7 7 4 CHEMISTS. 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. Lbwy 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 Pfiiiger 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 
dog, it causes no hind- 
hlG. 71. — rnuger s lung catheter. & ' 
ranee to the tree 
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the alveolar air can be withdrawn through the inner tube of the lung 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 off 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 oxyhamioglobin. 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 oxyhemoglobin undergoes dissociation. 
1 Skandin. Arch. f. Physiol., Leipzig, 1891, Bd. ii. S. 236. 
- Arch. f. d. gcs. Physiol., Bonn, 1894, Bd. lviii. S. 416 ; " Untersuch. u. d. Respira- 
tion und Circulation," 1895, S. 26. 
3 Arch. f. d. gcs. Physiol., Bonn, 1S72, Bd. vi. S. 43. 
4 Ibid., 1871, Bd. iv. S. 465 ; 1872, Bd. vi. S. 23. 
5 Ibid., 1873, Bd. vii. S. 296. 
