266 RESPIRATION 



results to the Physiological Society in 1915. We had been engaged 

 in measuring directly both the true venous CO 2 pressure and 

 oxygen pressure just after forced breathing, so as to discover the 

 effects of lowered CO 2 pressure on the circulation. We found that 

 the apparent venous oxygen pressures were incredibly high 

 70 mm. or even more. On further investigation it became evident 

 that after a single deep expiration, followed by a single deep in- 

 spiration of the gas mixture, the air in the alveoli was not properly 

 mixed. At the end of the forced breathing there would be nearly 

 20 per cent of oxygen in the alveolar air. With one deep inspira- 

 tion of the mixture, the air in the air-sac system of alveoli was 

 mingled with air from the inspired mixture, but an even mixture 

 in all parts of the alveolar system was not obtained, so that the 

 air-sac alveoli contained considerably more oxygen than the rest 

 of the alveoli. As a consequence the second alveolar air sample, 

 taken more exclusively from the air-sac alveoli, contained more 

 oxygen than the first, in spite of the fact that it had remained 

 longer in the lungs. It was evidently necessary, therefore, to take 

 two or, in the case of forced breathing, three successive deep 

 breaths of the mixture before holding the breath and taking 

 the samples. When this was done the results were quite consistent, 

 and showed that the venous CO 2 pressures as determined directly 

 during rest confirmed the calculated values previously obtained; 

 while the venous oxygen pressures, when interpreted in the light 

 of the thin-line curve of Figure 28, corresponded very closely 

 with the percentage oxygen loss of the blood as calculated indi- 

 rectly from the venous CO 2 pressure. Moreover, not only the 

 venous CO 2 pressure, but also the venous oxygen pressure, was 

 considerably lower at the end of forced breathing. 



The following are examples of two typical experiments carried 

 out on myself at the end of ten minutes' rest on a chair. 



No. i, 26/2/15. Bar. 762 mm. 



Mixture used contained 6.21 per cent of CO 2 and 5.73 per cent of 

 oxygen. 



First alveolar sample 2" after last deep inspiration, 6.43 per cent 

 of CO 2 and 6.18 per cent of oxygen. 



Second alveolar sample 5" after first sample, 6.47 per cent of CO 2 

 and 6.22 per cent of oxygen. 



Therefore venous CO 2 pressure = 6.47 per cent = 46.16 mm. and 

 oxygen pressure 6.22 per cent = 44.5 mm. 



Normal alveolar CO 2 percentage (mean of inspiratory and expira- 

 tory samples) 5.64 per cent = 40.3 mm. 



