140 



RESPIRATION 



accumulates faster than in others. Hence in some the blood is 

 less perfectly oxygenated; and if the breath is held for a time 

 this imperfect oxygenation becomes more and more marked till 

 at last the mixed arterial blood is very considerably short of oxy- 

 gen, just as when the breathing is very shallow. Hence the oxygen 

 percentage of the mixed alveolar air becomes altogether deceptive 

 as an index of the degree of oxygenation of the mixed arterial 

 blood, although the CO 2 percentage remains, for the reasons al- 

 ready given, a reliable index of the degree of saturation of arterial 

 blood with CO 2 . The results of these experiments on holding the 

 breath are thus very valuable as furnishing evidence that, even 

 with normal or increased inspirations, the relation between blood 

 supply and air supply varies considerably in different alveoli. 



That the arterial blood does actually become imperfectly oxy- 

 genated when the breath is held has been quite recently demon- 

 strated by Meakins and Davies. 5 They found that on holding a 

 deep breath of air for 40 seconds, the haemoglobin of the arterial 



Figure 46. 

 Alveolar CO 2 during breath holding after inhalation of oxygen. 



blood drawn from the radial artery was only 83.8 per cent satu- 

 rated with oxygen, although the mixed alveolar air contained 

 13.4 per cent of oxygen. Had air of this composition been dis- 

 tributed evenly throughout the alveoli the haemoglobin would 

 have been 97 per cent saturated with oxygen. 



A further series of experiments which Douglas and I performed 

 is very instructive in this connection. As already mentioned in 

 Chapter V, the alveolar CO 2 percentage rises high above its 

 normal value before the end of an apnoea after forced breathing 

 with extra oxygen. We observed how high the alveolar CO 2 pres- 

 sure went when there were varying pressures of oxygen in the 



'Meakins and Davies, Journ. of Pathol. and, Bacter., XXIII, p. 451,1920. 



