226 EFFECT OF PULMONARY VENTILATION 



vivo that the pulmonary epithelium has the power of forcing 2 into 

 the blood at a higher tension than pertains in the alveolar air. This 

 power, they say, continues to act when the barometric pressure is 

 lowered, and strives to maintain tension of oxygen at the high level 

 requisite for active life. It is weakened by fall of body temperature, 

 acute infections, pneumonia, &c. They say that animals whose blood 

 is | sat. with CO suffer at the same barometric pressure as normal 

 animals. Some of these results appear contrary to the deductions 

 of Zuntz, Loewy, &c., which are given in the text. It remains to 

 be seen whether their conclusions will hold. 



THE EFFECT OF INCREASED PULMONARY VENTILATION 



The alveolar air is made up of the residual and reserve air, 

 which together equal about 2-J- 1. ; the tidal air is 300-600 c.c. 

 Thus, about one- eighth of the alveolar air is changed at each respira- 

 tion. The mean percentage of oxygen in the expired air is 16- 5. 

 Men vary in regard to frequency of respiration from 510 to 

 18-25 per min. ; in volume of tidal air from 300-900 c.c. ; in 

 volume of air breathed per min. from 3 -5-8 1. 



The inspired air, which occupies the larger air- tubes, is expelled 

 by the next expiration unchanged in composition. This " dead 

 space " equals about 140 c.c. In superficial breathers, this may 

 equal one-half the expired air ; the deeper the breathing the more 

 closely does the alveolar air approximate to the composition of the 

 expired air. 



The composition of the alveolar air can be reckoned thus : 



In a given case the tidal air was 250 c.c. and the frequency 

 20. The expired air contained 3*5 per cent. C0 2 and 16 per cent. 

 2 . Of this, the 140 c.c. " dead space " air contained practically 

 no C0 2 . 



Thus the 110 c.c. alveolar air contained 8*75 c.c. C0 2 (3'5 

 per cent. C0 2 X 250 c.c.), that is, 7*9 per cent. C0 2 . 



The 140 c.c. " dead space " air contained 21 per cent. 2 , that 

 is, 29-4 c.c. 2 (21 per cent. X 140 c.c.). The 250 c.c. expired 

 air contained 16 per cent. 2 = 40 c.c. 2 (16 per cent. X 250). 

 Thus the 110 c.c. alveolar air contained 10-6 c.c. 2 (40-29-4), 

 that is, 9.6 per cent. On making the frequency 10 and the tidal 

 air 500 c.c. the C0 2 tension in the alveolar became 4-9 per cent- 



