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Fluid balance

From Wikipedia, the free encyclopedia
Not to be confused withWater balance.

Fluid balanceis an aspect of thehomeostasisoforganismsin which the amount ofwaterin the organism needs to be controlled, viaosmoregulationandbehavior,such that theconcentrationsofelectrolytes(saltsinsolution) in the variousbody fluidsare keptwithin 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 (viarespiration,perspiration,urination,defecation,andexpectoration) must equal the input (viaeatinganddrinking,or byparenteralintake).Euvolemiais the state of normal body fluid volume, includingblood volume,interstitial fluidvolume, andintracellular fluidvolume;hypovolemiaandhypervolemiaare 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 imbalanceproduces headache and fatigue if mild;illnessif moderate, and sometimes evendeathif severe. For example,water intoxication(which results inhyponatremia), the process of consuming too much water too quickly, can be fatal. Deficits tobody waterresult involume contractionanddehydration.Diarrheais a threat to bothbody watervolume and electrolyte levels, which is why diseases that cause diarrhea are great threats to fluid balance.

Implications

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Water consumption

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This section is an excerpt fromDaily consumption of drinking water.[edit]

The recommended daily amount ofdrinking waterfor humans varies.[1]It depends on activity, age,health,and environment. In the United States, theAdequate Intakefor total water, based on median intakes, is 4.0 litres (141 imp fl oz; 135 US fl oz) per day for males older than 18, and 3.0 litres (106 imp fl oz; 101 US fl oz) per day for females over 18; it assumes about 80% from drink and 20% from food.[2]TheEuropean Food Safety Authorityrecommends 2.0 litres (70 imp fl oz; 68 US fl oz) of total water per day for women and 2.5 litres (88 imp fl oz; 85 US fl oz) per day for men.[3]

The common advice to drink 8 glasses (1,900 mL or 64 US fl oz) of plain water per day is not scientific; thirst is a better guide for how much water to drink than is a specific, fixed amount.[4]Americans aged 21 and older, on average, drink 1,043 mL (36.7 imp fl oz; 35.3 US fl oz) of drinking water a day, and 95% drink less than 2,958 mL (104.1 imp fl oz; 100.0 US fl oz) [of what?] per day.[5]Exercise and heat exposure cause loss of water and therefore may induce thirst and greater water intake.[6]Active people in hot climates may need 6.0 litres (211 imp fl oz; 203 US fl oz) of water, or more, per day.[6]

How much drinking water contributes to the intake of mineral nutrients is unclear.Inorganicminerals generally enter surface water and groundwater viastormwaterrunoffand through the ground. Water treatment also adds some minerals, such ascalcium,zinc,manganese,phosphate,fluoride,andsodiumcompounds.[7]Water generated by thebiochemicalmetabolismof nutrients provides a significant part of the daily water needs for somearthropodsanddesertanimals, but provides only a small fraction of a human's necessary intake. There are trace elements in almost all potable water; some of these affect metabolism, such as sodium,potassium,andchloride,which are common in small amounts in most water. Other elements, such asfluoride,while beneficial in low concentrations, can cause dental and other problems at high levels.

Fluid balance is important to health. Profuse sweating can increase the need to replaceelectrolytes(salts).Water intoxication(the consumption of too much water too quickly) causeshyponatremia,which can cause death in minutes or hours.[8]Water makes up about 60% of the body weight in men and 55% of weight in women.[9]A baby is about 70% to 80%; old people are about 45% water.[10]

Medical use

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Effects of illness

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When a person is ill, fluid may also be lost throughvomiting,diarrhea,andhemorrhage.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

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Main article: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 innegative fluid balance.In this case, fluid is often givenintravenouslyto compensate for the loss.
  • On the other hand, apositive 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 (hypotension), the filtration rate in thekidneyswill 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

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Fluid can leave the body in many ways. Fluid can enter the body as preformed water,ingestedfoodanddrinkand to a lesser extent as metabolic water which is produced as a by-product of aerobic respiration (cellular respiration) and dehydration synthesis.[11]

Input

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A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration,sweatingandurination.Water generated from thebiochemicalmetabolismof nutrients provides a significant proportion of the daily water requirements for somearthropodsanddesertanimals, 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 fromaerobic respiration300 ml/day, totaling 2500 ml/day.[12]

Regulation of input

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Main article:Thirst

Input of water is regulated mainly through ingested fluids, which, in turn, depends onthirst.An insufficiency of water results in an increased osmolarity in theextracellular fluid.This is sensed byosmoreceptorsin theorganum vasculosum of the lamina terminalis,which trigger thirst. Thirst can to some degree be voluntarily resisted, as duringfluid restriction.

The humankidneyswill 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 becomedehydratedmore easily than someone who routinely drinks less.

Output

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  • The majority of fluid output occurs via theurine,approximately 1500 ml/day (approx 1.59 qt/day) in the normal adult resting state.[12][13]
  • Some fluid is lost throughperspiration(part of the body's temperature control mechanism) and as water vapor inexhaled 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,[12][14]while other sources put the minimum value at 800 ml (0.8 qt.).[15]In children, one calculation used for insensible fluid loss is 400 ml/m2body surface area.
  • In addition, an adult loses approximately 100 ml/day of fluid throughfeces.[12][16]
  • For females, an additional 50 ml/day is lost through vaginal secretions.

