322 



THE MAINTENANCE OF THE INDIVIDUAL 



body, especially the nerve centers, but the liver also splits off 

 hydrogen and unites it with carbon dioxide to produce a relatively 

 harmless substance called urea (CO(NH2)2), and water, thus 



2 NH3 + COo 



^0 = C 



NH2 

 NH5 



+ H0O 



which in turn is removed from the blood stream by the kidneys. 

 Other products which are eliminated by the hver include bile, its 

 pigments, as well as various salts, neutral fats, cholesterin, and 

 lecithin. 



Other Devices for Waste Elimination. There are parts of 

 other systems that should be mentioned in a consideration of the 

 phenomenon of excretion. These are the lungs, skin, and alimentary 

 canal. The former, as previously noted, excretes through the alveoli 

 most of the carbon dioxide produced in the body of man. This may 

 be indicated in tabular form ^ for man as follows : 



The skin serves a variety of purposes, one of the most important 

 being regulation of the elimination of small amounts of carbon dioxide. 

 When the kidneys are not functioning properly the skin may be 

 stimulated to excrete more waste substances. The alimentary canal 

 serves to rid the body of nondigested and nondigestible substances 

 which, through the processes of digestion, have yielded up their 

 content of foods. Furthermore, the alimentary canal actually excretes 

 waste products through its walls into the lumen of the canal. 



The Kidneys. We think of these structures as the principal organs 

 of excretion, and perhaps rightfully so. Nevertheless ehmination of 

 wastes is not the only important function of the kidneys. They help 



'From Kimber, Gray, and Stackpole, A Textbook of Anatomy and Physiology. By permission of 

 The Macmillan Company, publishers. 



