CHANGES OF AIR IN RESPIRATION. 21 1 



nearly acquires that of the blood before it is expelled from 

 the chest. 



2. The carbonic acid in respired air is always increased ; 

 but the quantity exhaled in a given time is subject to 

 change from various circumstances. From every volume 

 of air inspired, about 4 J per cent, of oxygen are abstracted ; 

 while a rather smaller quantity of carbonic acid is added 

 in its place. It may be stated, as a general average 

 deduced from the results of experiments by Valentin and 

 Brunner, that, under ordinary circumstances, the quantity 

 of carbonic acid exhaled into the air breathed by a healthy 

 adult man amounts to 1346 cubic inches, or about 636 

 grains per hour. According to this estimate, which cor- 

 responds very closely with the one furnished by Sir H. 

 Davy, and does not widely differ from those obtained by 

 Allen and Pepys, Lavoisier, and Dr. Ed. Smith, the weight 

 of carbon excreted from the lungs is about 173 grains per 

 hour, or rather more than 8 ounces in the course of twenty- 

 four hours. Discrepancies in the results obtained by 

 different experimenters may be due to the variations to 

 which the exhalation of carbonic acid is liable in different 

 circumstances ; for even in health the quantity varies accord- 

 ing to age, sex, diversities in the respiratory movements, 

 external temperature, the degree of purity of the respired 

 air, and other circumstances. Each of these deserves a 

 brief notice, because it affords evidence concerning either 

 the sources of carbonic acid exhaled, or the mode in which 

 it is separated from the blood. 



a. Influence of Age and Sex. According to Andral and 

 Gavarret the quantity of carbonic acid exhaled into the 

 air breathed by males, regularly increases from eight to 

 thirty years of age ; from thirty to forty it is stationary 

 or diminishes a little ; from forty to fifty the diminution is 

 greater ; and from fifty to extreme age it goes on diminish- 

 ing, till it scarcely exceeds the quantity exhaled at ten 

 years old. In females (in whom the quantity exhaled is 



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