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Fluid balance is the concept of human homeostasis that the amount of fluid lost from the body is equal to the amount of fluid taken in. Euvolemia is the state of normal body fluid volume.

Water is necessary for all life on Earth. Humans can survive for several weeks without food, but for only a few days without water.

Routes of fluid loss and gain

Fluid can leave the body in many ways. Fluid can enter the body in ingested food and drink.

Input

A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, sweating and urination. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake.

In the normal resting state, input of water through ingested fluids is approximately 1200 ml/day, from ingested foods 1000 ml/day and from metabolism 300 ml/day, totaling 2500 ml/day ^[1].

Regulation of input

Input of water is regulated mainly through ingested fluids, which, in turn, depends on thirst. An insufficiency of water results in an increased osmolarity in the extracellular fluid. This is sensed by osmoreceptors in the organum vasculosum of the lamina terminalis, which trigger thirst.

Output

• The majority of fluid output occurs via the urine^[2], approximately 1500 ml/day (approx 1.59 qt/day) in the normal adult resting state^[1]
• Some fluid is lost through perspiration and as water vapor 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 of 500 to 650 milliliters (.5 to .6 qt.) of water in adults,^[1][3] while other sources put the minimum value at 800 ml.(.8 qt.) ^[4] In children the calculation used for insensible fluid loss is 400mls/m2 surface area. For females, an additional 50 milliliters is lost each day through vaginal secretions. In addition, an adult loses approximately 100ml/day of fluid through feces ^[1].^[5]

These outputs are in balance with the input of ~2500 ml/day^[1].

Regulation of output

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.

Antidiuretic hormone

• If the body is becoming fluid-deficient, this will be sensed by osmoreceptors in the organum vasculosum of lamina terminalis and subfornical organ^[6]. These areas project to the supraoptic nucleus and paraventricular nucleus, which contain neurons that secrete the antidiuretic hormone, vasopressin, from their nerve endings in the posterior pituitary. Thus, there will be an increase in the secretion of antidiuretic hormone, causing fluid to be retained by the kidneys and urine output to be reduced.

Aldosterone

A fluid-insufficiency causes a decreased perfusion of the juxtaglomerular apparatus in the kidneys. This activates the renin-angiotensin system. Among other actions, it causes renal tubules (i.e. the distal convoluted tubules and the cortical collecting ducts) to reabsorb more sodium and water from the urine. Potassium is secreted into the tubule in exchange for the sodium, which is reabsorbed. The activated renin-angiotensin system stimulates zona glomerulosa of the adrenal cortex which in turn secretes hormone aldosterone. This hormone stimulates the reabsorption of sodium ions from distal tubules and collecting ducts. Water in the tubular lumen follows the sodium reabsorption osmotically.

Effects of drugs and illness

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.)

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.

• On the other hand, 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.

Trace elements

There are a variety of trace elements present in virtually all potable water, some of which play a role in metabolism; for example sodium, potassium and chloride are common chemicals found in very small amounts in most waters, and these elements play a role (not necessarily major) in body metabolism. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.

References

1. ^ ^{a b c d e} Walter F., PhD. Boron (2005). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3.   Page 829
2. ^ Physiology at MCG 7/7ch08/7ch08p33
3. ^ Physiology at MCG 7/7ch08/7ch08p28
4. ^ http://www.anaesthesiamcq.com/FluidBook/fl3_2.php 3.2 Insensible Water Loss]
5. ^ Physiology at MCG 7/7ch08/7ch08p32
6. ^ M.J. McKinley and A.K. Johnson (2004). "The Physiological Regulation of Thirst and Fluid Intake". News in Physiological Sciences 19 (1): 1–6. doi:10.1152/nips.01470.2003. PMID 14739394. http://physiologyonline.physiology.org/cgi/content/full/19/1/1. Retrieved 2006-06-02.