188 



METABOLISM 



take place within the cells themselves. No matter what kind 

 of food is taken into the body, it must first be broken down 

 into a soluble form, and assimilated by the cell, before it can 

 be converted into the kind of tissue needed. 



Metabolism of carbohydrates. All carbohydrates are reduced 

 to simple sugars, glucose or galactose, in the digestive process, 

 by means of enzymes. They are then absorbed by the capillaries 

 in the villi and sent to the liver by way of the portal vein. The 

 excess sugar is taken in by the liver and stored there as an in- 

 soluble form of carbohydrate, known as glycogen or animal starch. 

 Muscle cells, too, store up small quantities of sugars in the form 

 of glycogen. This glycogen is then con- 

 verted, as it is needed, into glucose and 

 given to the blood. Thus the percentage 

 of sugar in the blood is kept constant at 

 0.07 to 0.15 per cent. If more carbohy- 

 drates are eaten than can be used or stored, 

 the surplus tends to be converted into body 

 fat. As the sugar circulates, it is absorbed 

 in the tissue fluid and cells by means of 

 osmosis. Here it is oxidized to release 

 energy in the form of muscular work or heat 

 to keep the temperature of the body con- 

 stant. In the oxidation process, sugar is 

 converted into carbon dioxide and water. 

 These wastes are carried by the blood to 

 the lungs, skin, and kidneys for elimination 

 from the body. 



Metabolism of protein. Proteins are de- 

 composed by enzymes in the digestive tract 

 into the simple compounds, amino-acids. 



These are absorbed into the blood and from there into the tissue 

 fluid and cells. The body uses the various amino-acids for build- 



The food burned in a bomb 

 calorimeter and the energy 

 released is carefully deter- 

 mined. The number of cal- 

 ories shown in the food tables 

 is found in this manner. 



