REGULATION OF RESPIRATORY MOVEMENTS 1207 



per cent, of an atmosphere, a figure almost identical with, those 

 given in the last column of the Table above. 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 -=r = 



5*2 per 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 



f 600 

 I 



560 



520 

 480 

 440 



3000 2600 



2200 1800 UOO 



air pressure mm Hy 



600 



FIG. 504. Effects of alterations in the barometric pressure on the alveolar 

 ' C0 2 tension, the alveolar C0 2 percentage, and in the alveolar 2 tension, 

 Note that the excitant effects of 0. lack are not seen until the pressure 

 falls below 500 mm. Hg. (BOYCOTT and HALDANE.) 



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. 



Very different are the phenomena observed on alteration of the 

 partial pressure of oxygen (Fig. 504). Here, within wide limits, the 

 partial pressure of oxygen in the alveolar air is determined by its pres- 

 sure in the inspired air. Thus, if we take the same series of observations 

 with a pressure of 646 mm., the percentage of oxygen in the alveolar 



646 

 air was 13' 19, corresponding to a tension of 13*19 X j^ = 10'4 



per cent. At an atmospheric pressure of 755 mm. the percentage 

 of oxygen in the alveolar air was 13" 97, corresponding to a tension 

 of 13-06 per cent., which we may take as the normal figure at the 



