6 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



rendered excellent service by the clearing up of various problems, but 

 more complete determinations are now largely taking their place. 



The Respiratory Quotient. 



A determination of the nature of substances catabolized is possible 

 within certain limits by means of the respiratory quotient, the relation 

 by volume of the carbon dioxide eliminated to the oxygen absorbed 

 or by weight of the O 2 contained in the eliminated CO 2 to the O 2 

 absorbed, as first shown by Regnault and Reiset. 



This possibility depends on the fact that a definite relation exists 

 for each substance between the oxygen consumed and the carbon 

 dioxide formed in its catabolism. 



For all carbohydrates the respiratory quotient is I. 



C 6 H 12 O 6 + 6 O 2 = 6 CO 2 + 6 H 2 O. 



For fats the respiratory quotient is not absolutely the same for all 

 as the composition varies, but the differences are small. The average 

 composition of fat is 76*5 per cent. C, 12 per cent. H, and 11-5 per 

 cent. O. The catabolism takes place according to the equation 



ioo gr. fat + (76-5?! + 12 V - 11-5) gr. O 2 = 76-5$$ gr. CO 2 + 12 V gr. water, 

 or 



ioo gr. fat + 288-5 gr. O 2 = 280-5 gr. CO 2 + 108 gr. water. 



The 280-5 g r - CO 2 contain 28o'5ff = 204 gr. O 2 and the respira- 

 tory quotient is therefore f-f^ = 0707. 



Similar computations can be made for proteins, but as the per- 

 centage composition of different proteins is not the same, and as the 

 catabolism is incomplete and to a certain extent variable, the results 

 may vary from about 078 to 0-82. The average respiratory quotient 

 of protein is generally taken as 0*80. 



The respiratory quotient of alcohol is 0-667. 



The respiratory quotients met with in animals usually lie between 

 0*97 and 0-72. When nothing further has been determined a low 

 respiratory quotient will indicate qualitatively that the material cata- 

 bolized is chiefly fat and protein, and a high that it is chiefly car- 

 bohydrate and protein ; but quantitative results cannot be obtained. 

 When the amount of protein catabolized is known, generally in the 

 form of the quantity of nitrogen in the corresponding urine, the corre- 

 sponding quantities of O 2 and CO 2 , which have been given by Zuntzas 

 8-471 gr. = 5-923 litre O 2 and 9-347 gr. = 4754 litre CO 2 per gr. N 



