THE EXCRETORY SYSTEM l8l 



the general term metabolism; the constructive changes are 

 termed anabolic; the destructive, katabolic. 



The final products, then, of the metabolism of the body 

 will be certain waste matters; we shall now proceed to 

 describe the mechanism of the organs by means of which 

 these waste products are eliminated from the body. 



The Elimination of Waste Products. In passing 

 through the blood and tissues of the body the proteids, 

 fats, and carbohydrates are transformed into urea, or 

 some allied product, carbon dioxid, and water, the nitro- 

 gen of the urea being supplied by the proteids alone, 

 Many of the proteids contain sulphur, and in some phos- 

 phorus in some combination is also present; many of the 

 fats taken as food contain phosphorus. By oxidation 

 these elements are converted into phosphates and sul- 

 phates, and are excreted in company with other salts of 

 the body. 



Broadly speaking, then, the waste products of the body 

 consist of urea, carbon dioxid, salts, and water. These 

 leave the body by one of three main channels the lungs 

 the skin, or the kidneys. A portion, it is true, leaves 

 the body by the bowels, for, as we have seen, the feces 

 contain, besides undigested portions of food, substances 

 that have been excreted into the bowels, and are therefore 

 waste products; this amount is, however, very small, and, 

 except in diseased conditions, is not of any special import- 

 ance. The waste matter discharged relatively by the 

 lungs, skin, and kidneys may be stated to be as follows : 



By the lungs: The greater part of the carbon dioxid; a 

 considerable quantity of water. 



By the skin: A variable but, on the whole, large quantity 

 of water, a little carbon dioxid, and a small amount of salts. 



By ike kidneys: All or nearly all the urea and allied 

 bodies; the greater part of the salts; a large amount of 

 water and a very small quantity of carbon dioxid. 



