150 GENERAL METABOLISM 



of each kind of food which is being metabohsed we can 

 calculate the energy Hberated without recourse to a calori- 

 meter. This can be done even without a previous analysis 

 of the food administered, the only data required being : 

 (1) TJie total respiratory exchange, (2) the amount of nitrogen 

 excreted. 



The Respiratory Exchange : Respiratory Quotient 



The various methods which have been adopted for 

 estimating the oxygen intake and COg output fall into two 

 groups : {a) The animal is placed inside a chamber through 

 which air deprived of COg and water- vapour is pumped. 

 The total volume of air passing through is measured, and 

 the oxygen and CO2 passing out of the chamber estimated. 

 (6) This method, more suitable for experiments upon man, 

 consists in making the individual breathe through a suit- 

 able mask into a chamber which is supplied with a constant 

 stream of oxygen, the oxygen admitted and the CO. 

 expired and absorbed being estimated. 



The relation between the amounts of CO2 expired and 

 of oxygen absorbed during the same period," expressed as 



the former divided by the latter (^ A is termed the 



Respiratory Quotient (R.Q.). Its value varies according 

 to the amount of oxygen already present in the food 

 molecule undergoing combustion. "This will be seen from 

 the followng equations — 



Carbohydrate : 



CcHiaOe -f GO2 = 6CO0 + B^O. 



ca_6_ 



O2 ~ 6 ~ ^• 



Fat : 



C57H110O6 -f 8IIO2 = 57COo -f 55H2O. 



^=^-0.7 

 O2 81J~"^- 



2 



