RESPIRATION 225 



derived from the alimentary canal, either directly or after 

 absorption into the blood. It is entirely free from floating 

 matter (dust), which is always present in the inspired air. 

 The volume of the expired air, owing to its higher tempera- 

 ture and excess of watery vapour, is somewhat greater than 

 that of the inspired air, but if it be measured at the tem- 

 perature and degree of saturation of the latter, the volume 

 is somewhat less. Since the oxygen of a given quantity of 

 carbon dioxide would have exactly the same volume as the 

 carbon dioxide itself at a given temperature and pressure, it 

 is clear that the deficiency is due to the fact that all the 

 oxygen which is taken up in the lungs is not given off as 

 carbon dioxide ; some of it, going to oxidize hydrogen, 

 reappears as water a small amount of it unites with the 

 sulphur of the proteids (p. 392). The quotient of the volume 

 of oxygen given out as carbon dioxide by the volume of 

 oxygen taken in is the respiratory quotient. It shows what 

 proportion of the oxygen is used to oxidize carbon. It may 

 approach unity on a carbo-hydrate diet, which contains 

 enough oxygen to oxidize all its own hydrogen to water. 

 With a diet rich in fat it is least of all ; with a diet of lean 

 meat it is intermediate in amount. For ordinary fat con- 

 tains no more than one-sixth, and proteids not one-half, 

 of the oxygen needed to oxidize their hydrogen. In man 

 on a mixed diet the respiratory quotient may be taken 

 as *8 or "9. So long as the type of respiration is not 

 changed, the respiratory quotient may remain constant for 

 a wide range of metabolism. In hibernating animals, how- 

 ever, the respiratory quotient becomes very small during 

 winter sleep (as low as '4), the output of carbon dioxide falling 

 far more than the consumption of oxygen. On the other 

 hand, in excised mammalian muscles at a low temperature 

 the consumption of oxygen is lessened to a greater extent 

 than the production of carbon dioxide, and the respiratory 

 quotient may be as high as 3-2 (Rubner). Muscular work 

 increases the respiratory quotient, because carbo-hydrates 

 are chiefly used up. In starvation the respiratory quotient 

 diminishes, the production of carbon dioxide fallic^g off at 

 a greater rate than the consumption of oxygen, for the 



15 



