Mechanisms which maintain intracellular fluid tonicity

Water will move between compartments until their osmolality is the same

Water will move between compartments until their osmolality is the same

The cells will shrink because water will leave them to equilibrate the osmolality. Occasionally, this is useful.

Cells will compensate for chronic changes in extracellular tonicity.

The cells will secrete new osmotically active substances ("idiogenic osmoles") to counteract this fluid shift.

idiogenic osmoles being secreted

The production of idiogenic osmoles can also be downregulated to adjust for chronic hypotonic states

idiogenic osmoles being lost

Neurons are notorious for this activity. This the mechanism of coping with chronic hyponatremia: the drop in serum tonicity causes the neurons to swell, which then causes the neurons to downregulate their idiogenic osmole production, and thus to compensate for the drop in ECF tonicity with a drop in intracellular tonicity.

References

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