CHEMICAL COMPOSITION OF THE HUMAN BODY 45 



proteid combustion. Other nitrogenous end-products are 

 present in but small quantities. The amount of urea ex- 

 creted depends, therefore, upon the extent of proteid meta- 

 bolism. 



A large portion of the urea is formed in the animal body 

 synthetically from the combustion products of proteids, 

 namely, carbon dioxide and ammonja. That this formation 

 is possible is proven by the following facts : 



Certain ammonia salts, especially ammonium carbonate, 

 introduced into the body do not appear in the urine as 

 ammonia salts, but the amount of urea is increased in pro- 

 portion to the ammonia salts taken. This is also true for 

 some substituted ammonia compounds, such as amido acids 

 (leucine, glycbcoll, tyrbsine and others). 



That the ingested ammonia salt is really changed to urea and 

 does not merely increase proteid decomposition and thus the 

 excretion of urea, is proven by the fact that if a substituted 

 ammonia salt is ingested, the corresponding substituted urea is 

 formed. If meta-amido-benzoic acid, NH 2 .C 6 H 4 .COOH, is in- 

 gested, we find uramidobenzoic acid, NH 2 .CO.NH.C 6 H 4 .COOH. 



The place where the synthetical formation of urea from 

 ammonia takes place is the liver. If defibrinated blood con- < 

 taining ammonium carbonate is passed through an excised 

 liver, into the portal vein and out of the hepatic vein, the 

 ammonium carbonate decreases, while the urea increases, in 

 the blood. If the liver is artificially cut off from the circula- . 

 tion, the amount of urea is decreased, while the amount of 

 ammonia and also of amido acids (leucine, tyfosine) in the 

 urine is increased. This is also true for many diseases of 

 the liver. 



The nature of the substituted urea shows the part played by the 

 carbamic acid in urea formation. Perhaps carbamic acid and 

 ammonia are formed by the proteid metabolism and that these 

 substances are changed to urea in the liver. Hence urea must be 

 regarded as the amid of carbamic acid. 



The object of the formation of urea from ammonia salts 



