Pulmonary Physics
The primary force applied in the respiratory tract is supplied by atmospheric pressure. Total atmospheric pressure at sea level is 760 mmHg (101 kPa), with oxygen (O2) providing a partial pressure (pO2) of 160 mmHg (21 kPa), 21% by volume, at the entrance of the nares, a partial pressure of 150 mmHg (20 kPa) in the trachea due to the effect of partial pressure of water vapor, and an estimated pO2 of 100 mmHg (13 kPa) in the alveolar sac. The pressure drop is due to conduction loss as oxygen travels along the transport passageway. Atmospheric pressure decreases as altitude increases, making effective breathing more difficult at higher altitudes. Higher BPG levels in the blood are seen at higher elevations, as well.
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