SECTION IV. 



THE END PRODUCTS OF METABOLISM. 

 EXCRETIONS. ENERGY BALANCE. 



CHAPTER XIX. 

 THE NITROGENOUS EXCRETION. URINE. 



Having considered in the foregoing pages the substances 

 used in the nutrition of the body, the agencies of nutrition, 

 and the general character of the products formed, we come 

 now to a short study of the waste products rejected by the 

 body after it has assimilated and used the nutritives furnished 

 to it. The food-stuffs which the animal can utilize are com- 

 paratively complex, but consist essentially of the members of 

 the three groups, the fats, carbohydrates and proteins. The 

 theoretically simplest waste or oxidation products of these are 

 nitrogen, carbon dioxide and water, but in the animal organ- 

 ism the breaking down does not go so far. While from fats 

 and carbohydrates essentially only water and carbon dioxide 

 are formed, the protein metabolism is not carried to the elim- 

 ination of nitrogen, but ends with the formation, largely, of 

 urea, a body in a way related to the theoretical end products, 

 but which would call for three more atoms of oxygen to com- 

 plete oxidation. 



The nitrogen metabolism involves some extremely interest- 

 ing problems which are still far from complete solution. 

 From the older point of view urea was considered the one nor- 

 mal end point in the chain of katabolic reactions, and the other 

 nitrogenous bodies found in the urine, such as uric acid and 

 creatinine, were looked upon as substances which in some way 

 had accidentally escaped the fate due them. This view is 

 doubtless incorrect, as we have good reason to believe that 



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