USE OF THE DIGESTED FOOD 147 



not eliminated from the body, they cause results of a 

 most serious character, as, for instance, when an accumu- 

 lation of urea in the body produces ursemic poisoning. 

 The blood therefore not only carries to the tissues the 

 necessary nutrients and oxygen, but it has laid upon it 

 the burden of taking into its currents the waste products 

 of combustion and growth and carrying them to the 

 points where they are thrown off. (See Par. 77.) 



211. Elimination of urea. One of the branches of the 

 arterial system of blood vessels runs to the kidneys, and, 

 by repeatedly rebranching, traverses all their substance. 

 The main function of the kidneys is to eliminate certain 

 products through the urine. It is in this way that 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 breaking down of pro- 

 tein, and in a most wonderful manner these compounds 

 are filtered out so that they are not present in the out- 

 going blood. An excess of soluble mineral matters, espec- 

 ially the alkaline salts is also removed by the kidneys, as 

 well as the bile compounds which are absorbed from the 

 alimentary canal. 



212. Elimination of carbon dioxid. The carbon 

 dioxid must in some way also be eliminated from the 

 body. This is not accomplished to any extent until the 

 blood containing it reaches the lungs, where it is ex- 

 changed 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." 



213. Elimination of water. Water may be regarded 

 from one point of view as a waste, for it is produced in 



