ABSTRACT
Maintenance of a constant volume in the face of extracellular and intracellular osmotic perturbations is a critical problem faced by all cells. Most cells respond to swelling or shrinkage by activating specific membrane transport or metabolic processes that serve to return cell volume to its normal resting state. These processes are essential for normal cell function and survival. This chapter provides an overview of the cellular and molecular events underlying cell volume homeostasis. The bulk movement of water across a semipermeable membrane is termed osmosis. An ideal semipermeable membrane is one that is permeable only to water. Water is effectively in thermodynamic equilibrium across the plasma membrane of animal cells. In other words, the osmotic concentration of cytoplasmic and extracellular fluids are equal under steady-state conditions. Cells respond to volume perturbations by activating volume-regulatory mechanisms. The processes by which swollen and shrunken cells return to a normal volume is termed regulatory volume decrease and regulatory volume increase, respectively.
