388 TEXTBOOK OP ZOOLOGY 



Absorption and Utilization of Food Materials. — The soluble prod- 

 ucts of digestion are absorbed through the semipermeable epithelial 

 lining of the intestine into the blood of the adjacent capillaries, or in 

 the case of fats into the lacteal IjTnphatics and from here into the sub- 

 clavian vein. The blood supplying the intestine is collected by the 

 hepatic portal vein and delivered to the liver. 



The two functions of proteins in the body are : to rebuild debili- 

 tated protoplasm; and help supply heat and energy to the body by 

 oxidation. They serve first and best for the purpose mentioned first. 

 Carbohydrates and then fats are more economical and efficient as 

 sources of fuel for production of heat and energy. Oxidation of pro- 

 tein requires the disposal of much more waste products. The com- 

 parative heat production values of the three are as follows : 



One gram of protein z= 4.100 Calories* 



One gram of carbohydrate z= 4.100 Calories 

 One gram of fat = 9.305 Calories 



Some portion of the dextrose is distributed and oxidized directly 

 for immediate energy, but much of it is transformed into glycogen 

 by the enzyme glycogenase in the liver. This may be stored here or 

 in the muscles to be reconverted into dextrose for oxidation by the 

 tissues as needed. Normally there is a constant supply of dextrose 

 (0.1 to 0.15 per cent) in the blood and this level must be maintained. 

 The final oxidation products of carbohydrates in the body are heat, 

 kinetic energy, water, and carbon dioxide. The last two are dis- 

 charged from the body as waste products. Fat is converted to dex- 

 trose and oxidized to produce heat and kinetic energy. It is usually 

 stored as a reserve fuel supply in adipose tissue over the body. Car- 

 bohydrates in excess may be converted to fat, and stored. 



Vitamins and Their Functions. — Besides proteins, carbohydrates, 

 fats, inorganic salts, and water there is another indispensable class 

 of food material, the vitamins. They are natural substances found 

 in relatively small quantities in a number of different foods. In 

 general, their function is regulatory. They are recognized usually 

 through the abnormal condition brought on by their deficiency. 

 There is little danger of vitamin deficiency for adults living on a 

 balanced and mixed diet. Much of our knowledge concerning the 

 symptoms brought on by lack of different substances has been 



•A Calorie equals the amount of heat necessary to raise one liter of water one 

 degree centigrade under standard conditions. 



