262 EXCRETION OF CARBONIC ACID BY THE LUNGS. 



ditions. Part of the C0 2 forms a loose or feeble chemical compound, 

 while another portion is more firmly combined. The former is obtained 

 by those means which remove gases from fluids containing them 

 in a state of absorption, so that in removing the CO 2 from the blood it 

 is difficult to determine whether the C0 2 , so removed, obeyed the law 

 of diffusion, or if it was expelled by chemical means. 



Although it is convenient to represent the excretion of C0 2 from the 

 blood into the air-vesicles of the lung, as due to equilibration of the 

 tension of the C0 2 on opposite sides of the alveolar membrane, i.e., to 

 diffusion nevertheless, chemical processes play an important part in this 

 act. The absorption of O by the coloured corpuscles acts, at the same 

 time, in expelling C0 2 . This is proved by the fact that the expulsion 

 of C0 2 from the blood takes place more readily when is simultaneously 

 admitted (Ludwig and Holmgren). 



The free supply of not only favours the removal of the C0 2 , which 

 is loosely combined, but it also favours the expulsion of that portion of 

 the C0 2 which is more firmly combined, and which can only be 

 expelled by the addition of acids to the blood (Ludwig, Schoffer and 

 Sczelkow). That the oxygenated blood-corpuscles (i.e., their oxyhaemo- 

 globin) are concerned in the removal of C0 2 , is proved by the fact that 

 C0 2 is more easily removed from serum which contains oxygenated 

 blood-corpuscles than from serum charged with 0. 



[The following scheme may serve to illustrate the extent to which 

 diffusion comes into play. The O must pass through the alveolar 

 membrane, A B including the alveolar epithelium and the wall of the 

 capillaries as well as the blood-plasma, to reach the haemoglobin of 

 the blood-corpuscles. Similarly, the C0 2 must leave the salts of the 

 plasma with which it is in combination, and diffuse in the opposite 

 direction, through the wall of the capillaries, the alveolar membrane 

 and epithelium, to reach the air- vesicles. Let AB represent the 



CO 2 O 



Partial pressure of air in \ 



alveoli of lung. ) 27 27 '44 



A T I H 



~~( r -JT~ 



Tension of gases in venous 1 41 22 



blood of lung. ) 



I C0 2 . . . . . O 



alveolar membrane; on the one side of it is represented the partial 

 pressure of the C0 2 and in the air- vesicles; and on the other, the 

 partial pressure of the C0 2 and in the venous blood entering the 

 lung. The arrows indicate the direction of diffusion.] 



Theories. Various theories have been proposed to account for the expulsion of 

 the C0 2 from its state of chemical combination in the blood due to the action of 

 the oxygenated blood-corpuscles, (a.) It is possible that the C0 2 in the blood- 



