space resources for teachers WILLIAM H. BOWMANL RICHARD M. LAWRENCE
The Cabin Atmosphere in Manned 1 81 . S t o k Univooity Muncis, Indiana 47306
Space Vehicles
Manned explorations a~wvayshave been hampered by man's own physiological limitations. In choosing to extend his explorations into the near vaonum of outer space, man is placing even greater demands on his knowledge of his physiological needs and on his technological capabilit,y of meeting them. In resolving this dilemma of physiological needs versus technological capabilit,y, compromise solutions have sometimes been necessary. A case in point is the seiection of the cabin atmosphere for the Mercury, Gemini, and Apollo space vehicles. Man is adapted to an atmosphere composed principally of nitrogen and oxygen gases at a total pressure of approximately one atmosphere (see table). Man's need for oxygen is absolute; an insufficient supply quickly results in unconsciousness and eventually in death. When the ambient pressure of air drops as low as 480 torr, equivalent to an akitude of about 12,000 ft, the part.ial pressure of oxygen is 100 t o n and borderline hypoxia (oxygen deficiency) occurs (1). What pressure of oxygen must be maintained in a cabin atmosphere to sustain normal metabolism can be estimated by a simplified analysis of respiration. During inhalation air flows into the lungs unt,il the internal pressure equals the ambient. pressure. The total pressure in the lungs, however, is not entirely t,he result of the inhaled air. Carbon dioxide and water vapor in sufficient quantities to maintain relatively constant partial pressures of 40 ton- and 47 torr, respect,ively, are also found in the lungs. At an at.mospheric pressure of 760 ton, the inhaled air thus exerts a partial pressure of 673 torr. Because 21% of the inhaled air is oxygen, one would expect the normal partial pressure of oxygen in the lungs to be 141 tom.
Composition of
Gas
Dry Air Near Sea
Percent Composition (by volume)
Level
Partial Pressure (ton.)
Oxygen, however, dissolves in the blood in accordance with Henry's law2, and its partial pressure is less than this value. Almost as rapidly .as it dissolves, oxygen combines with the pigment hemoglobin. This reaction effectively removes the oxygen from physical solution, allowing additional oxygen to be dissolved. The overall process may be represented as follows (Rb represents hemoglobin)
Both of these reactions are readily reversible, and t,he position of each a t equilibrium is determined by the partial pressure of oxygeQ in contact with the blood. As the blood leaves the lungs, it has an oxygen tension8 of approximately 108 torr which is sufficient to produce 97-98yo saturation of the hemoglobin (see figure). Man thus requires a supply of oxygen a t sufficient
This article is one of n series of arlieler based on resource units , II., "Spsre Resonrre~for in L.\rn~xcl:,I
