Fluid balance is an aspect of the
homeostasis of living
organisms in which the amount of
water in the organism needs to be controlled, via
osmoregulation and
behavior, such that the
concentrations of
electrolytes (
salts in
solution) in the various
body fluids are kept
within healthy ranges. The core principle of fluid balance is that the amount of water lost from the body must equal the amount of water taken in; for example, in
human homeostasis, the output (via
respiration,
perspiration,
urination,
defecation, and
expectoration) must equal the input (via
eating,
drinking, and
parenteral intake).
Euvolemia is the state of normal body fluid volume, including
blood volume,
interstitial fluid volume, and
intracellular fluid volume;
hypovolemia and
hypervolemia are imbalances. Water is necessary for all life on Earth. Humans can survive for 4 to 6 weeks without food but only for a few days without water.