CHAPTER X 

 NUTRITION 



BETWEEN the absorption of food and the removal of the 

 waste products of its oxidation are certain chemical 

 changes which form part of the subject of metabolism. 

 In order to understand how cells perform their various pro- 

 cesses we must know what they receive and what becomes of 

 their incomings ; that is we must examine the material and 

 energy exchanges of cells. 



In the present chapter we shall discuss the conservation of 

 mass and of energy in the body and in the next chapter we 

 shall deal with the processes by which oxygen is obtained from 

 the atmosphere and transported to the tissues and by which 

 carbon dioxide is removed from the tissues and given off to 

 the atmosphere. 



Let us commence by an examination of the income and 

 expenditure of the whole body. We do this by what is termed 

 the balance-sheet method, in which the income and expendi- 

 ture are balanced against [each [other, the gain or loss 

 being included as one of the items in the statement. 



We need a balance for materials and a balance for energy. 

 Numerous experiments have shown that the living organism 

 obeys the laws of conservation of mass and of energy, so that 

 any surplus output of energy over intake is derived from a loss 

 in body substance and the converse. 



In making out a balance-sheet we need to know the income. 

 This is subdivided into gross income and nett income. There- 

 fore we must know not only the amount of food taken into 

 the body but also the amount of such food that is unabsorbed 

 and passes out by the faeces. 



The expenditure is measured by the materials excreted in 

 the urine and the carbon dioxide which escapes by the breath ; 

 the small amount of loss by the skin must also be included. 



NITROGENOUS EQUILIBRIUM 



Nitrogen is taken in mainly in the form of protein, which 

 has an average content of 16 per cent, of nitrogen. There- 



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