Figure 4-1 gives the approximate concentrations of important electrolytes and
other substances in the extracellular fluid and intracellular fluid. Note that the extracellular fluid contains a large amount of sodium but only a small amount of potassium.
Exactly the opposite is true of the intracellular fluid.
Also, the extracellular fluid contains a large amount of
chloride ions, whereas the intracellular fluid contains very
little. But the concentrations of phosphates and proteins in
the intracellular fluid are considerably greater than those
in the extracellular fluid. These differences are extremely
important to the life of the cell. The purpose of this chapter
is to explain how the differences are brought about by
the transport mechanisms of the cell membranes.