CHAPTER IV 



THE INTAKE OF MATERIALS AND ENERGY 



As long as the body is alive, it requires a constant supply of materials 

 to provide energy and to take care of growth and repair. All body energy 

 is derived from the slow burning (oxidation) of certain food substances 

 within the cells of the body. The materials needed for growth and repair 

 include these same foods and, in addition, such other component parts 

 of protoplasm and body fluids as water, various inorganic salts, and a 

 group of special substances known as vitamins. 



All the materials required by the body, except oxygen, are taken into 

 it by way of the digestive system; the intake of oxygen is provided for by 

 the respiratory system. 



WATER 



Surprisingly large amounts of water are required for the functioning 

 of the body. Every 24 hours about 1,500 cc. of water is secreted in the 

 saliva, about 1 liter in the pancreatic juice, another liter in the bile, 3 

 liters in the intestinal juices, and 2 liters in the gastric juice. Besides these 

 amounts, 170 liters is required in the kidneys in 24 hours, of which }4 to 

 1 or more liters is lost as urine. From ^ to 2% liters is given off by the 

 skin in perspiration, and % liter by the lungs in respiration. The total 

 physiological requirement for these processes is more than 180 liters, or 

 almost 200 quarts per day. Most of this amount is recaptured and re- 

 utilized, but the daily loss of water amounts to several quarts, which 

 must be made good in part by drinking water and in part by water pro- 

 duced by oxidation of the hydrogen contained in food. 



FOOD MATERIALS 



Food may be so defined as to include all the essential materials taken 

 into the digestive system. In this sense the term includes proteins, carbo- 

 hydrates, and fats, also water, a considerable list of inorganic salts, and 

 the vitamins. In a narrower sense, we may restrict the term food to those 

 substances which not only are required for the building up of protoplasm 

 but which are also capable of being oxidized to provide energy. It is the 

 substances of this last class, comprising the proteins, carbohydrates, and 

 fats, that commonly occur in nature in a condition unsuitable for direct 



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