RESPIRATION 



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always very distinctly below the oxygen pressures at the same 

 time in the alveolar air; while the arterial CO2 pressures were 

 sensibly equal to those in the alveolar air. There was never any 

 approach to excess of arterial over alveolar oxygen pressure, or 

 of alveolar over arterial CO2 pressure, even when these pressures 

 were varied considerably by altering the composition of the in- 

 spired air. Krogh, therefore, rejected Bohr's conclusions that 

 there is active secretion of oxygen or CO2 in the lungs, and con- 

 cluded in favor of Pfliiger's view that the exchange of gases in 

 the lungs is entirely due to diffusion. The following table shows 

 the results of a typical experiment in which the alveolar oxygen 

 pressure was varied during the experiment, the alveolar air and 

 blood samples being taken nearly simultaneously. 



Before following this long controversy further, I should like 

 to point out a fallacy in the interpretation of the aerotonometer 

 results. The conclusion of Pfliiger that diffusion alone explains 

 the giving off of CO2 in the lungs was wholly fallacious, as has 

 already been shown in Chapter V. The oxygen reaching the 

 blood in the lungs helps to drive out CO2; and under certain con- 

 ditions which are very apt to occur during physiological experi- 

 ments on animals, and may easily be produced in man, the venous 

 CO2 pressure may be lower than that of the alveolar air, although 

 no secretion at all may be occurring. In the lung-catheter experi- 

 ments the oxygen supply to the lungs was blocked off, so that the 

 blood could not take up oxygen. As a consequence the CO2 pres- 

 sure in the confined air must have been considerably lower than if 

 oxygen had been present. In reality Ludwig was right, and 

 Pfliiger was wrong. This source of fallacy does not in any way 

 invalidate Krogh's conclusion that the arterial CO2 pressure is 

 not, under normal conditions, lower than the alveolar CO2 pres- 

 sure. I think this conclusion is correct ; and it agrees, as he points 

 out, with all the indications given by the work of Priestley and 

 myself on the regulation of breathing in accordance with the 

 alveolar CO2 pressure. 



