47 Total 523 mmHg
(Note: diagramatic representation not to scale)
To prevent this deterioration when flying, a method has to be found of augmenting the oxygen content of the respired air such that, by gradual displacement of nitrogen, the alveolar oxygen tension is not allowed to fall below the ideal level of 103mmHg. This can be done by supplying oxygen from a source through a mask to the subject's respiratory tract with increasing concentration, such that the inspired mixture has the correct amount of oxygen to give a partial pressure of oxygen of 103mmHg in the lung. An alternative method is to pressurise the aircraft cabin with engine-bleed air to give an effective altitude of less than 10 000 feet (3000 m).
Clearly the ideal situation of breathing through a mask has a limit; sooner or later the ever-increasing concentration of oxygen necessary to supplement the inspired air will mean that to maintain the ideal value in the lungs it will be necessary for the individual to breath 100% oxygen. It can be calculated that this will occur at the altitude where the barometric pressure equals the alveolar oxygen pressure of 103mmHg plus the pressure of water vapour (47 mmHg) and the pressure of alveolar carbon dioxide (40 mmHg) (which, of course, remain constant). The altitude in feet at which this will happen is the altitude at which the barometric pressure is 190 mmHg, which coincides with 33 700 feet (10 250 m) in the standard atmosphere. This can be visualised as follows:
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