CHAPTER VIII. 



METABOLISM. 



The Energy Balance. 



Introductory. The object of digestion, as we have seen, is 

 to render the food capable of absorption into the circulatory 

 fluids, the blood and lymph. The absorbed food products are 

 then transported to the various organs and tissues of the body, 

 where they may be either used or stored away against future 

 requirements. After being used, certain substances are produced 

 as waste products, and these pass back into the blood to be car- 

 ried to the organs of excretion, by which they are expelled from 

 the body. By comparison of the amount of these excretory prod- 

 ucts with that of the constituents of food, we can tell how much 

 of the latter has been retained in the body, or lost from it. This 

 constitutes the subject of general metabolism. On the other 

 hand, we may direct our attention, not to the balance between 

 intake and output, but to the chemical changes through which 

 each foodstuffs must pass between its absorption and excretion. 

 This is the subject of special metabolism. In the one case we 

 content ourselves with a comparison of the raw material which 

 is acquired and the finished product which is produced by the 

 animal factory ; in the other, we seek to learn something of the 

 particular changes to which each crude product is subjected^ be- 

 fore it can be used for the purpose of driving the machinery of 

 life or of repairing the worn out parts of the body. 



In drawing up such a balance sheet of general metabolism, we 

 must select for comparison substances which are common to both 

 intake and output. In general the intake comprises, besides oxy- 

 gen, the proteins, fats and carbohydrates, and the output, carbon 

 dioxide, water and the various nitrogenous constituents of urine. 

 This dissimilarity in chemical structure between the substances 

 ingested and those excreted limits us, in balancing the one 



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