Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Fluid balance is the concept that the amount of fluid lost from the body is equal to the amount of fluid taken in.
Routes of fluid loss and gain
Fluid can leave the body in many 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. Some sources say it accounts for a daily loss 500 to 650 milliliters of water, while other sources put the minimum value at 800 ml.
- In addition, fluid is lost through urine and in faeces.
Fluid can enter the body in ingested food and drink.
The body's homeostatic control mechanisms, which maintain a constant internal environment, ensure that a balance between fluid gain and fluid loss is maintained. The hormones ADH (anti-diuretic hormone, also known as vasopressin) and Aldosterone play a major role in this.
- If the body is becoming fluid-deficient, there will be an increase in the 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.
Effects of drugs and illness
Drugs such as caffeine and alcohol suppress the secretion of ADH. This reduces the amount of water that is reabsorbed by the body in the kidneys, causing an increase in urine output and leading 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.)
Fluid balance in an acute hospital setting
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 (for example if the patient vomits and has diarrhea), the patient is said to be in negative fluid balance. In this case, fluid is often given intravenously to compensate for the loss.
- But a positive fluid balance (where fluid gain is greater than fluid loss) might suggest a problem with either the renal or cardiovascular system.
If blood pressure is low (hypotension), the filtration rate in the kidneys will lessen, causing less fluid reabsorption and thus less urine output.
An accurate measure of fluid balance is therefore an important diagnostic tool, and allows for prompt intervention to correct the imbalance.
- ↑ Essentials of Human Physiology by Thomas M. Nosek. Section 7/7ch08/7ch08p28.
- ↑ http://www.anaesthesiamcq.com/FluidBook/fl3_2.php
- ↑ Essentials of Human Physiology by Thomas M. Nosek. Section 7/7ch08/7ch08p33.
- ↑ Essentials of Human Physiology by Thomas M. Nosek. Section 7/7ch08/7ch08p32.