12 PHYSICOCHEJMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



chemist works with pure solutions, while the physiologist has to use 

 fluids that are always complicated and frequently very variable in com- 

 position. We must simplify the problem as far as possible by having 

 clearly before us the exact nature of the biological problem which a com- 

 parison of physicochemical values, such as osmotic pressure, may ena- 

 ble us to elucidate, and we must consider the other physical forces 

 which may assist or modify the particular one we are investigating. 



In the physical experiments described above, the semipermeable mem- 

 brane may be conceived of as composed of pores of such a size that 

 they permit only the smallest of molecules those of water to pass 

 through them. Semipermeable membranes with larger pores may, how- 

 ever, exist that is, membranes which permit water molecules and mole- 

 cules of simple chemical substances to pass, but hold back those com- 

 posed of large complex molecules. Such a semipermeable membrane 

 would allow the saline constituents but not the proteins of blood serum 

 to pass. It is, however, no longer semipermeable towards all of the dis- 

 solved substances, and the process of diffusion through it is more gener- 

 ally designated as one of dialysis than of osmosis. 



Since the passage of dissolved molecules through membranes de- 

 pends upon the principle of diffusion, its rate will be proportional to 

 the osmotic pressures of the solutions on the two surfaces of the mem- 

 brane and to the size of the molecules, small molecules diffusing more 

 quickly than large ones. Suppose a membrane permeable to sodium 

 chloride and water is placed between two fluids containing sodium 

 chloride in solution, but in greater concentration in one of them than 

 in the other: although the sodium chloride will diffuse from the stronger 

 to the weaker solution, the water will tend to diffuse still more quickly (be- 

 cause its molecules are smaller) in the opposite direction, until the number 

 of sodium-chloride molecules in a given volume of solution is equal on 

 both sides of the membrane. For a time, therefore, the volume of the 

 stronger solution will increase. The differences which exist in the dif- 

 fusibility of dissolved molecules are analogous to those which have 

 long been known to exist in the diffusibility of gases, but the relation 

 between rate of diffusibility and molecular weight is not so simple as 

 the ratio between these two quantities in gases. These relationships, 

 however, indicate several further possibilities in the explanation of the 

 mechanism of exchange of substances through membranes, and must not 

 be overlooked, as they often are, in the interpretation of physiological 

 phenomena. An excellent review of the possible conditions is given 

 by Starling in his " Human Physiology." 4 For example, let us suppose 

 the substances dissolved in the fluid on the two sides of a semipermeable 

 membrane, such as the peritoneum, to be different in diffusibility, as cane 



