1132 



PHYSIOLOGY 



alveolar air. under different atmospheric pressures. Thus, when the subject 

 of the experiments from which the above Table was derived, was placed in 

 an air-chamber compressed to a pressure of 1261 mm., the mean percentage 

 of C0 2 in the alveolar air was 342, corresponding however to a tension of 



342 X = 5-6 per cent, of an atmosphere, a figure almost identical 



i t)U 



with those given in the last column of this Table. At the top of Ben 

 Nevis, where the barometric pressure was 646 mm., the percentage of C0 2 in 



646 

 the alveolar air was 6-6, corresponding to a tension of 6-6 x = 5-2 per 



4 OU 



f 600 

 I 



560 



520 



BO 



52 



28 



3000 2600 



2200 1800 1400 -1000. 

 air pressure mm Hq 



600 



200 



Fia. 519. Effects of alterations in the barometric pressure on the alveolar C0 2 

 tension, the alveolar CO 2 percentage, and in the alveolar O 2 tension. Note 

 that the excitant effects of O. lack are not seen until the pressure falls below 

 500 mm. Hg. (BOYCOTT and HALDANE.) 



cent, of an atmosphere, i. e. of 760 mm. Thus the pressure of C0 2 in alveolar 

 air remains practically constant with widely varying limits of atmospheric 

 pressure and with very different percentages of C0 2 in the inspired air, 

 showing that the reactions of the organism are directed so as to maintain, 

 by alterations in the respiratory depth and rhythm, a constant tension of this 

 gas in the alveoli and therefore in the arterial blood. 



o 



Very different are the phenomena observed on alteration of the partial 

 pressure of oxygen (Fig. 519). Here, within wide limts, the partial pressure 

 of oxygen in the alveolar air is determined by its pressure in the inspired air. 

 Thus, if we take the same series of observations with a pressure of 646 mm., 

 the percentage of oxygon in the alveolar air was 13-19, corresponding to a 



tension of 13-19 X - == 10-4 per cent. At an atmospheric pressure of 



7.V) mm. 1h<' pen-nil :i<jr f nxvuvn in the alveolar air was 13-97, corresponding 

 to a tension of 13-OG por cent., which wo may take as the normal figure at the 



