812 



CHANOE OF (OMPOSITiON OF ALVEOLAR AIR, 



accompanying figures, however, give the results of experiments 

 in whicli the alveolar percentages of oxygen, as well as of carbon 

 dioxide, were determined aftei- holding the breath for about 20 

 seconds under various pressures below that of the atmosphere. 



Table viii. 

 Efftct of negative pressure on respiriilc y exchange in hmgs. 



These figures show that higher percentages of carbon dioxide 

 are accompanied by lower percentages of oxygen in the same 

 way, when the breath is held under negative pressures, as when 

 the air of the lungs is rebreathed from a bag. 



The results of these experiments indicate that the move- 

 ments of In-eathing, or the negative variations of the intra- 

 pulmonary pressure which accompany them, accelerate, under 

 certain conditions, the respiratory exchange of gases in the 

 lungs. This acceleration is brought about not only by increase 

 of the rates at which the alveolar tensions of carbon dioxide and 

 of oxygen tend toward certain final (venous) tensions, but by a 

 seeming alteration of these final tensions themselves. Negative 

 intrapulmonary pressures increase the eff"ective gradient of ten- 

 sion between the gases of the alveolar air and those of the venous 

 blood entering the lungs. It is unlikely that the movements of 

 breathing, or negative pressures in the chest, have any actual 

 effect on the tensions of the gases of the venous blood. 'J'hese 

 factors also can have unly a very slight effect on the partial 

 tensions of the gases of the alveolar air. It seems, therefore, 

 that, in the lungs, some mechanism must exist by which the 



