CHAPTER XXI. 

 THE RESPIRATION. 



Oxygen is one of the essential substances required by every 

 living organism, in the cells of which it combines with the carbon 

 to form carbon dioxide, and with hydrogen to form water. All. 

 the phenomena accompanying the supply and utilization of oxy- 

 gen and the excretion of carbon dioxide are included under the 

 subject of respiration. 



In the simplest forms of life the exchange of oxygen and car- 

 bon dioxide gas occurs directly with the air, but in more complex 

 organisms this sort of exchange is impossible since practically 

 none of the cells composing the organism is in direct communi- 

 cation with the air. Some sort of respiratory apparatus becomes 

 necessary, so that each cell may be supplied with oxygen and 

 have its carbon dioxide removed. In the higher animals this is 

 accomplished through the agency of the 'blood, which is well 

 adapted thus to transport the oxygen and carbon dioxide, first 

 because it contains chemical bodies with which the gases can 

 unite, and secondly because it comes in close contact with the 

 tissue cells in the peripheral portions of the body, and with the 

 atmospheric air in the capillaries of the lungs. The study of 

 the respiratory function therefore includes the mechanism of 

 the gas exchange between the tissues and the blood, or internal 

 respiration, and also that between the lungs and the blood, or 

 external respiration. 



Internal Respiration. 



The energy which the body expends in the performance of the 

 functions of life, including the heat which is required to main- 

 tain the body temperature, is produced in the cellular chemical 

 reactions, in which the oxygen of the air combines with the 



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