THE ALVEOLAR AIR OF THE LUNGS 539 



numerous, as many as 404 millions are present, according to Aeby's 

 estimate, in the lungs of a man ; their individual size varies con- 

 siderably, and as an average the following dimensions may be 

 given, O37, 0-22, and O16 mm., for the undistended alveolus. 

 Owing to the elasticity of their walls the capacity of the alveoli 

 can be doubled by distension with air during inspiration. Cal- 

 culated from such figures, the total surface of the alveoli will be 

 about 50 square metres at the end of a deep expiration and 130 

 square metres after a very deep inspiration. From other data 

 Zuntz calculates that there are 725 millions of alveoli, with a total 

 surface of about 90 square metres. 



The capillary network contains numerous vessels placed very 

 close together and covered by an extremely thin layer of the 

 membranous wall of the alveoli, and the layer of the alveolar 

 epithelium. The thickness of this double layer which separates 

 the capillaries from the alveolar air is barely O'OOl mm. thick. 



The average composition of the air in the alveoli must now 

 be considered. From the numerous analyses of the expired air 

 it is possible to form only a rough estimate of the alveolar air, 

 for the expired air is mixed with some of the air from the " dead 

 space " of the respiratory tract, which lies between the nose and 

 the alveoli. Loewy measured the volume of casts of the respira- 

 tory passages and estimated the " dead space " at about 140 c.c. ; l 

 from the composition of the expired air, its probable dilution 

 with air from this " dead space," and. with 6 per cent, of aqueous 

 vapour, for the air of the alveoli is saturated with moisture at 

 the temperature of the body, he calculated that the alveolar air 

 contained 12' 6 to 13-5 per cent, of oxygen at the atmospheric 

 pressure. 



In an ordinary expiration the first portion of the air which 

 leaves the nose or mouth is from the " dead space," then mixed 

 air, and finally air from the alveoli. This is on the supposition 

 that the lobes of the lungs contract simultaneously, and is pro- 

 bably correct, although some observers maintain that in some 

 cases all portions of the lungs are not in the same phase at the 

 same time. At the end of a normal expiration there will be a 



1 Haldane and Priestley calculated the value of the "dead space" in their 

 own cases, and found for the mean results 189 c.c. and 142 c.c., or about 30 per 

 cent, of the volume of a normal respiration during rest. 



