RESPIRATION 



troversy. Is the process a purely physical one, in which 

 the epithelium of the alveoli which separates the air 

 from the lung acts merely as a permeable membrane, 

 or have the epithelial cells themselves something to say 

 in the process ? Are they able actively to pick up oxygen 

 out of the air, and to excrete C0 2 from the blood, irre- 

 spective of such considerations as the tension of these 

 gases ? In other words, is the process a purely physical 

 one, or is it to use the only available word in part at 

 least vital? This question is one, it need hardly be 

 said, of great theoretical importance ; it is one of the 

 fundamental problems of physiology which goes to the 

 root of our conceptions of living function. But it has 

 also practical bearings. For if the epithelial cells of 

 the alveoli really do play an active part in the process 

 of exchange in the lungs, it is conceivable that the dis- 

 order of this exchange and the imperfect purification of 

 the blood which results from it, as seen in acute disease 

 of the lungs, may be due, in part at least, to disorganiza- 

 tion of the alveolar epithelium. Asphyxia would then 

 be the result of an inability of the epithelium of the air 

 cells to excrete C0 2 , just as uraemia is the result of a 

 failure of the renal epithelium to excrete the constituents 

 of the urine. That such interference with function does 

 occur in acute disease there is some experimental evidence 

 to prove. Lorrain Smith,* for example, concludes that 

 an interference with active absorption through the lung 

 epithelium is an integral part of many conditions of 

 disease directly or indirectly associated with the lungs. 

 Meanwhile it is interesting to note that most physiolo- 



* Journ. ofPhysiol, 1897-98, xxii. 807. 



142 



