Cells
The tendency of biologic tissues and processes to maintain a constancy of environment consistent with their vitality and well being. For cells to maintain their stability or equilibrium, the cell membranes must be in continuous interaction with both the internal (intracellular) environment and the external (extracellular) environment. When the equilibrium of any component is disturbed, the interaction permits automatic readjustment by giving rise to stimuli that result in restoration of the equilibrium.