Fluid balance
Encyclopedia
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 4–6 weeks without food, but for only a few days without water.
The amount of water varies with the individual, as it depends on the condition of the subject, the amount of physical exercise, and on the environmental temperature and humidity. In the US, the reference daily intake
(RDI) for water is 3.7 litres per day (l/day) for human males older than 18, and 2.7 l/day for human females older than 18 including water contained in food, beverages, and drinking water. The common misconception that everyone should drink two litres (68 ounces, or about eight 8-oz glasses) of water per day is not supported by scientific research. Various reviews of all the scientific literature on the topic performed in 2002 and 2008 could not find any solid scientific evidence that recommended drinking eight glasses of water per day. For example, people in hotter climates will require greater water intake than those in cooler climates. An individual's thirst provides a better guide for how much water they require rather than a specific, fixed number. A more flexible guideline is that a normal person should urinate 4 times per day, and the urine should be a light yellow color.
A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, perspiration and urination
. Food contributes 0.5 to 1 l/day, and the metabolism of protein
, fat
, and carbohydrates produces another 0.25 to 0.4 l/day, which means that 2 to 3 l/day of water for men and 1 to 2 l/day of water for women should be taken in as fluid, i.e. drunk, in order to meet the Recommended Daily Intake (RDI). In terms of mineral nutrients intake, it is unclear what the drinking water contribution is. However, inorganic
minerals generally enter surface water and ground water via storm water
runoff
or through the Earth's crust. Treatment processes also lead to the presence of some minerals. Examples include calcium
, zinc
, manganese
, phosphate
, fluoride
and sodium
compounds. Water generated from the biochemical
metabolism
of nutrients provides a significant proportion of the daily water requirements for some arthropod
s and desert
animals, but provides only a small fraction of a human's necessary intake. 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 small quantities in most waters, and these elements play a role (not necessarily major) in body metabolism. Other elements such as fluoride, while beneficial in low concentrations, can cause dental problems and other issues when present at high levels. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.
Profuse sweating can increase the need for electrolyte
(salt) replacement. Water intoxication
(which results in hyponatremia
), the process of consuming too much water too quickly, can be fatal.
The human kidney
s 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.
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.
. Water generated from the biochemical
metabolism
of nutrients provides a significant proportion of the daily water requirements for some arthropod
s 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.
. An insufficiency of water results in an increased osmolarity in the extracellular fluid
. This is sensed by 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.
These outputs are in balance with the input of ~2500 ml/day.
play a major role in this.
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.
, 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.)
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.
, potassium
and chloride
are common chemicals found in very small amounts in most waters, and these elements play a major role in body metabolism. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.
Human homeostasis
Human homeostasis is derived from the Greek, homeo or "same", and stasis or "stable" and means remaining stable or remaining the same.The human body manages a multitude of highly complex interactions to maintain balance or return systems to functioning within a normal range...
that the amount of fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
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 4–6 weeks without food, but for only a few days without water.
The amount of water varies with the individual, as it depends on the condition of the subject, the amount of physical exercise, and on the environmental temperature and humidity. In the US, the reference daily intake
Reference Daily Intake
The Reference Daily Intake or Recommended Daily Intake is the daily intake level of a nutrient that is considered to be sufficient to meet the requirements of 97–98% of healthy individuals in every demographic in the United States .The RDI is used to determine the Daily Value of foods,...
(RDI) for water is 3.7 litres per day (l/day) for human males older than 18, and 2.7 l/day for human females older than 18 including water contained in food, beverages, and drinking water. The common misconception that everyone should drink two litres (68 ounces, or about eight 8-oz glasses) of water per day is not supported by scientific research. Various reviews of all the scientific literature on the topic performed in 2002 and 2008 could not find any solid scientific evidence that recommended drinking eight glasses of water per day. For example, people in hotter climates will require greater water intake than those in cooler climates. An individual's thirst provides a better guide for how much water they require rather than a specific, fixed number. A more flexible guideline is that a normal person should urinate 4 times per day, and the urine should be a light yellow color.
A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, perspiration and urination
Urination
Urination, also known as micturition, voiding, peeing, weeing, pissing, and more rarely, emiction, is the ejection of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans the process of urination is under voluntary control...
. Food contributes 0.5 to 1 l/day, and the metabolism of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
, fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
, and carbohydrates produces another 0.25 to 0.4 l/day, which means that 2 to 3 l/day of water for men and 1 to 2 l/day of water for women should be taken in as fluid, i.e. drunk, in order to meet the Recommended Daily Intake (RDI). In terms of mineral nutrients intake, it is unclear what the drinking water contribution is. However, inorganic
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
minerals generally enter surface water and ground water via storm water
Stormwater
Stormwater is water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt that enters the stormwater system...
runoff
Surface runoff
Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...
or through the Earth's crust. Treatment processes also lead to the presence of some minerals. Examples include calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
, manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
, phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
, fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
and sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
compounds. Water generated from the biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
of nutrients provides a significant proportion of the daily water requirements for some arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
animals, but provides only a small fraction of a human's necessary intake. 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
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
and chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
are common chemicals found in small quantities in most waters, and these elements play a role (not necessarily major) in body metabolism. Other elements such as fluoride, while beneficial in low concentrations, can cause dental problems and other issues when present at high levels. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.
