8 The Chemistry of the Injured Cell 



tion by the normal cell, i.e. the maintenance -of ionic gradients. 



2. Free permeability of the normal cell w>rfTto water, sodium, po- 

 tassium and chloride in the sense indicated above. 



3. The existence of a rapid sodium and potassium exchange be- 

 tween the cell and the extracellular fluid not dependent upon 

 the expenditure of energy. 



4. The existence of intracellular isotonicity. 



5. The existence of an electrical potential across the normal cell 



6. The loss of ionic gradients by the injured cell and their restora- 

 tion with recovery when injury is reversible. 



The classical and still widely accepted explanation is based on 

 the concept of a "Sodium Pump." The theory suggests that intra- 

 cellular sodium exchanges freely with extracellular sodium on a 

 one-for-one basis but that there exists in addition a "leak" of 

 sodium ion into the cell from the higher concentration outside the 

 cell wall. The hypothesis postulates an active extrusion of sodium 

 (the Sodium Pump) sufficient to counterbalance this leak and de- 

 pendent upon the expenditure of energy. As a result of this ex- 

 trusion of sodium there is a potential excess of intracellular anions. 

 It is postulated that potassium is the only cation available to neu- 

 tralise this anionic excess and that the existence of a concentration 

 gradient for potassium is therefore part of Donnan equilibrium and 

 a result of the active extrusion of sodium. The "Sodium Pump" is 

 thought also to account for the existence of an electrical potential 

 across the cell wall. It is held that the exclusion of chloride ion from 

 the cell is again a consequence of the electrical potential caused by 

 the extrusion of sodium, the cell interior being relatively electro- 

 negative. 



It is convenient at this point to consider what picture emerges 

 of the normal and injured cell when the interpretations summar- 

 ised above are accepted. Putting the concept in its simplest possible 

 terms, the cell is seen as a sac bounded by a semipermeable mem- 

 brane. Within the cell electrolyte ions are in a freely diffusible 

 state and there is a rapid and continuous exchange between the in- 

 tracellular ions and their extracellular counterpart. The intracellu- 



