MILK, BLOOD AND URINE 227 



where this oxidation takes place, but that some of it is still 

 retained and locked up, as it were, in the urine excretion prod- 

 ucts and thus lost to the body. This will be explained in 

 detail when we study food values (Chapter XVII). 



Constituents 



Nitrogen Compounds. — The most important constituents of 

 urine in connection with normal metabolism are the nitrogen 

 compounds. The most abundant compounds of this class are 

 urea, uric acid, creatinine and the purine bases ; xanthine, etc. 

 The chemical constitution and relationship of these com- 

 pounds has been already considered, so far as is necessary, in 

 Section L 



Urea. — The chief constituent of urine and the principal 

 nitrogen product of protein katabolism is urea. Under normal 

 conditions of diet and health the urea represents about ^^ per 

 cent of the total nitrogen in urine. This relative amount may, 

 however, be changed very decidedly by changes in the total 

 amount of protein metabolized. Urea is more abundant in 

 the urine of man and flesh-eating animals than in herbivorous 

 animals. It is also present in most of the fluids of the animal 

 body, as in blood, as already mentioned. In man the total 

 amount per day is about 30 grams. As this represents about 

 88 per cent of the total nitrogen in urine which, in turn, is the 

 whole of the nitrogen of metabolized protein, it means that the 

 total nitrogen metabolism per day is about 16.0 grams, equiv- 

 alent to 100 grams of protein. As we shall find under food 

 requirements this is the normal amount of protein required by 

 man per day. 



Isolation and Determination. — The isolation of urea from 

 urine is easily accomplished by converting it first into a nitric 

 acid salt, crystallizing this, and then setting the urea free by 

 the action of a weak base (barium carbonate). This will be 

 described in the experiment following. The approximate 

 determination of urea in urine is also easily made, and as the 



