Respiration in High Flying: (United Services Section).
Atmospheric pressure falls, as height increases, to about one-ninth of its sea-level value at 50,000 feet. The intake of oxygen into the blood depends on the partial pressure of oxygen in the inspired air, which is about one-fifth of the atmospheric pressure. But since the gaseous content of the lungs is saturated with water vapour at body temperature, 47 mm. Hg. of the atmospheric pressure in the lungs is due to water vapour and is therefore not available for oxygen or other gases, while the alveolar air contains also an almost constant pressure of 40 mm. CO(2).Mental and physical output demand an adequate partial pressure of O(2); they begin to be limited as soon as this falls, and at heights above 18,000 feet are seriously reduced. Consequently in order to fly higher than about 15,000 feet it is necessary to increase the partial pressure of oxygen in the inspired air. Up to about 44,000 feet this can be done by merely raising the percentage of oxygen, usually by allowing a regulated stream of oxygen to enter a small naso-buccal mask, but preferably by a closed system in which the negative pressure of inspiration opens a valve and allows oxygen to enter a bag from which it is inspired.Beyond 44,000 feet as a limit (and a lesser height for safety) it is necessary to create a local atmospheric pressure around the pilot higher than that of the surrounding air, by enclosing him in an airtight sit or cabin in which a relatively increased pressure with a maximum value of about 2(1/2) lb. per square inch is maintained, while he breathes pure oxygen. This device was used in the recent British world record high flight, when a height of 50,000 feet was attained. The pressure-suit used by the pilot on this occasion and the decompression chamber recently built at Farnborough are described in detail.
Marshall
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