EDEMA 181 



is a shift in the equilibrium back towards normal. The conception of 

 edema as a disturbed equilibrium has been emphasized by Meltzer(a) and 

 others. 



The occurrence of edema depends then on a derangement of the 

 function which regulates the volume of the tissues. Henderson (e) (1916), 

 in a theoretical discussion of volume in the animal organism, points out 

 that there is an external regulatory mechanism for the total volume, which 

 operates chiefly by regulating the volume of 'urine excreted, and an in- 

 ternal mechanism, which involves the exchange of water between the "in- 

 finite assemblage of phases which make up the organism." It is this in- 

 ternal mechanism which we have just discussed. Disturbances in the 

 external mechanism, through their indirect influence on the internal 

 mechanism, are also possible causes of edema. It may be pointed out 

 that just as in regulating volume a disturbance in this function may 

 result in edema, so also may a disturbance, in the opposite direction, 

 result in a loss in volume from the tissues. 



Physico-Chemical Factors in Lymph Production and Edema. The 

 equilibrium involved in the exchange of fluid, with its dissolved and 

 suspended substances, which is responsible for the production of lymph 

 and the regulation of the volume of the tissues, is maintained chiefly by 

 the movement of fluid and dissolved substances through membranes. This 

 movement is known as transpiration, and it may occur in response to a 

 variety of forces, in each case a difference in potential on the two sides 

 of the membrane being necessary. The forces affecting the movement of 

 dissolved substances are not necessarily those governing the movement of 

 fluids. Actually movement of fluid may occur in one direction and move- 

 ment of dissolved substances in the other, and equilibrium is then ap- 

 proached through a combination of the two effects. So far as we know, 

 there are no membranes in the body which are permeable to water and 

 impermeable to all dissolved substances, although there are probably 

 different degrees of permeability. 



On the basis of the theoretical considerations in a previous paragraph, 

 we may analyze the forces which result in transpiration. In each case, 

 it is clear that any force is active only so long as there is a difference 

 in potential, so far as that particular force is concerned, on the two sides 

 of the membrane, and that when true equilibrium is reached movement 

 no longer occurs under the influence of that force. 



The forces which are known to affect the movement of solvents or 

 solutes through membranes are hydrostatic pressure (filtration pressure), 

 diffusion pressure of the solvent, diffusion pressure of the solute, and elec- 

 trostatic pressure. The influence of each of these forces on the movement 

 of fluid in the animal body is considered separately. 



Hydrostatic Pressure. This is the pressure exerted by a fluid by vir- 

 tue of its specific gravity or of pressure transmitted to it by compression. 



