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 processes 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 compared 

 with the amount of energy set free in the body. 



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