250 RESPIRATION 



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 

 other gases will diffuse through the plasma; but during secretion 

 of oxygen the living substance behaves like the protoplasm of the 

 swim bladder, taking up oxygen on one side of the cell, and giv- 

 ing it off at a higher pressure on the other. The oxygen will tend 

 to diffuse backwards if, as in experiments with a high percentage 

 of CO, the oxygen pressure becomes higher in the blood than in 

 the alveolar air; but some, at least, of this oxygen will be caught 

 on its way and returned. 



This general conception throws light in other directions. For 

 let us suppose the direction of the oxygen secretion to be reversed, 

 so that the lung epithelium, instead of absorbing oxygen, hinders 

 its passage. Nitrogen and other inert gases will still be able to pass 

 inwards freely by diffusion. We shall thus have nitrogen going 

 through, without oxygen. Now let us suppose that the epithelium 

 has an excretory function; and let us apply the general concep- 

 tions, above set forth, to the glomerular epithelium of the kidney. 

 We can imagine the living substance of this epithelium holding 

 back, by an active process, all the normal constituents of blood, 

 particularly water, if their normal diffusion pressures are not ex- 

 ceeded, but otherwise letting them through. All the known facts 

 seem to confirm Bowman's original conclusion that the water of 

 the urine is usually almost entirely separated in the glomeruli. It 

 seems also clear that as shown by Ludwig and his pupils, the 

 process of separation is dependent on blood pressure, like a filtra- 

 tion process. If we suppose that the passages through which the 

 liquid is filtered are not permeable by the proteins of the blood, 

 we have an explanation, as pointed out by Starling, of why a cer- 

 tain minimum blood pressure is needed. The liquid separated might 

 be little different from pure water, whereas the blood plasma 

 contains salts in considerable amount. Such a liquid could not be 

 separated by simple filtration, and numerous other facts are 

 against the simple filtration theory. I think that all the facts con- 

 form with the theory that the glomerulus is a filter, but with a 

 living framework, and that the action of this living framework, 

 is to pick out and return to the blood what belongs to its normal 