Profuse sweating can increase the need for electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
(salt) replacement. Water intoxication
Water intoxication
Water intoxication, also known as water poisoning, is a potentially fatal disturbance in brain functions that results when the normal balance of electrolytes in the body is pushed outside of safe limits by over-consumption of water....
(which results in hyponatremia
Hyponatremia
Hyponatremia is an electrolyte disturbance in which the sodium concentration in the serum is lower than normal. In the vast majority of cases, hyponatremia occurs as a result of excess body water diluting the serum sodium and is not due to sodium deficiency. Sodium is the dominant extracellular...
), the process of consuming too much water too quickly, can be fatal.
The human kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s 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
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
more easily than someone who routinely drinks less.
Routes of fluid loss and gain
Fluid can leave the body in many ways. Fluid can enter the body as preformed water, ingested foodFood
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...
and drink
Drinking
Drinking is the act of consuming water or a beverage through the mouth. Water is required for many of life’s physiological processes. Both excessive and inadequate water intake are associated with health problems.-Physiology:...
and to a lesser extent as metabolic water which is produced as a by-product of aerobic respiration (cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
) and dehydration synthesis.
Input
A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, sweating and urinationUrination
Urination, also known as micturition, voiding, peeing, weeing, pissing, and more rarely, emiction, is the ejection of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans the process of urination is under voluntary control...
. Water generated from the biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
of nutrients provides a significant proportion of the daily water requirements for some arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
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
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
300 ml/day, totaling 2500 ml/day.
Regulation of input
Input of water is regulated mainly through ingested fluids, which, in turn, depends on thirstThirst
Thirst is the craving for 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 and/or an increase in the concentration of certain osmolites, such as salt...
. An insufficiency of water results in an increased osmolarity in the extracellular fluid
Extracellular fluid
Extracellular fluid usually denotes all body fluid outside of cells. The remainder is called intracellular fluid.In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma...
. This is sensed by osmoreceptor
Osmoreceptor
An osmoreceptor is a sensory receptor primarily found in the hypothalamus of most homeothermic organisms that detects changes in osmotic pressure. Osmoreceptors can be found in several structures, including the organum vasculosum of the lamina terminalis and the subfornical organ...
s in the organum vasculosum of the lamina terminalis
Organum vasculosum of the lamina terminalis
The organum vasculosum of the lamina terminalis is one of the circumventricular organs of the brain. Other circumventricular organs are the subfornical organ and the area postrema in the brainstem.-AV3V region:...
, which trigger thirst. Thirst can to some degree be voluntarily resisted, as during fluid restriction.
Output
- The majority of fluid output occurs via the urineUrineUrine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
, approximately 1500 ml/day (approx 1.59 qt/day) in the normal adult resting state. - Some fluid is lost through perspiration (part of the body's temperature control mechanism) and as water vapor in expired airExpired Air- The Facts :Expired air is breathed out through the nose or mouth, in volume it normally consists of:78.04% Nitrogen13.6% - 16% Oxygen4% - 5.3% Carbon dioxide1% Argon and other gasesIn contrast, air generally consists of:78.08% Nitrogen...
. 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, while other sources put the minimum value at 800 ml (0.8 qt.). In children, one calculation used for insensible fluid loss is 400ml/m2 body surface areaBody surface areaIn physiology and medicine, the body surface area is the measured or calculated surface of a human body. For many clinical purposes BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal adipose mass...
. - In addition, an adult loses approximately 100ml/day of fluid through fecesFecesFeces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
. - For females, an additional 50 ml/day is lost through vaginal secretions.
These outputs are in balance with the input of ~2500 ml/day.
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 AldosteroneAldosterone
Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...
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 organSubfornical organThe subfornical organ, situated on the ventral surface of the fornix, at the interventricular foramina , is one of the circumventricular organs of the brain.-Relations with other circumventricular organs:...
. These areas project to the supraoptic nucleusSupraoptic nucleusThe supraoptic nucleus is a nucleus of magnocellular neurosecretory cells in the hypothalamus of the mammalian brain. The nucleus is situated at the base of the brain, adjacent to the optic chiasm...
and paraventricular nucleusParaventricular nucleusThe paraventricular nucleus is a neuronal nucleus in the hypothalamus. It contains multiple subpopulations of neurons that are activated by a variety of stressful and/or physiological changes. Many PVN neurons project directly to the posterior pituitary where they release oxytocin or vasopressin...
, which contain neurons that secrete the antidiuretic hormone, vasopressin, from their nerve endings in the posterior pituitaryPosterior pituitaryThe posterior pituitary comprises the posterior lobe of the pituitary gland and is part of the endocrine system. Despite its name, the posterior pituitary gland is not a gland, per se; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior...
. 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 illness
When a person is ill, fluid may also be lost through vomitingVomiting
Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...
, diarrhea
Diarrhea
Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...
, 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 diarrhoea), 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
Hypotension
In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...
), 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 sodiumSodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
and chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
are common chemicals found in very small amounts in most waters, and these elements play a major role in body metabolism. Water is essential for the growth and maintenance of our bodies, as it is involved in a number of biological processes.