22 RESPIRATION 



CO 2 percentage. We found that about 0.23 per cent increase in 

 the alveolar CO 2 gives 100 per cent increase in the resting alveolar 

 ventilation. For instance with 4.16 per cent of CO 2 in the inspired 

 air, the alveolar CO 2 percentage would rise to about 6.06 per 

 cent, if it had been about 5.6 per cent when pure air was breathed. 

 As the difference between 4.16 and 6.06 is only a third of the 

 difference between o.o and 5.6, it follows that the alveolar ventila- 

 tion is thrice as great with the slightly raised alveolar CO 2 per- 

 centage. 



A more precise measure of the effects of raising the alveolar 

 CO 2 percentage on the lung ventilation has more recently been 

 obtained by Campbell, Douglas, and Hobson, 7 who found that for 

 an increase of 10 liters per minute in the volume of air breathed 

 there was an increase of 0.28 per cent (or 2 mm. of mercury 

 pressure) in the alveolar CO 2 . An increase of 0.17 per cent was 

 sufficient to double the alveolar ventilation during complete rest 

 in a deck chair. 



If an increase of 0.2 per cent in the alveolar CO 2 is sufficient 

 to double the alveolar ventilation it might be expected that a 

 decrease of 0.2 per cent would cause the breathing to cease. As 

 already mentioned, forced breathing or excessive artificial respira- 

 tion causes temporary cessation of natural breathing, or apnoea. 

 After forced breathing for about a minute the subsequent apnoea 

 commonly lasts for about 1^2 minutes in man. The alveolar CO 2 

 percentage is markedly diminished for a few seconds by even a 

 single extra deep breath of pure air, and correspondingly in- 

 creased by a breath of air containing more than 5 or 6 per cent of 

 CO 2 . It is easy to show, however, that the full effect of the dimin- 

 ished or increased percentage of CO 2 on the respiratory center is 

 not immediate. This is just what might be expected. The arterial 

 blood leaving the lungs at any moment is doubtless saturated with 

 CO 2 to a point corresponding with the existing percentage of CO 2 

 in the alveolar air; but when this blood reaches the tissues it 

 comes in contact with tissue and lymph saturated with CO 2 to the 

 normal extent, but possessing a considerable capacity for absorbing 

 more CO 2 . In consequence of this the tissues, including the res- 

 piratory center, take some time to get into equilibrium with the 

 new level of saturation with CO 2 in the arterial blood. Hence in 

 order to measure the real effect of any increase or diminution in 

 the alveolar CO 2 percentage, it is necessary to maintain this per- 

 centage constant for some time. When air containing an excess 



'Campbell, Douglas, and Hobson, Journ. of Physiol., XLVIII, p. 303, 1914. 



