Fluid balance

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Fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behavior, such that the concentrations of electrolytes (salts in solution) in the various body fluids are kept 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 respiration, perspiration, urination, defecation, and expectoration) must equal the input (via eating and drinking, or by parenteral intake). Euvolemia is the state of normal body fluid volume, including blood volume, interstitial fluid volume, and intracellular fluid volume; hypovolemia and 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.

Contents

Profuse sweating can increase the need for electrolyte replacement. Water-electrolyte imbalance produces headache and fatigue if mild; illness if moderate, and sometimes even death if severe. For example, water intoxication (which results in hyponatremia), the process of consuming too much water too quickly, can be fatal. Deficits to body water result in volume contraction and dehydration. 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

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

The recommended daily amount of drinking water for humans varies. [1] It depends on activity, age, health, and environment. In the United States, the Adequate Intake for 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] The European Food Safety Authority recommends 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. Inorganic minerals generally enter surface water and groundwater via stormwater runoff and through the ground. Water treatment also adds some minerals, such as calcium, zinc, manganese, phosphate, fluoride, and sodium compounds. [7] Water generated by the biochemical metabolism of nutrients provides a significant part of the daily water needs for some arthropods and desert animals, 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, and chloride, which are common in small amounts in most water. Other elements, such as fluoride, 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 replace electrolytes (salts). Water intoxication (the consumption of too much water too quickly) causes hyponatremia, 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

Effects of 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.)

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 (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 drink and to a lesser extent as metabolic water which is produced as a by-product of aerobic respiration (cellular respiration) and dehydration synthesis. [11]

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 aerobic respiration 300 ml/day, totaling 2500 ml/day. [12]

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. 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 dehydrated more easily than someone who routinely drinks less.

Output

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

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 vasopressin (ADH) 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 vascular organ of lamina terminalis and subfornical organ. [17] 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 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

This section is an excerpt from Fasting.[ edit ]

Fasting is the abstention from eating and sometimes drinking. From a purely physiological context, "fasting" may refer to the metabolic status of a person who has not eaten overnight (see "Breakfast"), or to the metabolic state achieved after complete digestion and absorption of a meal. [18] Metabolic changes in the fasting state begin after absorption of a meal (typically 3–5 hours after eating).

A diagnostic fast refers to prolonged fasting from 1 to 100 hours (depending on age) conducted under observation to facilitate the investigation of a health complication, usually hypoglycemia. Many people may also fast as part of a medical procedure or a check-up, such as preceding a colonoscopy or surgery, or before certain medical tests. Intermittent fasting is a technique sometimes used for weight loss that incorporates regular fasting into a person's dietary schedule. Fasting may also be part of a religious ritual, often associated with specific scheduled fast days, as determined by the religion, or by applied as a public demonstration for a given cause in a practice known as a hunger strike.

See also

Related Research Articles

In biology, homeostasis is the state of steady internal, physical, chemical, and social conditions maintained by living systems. This is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance, being kept within certain pre-set limits. Other variables include the pH of extracellular fluid, the concentrations of sodium, potassium, and calcium ions, as well as the blood sugar level, and these need to be regulated despite changes in the environment, diet, or level of activity. Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life.

<span class="mw-page-title-main">Diabetes insipidus</span> Condition characterized by large amounts of dilute urine and increased thirst

Diabetes insipidus (DI), alternately called arginine vasopressin deficiency (AVP-D) or arginine vasopressin resistance (AVP-R), is a condition characterized by large amounts of dilute urine and increased thirst. The amount of urine produced can be nearly 20 liters per day. Reduction of fluid has little effect on the concentration of the urine. Complications may include dehydration or seizures.

<span class="mw-page-title-main">Dehydration</span> Deficit of total body water

In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mild dehydration can also be caused by immersion diuresis, which may increase risk of decompression sickness in divers.

Hyponatremia or hyponatraemia is a low concentration of sodium in the blood. It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L. Symptoms can be absent, mild or severe. Mild symptoms include a decreased ability to think, headaches, nausea, and poor balance. Severe symptoms include confusion, seizures, and coma; death can ensue.

