Water and Electrolytes 9 



lar colloids exert oncotic pressure which would lead to intracellular 

 accumulation of water and consequent cellular swelling were it not 

 for the active extrusion of sodium. 



When the cell is injured, energy may no longer be available 

 for the extrusion of sodium. As a result, the extracellular sodium 

 which continues to "leak" into the cell accumulates therein. These 

 sodium ions neutralise anions e.g. in protein molecules, thus 

 disturbing the Donnan equilibrium and allowing potassium to 

 leave the cell. In addition, chloride ion enters the cell partly be- 

 cause of loss of polarity by the cell membrane. As a result, more 

 sodium enters to maintain electrical neutrality. Thus the entry of 

 sodium and chloride exceeds the loss of potassium, hence there is 

 a net uptake of water by the cell which consequently undergoes 

 swelling. 



The classical view of cellular water and electrolyte metabolism 

 and the derived explanation of the water and electrolyte movements 

 seen in cell injury appear satisfactory. Unfortunately, rigorous ex- 

 periments, especially those measuring isotope exchange, have in- 

 dicated that in many cases the observed facts are not explicable on 

 the basis of a "sodium pump" alone. As a result, it has proved 

 necessary to postulate the existence of separate energy-dependent 

 transport mechanisms for potassium, chloride and water, in addi- 

 tion to the "sodium pump" (Leaf, 1959; Rothstein, 1959) . The 

 proposed mechanisms for these multiple transport systems include 

 contractile and re-orientating proteins and a variety of carrier 

 systems, diffusible and fixed. 



The point of such modifications of the classical hypothesis is to 

 fit a view of the cell as a water-filled sac containing electrolyte ions 

 in a freely diffusible state. In recent years this picture has been 

 challenged by an alternative concept. 



The classical view of the cell considered that such barrier as 

 existed between ions e.g. K ions of cell and environment was 

 merely a thin membrane, the cell wall, and that within the cell the 

 ions moved freely. Actual determinations of the mixing of isotopi- 

 cally labelled K ions revealed, however, that the situation was more 

 complex and that mixing did not proceed in as uncomplicated 

 fashion as it should were this concept of the cell a valid one. To 



