RESPIRATION 261 



lung which remained connected with fresh air was receiving 

 much more fresh air than usual, so that the proportion of COg 

 in the arterial blood from this lung would be reduced practically 

 in proportion to its increased ventilation. This blood would mix 

 with the venous blood from the other lung, and in this way form 

 a mixture in which the proportion of CO2 was about normal. The 

 arterial blood from the ventilated lung would, in virtue of the 

 higher pressure of oxygen and lower pressure of CO2, contain 

 slightly more oxygen than usual, while the blood from the un- 

 ventilated lung would contain considerably less. The result would 

 be a mixture containing an abnormally low proportion of oxy- 

 gen, but not sufficiently low to cause any marked immediate dis- 

 turbance. Even with a whole lung blocked off, the haemoglobin 

 of the mixed arterial blood would be at least 85 per cent saturated 

 with oxygen instead of 95 per cent, so that the effect on the breath- 

 ing would be no greater than the probable effect, hardly notice- 

 able at the time, of breathing air containing 14 per cent of oxy- 

 gen, or ordinary air at a height of about 1 1,000 feet. 



Analyses of the air in the blocked lung showed that after a 

 comparatively short interval of time the percentages of oxygen 

 and CO2 became steady, and were, in different individuals, about 

 5.3 per cent of oxygen and 6.0 per cent of CO2, corresponding 

 respectively to 37.5 mm. and 42 mm. These values are evidently 

 the pressures of oxygen and CO2 in the venous blood. The low 

 value of the venous CO2 pressure was quite unintelligible at the 

 time, since the average arterial CO2 pressure is about 40 mm. as 

 shown above. The experiments of Christiansen, Douglas, and 

 myself (Chapter V) showed, however, that the true venous CO2 

 pressure is in reality only a little higher than the arterial COg 

 pressure; and if we allow for the fact that the breathing was 

 presumably slightly increased by the stimulus of want of oxygen 

 the result is just what might be expected. The venous oxygen 

 pressure would be somewhat lower than usual, since the arterial 

 blood was incompletely saturated with oxygen. Hence both the 

 oxygen pressure and the CO2 pressure would be below normal. 

 The results of these experiments were nevertheless of the highest 

 interest. 



It is evident that if by any means we can measure the rate of 

 blood flow through the lungs, and at the same time measure the 

 intake of oxygen and discharge of CO2 from the blood, we can 

 calculate how much oxygen a given volume of the blood gains, 

 and how much CO2 it loses, in the lungs ; and in this way we can 



