Fluid balance is an aspect of the homeostasis of 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 humans, the output (via respiration, perspiration, urination, defecation, and expectoration) must equal the input (via eating and drinking, or by 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.
Profuse sweating can increase the need for electrolyte replacement. Water-electrolyte imbalance produces headache and fatigue if mild; illness if moderate, and sometimes even death if severe. For example, water intoxication (which results in hyponatremia), the process of consuming too much water too quickly, can be fatal. Deficits to body water result in volume contraction and dehydration. Diarrhea is a threat to both body water volume and electrolyte levels, which is why diseases that cause diarrhea are great threats to fluid balance.
When a person is ill, fluid may also be lost through vomiting, diarrhea, and hemorrhage. An individual is at an increased risk of dehydration in these instances, as the kidneys will find it more difficult to match fluid loss by reducing urine output (the kidneys must produce at least some urine in order to excrete metabolic waste.)
Oral rehydration therapy
Oral rehydration therapy (ORT), is type of fluid replacement used as a treatment for dehydration. In an acute hospital setting, fluid balance is monitored carefully. This provides information on the patient's state of hydration, kidney function and cardiovascular function.