RESPIRATION 



137 



blood. To see what the results of this mixture will be, we must 

 refer to the respective dissociation curves for the oxygen and the 

 CO 2 present in blood, taking also into account the action of oxygen 

 in expelling CO 2 from venous blood, as shown on the curve in 

 Figure 26. For convenience' sake the two relevant curves are 

 plotted together in Figure 45, taken from our paper. It will at 

 once be seen that the over-ventilation in some parts of the lungs 

 will wash out CO 2 from the blood in the same proportion as the 

 under-ventilation fails to wash it out. The mixed arterial blood 

 will thus be normal as regards its content of CO 2 if the total al- 

 veolar ventilation is normal. On the other hand, the over- ventila- 

 tion will hardly increase at all the charge of oxygen in the blood 

 from the over- ventilated alveoli, since this blood is on the flat part 

 of the curve with the alveolar oxygen pressure at perhaps 16 or 

 1 8 per cent of an atmosphere. The under-ventilation, on the other 

 hand, will leave the venous blood nearly venous and on the steep 

 part of the oxyhaemoglobin curve, with a large deficiency of oxy- 

 gen. The mixed arterial blood will, therefore, be seriously deficient 

 in oxygen, and symptoms of anoxaemia will consequently be pro- 

 duced. As one of these symptoms is increase in the breathing, there 

 will be some compensation, and the CO 2 percentage of the mixed 

 alveolar air will fall somewhat, there being a corresponding rise 

 also in the oxygen percentage, as was actually found in our ex- 

 periments. 



There is thus a very complete explanation of our experimental 

 results, and also of the symptoms of anoxaemia in the neurasthenic 

 cases ; but clearly it is necessary to modify radically the idea that 

 the alveolar oxygen pressure gives the oxygen pressure of the 

 mixed arterial blood. We have no guarantee that even during 

 quite normal breathing the distribution of air in the individual 

 lung alveoli corresponds exactly with the distribution of blood to 

 them. Unless this correspondence is exact some alveoli will re- 

 ceive more air in proportion to their blood supply than others, 

 and as a consequence the mixed arterial blood will be a mixture 

 of more and less fully arterialized blood, with some of the con- 

 sequences first discussed. It is probable indeed that in some way 

 or other the air supply is proportioned to the blood supply, 

 whether by regulation through the muscular coats of the bron- 

 chioles or regulation of the blood distribution ; but it is also certain 

 that this proportioning is only an approximation. The fact that 

 in animals the aerotonometer gives a lower arterial oxygen pres- 

 sure than the alveolar oxygen pressure (Chapter IX) is most 



