242 A MANUAL OF PHYSIOLOGY 



constitutes a substantial fraction of the tidal air. The carbon 

 dioxide percentage in the alveolar air at the end of expiration, 

 with the body at rest, is remarkably constant in one and the same 

 individual at constant atmospheric pressure (p. 257). There are 

 in addition in expired air small quantities of hydrogen and marsh- 

 gas derived from the alimentary canal, either directly from eructa- 

 tion or after absorption into the blood. Sometimes a trace of 

 ammonia can be detected in the air of expiration, but this is due 

 to decomposition of proteins taking place in the mouth, especially 

 in carious teeth, or in the air-passages and lungs in disease of these 

 organs. It has indeed been shown that the lungs are practically 

 impermeable for ammonia. Expired air 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 temperature 

 and excess of watery vapour, is somewhat greater than that of the 

 inspired air, but if it be measured at the temperature 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 proteins (p. 444). 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 rich diet in fat it is least of all ; 

 with a diet of lean meat it is intermediate in amount. For 

 ordinary fat contains no more than one-sixth, and proteins not 

 one-half, of the oxygen needed to oxidize their hydrogen. In man 

 on a mixed diet the respiratory quotient may be taken as 0-8 or 

 0-9. So long as the type of respiration is not changed, the respira- 

 tory quotient may remain constant for a wide range of meta- 

 bolism. In hibernating animals, however, the respiratory 

 quotient may become very small during winter sleep (as low as 

 0*25), both the output of carbon dioxide and the consumption of 

 oxygen falling enormously, but the former in general more 

 than the latter. This has been explained on the assumption 

 that oxygen is stored away in winter sleep in the form of incom- 

 pletely oxidized substances. On the other hand, in dyspnrea 

 accompanying muscular exertion the respiratory quotient 

 has been found as high as 1-2. It must be remembered 

 that even a voluntary increase in the respiratory movements 

 causes an immediate temporary increase in the respiratory 

 quotient, owing to the ' washing out ' of carbon dioxide 



