CHAPTER XI 

 EXCBETION 



1. The organs of excretion. The student now realizes that 

 the work of the body is accompanied by the production of 

 wastes and also understands the necessity for their removal. 

 The most abundant waste product of the body, carbon dioxide, 

 is a gas and is excreted by the lungs ; others, notably urea 

 and other waste products of the proteins, are dissolved solids 

 and are removed from the blood to some extent by the 

 intestine and the sweat glands of the skin, but chiefly by 

 the kidneys. 



A number of organs thus perform the work of excretion, 

 but four of them namely, the lungs, the kidneys, the in- 

 testine, and the skin are of greater importance than all 

 others. Of these four the lungs and kidneys are far more 

 important than the intestine, and all three of these are more 

 important than the skin. 



2. Essential and incidental excretion by organs. An organ 

 may be essential to the proper removal of a given waste, or 

 it may remove the waste product only incidentally in per- 

 forming its essential functions. Thus the skin removes a 

 small amount of carbon dioxide from the body merely be- 

 cause a certain amount of this gas diffuses from the blood 

 as it flows through the skin. It is not necessary to the 

 health of the body that the skin should excrete this carbon 

 dioxide, for the lungs are quite capable of doing the work 

 and would do so if for any reason such excretion through 

 the skin were prevented. Without the lungs, on the other 

 hand, the carbon dioxide would rapidly accumulate in the 



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