7 5 4 CHEMISTR \ T OF RESPIRA TION. 
The general conclusion to be drawn from these and other similar data is 
that the larger animals respire more slowly than the smaller animals of a 
similar class. It has been shown that a similar difference obtains in the out- 
put of carbon dioxide and the intake of oxygen. 
The alveolar surface of the human lungs. — The volume of the lungs 
in the mean phase of respiration is about 3500 c.c; the diameter of a single 
alveolus is about 0"2 mm., its volume "004 c.mm., and its surface 0T26 s.mm. 
In order to contain the air in the lungs, there must be 725 millions of alveoli, 
with a surface of about 90 sq. metres. 1 The above calculation is the one given 
by Zuntz. 2 
The changes in the composition of the air during respiration. — 
The fresh air taken into the lungs during respiration has the following 
composition, when it is dry and measured at 0° and 760 mm. pressure, 
20 - 96 volumes per cent, oxygen, 79'02 nitrogen, 3 and about - 03 carbon 
dioxide, or by weight per cent., 23 - 015 oxygen, and 76 - 985 nitrogen. 
Under ordinary conditions, the air contains a quantity of aqueous 
vapour, which is liable to considerable variations according to the 
temperature and other atmospheric conditions ; the carbon dioxide, 
moreover, may in badly-ventilated rooms rise considerably above the 
amount just given. 
The inspired air is warmed and moistened in passing through the 
nose, pharynx, trachea, and bronchi, and rapidly mixes and diffuses with 
the air retained in the alveoli of the lungs. The passage of the air 
through the nose alone raises the temperature of the air considerably ; 
thus Bloch 4 found that, when the temperature of the external air was 
„8° 5 -0°-5 to 3°-5, 12° to 16° and 18°, that of the air entering the pharynx 
from the nose was respectively 24°-5, 26°, 30°, and 31°. With a 
moderate external temperature the air becomes about one-third saturated 
with moisture during its passage through the nasal cavity. The rapidity 
of the processes of mixture and diffusion will vary according to the 
frequency and depth of breathing and the capacity of the lungs. The 
air expired will likewise vary in composition, and under normal condi- 
tions will never represent the alveolar air. 
The expired air. — The earliest determinations of the composition of 
the expired air of man were made by Menzies, 5 Lavoisier and Seguin, 6 
H. Davy, 7 Allen and Pepys, 8 and Prout. 9 The following table gives the 
more exact results of recent investigations, but at the same time it is 
important to remember that it is impossible to give figures which shall 
exactly represent the average composition of the expired air ; the 
percentage of oxygen and of carbon dioxide varies according to the 
frequency and depth of breathing, and is influenced by various condi- 
tions which affect the metabolism of the body, such as muscular 
activity, temperature, and food. For these reasons the respiratory ex- 
change of an animal should be estimated by the direct determination 
of the intake of oxygen and the output of carbon dioxide and water in 
1 1 mm. = 0-03937 in., and 1 metre = 39-37079 in. 
2 Arch. f. d. gcs. Physiol., Bonn, 1888, Bd. xlii. S. 410. 
3 This includes a small quantity of argon, but it appears to have no physiological 
importance. 
4 Ztschr.f. Ohrerih., Wiesbaden, Bd. xviii. S. 215, 354. 
5 "Essay on Respiration," Edinburgh, 1796, p. 50. 
6 Ann. de chim., Paris, 1814, tome .\ci. p. 318. 
7 "Researches," London, 1808, p. 331. 
8 Phil. Trans., London, 1808, 1809. 
9 Ann. Phil., London, 1813, vol. ii. p. 328. 
