CHAPTER XLII 



RESPIRATORY SYSTEM 



Need of air. We already know that we must have 

 air in order to live. If a man is shut off from a supply 

 of air for even a relatively short time he dies. More- 

 over, we constantly need a large amount of air. From 

 fifteen to eighteen times every minute each one of us 

 is taking in a new supply of air by a process which 

 we call inhaling. Each time this is followed by exhaling, 

 in which air is given off from the body. The rate at 

 which we breathe varies. If we are exercising we 

 breathe faster than we do w r hen at rest. 



Oxidation. The element in the air essential for our 

 bodies is oxygen. This we know combines readily 

 with other elements, and in so doing produces energy 

 in the form of light or heat. It is precisely this 

 property of oxygen which makes it so necessary to the 

 cells in our body. In order to move, to think, and to 

 keep the body warm, we must have energy. This is 

 produced by the oxidation of the cells of the body. As 

 a result of oxidation, waste products, especially carbon 

 dioxide, water, and nitrogenous waste in the form of 

 urea, are formed. 



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