418 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of in the water, will pass to the water ; if the partial pressure of CO, in 

 the air be less than the tension of CO a in the water, CO, will pass to the air. 

 It' now we interpose an animal membrane between the atmosphere and the 

 water, the interchange of gases will continue as before. In this ease we have 

 conditions analogous to those which exisl in the living organism : In the alveoli 

 there i~ an atmosphere consisting of ().<'<)_,. and X ; each gas is under a par- 

 tial pressure proportional to its volume per cent, of the mixture; the pul- 

 monary membrane and the walls of the capillaries may he regarded as a simple 

 animal membrane separating the air in the alveoli from the blood ; finally, the 

 blood contains O, C0 2 , and X, each of which exists under a definite and 

 independent degree of tension. Whether or not any or all of these gases will 

 pass in one direction or the other must obviously depend upon the conditions 

 of partial pressure ami tension of each gas on the two sides of the membrane. 

 The tension of O in venous Mood, as above stated, is 22.04 millimeters of 

 Hg, and of CO,, 41.04 millimeters. What are the partial pressures of these 

 oases in the alveoli'.' The precise pressures are not known, but it is esti- 

 mated that the partial pressure of O is about 100 millimeters, and of C0 2 

 aboul 2.'! millimeters. 



Comparing the partial pressures and the tensions, as generally accepted, 

 of these two gases in the alveoli and the blood respectively, it is obvious that 

 the conditions on the two side- of the membrane are favorable to the diffusion 

 of O and CO,, and in definite but opposite direetions. This is illustrated in 

 the following diagrammatic presentation : 



o. co 2 . 



Partial pressures in alveolar air 100.00 23.00 



Pulmonary membrane 1 4— — 



Tensions in venous blood 22.04 41.04 



Sine*' gases diffuse from the point of higher pressure or tension to that of 

 lower pressure or tension, () passes from the alveoli to the blood, while C0 2 

 passes from the blood to the alveoli. 



It i-, however, impossible under certain conditions, and possibly under 

 ordinary conditions, to account for the transmission of all of either the O or 

 the CO, by the laws of diffusion. Bohr ' found in experiments upon dogs 

 that the ten-ion of oxygen in arterial blood is almost invariably higher than 

 the partial pressure of oxygen in the lungs, and in some instances consider- 

 ably higher. His records a- regards C0 2 , while lacking uniformity, are of 

 like import, and indicate that the tension of CO, in the blood is lower than 

 the partial pressure of this gas in the lungs. Although Bohr's results have 

 met with much adverse criticism, they have received substantial support in 

 the recent researches of Haldane and Smith"' on mice, birds, dogs, and other 

 animals. They found that the normal oxygen tension in arterial blood is 

 always higher than in alveolar air, and they were consequently led to conclude 



1 Skcmdinavischea Archivfiir Physiologie, 1891, Bd. ii. S. 236. 

 ./ nirnal <>/ Physiology, 1*07, vol. xxii. p. 231. 



