22 RESPIRATION 



CO2 percentage. We found that about 0.23 per cent increase in 

 the alveolar CO2 gives 100 per cent increase in the resting alveolar 

 ventilation. For instance with 4.16 per cent of CO2 in the inspired^ 

 air, the alveolar CO2 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 0.0 and 5.6, it follows that the alveolar ventila- 

 tion is thrice as great with the slightly raised alveolar CO2 per- 

 centage. 



A more precise measure of the effects of raising the alveolar 

 CO2 percentage on the lung ventilation has more recently been 

 obtained by Campbell, Douglas, and Hobson,'' 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 CO2. 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 CO2 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 i^ minutes in man. The alveolar CO2 

 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 

 CO2. It is easy to show, however, that the full effect of the dimin- 

 ished or increased percentage of CO2 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 

 CO2 to a point corresponding with the existing percentage of CO2 

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

 comes in contact with tissue and lymph saturated with CO2 to the 

 normal extent, but possessing a considerable capacity for absorbing 

 more COg. 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 CO2 in the arterial blood. Hence in 

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

 the alveolar CO2 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, 19 14. 



