774 CHEMISTR Y OF RESPIRA TION. 



The evidence, however, in support of this explanation must be 

 examined, for of late it has been challenged, especially by Bohr. 1 In 

 the first place, it is necessary to remember that the composition of the 

 alveolar air is not represented by that of the air expired. The composi- 

 tion of the inspired and of the expired air and the tension of their 

 component gases can be readily determined. The tension of oxygen in 

 the inspired air is 159 mm., under the mean pressure of an atmosphere, 

 760 mm. It is difficult, however, to obtain with accuracy similar data 

 for the air of the alveoli. From the numerous analyses of expired air 

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

 it probably contains 5 to 6 per cent, of carbon dioxide, and 14 to 15 

 per cent, of oxygen ; and the tension of the former would be about 

 36 mm., and of the latter about 114 mm. Lowy 2 calculates that the 

 tension of oxygen in the alveoli of the human lungs is from 12'6 to 13'5 

 per cent, of an atmosphere, or about 99 mm. of mercury. 



In animals, direct determinations of the composition of the alveolar 

 air of an occluded portion of the lungs have been made. For the 

 collection of this air Pniiger 3 constructed a special catheter (Fig. 71). 



It consists of an 

 ordinary fine elastic 

 catheter, surrounded, 

 except at its extrem- 

 ities, by a tube with 

 a rubber enlargement 

 towards the free end 

 of the catheter. The 

 instrument is so small 

 that, when introduced 

 through the trachea 

 into a bronchus of a 



FIG. 7l.-Pfluger's lung catheter. d S> {i CaUS6S ? 



ranee to the free 



passage of air into the other parts of the lungs. The rubber enlarge- 

 ment is now inflated, and shuts off' a portion of the lungs, from which 

 the alveolar air can be withdrawn through the inner tube of the lung 

 catheter. In such experiments Wolff berg 4 and Nussbaum 5 found 

 that the alveolar air of a dog contained 3'5 per cent, of carbon dioxide, 

 whereas the expired air yielded 2 '8 volumes per cent. It is to be noted 

 that this value for the alveolar air is higher than the normal, for the air 

 in the alveoli was shut off' from the tidal air, and, in fact, represents the 

 air after an equilibrium had been established with the gases of the blood 

 passing through that portion of the lung shut oft' by the catheter. 



In the next place, it is necessary to consider the tension of the 

 oxygen and carbon dioxide present in the blood, and this involves a 

 preliminary study of the dissociation of oxyhrenioglobin. Under the 

 ordinary tension of oxygen in the air, haemoglobin readily combines 

 with oxygen, but if the external pressure be lowered sufficiently, then 

 oxygen is given off, and the oxyhujinoglobin undergoes dissociation. 



1 Skandin. Arch. f. PhysioL, Leipzig, 1891, Bd. ii. S. 236. 



2 Arch. f. d. ges. PhysioL, Bonn, 1894, Bd. Iviii. S. 416; " Untersuch. u. d. Respira- 

 tion und Circulation," 1895, S. 26. 



3 Arch. f. d. ges. PhysioL, Bonn, 1872, Bd. vi. S. 43. 



4 Ibid., 1871, Bd. iy. S. 465 ; 1872, Bd. vi. S. 23. 



5 Ibid., 1873, Bd. vii. S. 296. 



