THE CHEMISTRY OF RESPIRATION 489 



mass and (h) the atomic movement of the constituents of the air in the 

 deeper recesses of the lung. Thus, we have, on the one hand, an alter- 

 nate inward and outward movement of definite quantities of air and, 

 on the other, an atomic interchange of the gases between the tidal 

 air and the blood directly through the walls of the alveoli and capillaries. 

 The former is a movement of a definite mass of air as a whole and the 

 latter, a progression of the atoms of the gases in accordance with 

 their diffusion pressures and other properties. 



The interchange of the gases between the tidal air and the blood 

 has been explained in a physical and in a chemical way. The former 

 explanation, which is commonly accepted to-day, is based upon the 

 ordinary physical laws of the diffusion of gases, while the latter neces- 

 sitates the assumption that the cells lining the alveoli possess a 

 definite vital activity, leading to a secretion of the gases through this 

 membrane. 



The physical theory, first of all, recognizes the fact that the gases 

 in the minute air spaces and in the blood are separated from one 

 another by a permeable membrane formed by the lining cells of the 

 alveoli and capillaries. If it is now assumed that the partial pressures 

 of these gases are the same on the two sides of this membrane, an 

 equilibrium must exist which renders the diffusion equal in both direc- 

 tions. But in as much as the body makes constant use of the oxygen 

 and yields in turn carbon dioxid, the region on the inner side of this mem- 

 brane must give lodgment to relatively miich smaller amounts of oxygen 

 and much larger amounts of carbon dioxid than the outer region. 

 Consequently, the partial pressure of the oxygen in the blood must be 

 considerably below that in the alveoli and adjoining larger air passages, 

 while the tension of the carbon dioxid must be greater in the blood. 

 Obviously, therefore, the atoms of oxygen must progress from without 

 to within, while the molecules of carbon dioxid must flow from within to 

 without. Inasmuch as the body does not make use of the nitrogen, 

 this gas remains "stationary, " and serves mostly as the medium for the 

 diffusion of the other two gases. It should be remembered, however, 

 that the term "stationary" is only a relative one, because an actual 

 standstill of the atoms of nitrogen, or of any other gas, is not in accord 

 with our modern conception of the behavior of gases. Even when 

 resting, their atoms move about constantly, although they do not ad- 

 vance in large numbers in any one particular direction. 



On further inquiry into the conditions prevailing in the intrapul- 

 monic spaces, it is found that the capacity of the bronchial tree is 

 only 140 c.c. and that the air contained therein possesses practically 

 the same composition as the atmospheric. Consequently, the partial 

 pressure of the oxygen in these spaces must amount to 152 mm. Hg 

 and that of the carbon dioxid to practically zero. Keeping these facts 

 firmly in mind, let us see how great a partial pressure these gases 

 exert in the alveoli and in the blood entering the lungs. These values 

 can only be ascertained by a chemical analysis of the air resident 



