THE CHEMISTRY OF EXTERNAL RESPIRATION 2 4 i 



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. 



Alveolar Air. The percentage of carbon dioxide in the air of the 

 alveoli is of course greater and that of oxygen less than in the 

 ordinary expired air. For the relatively pure air of the dead-space 

 makes up a substantial fraction of the tidal air. The mean of the 

 oxygen or carbon dioxide percentages in samples taken from the 

 last portions of the air of two deep expirations, one following an 

 ordinary inspiration and the other following an ordinary expiration, 

 corresponds fairly well with the mean percentage in the alveoli 

 during ordinary breathing. This method becomes untrustworthy 

 during muscular work. The average percentage of oxygen may 

 be taken as 14*5, corresponding to a partial pressure (p. 248) 

 of 109 mm. of mercury. The percentage of carbon dioxide in 

 the alveolar air with the body at rest is remarkably constant in one 

 and the same individual at constant atmospheric pressure. It 

 varies in different men from 4-6 to 6*2 (mean 5-5) per cent, of the dry 

 alveolar air. In women and in children of both sexes it is less than 

 in men. From this we conclude that in men the partial pressure 

 of carbon dioxide in the alveoli may be at least one-eighteenth of 

 an atmosphere, or 42 mm. of mercury (Fitzgerald and Haldane). 



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 .ox^gyen 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. 620). 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 

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

 change has no metabolic significance. Indeed, the determination 



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