View source for Chemistry:Fluid balance

{{Short description|Aspect of homeostasis concerning control of the amount of water in an organism}}

'''Fluid balance''' is an aspect of the [[Homeostasis|homeostasis]] of organisms in which the amount of [[Chemistry:Water|water]] in the organism needs to be controlled, via [[Biology:Osmoregulation|osmoregulation]] and behavior, such that the concentrations of [[Physics:Electrolyte|electrolyte]]s ([[Chemistry:Salt|salts]] in [[Chemistry:Solution|solution]]) in the various [[Biology:Body fluid|body fluid]]s are kept [[Reference range|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 [[Physics:Breathing|respiration]], [[Medicine:Perspiration|perspiration]], [[Biology:Urination|urination]], [[Biology:Defecation|defecation]], and [[Biology:Spitting|expectoration]]) must equal the input (via [[Biology:Eating|eating]] and [[Biology:Drinking|drinking]], or by parenteral intake). '''Euvolemia''' is the state of normal body fluid volume, including [[Medicine:Blood volume|blood volume]], [[Biology:Interstitial fluid|interstitial fluid]] volume, and [[Biology:Cytosol|intracellular fluid]] volume; [[Medicine:Hypovolemia|hypovolemia]] and [[Medicine:Hypervolemia|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; [[Disease|illness]] if moderate, and sometimes even [[Biology:Death|death]] if severe. For example, [[Medicine:Water intoxication|water intoxication]] (which results in [[Medicine:Hyponatremia|hyponatremia]]), the process of consuming too much water too quickly, can be fatal. Deficits to body water result in [[Biology:Volume contraction|volume contraction]] and [[Medicine:Dehydration|dehydration]]. [[Medicine:Diarrhea|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.

==Implications==

===Water consumption===
{{excerpt|Daily consumption of drinking water}}

===Medical use===

====Effects of illness====
When a person is ill, fluid may also be lost through [[Medicine:Vomiting|vomiting]], [[Medicine:Diarrhea|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====
{{main|Medicine: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.

* 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 kidney or cardiovascular system.

If blood pressure is low ([[Medicine:Hypotension|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.

==Routes of fluid loss and gain==
Fluid can leave the body in many ways. Fluid can enter the body as preformed water, ingested food and [[Biology:Drinking|drink]] and to a lesser extent as metabolic water which is produced as a by-product of aerobic respiration ([[Biology:Cellular respiration|cellular respiration]]) and dehydration synthesis.<ref>Saladin, Kenneth S. Water, Electrolyte, and Acid-Base Balance (New York: McGraw-Hill Companies, Inc., 2010), 943-944.</ref>

===Input===
A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, [[Medicine:Perspiration|sweating]] and [[Biology:Urination|urination]]. Water generated from the [[Biology:Biochemistry|biochemical]] [[Biology:Metabolism|metabolism]] of nutrients provides a significant proportion of the daily water requirements for some [[Biology:Arthropod|arthropod]]s and [[Earth:Desert|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 [[Biology:Cellular respiration|aerobic respiration]] 300 ml/day, totaling 2500 ml/day.<ref name=boron829>{{cite book |first=Walter F. |last=Boron |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |year= 2005|isbn=1-4160-2328-3 |page=829}}</ref>

====Regulation of input====
{{Main|Medicine:Thirst}}
Input of water is regulated mainly through ingested fluids, which, in turn, depends on [[Medicine:Thirst|thirst]]. An insufficiency of water results in an increased osmolarity in the [[Biology:Extracellular fluid|extracellular fluid]]. This is sensed by [[Biology:Osmoreceptor|osmoreceptor]]s in the organum vasculosum of the lamina terminalis, which trigger thirst. Thirst can to some degree be voluntarily resisted, as during fluid restriction.

