490 



EESPIRATION 



in the alveoli themselves, because as the air from the deeper recesses 

 of the lungs moves outward, it intermingles with that contained in 

 the outer passages, and gives rise to a disproportional relationship of 

 the gases. For this reason, an analysis of ordinary expiratory ah* 

 cannot yield exact results. It is possible, however, to determine its 



mean oxygen and carbon dioxid content 

 by collecting the last portions of the air 

 expelled by two forced expirations, one 

 of which follows a normal inspiration 

 and the other, an ordinary expiration 

 (Haldane). 



Zuntz and Loewy have calculated 

 the composition of alveolar air by con- 

 trasting the capacity of the bronchial 

 tree with that of the alveoli. Thus, if 

 the volume of the expiratory air is 

 reckoned at 500 c.c., 140 c.c. of this 

 amount must be derived from the bron- 

 chial tree and 360 c.c. from the deeper 

 recesses of the lung. Furthermore, if 

 the expired air contains 4.38 per cent, 

 of carbon dioxid, the alveolar air must 

 embrace 4.38 -f- 1 ^ 5 , or 6 per cent, of 

 this gas. Actual analyses upon human 

 beings have not yielded absolutely con- 

 stant values, but show variations be- 

 tween 11 and 17 per cent, of an atmos- 

 phere for oxygen and between 3.7 and 

 6.2 per cent, of an atmosphere for carbon 

 dioxid. The average percentage of 

 oxygen, therefore, may be estimated at 

 14.5, that of carbon dioxid at 5.5, and 

 that of nitrogen at 80. Thus, it will be 

 seen that the oxygen tension in alveolar 

 air amounts to 109 mm. Hg and that of 

 carbon dioxid to 40 mm. Hg. If these 

 figures are now compared with those 

 given previously for the air in the bron- 

 chial tree (tidal air), it is evident that 

 the atoms of oxygen must flow from with- 

 out to within, and the molecules of car- 

 bon dioxid from within to without. 



FIG. 252. DIAGRAM TO SHOW 

 THE PRINCIPLE OF THE AEROTONO- 

 METER. 



A, the tube containing a known 

 mixture of gases, O, CO2, N; C, 

 the outside jacket for maintain- 

 ing a constant body temperature. 

 When stopcock b is open the 

 blood trickles down the sides of A 

 and enters into diffusion relations 

 with the contained gases. After 

 equilibrium is reached the stop- 

 cock b is closed and a is opened. 

 By means of the mercury bulb the 

 gases can then be forced out of A 

 into a suitable receiver for analy- 

 sis. (Howett.) 



In further analysis of this subject matter let us now ascertain whether this 

 relationship also prevails between the alveolar air and the blood. The determina- 

 tion of the tension of the gases in the blood presents several difficulties, because 

 it requires the bringing together of the latter with different gases possessing 

 known tensions, until one is found with which it is in equilibrium. This end is 



