GENERAL ZOOLOGY 



dissolved oxygen varies, being affected by temperature, degree of agitation, 

 presence of photosynthetic plants, salt content, and other factors. As with 

 temperature, different species of aquatic animals are commonly adapted to 

 specific ranges of oxygen concentration, between tolerable maxima and 

 minima, and are limited to environments where these favorable conditions 

 obtain. Under normal circumstances, disregarding anaerobic forms, a species 

 is more likely to be limited in its distribution by lack of oxygen than by an 

 excess of oxygen. 



Air-breathing animals, whether terrestrial or aquatic, are rarely subjected 

 to significant variations in the amount of free oxygen for breathing. The 

 oxygen content of atmospheric air is remarkably constant over the surface of 

 the earth at all altitudes normally supporting animal life. There may be 

 slight temporary or local variations — for example, near active volcanoes or in 

 extensive industrial areas — but air normally contains about 21 per cent 

 oxygen. This percentage does not change appreciably with increasing alti- 

 tude, yet it is common knowledge that man, for example, cannot survive for 

 any length of time under normal conditions at altitudes much above 20,000 

 feet. The explanation of this seeming paradox demonstrates the effect of 

 barometric pressure on the availability of atmospheric oxygen. The respira- 

 tory exchange mechanisms of man and other mammals are physiologically 

 adjusted to extract oxygen from air at barometric pressures characteristic of 

 sea level, about 760 mm of mercury. With increasing altitude, barometric 

 pressure steadily decreases, with a consequent decrease in the density of the 

 air. At 18,000 feet, for example, barometric pressure is approximately half 

 the sea-level value; here a given volume of air contains only half as many 

 molecules as the same volume at sea level. Although 21 per cent of these 

 are oxygen molecules, as at sea level, it is obvious that the absolute amount of 

 oxygen has been reduced by one-half. Under such conditions, and increas- 

 ingly at higher altitudes, the metabolic requirements for oxygen exceed the 

 available supply; exhaustion, and in extreme cases unconsciousness and death, 

 results. In high-altitude mountaineering, and in some military aircraft, 

 auxiliary supplies of oxygen are commonly used. In commercial aircraft 

 designed for operation at high altitudes, the air in the cabin is artificially 

 maintained at approximately sea-level pressure. 



It has been demonstrated experimentally that many terrestrial inverte- 

 brates, such as insects, can survive for long periods under conditions of 

 reduced barometric pressure equivalent to altitudes of as much as 15 miles 

 above sea level. It is questionable whether in these experiments the upper 

 limits of survival are imposed by oxygen lack, by desiccation, or by some 

 more direct effect of reduced pressure itself. There are clear indications, 

 however, that the absence of insects from high mountain ranges is not 

 dictated by oxygen deficiency; the experiments demonstrate that insects are 

 tolerant of more extreme oxygen deficiencies than any found naturally on the 

 surface of the earth. The temperature conditions at high altitudes are 

 probably the limiting factor in this case; temperature decreases with increas- 



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