<span class="mw-page-title-main">Renin–angiotensin system</span> Hormone system

The renin–angiotensin system (RAS), or renin–angiotensin–aldosterone system (RAAS), is a hormone system that regulates blood pressure, fluid and electrolyte balance, and systemic vascular resistance.

Diuresis is the excretion of urine, especially when excessive (polyuria). The term collectively denotes the physiologic processes underpinning increased urine production by the kidneys during maintenance of fluid balance.

<span class="mw-page-title-main">Collecting duct system</span> Kidney system

The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and vasopressin.

<span class="mw-page-title-main">Thirst</span> Craving for potable fluids experienced by animals

Thirst is the craving for potable fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids or an increase in the concentration of certain osmolites, such as sodium. If the water volume of the body falls below a certain threshold or the osmolite concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst.

<span class="mw-page-title-main">Renal physiology</span> Study of the physiology of the kidney

Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.

<span class="mw-page-title-main">Electrolyte imbalance</span> Medical condition

Electrolyte imbalance, or water-electrolyte imbalance, is an abnormality in the concentration of electrolytes in the body. Electrolytes play a vital role in maintaining homeostasis in the body. They help to regulate heart and neurological function, fluid balance, oxygen delivery, acid–base balance and much more. Electrolyte imbalances can develop by consuming too little or too much electrolyte as well as excreting too little or too much electrolyte. Examples of electrolytes include calcium, chloride, magnesium, phosphate, potassium, and sodium.

Hypernatremia, also spelled hypernatraemia, is a high concentration of sodium in the blood. Early symptoms may include a strong feeling of thirst, weakness, nausea, and loss of appetite. Severe symptoms include confusion, muscle twitching, and bleeding in or around the brain. Normal serum sodium levels are 135–145 mmol/L. Hypernatremia is generally defined as a serum sodium level of more than 145 mmol/L. Severe symptoms typically only occur when levels are above 160 mmol/L.

The syndrome of inappropriate antidiuretic hormone secretion (SIADH), also known as the syndrome of inappropriate antidiuresis (SIAD), is characterized by a physiologically inappropriate release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an abnormal non-pituitary source. Unsuppressed ADH causes a physiologically inappropriate increase in solute-free water being reabsorbed by the tubules of the kidney to the venous circulation leading to hypotonic hyponatremia.

Hyperchloremia is an electrolyte disturbance in which there is an elevated level of chloride ions in the blood. The normal serum range for chloride is 96 to 106 mEq/L, therefore chloride levels at or above 110 mEq/L usually indicate kidney dysfunction as it is a regulator of chloride concentration. As of now there are no specific symptoms of hyperchloremia; however, it can be influenced by multiple abnormalities that cause a loss of electrolyte-free fluid, loss of hypotonic fluid, or increased administration of sodium chloride. These abnormalities are caused by diarrhea, vomiting, increased sodium chloride intake, renal dysfunction, diuretic use, and diabetes. Hyperchloremia should not be mistaken for hyperchloremic metabolic acidosis as hyperchloremic metabolic acidosis is characterized by two major changes: a decrease in blood pH and bicarbonate levels, as well as an increase in blood chloride levels. Instead those with hyperchloremic metabolic acidosis are usually predisposed to hyperchloremia.

<span class="mw-page-title-main">Water intoxication</span> Potentially fatal overhydration

Water intoxication, also known as water poisoning, hyperhydration, overhydration, water toxemia or hyponatremia is a potentially fatal disturbance in brain functions that can result when the normal balance of electrolytes in the body is pushed outside safe limits by excessive water intake.

<span class="mw-page-title-main">Bartter syndrome</span> Medical condition

Bartter syndrome (BS) is a rare inherited disease characterised by a defect in the thick ascending limb of the loop of Henle, which results in low potassium levels (hypokalemia), increased blood pH (alkalosis), and normal to low blood pressure. There are two types of Bartter syndrome: neonatal and classic. A closely associated disorder, Gitelman syndrome, is milder than both subtypes of Bartter syndrome.

