RESPIRATION 109 



reduced atmospheric pressure. In either case the partial pressure 

 of the oxygen breathed is reduced, and the haemoglobin tends to 

 become imperfectly saturated with oxygen in the lungs in cor- 

 respondence with the dissociation curve for the oxygen in human 

 blood (Figure 20). 



The effects on the breathing have already been touched upon 

 in Chapter II, but must now be discussed fully. In most persons 

 the percentage of oxygen in the air breathed, or the barometric 

 pressure, must be reduced by about a third before any evident 

 effect on the breathing is produced at the time; and this effect 

 differs according as the reduction is produced rapidly or slowly. 

 With a greater reduction the contrast in this latter respect is still 

 more marked. With rapid reduction there is at first a quite notice- 

 able increase in the depth, and also in the frequency, of breathing. 

 In the course of several minutes, however, the increase diminishes 

 markedly. This phenomenon and the causes of it were described 

 and investigated by Poulton and myself.^ We found that the in- 

 creased breathing causes, as could be anticipated, a distinct fall 

 in the alveolar CO2 pressure. As a consequence, more CO2 than 

 usual is washed out of the blood, and the respiratory quotient, or 

 ratio of the volume of CO2 given off to that of oxygen absorbed, 

 is increased. Thus it increased from the normal of about 0.8 to as 

 much as 2.8 when there was sudden and considerable oxygen de- 

 ficiency. Soon, however, the extra discharge of CO2 from the 

 blood began to cease and there was only a slight further fall in 

 the alveolar CO2 pressure^Pan passu the breathing quieted down 

 so as, in spite of the diminished discharge of CO2, to maintain a^ 

 certain level of alveolar CO2 pressure, this level being of course 

 below the normal level. At the same time the alveolar oxygen 

 pressure dropped, since the lung ventilation had diminished while 

 the rate of absorption of oxygen remained undiminished. The 

 drop in alveolar oxygen pressure tended, of course, to increase the 

 symptoms of want of oxygen and thus prolong the period of in-' 

 creased breathing; but finally a balance was struck, for the time 

 at any rate. When the deficiency of oxygen was produced quite 

 gradually the initial marked increase of breathing was not notice- 

 able, as the extra CO2 was washed out gradually. 



By further experiments, we found that the new and lower 

 level of alveolar CO2 pressure had become the regulating level 

 for the atmosphere breathed. That is to say, a small increase above 

 this level caused a great increase in the breathing, while a small 



*Haldane and Poulton, Journ. of Physiol., XXXVII, p. 390, 1908. 



