EXCRETION. 473 



been shown to be the salts of ammonia the carbonate, carbamate, and 

 lactate. The increase in the ammonia of the urine simultaneously with 

 the decrease in the urea renders it extremely probable that these salts are 

 antecedents of urea and that the transformation takes place in the liver 

 cells. The chemic change that takes place is simply the abstraction of two 

 molecules of water as shown in the following formula: 



(NH 4 ) 2 C0 3 - 2 H 2 = CON 2 H 4 . 



The source of the ammonia is probably in part the intestine as this 

 compound is one of the products of the hydrolysis and cleavage of the proteins 

 during digestion. That this is the case is apparent from the fact that the 

 blood of the portal vein always contains more ammonia that the blood of 

 any other region of the vascular apparatus. The advantage to the body 

 that results from the conversion of ammonia to urea is that it prevents an 

 ammonia intoxication with its attendant evils that would otherwise arise. 



The amino-acids, as tyrosin,leucin, glutamic, and aspartic acids, diamino- 

 acids and bases, as lysin, arginin, histidin which are also products of the hy- 

 drolysis of proteins during digestion are capable of being absorbed as such 

 by the epithelial cells of the villi and mucous membrane, in which they 

 undergo a cleavage into an NH 2 portion and an organic portion; the former 

 is then converted to ammonia and subsequently to urea by the liver cells, 

 the latter the organic portion contributes to the production of fat or sugar, 

 which are in due time oxidized and thus contribute to the store of body heat. 

 A portion of the ammo-acids, such as is not used in the formation of tissue 

 protein is apparently disposed of in this way. 



From the foregoing facts it is evident that given the presence of am- 

 monia salts and ammo-acids in the blood of the portal vein the appearance of 

 urea in the urine is readily accounted for. However it must be remembered 

 that though the intestine is a source of the ammonia and the amino-acids it is 

 probably not the only source for there is evidence that the proteins that 

 enter into the composition of all tissues and tissue fluids, are undergoing at 

 all times a hydrolysis under the influence of enzymes whereby products 

 are produced similar to if not identical with those produced in the intestine. 

 These after their discharge into the blood stream are carried to the liver 

 where they undergo the same change as though derived from the intestine. 



The question arises however as to what percentage of the urea is derived 

 from the products of the metabolism of the tissue proteins (endogenous 

 urea) and what percentage is derived from the products of the metabolism 

 of the food proteins in the alimentary canal (exogenous urea). The answer 

 to this question is connected with the further question as to the amount of 

 protein necessary to keep the body in nitrogen equilibrium at its lowest 

 level compatible with health and efficiency. If this amount be from 30 to 

 50 grams as recent experiments would seem to show then the endogenous 

 urea would be approximately from 10 to 17 grams. Accordingly as this 

 lower level is raised will the amount of the endogenous urea be increased. 



Uric Acid. Uric acid is one of the constant ingredients of the urine. 

 It is a crystalline nitrogen-holding body closely resembling urea, its formula 

 being C 5 H 4 N 4 O 3 . The total quantity excreted daily varies from 0.2 to i 

 gram. It is doubtful if uric acid exists in a free state in the urine, the indi- 



