II 



THE ACCESS TO OXYGEN 



In nature we find animals in all kinds of environments where 

 oxygen may sometimes be quite difficult to obtain and where 

 the concentration of carbon dioxide may rise so as to seriously 

 obstruct the elimination of this product of metabolism. It 

 will be convenient to give at this stage a brief survey of oeco- 

 logical conditions so far as the respiratory gases are concerned. 



The access to oxygen in air. The composition of the atmos- 

 phere is constant and can be taken as 20.948% 2 and 

 0.030% C0 2 (Krogh, 1919) while the rest is a mixture of 

 gases — inert from our point of view — and lumped together 

 as nitrogen which in reality makes up about 78% while 

 0.94% is argon. 



The changes in composition caused by respiratory or com- 

 bustion processes are astonishingly small. In the streets of a 

 large city the C0 2 may rise to 0.04 and the 2 be lowered by 

 a corresponding amount, and even in crowded rooms the 

 changes are usually within 0.1% and practically always 

 within 1%. They have no effect upon man or any air- 

 breathing animal. 



At the normal sea level barometric pressure of 760 mm 

 mercury, the oxygen pressure corresponds to about 155 mm 

 with small variations due to the varying content of water 

 vapour. At higher altitudes the composition remains un- 

 altered, but the pressure drops and at a height of 5000 m 

 ( = 16,000 feet) the oxygen pressure is 88 mm, which seriously 

 affects the power of man and most warm-blooded animals to 

 obtain the necessary oxygen from the atmosphere and strictly 

 limits the ability to do muscular work. 



Because the total pressure is variable it may be misleading 

 to express the concentration of a gas in per cent of the atmos- 



9 



