In health, the amount of fluid lost from the body is equal to the amount of fluid taken in. This is known as fluid balance. Fluid can be lost from the body in a variety of ways. Some fluid is lost through perspiration and as water vapour in expired air. This is part of the body's temperature control mechanism and is termed insensible loss: it cannot be easily measured. In addition, fluid is lost through urine and in faeces. Conversely, fluid is taken in through ingested food and drink.

In health, the body's homeostatic control mechanisms (concerned with maintaining a constant internal environment) ensure that a balance between fluid gain and fluid loss is maintained. The hormones ADH (Anti-Diuretic Hormone) and Aldosterone play a major role in this. If the body is becoming fluid-deficient, there will be an increase in secretion of these hormones - causing fluid to be retained by the kidneys and urine output to be reduced. Conversely, if fluid levels are excessive, secretion of these hormones is suppressed, resulting in less retention of fluid by the kidneys and a subsequent increase in the volume of urine produced.

Drugs such as caffeine and alcohol suppress the secretion of ADH. This reduces the amount of water reabsorbed by the body in the kidneys, causing an increase in urine output. This leads to the dehydration associated with these drugs.

In illness the situation is more complex. Fluid may also be lost through vomiting, diarrhea and haemorrhage. 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).

In an acute hospital setting, fluid balance is monitored carefully. This provides information on the patient's state of hydration, renal function and cardiovascular function. If fluid loss is greater than fluid gain (if for example the patient has vomiting and diarrhea), the patient is said to be in negative fluid balance. In this case, fluid is often given intravenously to compensate. Conversely, a positive fluid balance (where fluid gain is greater than fluid loss) may suggest a problem with either the renal or cardiovascular system. If blood pressure is low (hypotension) the filtration rate in the kidneys will be reduced, leading to reduced fluid reabsorption and thus a decreased urine output.

An accurate measure of fluid balance is therefore an important diagnostic tool, and allows for prompt intervention to correct the imbalance. This usually involves correcting the underlying problem.