364 



ESSENTIALS OF PHYSIOLOGY. 



The liver plays an important part in the metabolism of all three 

 classes of food-stuffs, especially in the preliminary changes which they 

 undergo after absorption. In the first place, it serves as a store -house 

 for glycogen, which it forms from the carbohydrate absorbed during 

 digestion, and which it converts into dextrose and returns to the blood 

 stream in order to keep constant the percentage of sugar in the blood. 

 Secondly, it desaturates the fatty acids reaching it from the fat depots, 

 and prepares them for the further metabolic changes which occur in 

 the tissues. Thirdly, it removes the amino-group from part of the 

 ammo-acids absorbed from the digestive tract, converting them into 

 keto- or oxy-acids and transforming the ammonia thus set free, and 

 also that reaching it from the portal vein, into urea. 



Interference with these functions, which sometimes occurs in 

 extensive disease of the liver in man, leads to the appearance of inter- 

 mediate metabolic products such as leucine, tyrosine, and other sub- 

 stances in the urine, and to disturbance of the normal course of the 

 metabolism of fat; similar effects are seen in animals poisoned with 

 phosphorus, which greatly reduces the metabolic activity of the liver. 



The importance of the liver is further shown by the fact that the 

 complete cutting off of its blood supply in mammals is followed by 

 death within a few hours. 



In birds, however, the portal system communicates with branches of 

 the renal vein, and so with the systemic venous system ; and birds may 

 live for three or four days after the removal of the liver. In these 

 circumstances uric acid, which is the normal end product of nitrogenous 

 metabolism in birds, is largely replaced in the urine by ammonia and 

 lactic acid, which in these animals are the precursors of uric acid. 



Apart from its metabolic activities, the other functions of the liver 

 are (1) the secretion of bile, and (2) the conversion of the blood 

 pigment into bile pigment. 



