248 RESPIRATION 



after exertion is far greater than could be accounted for by ex- 

 perimental errors. 



As already mentioned, the aerotonometer experiments of Krogh 

 indicated that the arterial CO 2 pressure is the same as that of the 

 alveolar air. The manner in which the respiratory center responds 

 to the slightest increase or diminution in the alveolar CO 2 pres- 

 sure, and the quantitative correspondence between rise in alveolar 

 CO 2 pressure and response of the respiratory center, point most 

 clearly to the conclusion that within pretty wide limits there is no 

 active secretion of CO 2 outwards in the lung, or active retention 

 of CO 2 when the lungs are over-ventilated. In individual experi- 

 ments Bohr obtained results which seemed to point to active 

 secretion of CO 2 outwards. The latest of these were made with 

 Krogh's small aerotonometer; but Krogh has pointed out how 

 easily errors may arise with this instrument; and in view of all 

 the facts I think his criticism of Bohr's experiments is probably 

 correct. 



If we calculate, by Bohr's method, the rate of diffusion of CO 2 

 from the alveolar air into the blood, the result is that for the same 

 difference in partial pressure CO 2 , in consequence of its much 

 greater solubility, must diffuse outwards about 20 times as rapidly 

 as oxygen diffuses inwards. Against this, however, must be set the 

 fact that the initial difference in CO 2 pressure between the venous 

 blood and alveolar air is only about a tenth of the corresponding 

 difference in oxygen pressure. On balance, however, there is prob- 

 ably little hindrance, even during hard work, to the establishment 

 by diffusion of practical equilibrium in CO 2 pressure between the 

 alveolar air and arterial blood. We have already seen that the 

 giving off of CO 2 in the lungs is dependent in great part on the 

 saturation of the haemoglobin with oxygen. Hence the giving off 

 of CO 2 is to a large extent under the control of oxygen absorp- 

 tion, and so of oxygen secretion when this occurs. 



Apart from this there seem to me to be strong reasons for sus- 

 pecting that although active secretion of CO 2 , like active secretion 

 of oxygen, does not occur under ordinary conditions, it does occur 

 when high pressures of CO 2 exist in the arterial blood, and the 

 body is threatened by the excess of CO 2 . As yet there is no direct 

 evidence on this subject ; but the reasons are as follows : ( I ) When 

 a small volume of oxygen is rebreathed as long as possible, or 

 even when the breath is held as long as possible after filling the 

 lungs with oxygen, the percentage of CO 2 in the alveolar air 

 mounts up much higher and more rapidly than can well be ac- 



