CHAPTER I. 



THE PHYSIOLOGICAL SIGNIFICANCE OF THE EXCHANGE OF OXYGEN 

 AND CARBON DIOXIDE. 



LEAVING out of account the mechanism by which the gas exchange 

 is brought about, its physiological significance lies in the catabolic pro- 

 cesses of which the gas exchange furnishes quantitative evidence. 



The study of the gas exchange has been utilized in three main 

 directions : 



(j) To establish the carbon balance of the organism ; 



(2) to determine the nature of the substances catabolized ; 



(3) to measure the total catabolism. 



In many cases the respiratory exchange appears to have been in 

 itself the object of quantitative physiological research, without further 

 thought about its significance. It will be shown, however, that such 

 determinations fall in reality under the third and sometimes the 

 second of the above heads, and are to be considered as more or less 

 approximate determinations of the amount of energy transformed from 

 a potential to a free state. 



The Carbon Balance of the Organism. 



In order to establish the carbon balance it is necessary only to 

 measure the output of carbon dioxide, which simplifies considerably 

 the technique of the respiration experiment. On the other hand the 

 experiments must be of long duration, and cover at least the greater 

 part of the twenty-four hours or any longer period for which it is de- 

 sired to establish the balance. The results of short experiments 

 are sometimes very misleading as pointed out repeatedly by Rubner. 

 The amount of carbon excreted in twenty-four hours is found from 

 the respiration experiment and analyses of urine and faeces, and 

 is compared with the intake of carbon in the food. 



Experiments of this sort were first made by Ranke, using Petten- 

 kofer's respiration apparatus [1862], and have since been extensively 

 used by Pettenkofer, Voit, Rubner, and their pupils. They have 



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