248 RESPIRATION 



CO2 pressure and response of the respiratory center, point most 

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

 active secretion of CO2 outwards in the lung, or active retention 

 of CO2 when the lungs are over-ventilated. In individual experi- 

 ments Bohr obtained results which seemed to point to active 

 secretion of CO2 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 CO2 

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

 difference in partial pressure CO2, 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 CO2 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 CO2 pressure between the 

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

 giving off of CO2 in the lungs is dependent in great part on the 

 saturation of the haemoglobin with oxygen. Hence the giving off 

 of CO2 is to a large extent under the control of oxygen absorption, 

 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 CO2, like active secretion 

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

 when high pressures of CO2 exist in the arterial blood, and the 

 body is threatened by the excess of COg. 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 CO2 in the alveolar air 

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

 counted for from any probable rise in the pressure of CO2 in the 

 venous blood. Examples of experiments in this direction are given 

 in the paper by Christiansen, Douglas, and myself. (2) It ap- 

 pears that men in good training and with the power of oxygen 

 secretion well developed are capable of standing a much higher 

 percentage of CO2 in the inspired and alveolar air than other 

 men. In my experience with self-contained mine-rescue apparatus, 



