CHANGES PRODUCED IN THE RESPIRED AIR 343 



are influenced by the altered frequency and extent of the respiratory move- 

 ments. As far as this latter question is concerned, numerous observations 

 teach us that augmented respiration increases the absolute output of C0 2 

 not in consequence of the greater exchange of air, but on account of the 

 increased work of the respiratory muscles. 



The percentage of oxygen in the expired air is of course less than that 

 of the inspired air, and in fact it decreases more as a rule than the percentage 

 of C0 2 increases. When carbon burns in oxygen, the volume of the gas does 

 not change. Since in respiration, however, the amount of oxygen which has 

 disappeared is greater than that of the carbon dioxide formed, it follows that 

 the oxygen is used in the body for other oxidations than that of carbon. The 



PO 



ratio between carbon dioxide formed and oxygen used -y- 2 is called the respir- 

 atory quotient. a 



The value of the respiratory quotient is very different under different 

 circumstances, and depends upon the kind of foodstuffs which at the time are 



FIG. 134. The amount of carbon dioxide measured in two-hour periods, expired by a woman 

 who slept during the entire time, and who for five days previously had eaten scarcely any- 

 thing. 



being burned in the body. The carbohydrates contain in their molecule just 

 as much oxygen as is necessary to completely utilize their hydrogen. The 

 total quantity of the inspired oxygen therefore can be used for the oxidation 

 of their carbon. Hence, if carbohydrates exclusively are being burned, the 

 value of the respiratory quotient will be 1. 



Fat and proteid require more oxygen than carbohydrates for their com- 

 plete oxidation because the oxygen contained in their molecule is not sufficient 

 for the complete saturation of their hydrogen. Consequently when these 

 substances are being burned the respiratory quotient will be less than 1 for 

 fats 0.71 and for proteid 0.78. (Fat contains on the average 76.5 per cent C, 

 12 per cent H, 11.5 per cent ; proteid (dry muscle) 50.5 per cent C, 7.6 

 per cent H, 15.4 per cent N and 20.97 per cent 0, of which 11.3 per cent C, 

 2.8 per cent H, 15.4 per cent N, and 11.44 per cent are eliminated in the 

 urine and faeces, leaving 39.2 per cent C, 4.8 per cent H, and 9.53 per cent 

 to be eliminated in the breath.) Since it only rarely happens that carbohy- 

 drates alone are burned in the body, the respiratory quotient as a rule, is 



