526 A ft AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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

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

 there is an atmosphere consisting of O, CO 2 , and N ; 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 be regarded as a simple 

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

 blood contains O, CO 2 , and N, each of which is in 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 and tension of each gas on the two sides of the membrane. The ten- 

 sion of O in venous blood, as above stated, is 22.04 millimeters of Hg, and of 

 CO 2 , 41.04 millimeters. What are the partial pressures of these gases in the 

 alveoli? The precise pressures are not known, but it is estimated that the 

 partial pressure of O is about 122 millimeters, and of CO 2 about 38 millimeters. 

 Comparing the partial pressures and the tensions of these two gases in the 

 alveoli and the blood respectively, it is obvious that the conditions on the two 

 sides of the membrane are favorable to the diffusion of O and CO 2 , and in 

 definite but opposite directions. This is illustrated in the following diagram- 

 matic presentation : 



o. co 2 . 



Tensions in alveolar air 122.00 38.00 



Pulmonary membrane 1 + 



Tensions in venous blood 22.04 41.04 



Since the gases diffuse from the point of higher pressure or tension to that 

 of lower pressure or tension, O passes from the alveoli to the blood, while CO 2 

 passes from the blood to the alveoli. 



It is, however, impossible under certain circumstances to account for the 

 transmission of all of either the O or the CO 2 by the laws of diffusion. In 

 regard to O, physical forces are active to the extent that they cause a diffusion 

 of O to the blood-plasma, where it is brought in contact with the hemoglobin 

 of the blood-corpuscles. The chemical union of O with haemoglobin takes 

 place at a low tension, hence the quantity of O taken up by the blood does 

 not vary materially with the amount of O in the air breathed, no more O 

 being taken up when pure O is respired than from atmospheric air, in which O 

 constitutes only about 20 volumes per cent. ; and Frankel and Geppert record 

 that the quantity of O in arterial blood is but little diminished even when the 

 air-pressure is reduced as low as 378-365 millimeters. But Bohr found in 

 experiments on dogs that the tension of O in arterial blood may even be higher 

 than its partial pressure in the alveolar air ; and Pfliiger long since determined 

 that when animals breathe pure N or H, no O passes from the blood into the 

 alveoli. It is apparent from these latter facts that the transmission of O may 

 not be entirely a matter of diffusion. In addition to the physical and chemi- 

 cal factors, it is possible, as suggested by Bohr, that the pulmonary tissue 

 takes an active part as a specific secretory membrane in this transmission. 



The problem in connection with CO 2 is also complex. It is commonly 



