804 CHANGE OF COMPOSITION OF ALVEOLAR AIR, 



empty bag, instead of being held in the closed chest, the alveolar 

 tension of carbon dioxide rises from its initial value of 38-0 mm. 

 Her at a lusrarithmicaliv decreasing rate such that, if the rise 

 continued in the same way, a final tension of 60-5 mm Hg would 

 be approached closely. During the period of the experiment, 

 tlie alveolar tension of carbon dioxide ri,ses to within GSmm Hg 

 of this final value. The initial difference of tension driving 

 carbon dioxide from the blood into the alveolar air is, in this 

 case, 22 5 mm.Hg, or about double that acting when the breath 

 is held. 



The alveolar tension of oxygen, when the air in the lungs is 

 rebreathed, falls from its initial value of 116"4 mm.Hg at a 

 logarithmically decreasing rate such that the final tension of 

 oxygen would approach zero, if the fall continued in the same 

 way. The initial difi'erence of tension driving oxygen from the 

 alveolar air into the blood is, in this case, 116-4 mm.Hg, again 

 practically double the efi'ective difi'erence existing when the 

 breath is held. During the period of the experiment, the alveo- 

 lar tension of oxvsen falls to a value which is still about 06 mm. 

 Hg above this final value. 



These experiments show that the rate and extent of the 

 exchange of gases between the blood and the alveolar air are 

 very much increased by the movements of breathing. 



In the experiments of Hill and Flack (loc. cil.), a similar 

 effect of respiration on the gas-eous exchange in the lungs is to 

 be observed. The experiments of these authors on the effect of 

 breathing from a bag are not strictly comparable with those of 

 the present work. Hill and Flack's subjects breathed from an 

 anaesthetic bag "filled" with expired air. The volume of air 

 with which the blood could exchange gases was very much 

 greater, therefore, than that present in the lungs alone, and the 

 period for which the experiments could be continued was cor- 

 respondingly extended to about two minutes, or three times as 

 long as when the breath was held. In the present experiments, 

 the bag was empty, and the volume of air in the lungs was 

 hardly added to. The period for which the experiment could 

 be continued was not greatly extended beyond that for which 

 the breath could be held in the ordinary way. 



