RESPIRATION 



249 



and similar devices, I have often been struck with the greater 

 sensitiveness to CO2 of myself and other sedentary workers in 

 comparison with men in good physical training, although nearly 

 pure oxygen was being breathed. These observations suggest very 

 strongly that along with the power of oxygen secretion the power 

 of secretion of CO2 is developed by muscular exertion. (3) In the 

 experiments of Paul Bert^^ on the blood gases when increasingly 

 high percentages of CO2 were breathed by animals, it appeared 

 that with increase in the CO2 percentage the CO2 in the arterial 

 blood often showed little or no increase. It seems very dif- 

 ficult to explain these results apart from active secretion of CO2 

 coming into play progressively, and particularly in view of the 

 experiments of Henderson and Haggard on the increased CO2- 

 absorbing capacity of the blood when excess of CO2 is breathed 

 (Chapter VIII). 



In view of the absence, as yet, of direct measurements, it seems 

 unnecessary to discuss this question further; but I may point out 

 that just as the opponents of the oxygen-secretion theory have 

 been mistaken in drawing general conclusions from experiments 

 in which oxygen secretion was either absent or could not be dem- 

 onstrated, it is very probable that they have been equally mistaken 

 over secretion of CO2. Bearing in mind Johannes Miiller's argu- 

 ment as to the analogy between secretory activity and ordinary 

 metabolic processes, it seems quite likely that the active transport, 

 not only of oxygen, but also of CO2, is a phenomenon which oc- 

 curs in all living cells. 



Not only do oxygen and CO2 diffuse through the lung epithe- 

 lium into or out of the blood, but also other gases, such as nitrogen, 

 hydrogen, methane, carbon monoxide, etc., so that their partial 

 pressures become exactly equal in the body and the alveolar air. 

 But how is it that oxygen is sometimes actively secreted inwards, 

 and that the oxygen pressure may be greater in the blood without 

 the oxygen leaking back by diffusion into the alveolar air just as 

 other gases leak in or out? We must, I think, suppose that the 

 structure of the alveolar epithelium is not homogeneous but may 

 be divided into a reticulum of living structure and a plasma filling 

 the interstices, just as is the case with the body as a whole. The 

 diffusion will take place through the plasma, while the living sub- 

 stance behaves as a solid towards diffusion, as in the case of the 

 secreting cells of the swim bladder. Not only oxygen but also 



"Paul Bert, La Pression barometriqtie, p. 985. 



