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 (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. Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg.