372 



HUMAN PHYSIOLOGY 



Only a small amount escapes by the bowels and skin, and 

 hence only when very accurate observations are desired is it 

 necessary to analyse the faeces and sweat. 



Since nucleo-proteids form so important a constituent of 

 living matter, it is sometimes desirable to study the chemical 

 changes which they are undergoing. To do this the excre- 

 tion of phosphorus and the purin bases must be investigated. 

 But it is difficult to arrive at reliable conclusions, because 

 there are other phosphorus- containing substances besides 

 nucleins in the body e.g. the bones ; and secondly, the 

 purin bodies all tend to be converted into urea before being 

 excreted. 



B. Metabolism of Fats. Proteids contain nearly three 

 and a half times as much carbon as nitrogen, and hence, 

 when broken down, for each grm. of nitrogen excreted, 

 3 '4 grms. of carbon are given off. 



The carbon is chiefly excreted from the lungs as carbon 

 dioxide, and in this form it may be collected and estimated. 



Any excess of carbon excreted over 3-4 times the amount 

 of nitrogen given oft', must be derived from the fats of the 

 body or from the fats and carbohydrates taken in the food. 

 Any carbon retained in the body, apart from that in proteids, 

 is stored ultimately as fat, and since carbon constitutes 

 76*5 per cent, of fats, the amount of fat is calculated by 

 multiplying the carbon by 1/3. 



The following tabular example of an investigation of the 

 metabolism may be given : 



Two grms. of nitrogen are retained as proteid ; that is, 2 x 

 6*25 = 12-5 grms. of proteid are being daily laid on. Thirty 

 grms. of carbon are also retained in the body, and of this 



