CHAPTER IX 

 Gas Secretion in the Lungs. 



IN the lungs the blood is separated from the alveolar air by two 

 layers of living tissue, namely the capillary endothelium and the 

 alveolar epithelium. What part in respiratory exchange is played 

 by these very thin layers of living tissue? Is this part purely me- 

 chanical? In other words, do these layers behave towards the 

 respiratory gases as any very thin non-living moist membrane 

 would behave? Or do the living membranes play an active part in 

 the process? We must now face this interesting, but also contro- 

 versial subject. 



There has been a tendency to assume that because these mem- 

 branes are very thin they cannot play any active part. But it is not 

 so long since even membranes consisting of cubical or columnar 

 epithelial cells were supposed only to play a passive part in the 

 separation of material; and the presumption that a thinner mem- 

 brane of flattened cells cannot play an active part has come down to 

 us from the time, about the middle of last century, when physico- 

 chemical theories became dominant in physiology, and secretion in 

 general was supposed to be a mere mechanical process like filtra- 

 tion or diffusion. Another prevalent presumption is that though 

 liquids or dissolved solids may be actively secreted, gases probably 

 pass through living membranes by simple diffusion. 



So little information about gas secretion is usually to be found 

 in physiological text books that it may be useful, before discussing 

 gas secretion by the lungs, to give some account of gas secretion 

 as it is now well known to exist in the swim bladder of fishes. 



The swim bladder is morphologically a diverticulum of the 

 alimentary canal, like the lungs. In some classes of fishes there is 

 an open duct from the swim bladder into the alimentary canal, 

 but in other classes this duct is closed. Quite evidently, the main 

 function of the swim bladder is to make the specific gravity of the 

 fish about equal to that of the water it displaces when the fish is 

 at a certain depth. With a certain amount of gas in its swim 

 bladder the fish will just float at a certain depth. It is, however, 

 in a position of unstable equilibrium : for any movement upwards 

 will cause expansion of the air, so that the fish will tend to rise 

 with increasing velocity towards the surface ; and any movement 



