234 RESPIRATION 



man corresponds exceedingly closely to the pressure of oxygen in 

 the alveolar air. In fact in no single instance does the value of the 

 arterial oxygen pressure differ from the alveolar by a greater 

 amount than can be accounted for by the experimental error of 

 the method. 



We then tested the effect of raising the alveolar oxygen pressure 

 considerably above the normal value by filling the respiration 

 apparatus with an atmosphere rich in oxygen. The results of the 

 experiments are also given in the table. Here again the figures 

 show that the arterial and alveolar oxygen pressures have prac- 

 tically identical values. In these experiments on man we were 

 content to use only a moderate increase of the alveolar oxy- 

 gen pressure, for the higher the oxygen pressure is raised the 

 less proportional difference is there between the inspired air and 

 the alveolar air. A point will therefore eventually be reached 

 when the determination of the difference of tint between the blood 

 withdrawn from the body and that saturated with the inspired 

 air in vitro will fall almost within the experimental errors of the 

 method. It should be noted that in these experiments the car- 

 bonic acid in the alveolar air had precisely its normal value, 

 namely 5.6 per cent when measured dry, and we have therefore 

 no reason to suppose that the alveolar air samples were other 

 than normal. 



Having obtained thus results which indicated that during rest 

 under normal conditions the transference of oxygen through the 

 pulmonary epithelium occurs without active secretory interven- 

 tion of the alveolar epithelium, we were naturally anxious to test 

 the matter further under conditions in which some amount of 

 deficiency of oxygen- might affect the subject. The necessary de- 

 ficiency of oxygen was obtained by exposing the subject to an 

 atmosphere containing a considerably lower percentage of oxy- 

 gen than the normal. The experimental procedure was precisely 

 the same as before, save that we filled the respiration apparatus 

 before the start with an appropriate atmosphere by the method 

 described above. The results of these experiments are collected 

 in the middle part of the table. 



The partial pressure of oxygen in the air breathed corresponded 

 to an altitude of 15,000 feet or over; yet we noted that a 23 per 

 cent saturation of the blood with carbon monoxide was tolerated 

 without inconvenience. One of the subjects was liable to head- 

 ache when his blood was saturated to 25 per cent or more with 

 carbon monoxide, but this was in no wise accentuated in these 



