848 PHYSIOLOGY OF DIGESTION AND SECRETION. 



decrease in the amount of urea in the urine and an increase in the 

 ammonia contents. In these remarkable experiments a fistula 

 (Eck fistula) was made between the portal vein and the inferior 

 vena cava, the result of which was that the whole portal circulation of 

 the liver was abolished, the organ receiving blood only by way of 

 the hepatic artery. If now the latter artery was ligated and the 

 liver was cut away as far as possible, the result was practically a 

 complete extirpation of the organ. Later investigations * showed 

 that in normal animals the ammonia contents of the blood of the 

 portal vein may be three to four times as great as in arterial blood, 

 but that after removal of the liver the ammonia in the general 

 circulation increases to a point equal to that observed for the 

 portal blood and produces symptoms of poisoning which may 

 result fatally. It would seem, therefore, that the liver protects the 

 body from the poisonous action of the ammonia compounds by 

 converting them to urea. Now in the normal digestive hydrolysis 

 of proteins brought about by the successive action of pepsin, tryp- 

 sin, and erepsin the protein material is split largely or entirely 

 into its constituent elements and its nitrogen appears mainly in 

 the form of the amino-acids, but to some extent also probably as 

 ammonia. In addition there is evidence that some ammonia is 

 formed in the large intestine, as the result of the action of the 

 putrefactive bacteria. The ammonia produced in these ways is 

 probably carried to the liver and there converted to urea. In 

 what form the ammonia exists in the blood is not positively known ; 

 it may be present as a carbonate or possibly, as some observers 

 have thought, as a carbamate. Ammonium carbamate might 

 be changed to urea according to the following reaction : 



TT r\ _ /-^ .-2 

 NH 2 ~ <NH 2 - 



Ammonia salts may arise similarly in the tissues of the body, 

 since the cells contain intracellular enzymes capable of causing 

 hydrolytic cleavage of the protein molecule. So far as ammonia is 

 produced in this way, it will be converted to urea by the action of 

 the liver and possibly by a similar action in other tissues. 



2. Urea arises from the monamino-acids by a process of deamin- 

 ization, whereby the NH 2 group- is converted to ammonia and 

 then probably to urea. In the digestion of protein in the alimen- 

 tary canal amino-acids are formed in quantity. The current belief 

 in physiology is that these amino-acids are carried in the blood to 

 the various tissues and are there resynthesized in part to form the 

 characteristic protein of the tissue. The tissues pick and choose the 

 several amino-acids necessary for their reconstruction or growth. 



* See Nencki and Pawlow, " Archives des sciences biologiques," v., 213. 



