Homeostasis--the key concept to physiological control.

Water in the body forms the fluid compartments inside and outside cells, but it is more than just a space filler. A continuous exchange of water between the compartments partly facilitates regulation of the compartment volume but also permits the exchange of substances dissolved within it. Some of these latter are ions that form the electrolyte environment in which cells must operate. Body fluid physiology is concerned with the actions and regulation of the major electrolyte constituents. The use of infusions provides clinicians with the means of supporting this physiology when homeostatic mechanisms have failed or are insufficient to meet demands.