This video explains the various mechanisms that insure that the intracellular osmolality equals the extracellular osmolality and the mechanisms that regulate the movement of molecules (ions and non-ions) through the plasma membrane. The video also illustrates that changes in cell volume may be physiological adaptations to changes in the extracellular environment (that is an allostatic mechanism). Changes in cell volume can also be pathophysiological events related to malfunction of the properties of the cell membrane.