Chap. 13 THE RLLEASE OF ENERGY RESPIRATION 225 



CARBON DIOXIDE 

 AND OTHER GASES 



Fig. 13.1. Proportions of gases in a relatively dry atmosphere at sea level. 

 Actually the average atmosphere contains a variable amount of water vapor, 

 usually one to five per cent, which slightly changes the proportions given here. 



dizziness and unconsciousness that have overcome many mountain climbers. 

 At the top of Mount Everest 29,002 feet, the highest mountain in the world, 

 the air pressure is only 4.4 pounds. 



The commonest way to adjust to high altitudes, especially in airplanes, is 

 to increase the oxygen content of the air by breathing through a mask con- 

 nected with an oxygen tank. Beyond 38,000 feet even breathing pure oxygen 

 is not enough because the atmospheric pressure is too low to drive any gas 

 into the blood. At this height it is necessary to have a hermetically sealed 

 plane, a pressure cabin, which confines the higher pressure caught in it at 

 lower levels. Those who have lived in low countries and later moved to high 

 mountains (14,000 feet or more) usually find that they are weak and short- 

 breathed. The red blood cells of the newcomers are too few. The usual im- 

 mediate reaction is in the spleen which contracts and forces its store of red 

 cells into the circulation, creating a sudden increase in the number of blood 

 cells in the peripheral blood. This is followed by further increases due to the 

 formation of blood cells in the red bone marrow. 



Of the other atmospheric gases only carbon dioxide is directly active in 

 respiration. A very minute amount of it in the blood is necessary to stimulate 

 the mechanism of breathing; more than that is a poison and is normally elimi- 

 nated. 



Nitrogen forms the great bulk of air, takes no part in respiration but is of 

 necessity inhaled and exhaled in breathing and is regularly present in the blood 

 as a dissolved inactive gas. When pressure on the body is suddenly lifted nitro- 

 gen comes out of solution and forms bubbles in the blood, in the joints and 



