torr) for a human at rest:
Partial pressure of oxygen (at sea level).
The alveolar oxygen partial pressure is lower than the atmospheric O2 partial pressure for two reasons. 
The alveolar pO2 is not routinely measured but is calculated from blood gas measurements by the alveolar gas equation.
Pathology.
The partial pressure of carbon dioxide, along with the pH, can be used to differentiate between metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis.
Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases above normal values – greater than 45mmHg. If pH is also less than 7.35 this is respiratory acidosis.
Hyperventilation exists when the same ratio decreases – less than 35mmHg. If the pH is also greater than 7.45 this is respiratory alkalosis.