ELEMENTARY PHYSIOLOGICAL CHEMISTRY 



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urinary substances are derived, the amounts of these vary enormously 

 according to the nature of the diet. The reason of this is that the 

 diet is the chief controlling factor of tissue metabolism. A proteid- 

 rich food leads to more active tissue metabolism than does a proteid- 

 poor one, as a consequence of which there are more solids excreted in 

 the urine during the former than during the latter diet. In studying 

 the composition of urine, therefore, we must bear this in mind before 

 deciding whether any constituent is increased or diminished in amount. 

 The average composition of the 24 hours urine during health in a 

 young man is approximately as follows : 



* Calculated here as the metal and not as the oxide. 



The Organic Constituents. With the exception of some of the 

 aromatic bodies absorbed from the intestine, the organic substances in 

 the urine all contain nitrogen, and if we add to these the small amount 

 excreted as ammonia, we can then account for over 90 per cent, of all the 

 nitrogen resulting from tissue metabolism, the remaining fraction being 

 lost mainly in the faeces (1 gramme per diem), and slightly in the 

 sweat as urea. It is most important, therefore, first of all to become 

 acquainted with some method by which the total nitrogen excreted in 

 urine can be estimated. This is furnished by Kjeldahl's method, the 

 principle of which is the following. 



