Disposition of the Wastes 149 



One of the branches of the arterial system of blood- 

 vessels runs to the kidneys, and, by repeatedly rebranch- 

 ing, traverses all their substance. The main function 

 of the kidneys is to secrete the urine, a liquid in which 

 all the waste nitrogen from the digested protein finds its 

 way out of the body in the form of urea and similar 

 bodies. The blood that enters them carries with it the 

 urea and uric acid which have resulted from a break- 

 ing down of protein, and in a most wonderful manner 

 these compounds are filtered out so that they are not 

 present in the outgoing blood. An excess of soluble 

 mineral matters such as common salt is also removed 

 by the kidneys, as well as the bile compounds which 

 are absorbed from the alimentary canal. 



The carbon dioxid must in some way also be elimi- 

 nated from the body. This is not accomplished to any 

 extent until the blood containing it reaches the lungs, 

 where it is exchanged for a new supply of oxygen and 

 passes off in the expired air. In the case of man, the 

 air "breathed out" is nearly a hundred times richer in 

 carbonic acid than the air "breathed in." 



Water may be regarded from one point of view as 

 a waste, for it is produced in the oxidation of the 

 food, and this passes off from the lungs as vapor, 

 through the skin as sensible or insensible perspiration, 

 and in considerable quantities through the kidneys. 



To summarize, it may be said that the blood is con- 

 stantly undergoing gain and loss. The gain comes 

 from the food (including water and oxygen), and the 

 loss consists of urea, carbonic acid and water given off 

 through various channels. 



