Pulmonary gas pressures

The factors that determine the values for alveolar pO₂ and pCO₂ are:

The pressure of outside air
The partial pressures of inspired oxygen and carbon dioxide
The rates of total body oxygen consumption and carbon dioxide production
The rates of alveolar ventilation and perfusion
Partial pressures

The partial pressures 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.

Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour reduces the oxygen partial pressure to about 150 mmHg.
The rest of the difference is due to the continual uptake of oxygen by the pulmonary capillaries, and the continual diffusion of CO₂ out of the capillaries into the alveoli.

The alveolar pO₂ is not routinely measured but is calculated from blood gas measurements by the alveolar gas equation.

Partial pressure of carbon dioxide

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.