240 RESPIRA TION 



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 float- 

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

 volume of the expired air, owing to its higher temperature and ex- 

 cess of watery vapour, is somewhat greater than that of the inspired 

 air, but if it be measured at the temperature and degree of satura- 

 tion 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. 478) . 

 Respiratory Quotient. 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 contains 

 no more than one-sixth, and proteins not one-half, of the oxygen 

 needed to oxidize their hydrogen (p. 608). 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 respiratory quotient may 

 remain constant for a wide range of metabolism. 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 incompletely oxidized substances. On the other hand, in 

 dyspnoea 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 imme- 

 diate temporary increase in the respiratory quotient, owing to the 

 1 washing out ' of carbon dioxide from the blood and tissues. This 

 change has no metabolic significance. Indeed, the determination 

 of the respiratory quotient for short periods has only a limited 

 value, and such observations must be interpreted with great care. 

 In starvation the respiratory quotient diminishes, the production 

 of carbon dioxide falling off at a greater rate than the consumption 



