Mechanisms which maintain the intracellular fluid volume

The First Gibbs-Donnan Effect

The first Gibbs-Donnan Effect

The intracellular fluid colloids attract water into the cell.

This is partly because of the Gibbs-Donnan effect; the anionic colloid proteins attract cations (mainly sodium) into the cell.

If sodium were allowed to diffuse easily into the cell along the electrochemical gradient, it would eventually reach an equilibrium where there is more sodium inside than outside. This would increase the osmolality of the intracellular fluid. Water would then follow the osmotic gradient into the cell.

Ultimately if this process were to continue the cells would explode. This is embarrassing for the organism and must be stopped

The Second Gibbs-Donnan Effect

the Second Gibbs-Donnan effect

The sodium is actively excluded from the cell.

Firstly, there is a semipermeable membrane which does not allow sodium to pass into the cell.

Secondly, the Na/K ATPase actively pumps sodium out of the cell.

Thus, there is a cation excess outside the cell, which sets up another Gibbs-Donnan effect: Anions are now as attracted to the extracellular fluid as the cations are to the intracellular.

In summary: The intracellular Gibbs-Donnan effect (due to anionic proteins) is opposed by the second Gibbs-Donnan effect (which is due to extracellular sodium), and the net effect is an osmotic equilibrium.

With equal tonicity set up by these two electrochemical gradients, the water doesn't want to migrate across the membrane; it is happy to stay put in whatever compartment it already happens to be in.

References, as always; my source for most of this information has been the Kerry Brandis Anaesthetic Viva book.  Everyone should have one.