1 14 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



nomical regulation of its distribution. The amount of sugar 

 in the circulation at any time is very small, and a single car- 

 bohydrate meal may, by the action of the liver be made to 

 supply the carbohydrate demands of the tissues for a consid- 

 erable period. 



Now, it was just said that the sugar of the blood is dex- 

 trose; if the dextrose of the portal blood is converted into 

 glycogen to be stored up, it must be reconverted into dex- 

 trose before it can leave the liver, since it leaves by the blood. 

 The cells do effect the second conversion, and this is the sec- 

 ond part of the glycogenic function. It may be that the liver 

 cells produce an enzyme corresponding to ptyalin, which con- 

 verts the glycogen. Dextrose does not normally exist in the 

 liver cells. At the very moment of its formation it is car- 

 ried away by the blood. 



The fact that the liver can form glycogen out of pro- 

 teids shows, of course, that nitrogen is eliminated from the 

 proteid molecule in some way. A carbohydrate molecule is 

 left to be oxidized in the usual manner. This is thought to 

 be the initial step in the final consumption of proteids in nu- 

 trition. The fats have no influence on glycogen formation. 



Glycogen also exists in other parts of the body, particu- 

 larly in the voluntary muscular substance. The cells of the 

 tissue in which it is found must also have a glycogenic func- 

 tion. 



Urea Formation. But the liver has another function be- 

 sides the production of bile and glycogen, and that is to form 

 urea. It will be seen later that the chief end product of pro- 

 teid metabolism is urea, and that it is eliminated almost en- 

 tirely by the kidneys. The liver is much more active in the 

 production of thfs substance when the portal blood is charged 

 with digested materials, but it also forms urea in fasting 

 animals. The liver must, therefore, be capable of forming 

 urea from some of the products of digested foods. With 

 reference to its formation in fasting animals, suffice it to say 

 here that it seems that as long as proteid metabolism goes 



