192 ABSORPTION OF OXYGEN IN THE LUNGS. 



proved that this is a chemical process. The gas-free (reduced) haemoglobin takes 

 up O to form oxyhemoglobin ( 15, 1). That this absorption has nothing to do 

 directly with the diffusion of gases, but is due to a chemical combination of the 

 atomic compounds, is shown by the fact that, when pure O is respired, the blood does 

 not take up more O than when atmospheric air is respired ; further, that animals 

 made to breathe in a limited closed space can absorb almost all the O even to 

 traces into their blood before suffocation occurs. Of course, if the absorption of 

 O were due to diffusion, in the former case more O would be absorbed, while in the 

 latter case the absorption of O could not possibly occur to such an extent as it does. 

 The law of diffusion comes into play in connection with the absorption of O to this 

 extent, viz., that the O diffuses from the air-cells of the lung into the blood-plasma, 

 where it reaches the blood-corpuscles floating in the plasma. The haemoglobin of the 

 blood-corpuscles forms at once a chemical compound (oxyhaemoglobin) with the O. 

 Even in very rarefied air, such as is met with in the upper regions of the atmosphere during 

 a balloon ascent, the absorption of still remains independent of the partial pressure. But a 

 much longer time is required for this process at the ordinary temperature of the body, so that 

 in rarefied air, the absorption of is greatly delayed, but it is not diminished. This is the 

 cause of death in aeronauts who have ascended so high that the atmospheric pressure is dimin- 

 ished to one-third {Sctsehenow). 



2. Excretion of C0 2 . With regard to the excretion of C0 2 from the blood, we 

 must remember that the C0 2 in the blood exists in two conditions. 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 C0 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 O is 

 simultaneously admitted. The free supply of O 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. That the oxygenated blood-corpuscles (i.e., their 

 oxyhaemoglobin) 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 O. 



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

 into play. The must pass through the alveolar membrane, AB 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 alveolar membrane ; on the one 

 side of it is represented the partial pressure of the C0 2 and O in the air-vesicles ; 

 and on the other, the partial pressure of the C0 2 and O in the venous blood entering 

 the lung. The arrows indicate the direction of diffusion.] 



Partial pressure of air in I C0 2 ...... O 



alveoli of lung. ) 27 27 '44 



A Jl- -J B 



Tension of gases m venous ) 41 22 



blood of lung. f CO O 



