CHAPTER IX. 

 ' METABOLISM (Cont'd). 

 The Material Balance of the Body. 



We must distinguish, between the balances of the organic and 

 the inorganic foodstuffs. From a study of the former we shall 

 gain information regarding the sources of the energy production 

 whose behavior under various conditions we have just studied. 

 From a study of the inorganic balance, although we shall learn 

 nothing regarding energy exchange for such substances can 

 yield no energy we shall become acquainted with several facts 

 of extreme importance in the maintenance of nutrition and 

 growth. ; \ 



To draw up a balance sheet of organic intake and output re- 

 quires an accurate chemical analysis of the food and of the 

 excreta (urine and expired air). Furnished with such analyses 

 we proceed to ascertain the total amount of nitrogen and carbon 

 in the excreta in a given time and to calculate from the known 

 percentage of nitrogen in protein how much protein must have 

 undergone metabolism. We then compute how much carbon this 

 quantity of protein would account for, and we deduct this from 

 the total carbon excretion. The remainder of carbon must have 

 come from the metabolism of fats and carbohydrates, and al- 

 though we cannot tell exactly which, yet we can arrive at a close 

 approximation by observing the respiratory quotient (R. Q.), 

 which is the ratio of the volume of carbon dioxide exhaled to 



CO 2 

 that of oxygen retained by the body in a given time, i. e., 



2 



When carbohydrates are the only foodstuff undergoing metabol- 

 ism, the quotient is one, that is to say, the C0 2 excretion and 2 

 intake are equal in volume. The reason for this is that a molecule 

 of carbohydrates consists of C. along with II. and 0. in the same 

 proportions as they exist in water ; therefore oxygen is required 



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