7 54 CHEMISTR Y 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 0'004 c.mm., and its surface 0"126 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 and 760 mm. pressure, 

 20*96 volumes per cent, oxygen, 7 9 -02 nitrogen, 3 and about 0'03 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, -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. ges. 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 xci. p. 318. 



7 "Researches," London, 1808, p. 331. 

 s Phil. Trans., London, 1808, 1809. 



9 Ann. Phil., London, 1813, vol. ii. p. 328. 



