334 FOOD AND DIGESTION 



and globulin of the blood. These are the forms of protein 

 which are transported through the body. The proteins 

 pass into the portal vein and through the liver, but are not 

 arrested there as are the carbohydrates. 



Water and inorganic salts which have been set free from 

 their organic combination in foods by the various processes 

 of digestion are absorbed through the villi into the capillaries. 

 Under ordinary conditions no water is absorbed by the 

 lymph vessels. The progress of water and inorganic salts 

 is then like that of carbohydrates and proteins, through the 

 portal vein and the liver. Fig. 82 shows the routes of the 

 absorbed material. 



239. Functions of Food Constituents. — Various constituents 

 of the food after absorption into the body, serve each a more 

 or less distinct function in the activity of the animal and 

 can be discussed separately. 



(a) Carbohydrate. — The only active form of carbohy- 

 drate in the body is dextrose and this material serves primarily 

 as a source of energy. Just as energy is derived in the steam 

 engine from the combustion of fuel so is the energy of the 

 body derived from combustion of fuel. This combustion 

 takes place within the tissues of the body and is caused 

 by enzymes, the final product being carbon dioxide and 

 water. Energy is set free in the form of heat and work. 

 When dextrose is present in larger quantities than is necessary 

 for fuel consumption it is transformed into fat and stored 

 away in the adipose tissue of the body. 



(b) Fat serves also as fuel for body energy and is 

 the most concentrated source of fuel in the body, yielding 

 more energy per unit of weight than any other form of fuel. 

 Some of the fat absorbed is deposited in adipose tissue, and 

 there is some evidence that dextrose may be formed from fat. 



(c) Protein. — The primary function of protein is, of 

 course, to supply the principal part of tissue material. It is 

 necessary for cell wall and protoplasmic contents of new 

 cells and also to replace worn out material in old cells. 

 Changes are constantly taking place in the body; old cells 

 wearing out, new cells being formed, in addition to increase 

 in the number of cells when an animal is growing. Protein 



