CHAPTER IX 



THE EXCHANGES OF MATTER AND ENERGY 

 IN THE BODY 



GENERAL METABOLISM 



ALL the energy which leaves the body as heat or work is derived from pro- 

 cesses of oxidation, the carbon, hydrogen, nitrogen, and sulphur of the food- 

 stuffs uniting with oxygen in the body and being eliminated in the form of 

 carbon dioxide, water, urea and other substances, and sulphates. In a 

 starving animal this discharge of energy must be associated with a loss of 

 body substance. The necessity for taking food is determined by the need of 

 replacing this loss. The food-stuffs cannot, like the coal or fuel of a steam- 

 engine, be utilised directly as a source of energy, but must be built up to a 

 greater or less degree into the structure of the living protoplasm. The total 

 amount of living material in the body, though maintained fairly constant 

 in the adult animal, may yet undergo alterations under varying conditions, 

 and these alterations are naturally more marked in the growing animal. We 

 have in this chapter to inquire into : 



(1) The nature and amount of the substances which may serve as food- 

 stuffs and are necessary for maintaining the weight of the body constant or 

 providing for its growth ; 



(2) The relation between the total amount of material taken up by the 

 body and the total amount given out ; 



(3) The variations in the total chemical exchanges determined by 

 variations in the output of energy by the body ; and 



(4) The significance of the various classes of food-stuffs as sources of 

 energy and in the replacing of tissue waste. 



We have therefore to make balance-sheets of two kinds, namely : (1) an 

 accurate comparison of the ingesta (food and oxygen) and the egesta (carbon 

 dioxide, water, urea, &c.) ; and (2) one showing the amount of potential 

 energy introduced into the body c jmpared with the amount of energy set 

 free in the body. 



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