USE OF THE DIGESTED FOOD 149 



the oxidation of the food, and this passes off from the 

 lungs as vapor, through the skin as sensible or insensible 

 perspiration, and in considerable quantities through the 

 kidneys. 



To summarize, it may be said that the blood is con- 

 stantly undergoing gain and loss. The gain comes from 

 the food (including water and oxygen), and the loss con- 

 sists of the compounds in the urine, carbonic acid, and 

 water given off through various channels. 



THE LIVER 



214. Regulation of carbohydrate use. — One part of 

 the arterial system of blood vessels runs to the stomach 

 and intestines and is distributed over their walls in fine 

 divisions. These connect with the capillaries of the portal 

 vein which leads to the liver (Fig. 10). During this passage 

 of the blood from one system to the other, part of the 

 digested food is taken up. The quantity of material thus 

 absorbed must vary greatly at different times according to 

 the nature and amount of food supply and the activity of 

 the digestive processes. If, therefore, the blood from the 

 alimentary canal were allowed to pass directly into the 

 general circulation, the supply to the tissues of the sugar 

 resulting from digestion would be very imeven. Just here 

 comes in a liver function. In that organ there is found a 

 starch-like body known as glycogen, which appears in 

 increased quantity following the abundant absorption 

 of sugar from the intestines. It is believed, because of 

 this and other facts, that the liver acts as a regulator of 

 the carbohydrate supply to the general tissues of the 

 body, storing a temporary excess of the sugar in the form 

 of glycogen and then gradually giving it up to the gen- 