The human body and even its individual body fluids may be conceptually divided into various fluid compartments, which, although not literally anatomic compartments, do represent a real division in terms of how portions of the body's water, solutes, and suspended elements are segregated. The two main fluid compartments are the intracellular and extracellular compartments. The intracellular compartment is the space within the organism's cells; it is separated from the extracellular compartment by cell membranes.

<span class="mw-page-title-main">Potomania</span> Medical condition

Potomania is a specific hypo-osmolality syndrome related to massive consumption of beer, which is poor in solutes and electrolytes. With little food or other sources of electrolytes, consumption of large amounts of beer or other dilute alcoholic drinks leads to electrolyte disturbances, where the body does not have enough nutrients known as electrolytes, namely sodium, potassium, and magnesium. The symptoms of potomania are similar to other causes of hyponatremia and include dizziness, muscular weakness, neurological impairment and seizures, all related to hyponatremia and hypokalaemia. While the symptoms of potomania are similar to other causes of hyponatremia and acute water intoxication, it should be considered an independent clinical entity because of its often chronic nature of onset, pathophysiology, and presentation of symptoms.

<span class="mw-page-title-main">Adipsia</span> Medical condition

Adipsia, also known as hypodipsia, is a symptom of inappropriately decreased or absent feelings of thirst. It involves an increased osmolality or concentration of solute in the urine, which stimulates secretion of antidiuretic hormone (ADH) from the hypothalamus to the kidneys. This causes the person to retain water and ultimately become unable to feel thirst. Due to its rarity, the disorder has not been the subject of many research studies.

<span class="mw-page-title-main">Diuretic</span> Substance that promotes the production of urine

A diuretic is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics increase the excretion of water from the body, through the kidneys. There exist several classes of diuretic, and each works in a distinct way. Alternatively, an antidiuretic, such as vasopressin, is an agent or drug which reduces the excretion of water in urine.

The recommended daily amount of drinking water for humans varies. It depends on activity, age, health, and environment. In the United States, the Adequate Intake for total water, based on median intakes, is 4.0 litres per day for males older than 18, and 3.0 litres per day for females over 18; it assumes about 80% from drink and 20% from food. The European Food Safety Authority recommends 2.0 litres of total water per day for women and 2.5 litres per day for men.

References

  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 from the original on 6 October 2011. Retrieved 5 December 2011.
  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"? Archived 20 April 2010 at the Wayback Machine Am 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 from the original (PDF) on 24 September 2015. Retrieved 24 May 2015.
  6. 1 2 "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. Retrieved 13 September 2017.
  7. ↑ World Health Organization Archived 19 January 2011 at the Wayback 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. PMID   16366205. S2CID   28370290.
  9. ↑ Miller, Thomas A. (2006). Modern surgical care physiologic foundations and clinical applications (3rd ed.). New York: Informa Healthcare. p. 34. ISBN   978-1-4200-1658-1. Archived from 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. ISBN   978-1-4496-4586-1. Archived from 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. 1 2 3 4 5 Boron, Walter F. (2005). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. p. 829. ISBN   1-4160-2328-3.
  13. ↑ Nosek, Thomas M. "Section 7/7ch08/7ch08p33". Essentials of Human Physiology. Archived from the original on 2015-05-12.
  14. ↑ Nosek, Thomas M. "Section 7/7ch08/7ch08p28". Essentials of Human Physiology. Archived from the original on 2016-03-24.
  15. ↑ 3.2 Insensible Water Loss
  16. ↑ Nosek, Thomas M. "Section 7/7ch08/7ch08p32". Essentials of Human Physiology. Archived from the original on 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. PMID   14739394 . Retrieved 2006-06-02.
  18. ↑ "fasting | Definition, Description, Types, Benefits, & Facts". Encyclopedia Britannica. Retrieved 2021-10-28.
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