272 HANDBOOK OF PHYSIOLOGY. 



while a rather smaller quantity, 4.0 of carbon dioxide is added in its 

 place: the expired air will contain, therefore, 400 vols. of carbon dioxide 

 in 10,000. The quantity of carbon dioxide exhaled into the air breathed 

 by a healthy adult man. calculating that 20 ccm. of the 500 com. of the 

 air breathed out at each expiration consists of carbon dioxide, and that 

 the rate of respiration is on an average 16, the total amount would be 

 about 460 litres in the 24 hours. From actual experiment this amount 

 seems to be too high, since from the average of many investigations the 

 total amount of carbon dioxide excreted per diem has been found to be 

 about 400 litres, weighing 800 grins., consisting of 218 grins, of C., and 

 582 grms. of 0. From this has to be deducted about 10 grms. excreted in 

 any other way than by the lungs, it leaves about 215 grms. as the amount 

 of C. excreted by the average healthy man by respiration each day and 

 night, that is about 7 oz., about half a pound. These quantities must 

 be considered approximate only, inasmuch as various circumstances, even 

 in health, influence the amount of carbon dioxide excreted, and, correla- 

 tively, tbe amount of oxygen absorbed. 



Circumstances influencing the amount of carbon dioxide excreted. a. Age 

 and Sex. The quantity of carbon dioxide exhaled into the air breathed by 

 males, regularly increases from 8 to 30 years of age ; from 30 to 50 the quantity, 

 after remaining stationary for a while, gradually diminishes, and from 50 to 

 extreme age it goes on diminishing, till it scarcely exceeds the quantity ex- 

 haled at ten years old. In females (in whom the quantity exhaled is always 

 less than in males of the same age) the same regular increase in quantity goes 

 on from the 8th year to the age of puberty, when the quantity abruptly ceases 

 to increase, and remains stationary so long as they continue to menstruate. 

 When menstruation has ceased, it soon decreases at the same rate as it does in 

 old men. 



b. Respiratory Movements. The quicker the respirations, the smaller is the 

 proportionate quantity of carbon dioxide contained in each volume of the expired 

 air. Although the proportionate quantity of carbon dioxide is thus diminished, 

 the absolute amount exhaled within a given time is increased thereby, ow r ing to 

 the larger quantity of air which is breathed in the time. The last half of a vol- 

 ume of expired air contains more carbonic acid than the half first expired ; a 

 circumstance which is explained by the one portion of air coming from the 

 remote part of the lungs, where it has been in more immediate and prolonged 

 contact w^ith the blood than the other has, which comes chiefly from the larger 

 bronchial tubes. 



c. External temperature. The observation made by Vierordt at various 

 temperatures between 3.4 23.8 C. (38 F. and 75 F.) show, for warm-blooded 

 animals, that within this range, every rise equal to 5.5 C. (10 F.) causes a 

 diminution of about 33 ccm. (2 cubic inches) in the quantity of carbonic acid 

 exhaled per minute. 



d. Season of the Year. The season of the year, independently of tempera- 

 ture, materially influences the respiratory phenomena ; spring being the season 

 of the greatest, and autumn of the least activity of the respiratory and other 

 functions. 



e. Purity of the Respired Air. The average quantity of carbon dioxide 



