xi EESPIEATOEY EXCHANGES 399 



molecule of the substances comprised in the carbo-hydrate group 

 contains oxygen enough to convert the whole of the hydrogen into 

 water. Accordingly for their complete combustion the only 

 oxygen required is that necessary for conversion of the carbon 

 into carbonic acid. Hence the volume of C0 2 formed is perfectly 



equal to the volume of 9 consumed, the quotient n ' 2 = l. 



^2 



For the perfect combustion of fats and proteins, on the 

 contrary, more O 2 is required, since in their molecules there is not 



enough O 2 to convert all the H 2 into water. The quotient -~ ~ 



^2 



is therefore less than 1. In the complete combustion of fats, the 

 quotient = 0*71 ; in the complete combustion of proteins = O78. 



It follows that if the substances introduced as food were 

 oxidised exclusively within the body, the respiratory quotient in a 

 pure carbohydrate diet would be = 1, in a fatty diet = 0'7l, in a 

 protein diet =0'78. But since not only do the food-substances 

 introduced into the body share in its oxidative processes, but the 

 various tissue-forming substances also take part, it seldom happens 

 that the respiratory quotient rises to 1, i.e. for the most part 

 it is represented by a variable proper fraction. It is only 

 under special circumstances that the respiratory quotient may 

 temporarily attain the value of 1, or even exceed it, as when fats 

 are formed from carbohydrates in the body, or when there 

 is a rapid diminution in the oxygen content of inspired air 

 (Rosenthal, 1902). An apparent increase of the quotient may 

 appear in certain birds in whose crop there is fermentation of the 

 food stored there, with production of C0. 7 . 



It is a very interesting fact that the respiratory quotient 

 oscillates with the substances that predominate in the food : in a 

 diet mainly composed of starch the value rises to 0'9 ; in a diet 

 that is chiefly fatty it drops to 0*55 : while lastly, in a diet 

 mainly consisting of meat it attains an intermediate value of 

 0'7-0*65. In an ordinary mixed diet the respiratory quotient is 

 about 0'8. 



When we consider the metabolism or material exchanges of the 

 body as a whole, we shall examine the importance of these and 

 other facts relating to the oscillations of the respiratory quotient ; 

 we shall discuss the different methods employed for animals, or 

 man, in the study of the absolute magnitude of the respiratory 

 gas exchanges ; we shall see that this magnitude changes with age, 

 sex, constitution, external temperature, work or rest, the different 

 hours of the day or night, etc. These investigations obviously 

 exceed the limits of the physiology of the respiratory apparatus, and 

 involves the functioning of the body as a whole. 