The human kidneys will normally adjust to varying levels of water intake. The kidneys will require time to adjust to the new water intake level. This can cause someone who drinks a lot of water to become [[Medicine:Dehydration|dehydrated]] more easily than someone who routinely drinks less.

===Output===
* The majority of fluid output occurs via the [[Biology:Urine|urine]], approximately 1500 ml/day (approx 1.59 qt/day) in the normal adult resting state.<ref name=boron829/><ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 7/7ch08/7ch08p33 |chapter-url=http://humanphysiology.tuars.com/program/section7/7ch08/7ch08p33.htm |archive-url=https://web.archive.org/web/20150512111106/http://humanphysiology.tuars.com/program/section7/7ch08/7ch08p33.htm|archive-date=2015-05-12}}</ref>
* Some fluid is lost through [[Medicine:Perspiration|perspiration]] (part of the body's temperature control mechanism) and as water vapor in [[Biology:Exhalation|exhaled air]]. These are termed "insensible fluid losses" as they cannot be easily measured. Some sources say insensible losses account for 500 to 650 ml/day (0.5 to 0.6 qt.) of water in adults,<ref name=boron829/><ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 7/7ch08/7ch08p28 |chapter-url=http://humanphysiology.tuars.com/program/section7/7ch08/7ch08p28.htm |archive-url=https://web.archive.org/web/20160324124828/http://humanphysiology.tuars.com/program/section7/7ch08/7ch08p28.htm|archive-date=2016-03-24}}</ref> while other sources put the minimum value at 800 ml (0.8 qt.).<ref>[http://www.anaesthesiamcq.com/FluidBook/fl3_2.php 3.2 Insensible Water Loss<!-- Bot generated title -->]</ref>  In children, one calculation used for insensible fluid loss is 400&nbsp;ml/m<sup>2</sup> [[Body surface area|body surface area]].
* In addition, an adult loses approximately 100&nbsp;ml/day of fluid through [[Biology:Feces|feces]].<ref name=boron829/><ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 7/7ch08/7ch08p32 |chapter-url=http://humanphysiology.tuars.com/program/section7/7ch08/7ch08p32.htm |archive-url=https://web.archive.org/web/20160324124828/http://humanphysiology.tuars.com/program/section7/7ch08/7ch08p32.htm|archive-date=2016-03-24}}</ref>
* For females, an additional 50 ml/day is lost through vaginal secretions.

These outputs are in balance with the input of ~2500 ml/day.<ref name=boron829/>

====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 anti-diuretic hormones [[Biology:Vasopressin|vasopressin]] (ADH) and [[Chemistry:Aldosterone|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 vascular organ of lamina terminalis and subfornical organ.<ref>{{cite journal
|first1=M.J. |last1=McKinley |first2=A.K. |last2=Johnson | title=The Physiological Regulation of Thirst and Fluid Intake
| journal=News in Physiological Sciences
| volume=19
| issue=1 
| year=2004
| pages=1–6
| url=http://physiologyonline.physiology.org/cgi/content/full/19/1/1
| access-date=2006-06-02
| pmid=14739394
| doi=10.1152/nips.01470.2003 }}</ref> 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=====
{{Main|Medicine:Renin–angiotensin system}}
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 the zona glomerulosa of the adrenal cortex which in turn secretes the hormone aldosterone. This hormone stimulates the reabsorption of sodium ions from distal tubules and collecting ducts. Water in the tubular lumen cannot follow the sodium reabsorption osmotically, as this part of the kidney is impermeable to water; release of ADH (vasopressin) is required to increase expression of aquaporin channels in the cortical collecting duct, allowing reabsorption of water.

==Effect on weight loss==
{{excerpt|Weight loss effects of water}}

==See also==
*Drinking water

==References==
{{reflist}}

{{Human homeostasis}}

{{DEFAULTSORT:Fluid Balance}}
[[Category:Electrolyte disturbances]]

de:Hypovolämie#Isovolämie, Normovolämie

{{Sourceattribution|Fluid balance}}