62 COLLOIDS IN BIOLOGY AND MEDICINE 



Na will diffuse out, K and Cl will diffuse in. Thus we obtain the 

 following equation if CNa(i) expresses the molar concentration of 

 Na in space (1). 



_ CK(I) _ Cci(i) _ Ci 4" 



_ _ _ 



CNa(2) CK(2) Cci(2) C 2 



If the concentration in the cell (Ci) is large compared with the outer 

 solution (C 2 ), then 



G! + C 2 Ci 

 C 2 ~ C 2 ' 

 If Ci is small, then 



Ci + C 2 

 C 2 



Let us consider the equation representing a condition that frequently 

 occurs physiologically. If Ci = 100 and C 2 = 1 it follows that 99 

 per cent of the Na originally present in (1) will remain in (1); only 

 1 per cent will diffuse into (2). 69 p?r cent of the K originally pres- 

 ent in (2) will diffuse into (1); only 1 per cent of the Cl originally 

 present in (2) will diffuse into (1). 



From this we may understand the hitherto inexplicable distribution 

 of salt in cells, e.g.,'m red blood corpuscles. A colloidal anion attracts 

 the foreign crystalloid cation and drives out the anions. Colloidal 

 cations act in the reverse way. 



In conclusion DONNAN derived formulas for the differences in 

 electric potential which must exist after the equilibria described 

 have been reached (membrane potentials). 



There are already many theories to explain the difference in poten- 

 tial in organs and the electrical currents which occur in the body 

 (muscle, nerve, electric-fish) . These theories have the error that 

 they require conditions which do not exist in the body, and in part 

 that much greater differences in potential arise in the body than 

 would be possible according to these theories. In this respect 

 DONNAN'S theory differs much from its predecessors. 



We shall not present his formula here but only a general explana- 

 tion: If there are two equally concentrated solutions of NaCl sep- 

 arated by a membrane, and we insert a piece of platinum foil into 

 each and connect them with a wire, no current will flow in the wire. 

 If the solutions are of different concentrations, theoretically, electric 

 energy will be evident until the differences in concentration disappear 

 as the result of diffusion. Such systems are called "concentration 

 couples/' 



A system consisting of colloid electrolyte, salt or water is a con- 

 centration couple which develops a current in passing from "initial 

 condition" to "equilibrium." This may be ascribed to the unequal 



