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VITAL ANIMAL LIFE PROCESSES 



Excretion 



In the complicated process of cell metabolism there are other wastes 

 besides carbon dioxide. These wastes must be removed from the cell 

 and from the animal body with regularity or the animal will die. Ex- 

 cretion is the name given to the process of the removal of these wastes. 



Fig. 5.1. Relationship of four vital life processes to metabolism in man. This diagram 

 shows how a cell in the shoulder region is dependent upon digestion, respiration, excre- 

 tion, and transportation. If any one of these fails to perform its functions properly, 

 metabolism stops and the cell dies. (The dash lines do not show the exact course of 

 transportation — they merely show the source and destination of the products.) 



If animals could live on a diet of pure carbohydrates and fats, there 

 would be no need for excretion, since these compounds contain no ele- 

 ments besides carbon, hydrogen, and oxygen ; and when they are 

 oxidized they produce only carbon dioxide and water as by-products. 

 The food of all animals, however, contains other elements ; it must 

 contain other elements, because protoplasm cannot be constructed of 

 only carbon, hydrogen, and oxygen. Nitrogen is one of the primary 



