356 ESSENTIALS OF PHYSIOLOGY. 



with carbonic acid to form ammonium carbonate, or carbamate, the 

 change taking place in the liver may be represented thus: 



Urea. 



/o\ ro ONH NH 



The formation of urea in the liver has been proved in several ways. 

 In the first place, when the liver of a recently killed animal is removed 

 from the body and perfused with oxygenated blood to which ammonium 

 carbonate or carbamate is added, the ammonium salt gradually dis- 

 appears from the blood, being replaced by urea ; this change does not 

 occur when blood containing ammonium salts is perfused through 

 other organs. 



Secondly, it is possible to unite the portal vein with the inferior 

 vena cava in such a way that the blood flowing from the digestive 

 tract along the portal vein is diverted into the vena cava, and thus 

 into the general circulation, without going through the liver ; the 

 fistula between the two veins is known as Eck's fistula. The liver is 

 still supplied, in this case, with blood through the hepatic artery. 

 Dogs in which such a fistula has been made remain well when their 

 diet consists chiefly of carbohydrate, but when they are fed on meat or 

 receive ammonium salts or amino-acids by the mouth, they become 

 convulsed ; during the convulsions their arterial blood contains four or 

 five times as much ammonia as is present in a normal animal, and the 

 percentage of ammonium salts in the urine rises. It may be concluded 

 that the liver, when supplied only with blood from the hepatic artery, 

 can no longer convert into urea the large amounts of ammonia entering 

 the blood after a protein meal. 



Thirdly, in extensive disease of the liver in man the amount of 

 ammonia in the urine is increased, and the amount of urea is corre- 

 spondingly diminished. It is thus evident that the formation of urea 

 from ammonium salts takes place solely in the liver, probably by the 

 action of a synthetic ferment. 



Urea can also be formed in other parts of the body and from other 

 substances than ammonium salts, the most important of these being 

 arginine and uric acid. Arginine occurs in the tissues, and can be 

 broken down by a ferment known as arginase into urea and ornithnin ; 

 this ferment is most abundant in the liver and kidneys. 