These outputs are in balance with the input of ~2500 ml/day.[12]

Regulation of output

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The body'shomeostaticcontrol mechanisms, which maintain a constant internal environment, ensure that a balance between fluid gain and fluid loss is maintained. The anti-diuretic hormonesvasopressin(ADH) andaldosteroneplay 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 areexcessive,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
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Main article:Antidiuretic hormone

If the body is becoming fluid-deficient,this will be sensed byosmoreceptorsin thevascular organ of lamina terminalisandsubfornical organ.[17]These areas project to thesupraoptic nucleusandparaventricular nucleus,which contain neurons that secrete the antidiuretic hormone, vasopressin, from their nerve endings in theposterior 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
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Main article: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 thecortical 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 andcollecting 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

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This section is an excerpt fromFasting.[edit]
A glass of water on an empty plate
Fastingis abstention from eating, and sometimes drinking. However, from a purelyphysiologicalcontext, "fasting" may refer to themetabolicstatus of a person who has not eaten overnight (before "breakfast"), or to the metabolic state achieved after completedigestionandabsorptionof a meal.[18]Metabolic changes in the fasting state begin after absorption of a meal (typically 3–5 hours after eating). Adiagnostic fastrefers to prolonged fasting from 1–100 hours (depending on age), conducted under observation, to facilitate the investigation of a health complication (usuallyhypoglycemia). Many people may also fast as part of amedical procedureor acheck-up,such as preceding acolonoscopyorsurgery,or before certain medical tests.Intermittent fastingis a technique sometimes used forweight lossor other health benefits that incorporates regular fasting into a person'sdietaryschedule. Fasting may also be part of areligious ritual,often associated with specific scheduled fast days, asdetermined by the religion,or be applied as apublic demonstrationfor a given cause, in a practice known as ahunger strike.

See also

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References

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  1. ^Ann C. Grandjean (August 2004)."3"(PDF).Water Requirements, Impinging Factors, & Recommended Intakes.World Health Organization. pp. 25–34.Archived(PDF)from the original on 22 February 2016.This 2004 article focuses on the USA context and uses data collected from the US military.
  2. ^"US daily reference intake values".Iom.edu. Archived fromthe originalon 6 October 2011.Retrieved5 December2011.
  3. ^EFSA Panel on Dietetic Products, Nutrition, and Allergies (2010)."Scientific Opinion on Dietary Reference Values for water".EFSA Journal.8(3): 1459.doi:10.2903/j.efsa.2010.1459.{{cite journal}}:CS1 maint: multiple names: authors list (link)
  4. ^H. Valtin,Drink at least eight glasses of water a day. "Really? Is there scientific evidence for" 8 × 8 "?Archived20 April 2010 at theWayback MachineAm J Physiol Regul Integr Comp Physiol 283: R993-R1004, 2002.
  5. ^Exposure Factors Handbook: 2011 Edition(PDF).National Center for Environmental Assessment. September 2011. Archived fromthe original(PDF)on 24 September 2015.Retrieved24 May2015.
  6. ^ab"Report Sets Dietary Intake Levels for Water, Salt, and Potassium To Maintain Health and Reduce Chronic Disease Risk".US Institute of Medicine, Food and Nutrition Board. 11 February 2004.Retrieved13 September2017.
  7. ^World Health OrganizationArchived19 January 2011 at theWayback Machine(WHO). Geneva, Switzerland. Joyce Morrissey Donohue, Charles O. Abernathy, Peter Lassovszky, George Hallberg."The contribution of drinking-water to total dietary intakes of selected trace mineral nutrients in the United States."Draft, August 2004.
  8. ^Noakes, Timothy D.; Goodwin, Neil; Rayner, Brian L.; Branken, Trevor; Taylor, Robert K.N. (2005)."Water Intoxication: A Possible Complication During Endurance Exercise☆".Wilderness & Environmental Medicine.16(4): 221–227.doi:10.1580/1080-6032(2005)16[221:WIAPCD]2.0.CO;2.PMID16366205.S2CID28370290.
  9. ^Miller, Thomas A. (2006).Modern surgical care physiologic foundations and clinical applications(3rd ed.). New York: Informa Healthcare. p. 34.ISBN978-1-4200-1658-1.Archivedfrom the original on 1 September 2017.
  10. ^Nancy caroline's emergency care in the streets(07 ed.). [S.l.]: Jones And Bartlett Learning. 2012. p. 340.ISBN978-1-4496-4586-1.Archivedfrom the original on 1 September 2017.
  11. ^Saladin, Kenneth S. Water, Electrolyte, and Acid-Base Balance (New York: McGraw-Hill Companies, Inc., 2010), 943-944.
  12. ^abcdeBoron, Walter F. (2005).Medical Physiology: A Cellular And Molecular Approaoch.Elsevier/Saunders. p. 829.ISBN1-4160-2328-3.
  13. ^Nosek, Thomas M."Section 7/7ch08/7ch08p33".Essentials of Human Physiology.Archived fromthe originalon 2015-05-12.
  14. ^Nosek, Thomas M."Section 7/7ch08/7ch08p28".Essentials of Human Physiology.Archived fromthe originalon 2016-03-24.
  15. ^3.2 Insensible Water Loss
  16. ^Nosek, Thomas M."Section 7/7ch08/7ch08p32".Essentials of Human Physiology.Archived fromthe originalon 2016-03-24.
  17. ^McKinley, M.J.; Johnson, A.K. (2004)."The Physiological Regulation of Thirst and Fluid Intake".News in Physiological Sciences.19(1): 1–6.doi:10.1152/nips.01470.2003.PMID14739394.Retrieved2006-06-02.
  18. ^"fasting | Definition, Description, Types, Benefits, & Facts".Encyclopedia Britannica.Retrieved2021-10-28.
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