GASES IN THE LUNGS AND TISSUES 309 



Elimination of Carbon Dioxide by the Blood and the Respiratory 

 Apparatus. The principles of absorption of gas by liquids discussed 

 in the preceding pages apply equally well for carbon dioxide with the excep- 

 tion that carbon dioxide is about three times as soluble in blood as is oxygen. 

 The carbon dioxide results from the oxidative processes going on in the tis- 

 sues, and this gas is present in large quantities in the tissues and their im- 

 mediately surrounding lymph. An analysis of the carbon- dioxide content 

 of venous blood reveals the presence of about 45 cc. of the gas in 100 cc. of 

 blood. This gas, like oxygen, is held in such large quantity by virtue of the 

 fact that it forms loose chemical combinations in the blood. Of the total 

 quantity not more than 5 per cent, is held in simple solution. From 10 to 

 15 per cent, of the total volume is found in firm combination in such forms 

 as carbonates, bicarbonates, etc. The remaining 85 and more volumes 

 per cent, is held in loose chemical combination, a combination which is broken 

 up under the same conditions of variation in carbon-dioxide tension as 

 were found to exist for oxygen in combination with hemoglobin. In the case 

 of carbon dioxide an analysis of plasma reveals the fact that the gas is in com- 

 bination with some compound of the plasma, probably a protein. In fact, 

 there is some evidence to show that carbon dioxide combines with the globu- 

 lin group. Carbon dioxide also forms loose chemical compounds with the 

 constituents of the red corpuscles, probably with the protein portion of the 

 hemoglobin molecule. The pressure relations of this gas as regards its 

 diffusion in the process of elimination are shown in the following table: 



Carbon-dioxide tension in the tissues 58 mm. of mercury 



! 



Carbon-dioxide tension in the venous blood 45 mm. of mercury 



1 



Carbon-dioxide tension in the alveolar air . . 23 to 38 mm. of mercury 



I 



Carbon-dioxide tension in the expired air ... 5.8 mm. of mercury 



Theories of Interchange of Gases in the Lungs and in the Tissues. 

 The above discussion is on the basis of the mechanical interpretation of the 

 transfer of gases in the lungs and in the tissues. By this theory it is assumed 

 that the oxygen passes from the air in the lungs through the moist pulmonary 

 membrane of the alveoli through the capillary walls and into the blood 

 plasma, obeying the physical laws of gas diffusion. Likewise in the tissues 

 this theory presupposes that the difference in the mechanical tension in the 

 capillary blood plasma, the lymph, and the living tissue will lead to diffusion 

 of the oxygen in the direction of lowest pressure, i.e., toward the tissues. 



Some facts have indicated that we cannot account for the transference 

 of oxygen by the purely mechanical theory. The idea has been advanced 

 that the living epithelial wall of the lung, as well as that of the capillaries, 

 exerts a distinct influence on the passage of oxygen of such nature that it 



