SIGNIFICANCE OF GAS EXCHANGE 



heat production of an animal, which is accurate to within some 3 per 

 cent., can be obtained from determinations of the oxygen consumption 

 alone, provided always that the catabolic processes comprise only the 

 usual oxidations of carbohydrates, fats and proteins, and that syntheses 

 of any of these substances do not take place or can be considered as 

 quantitatively insignificant. 



When both the oxygen consumption and the production of carbon 

 dioxide are determined, and the protein metabolism is either estimated 

 or measured from the nitrogen excretion, the respective amounts of carbo- 

 hydrate and fat catabolized can be calculated from the respiratory 

 quotient of the non-protein metabolism as mentioned above (p. 7, 

 Table I) ; and as the caloric value of each of these substances is pretty 

 accurately known the total heat production can be calculated with con- 

 siderable accuracy. In the table by Zuntz and Schumburg given 

 above, the caloric value of oxygen corresponding to the different re- 

 spiratory quotients of the non-protein metabolism is included. 



A number of experiments have been made on dogs (Rubner, 1 894) 

 and man (Atwater and Benedict ; Benedict, 1907 ; Benedict and Milner 

 [1907]), both during rest and during muscular work, in which the results 

 of direct and indirect calorimetry were compared, while the quality and 

 quantity of material catabolized were varied within very wide limits. 

 The results,^ of which a typical example is given in Table II from Bene- 

 dict's paper, usually agree to within I or 2 per cent., and on an average 

 for a large number of experiments the difference is in most series 

 only a fraction of I per cent. With higher animals the indirect calo- 

 rimetry is therefore in almost all circumstances completely justified, 

 when the respiratory quotient remains within the limits 071 to 0*99. 



TABLE II. INANITION EXPERIMENT ON MAN LASTING SEVEN DAYS. 



Bohr and Hasselbalch have compared the heat production and re- 

 spiratory exchange of the embryo of the common fowl during almost 



