184 FRANKLIN C. McLEAN 



tiate the diffusion pressure of the solute, and the diffusion pressure of the 

 solvent, which is related to what is known as osmotic pressure. 



Diffusion in the sense of movement of molecules of solute to regions 

 of lesser concentration, was considered by Ludwig to play a part in lymph 

 formation, and is generally recognized as an important factor in the 

 process. It certainly plays a large part, if not the major part, in the 

 transfer of oxygen from the blood to the tissues, and in the return of 

 carbon dioxid, in the form of carbonic acid, to the blood. It may be 

 considered as the major force concerned in the distribution of urea and 

 other non-protein nitrogenous substances, and it is also concerned in the 

 movement of electrolytes and their ions. Diffusion influences both the 

 movement of molecules through membranes and in fluids where membranes 

 are not concerned. Since diffusion pressure of solutes is not in itself 

 responsible for the movement of solvents it probably plays only a secondary 

 or indirect part in the production of edema. 



Osmosis, or the passage of a solvent through a membrane permeable 

 to it, but not to the dissolved substance, occurs as the result of the diffusion 

 pressure of the solvent, or the tendency of molecules of the solvent to 

 move to regions in which these molecules are in lesser concentration, or, in 

 other words, to regions in which the molecules of the solute are in greater 

 concentration. The property of a fluid, by virtue of which it can attract 

 molecules of water through a membrane, is generally known as osmotic 

 pressure. Osmotic pressure may be defined as the pressure which it is 

 necessary to apply to a fluid to prevent molecules of water from diffusing 

 into it through a semipermeable membrane. Osmotic pressure is, there- 

 fore, not a force in itself, but is rather a negative pressure, insofar as it 

 can be considered as a pressure at all. 



As has already been pointed out, there are no strictly semipermeable 

 membranes in the body. When a difference in osmotic pressure occurs on 

 two sides of such membranes as exist, equilibrium is restored by a com- 

 bination of osmosis and of diffusion of the solute, which, it will be seen, 

 must act in opposite directions. As osmosis involves the movement of 

 fluid, with a tendency to increase the pressure or volume of the fluid on 

 the side of the membrane on which there was originally the greater con- 

 centration, it is also opposed to some extent by the increase in hydrostatic 

 pressure which it produces. Actually, therefore, the movement of fluid, 

 through membranes which are permeable to water and to most dissolved 

 substances, occurs as the result of a balance between osmotic pressure on 

 the one hand and diffusion pressure of the solute on the other hand, and 

 the hydrostatic pressure on the two sides of the membrane. It is this 

 combination of forces which probably has the greatest influence on the 

 regulation of volume in the tissues. 



If osmotic pressure has an important part in the continuous move- 

 ment of fluids, there must be a source of energy resulting in the main- 



