1082 



PHYSIOLOGY 



react to the slightest increase of C0 2 tension in the blood, any increase in this 

 gas giving at once a compensatory increase in depth and frequency of 

 respiration, so that the alveolar C0 2 content may be maintained almost 

 constant. 



That it is the tension of C0 2 in the alveolar air, and therefore in the blood 

 bathing the centres, and not the percentage amount of this gas, which is the 

 determining factor is shown by a comparison of the composition of the 

 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 



30QO 2600 



2200 1800 UOO 

 air pressure mm Hy 



600 



200 



FIG. 506. Effects of alterations in the barometric pressure on the alveolar C0 2 

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

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

 500 mm. Hg. (BOYCOTT and HALDANE.) 



of C0 2 in the alveolar air was 3-42, corresponding, however, to a tension of 

 3*42 x =5-6 per cent, of an atmosphere, a figure almost identical 



i Oly 



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 



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



760 



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. 

 [[Very different are the phenomena observed on alteration of the partial 



