CHAPTER XXVIII 

 RESPIRATION 



If we hold our breath for a short time we soon experience 

 a sense of discomfort which increases the longer our breath- 

 ing is interrupted, until it becomes quite intolerable. 

 While we can go without water for some time and without 

 food for a much longer time, we would very quickly suc- 

 cumb (it would be a matter of a very few minutes at best) 

 if deprived of air. The element in the air upon which 

 we are so closely dependent for our life is oxygen, the 

 nitrogen being simply so much inert substance that plays 

 no important part in respiration. While we may be prone 

 to think of breathing or respiration as drawing air into the 

 lungs and forcing it out again, these processes are merely 

 subsidiary to the essential part of respiration which con- 

 sists in the assimilation of oxygen and the giving off of 

 carbon dioxide. Liquids tend to absorb gases when the 

 latter are present in considerable quantities, and they give 

 off gases when there is nothing to check their escape. 

 If a liquid such as water is separated from the air by a 

 permeable membrane it may absorb air through it and give 

 off any gas which it may contain in excess. Blood has 

 this property, like other liquids, and it has an especial 

 aptitude for absorbing and giving off unusually large 

 amounts of oxygen and carbon dioxide, the chief gases con- 

 cerned in respiration. Gases have the general property 

 of tending to become uniformly distributed. If a bladder 

 is filled with oxygen and suspended in an atmosphere of 

 carbon dioxide the oxygen will diffuse out of the bladder 



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