Cellular Aspects of Body Electrolytes and Water 19 



two main factors — the osmolarity of the plasma and the 

 activity of this Na+-extrusion mechanism. The passage of 

 water across the cell membrane is very rapid, so that the cell 

 responds to changes in osmolarity of the plasma by virtually 

 instantaneous changes in its water content; in this way it 

 may be said to respond passively to changes in the plasma, 

 and its changes of water content and salt concentration may 

 be said to be secondary to primary changes determined 

 principally by the kidney. The operation of the second factor 

 — the Na+-extrusion mechanism — will influence the amount 

 of material — salts and water — in the cell, and it would be by 

 virtue of this mechanism that this type of cell could exert a 

 primary influence on the water and electrolyte content of the 

 organism. Thus, if the Na+-extrusion mechanism operated 

 more rapidly than the influx under the electrochemical 

 gradient, there would be a net loss of Na+ and of anions, 

 namely Cl~ and HCOg" ; this would decrease the osmolarity 

 of the cell and water would be lost to the plasma. Such a 

 shrinkage of cells is easily demonstrable by allowing them to 

 recover from the effects of putting the Na+-extrusion mechan- 

 ism out of action. Thus, when the cells are cooled, the metabolic 

 processes supplying energy can no longer work; Na+ enters 

 the cells accompanied by anions and they swxll. When the cells 

 are warmed, the metabolic processes begin, and the extra Na+ 

 is excreted until the cells return to their normal volume. The 

 effects of agents that increase the permeability of the cell 

 membrane are of some interest; substances like alcohol or 

 urethane, in the appropriate concentration, can increase the 

 permeability of the cell membrane to Na+ and K+ to such an 

 extent that the Na+-extrusion mechanism is unable to keep 

 pace with the influx of this ion; thus, in spite of a normally 

 functioning metabolism the cell may swell; on removing the 

 agent it may return to its normal size. 



The erythrocytes of most species contain K+ as their 

 principal cation, so that the cell maintains large gradients 

 of Na+ and K+ (Fig. 3). The condition for an osmotically 

 stable system could be given by an impermeability of cations, 



