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Pressure as a function of the height above the sea level. The human body can perform best at sea level, [7] where the atmospheric pressure is 101,325 Pa or 1013.25 millibars (or 1 atm, by definition). The concentration of oxygen (O 2) in sea-level air is 20.9%, so the partial pressure of O 2 (pO 2) is 21.136 kilopascals (158.
The alveolar partial pressure of oxygen ) 107 mmHg (14.2 kPa) ... and the prevailing atmospheric pressure 47 mmHg (6.25 kPa) ... for air at sea level at 37 °C. ...
The atmospheric pressure is roughly equal to the sum of partial pressures of constituent gases – oxygen, nitrogen, argon, water vapor, carbon dioxide, etc.. In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. [1]
The alveolar oxygen partial pressure is lower than the atmospheric O 2 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.
This is because a person who is used to living at sea level needs about 0.20 bar (20 kPa; 2.9 psi) partial oxygen pressure to function normally and that pressure can be maintained up to about 40,000 ft (12,192 m) by increasing the mole fraction of oxygen in the air that is being breathed.
The concentration of oxygen (O 2) in air is 20.9% so the partial pressure of O 2 (PO 2) at sea level is about 21.2 kPa (6.3 inHg; 3.07 psi). In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells. [7]
The concentration of oxygen in the air (mmols O 2 per liter of air) therefore decreases at the same rate as the atmospheric pressure. [26] At sea level, where the ambient pressure is about 100 kPa, oxygen constitutes 21% of the atmosphere and the partial pressure of oxygen (P O 2) is 21 kPa (i.e. 21% of 100 kPa).
At sea level, alveolar partial pressure of oxygen is 104 mmHg, reaching 6000 meters above the sea level. This pressure will decrease up to 40 mmHg in a non-acclimated person, but in an acclimated person, it will decrease as much as 52 mmHg. This is because alveolar ventilation will increase more in the acclimated person. [1]