ABSTRACT
Sodium is necessary for the survival of most, if not all, animals. It is the most prevalent of extracellular solutes, and because it is largely excluded from the intracellular uid compartment by the cell plasma membrane, it in a sense comprises the “true grit” of the extracellular uid. The maintenance of near-ideal concentrations of intracellular constituents is necessary for optimum cellular tness and function. The concentration of impermeable solutes such as sodium in the extracellular uid compartment is the major determinant of water movement across the plasma membrane. A high concentration of sodium in the extracellular space causes water to move out of cells and produces dehydration of the intracellular uid compartment, whereas a low concentration of extracellular solute causes overhydration of intracellular space. In addition to its importance in establishing the distribution of uid across semipermeable membranes, the relative concentration of extra-and intracellular sodium can also inuence cellular functions. For example, the resting membrane potential of cells is in large part dependent on the relative concentrations of intracellular versus extracellular sodium, and the shape of action potentials generated by neurons is affected by the ratio of these two sodium concentrations.
