154 RESPIRATION 



a cubic foot of CO 2 ; or a cubic foot of CO 2 will yield, on dis- 

 sociation, a cubic foot of O. 



This being the case, if all the O consumed in the lungs were 

 exhaled therefrom in the form of CO 2 , the amount of CO 2 

 exhaled would just equal the amount of O consumed. But the 

 amount of consumed O is about 5 per cent, of the inspired air, 

 while the amount of exhaled CO 2 is only about 4 per cent, of 

 the expired air. It follows, therefore, that i per cent, of the vol- 

 ume of inspired air is not represented by the CO 2 exhaled from 

 the lungs and skin. The relation between the consumed O 

 and the exhaled CO 2 is usually expressed as the " respiratory 

 quotient" the division of the latter by the former giving the 

 quotient. This quotient is made to vary by many circum- 

 stances, though for any considerable period its average is about 

 the same. 



While it has been stated that the O absorbed and the CO 2 

 produced vary together usually, they are in a certain measure 

 independent of each other. For CO 2 does not result from the 

 immediate union of O with carbon of the carbohydrates and fats, 

 but may be stored in the shape of complex compounds, which 

 may later split up with the formation of CO 2 , either by oxida- 

 tion or by intramolecular cleavage. Furthermore, more O is 

 necessary to oxidize (that is, to form carbon dioxide) some mole- 

 cules than others. A fat requires considerably more O to pro- 

 duce CO 2 than does a carbohydrate; so that the kind of food in 

 store would also affect the respiratory quotient. 



With respect to the O which, in the long run, is not represented 

 in the CO 2 exhaled from the lungs and skin, it is certain that 

 when various of the food stuffs are broken down at least a 

 part of it is appropriated by hydrogen to form water. 



Source of Exhaled Carbon Dioxide. The increase of CO 2 

 in expired air over the small amount contained in inspired air is 

 derived from the venous blood circulating through the lungs. It 

 exists in that blood under a constant tension, as is demonstrated 



