Body water
Encyclopedia
In medicine, body water is the water
content of the human body
. A significant fraction of the human body is water.
Arthur Guyton 's Textbook of Medical Physiology states that "the total amount of water in a man of average weight (70 kilograms) is approximately 40 litres, averaging 57 percent of his total body weight. In a newborn infant, this may be as high as 75 percent of the body weight, but it progressively decreases from birth to old age, most of the decrease occurring during the first 10 years of life. Also, obesity decreases the percentage of water in the body, sometimes to as low as 45 percent". These figures are statistical averages, so are illustrative, and like all biostatistics, will vary with things like type of population, age and number of people sampled, and methodology. So there is not, and cannot be, a figure that is exactly the same for all people, for this or any other physiological measure. For example, Jackson's (1985) Anatomy & Physiology for Nurses gives a figure of 60% for the proportion of body-weight attributable to water, which approximates Guyton's 57%.
In diseased states where body water is affected, the compartment or compartments that have changed can give clues to the nature of the problem. Body water is regulated by hormone
s, including anti-diuretic hormone (ADH), aldosterone
and atrial natriuretic peptide
.
There are many methods to determine body water. One way to get a simple estimate is by calculation.
Per Netter's Atlas of Human Physiology, body water is broken down into the following compartments:
FA-MS
measurement of deuterium
abundance in breath samples from individuals. A known dose of deuterated water (Heavy water
, D2O) is ingested and allowed to equilibrate within the body water. The FA-MS instrument then measures the deuterium-to-hydrogen (D:H) ratio in the exhaled breath water vapour. The total body water is then accurately measured from the increase in breath deuterium content in relation to the volume of D2O ingested.
Different substances can be used to measure different fluid compartments:
Intracellular fluid may then be estimated by subtracting extracellular fluid from total body water.
(BIA). In the traditional BIA method, a person lies on a cot and spot electrodes are placed on the hands and bare feet. Electrolyte gel is applied first, and then a current of 50 kHz is introduced. This AC waveform allows the creation of a current inside the body via the very capacitive skin without causing a DC flow or burns, and limited in the ~20mA range current for safety.
BIA has emerged as a promising technique because of its simplicity, low cost, high reproducibility and noninvasiveness. BIA prediction equations can be either generalized or population-specific, allowing this method to be potentially very accurate. Selecting the appropriate equation is important to determining the quality of the results.
For clinical purposes, scientists are developing a multi-frequency BIA method that may further improve the method's ability to predict a person's hydration level. New segmental BIA equipment that uses more electrodes may lead to more precise measurements of specific parts of the body.
is a decrease in body fluid volume, with or without a concomitant loss of osmolyte
s. The loss of the body water component of body fluid is specifically termed dehydration
.
Na+ loss approximately correlates with fluid loss from extracellular fluid
(ECF), since Na+ has a much higher concentration in ECF than intracellular fluid (ICF). In contrast, K+ has a much higher concentration in ICF than ECF, and therefore its loss rather correlates with fluid loss from ICF, since K+ loss from ECF causes the K+ in ICF to diffuse out of the cells, dragging water with it by osmosis
.
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
content of the human body
Human body
The human body is the entire structure of a human organism, and consists of a head, neck, torso, two arms and two legs.By the time the human reaches adulthood, the body consists of close to 100 trillion cells, the basic unit of life...
. A significant fraction of the human body is water.
Arthur Guyton 's Textbook of Medical Physiology states that "the total amount of water in a man of average weight (70 kilograms) is approximately 40 litres, averaging 57 percent of his total body weight. In a newborn infant, this may be as high as 75 percent of the body weight, but it progressively decreases from birth to old age, most of the decrease occurring during the first 10 years of life. Also, obesity decreases the percentage of water in the body, sometimes to as low as 45 percent". These figures are statistical averages, so are illustrative, and like all biostatistics, will vary with things like type of population, age and number of people sampled, and methodology. So there is not, and cannot be, a figure that is exactly the same for all people, for this or any other physiological measure. For example, Jackson's (1985) Anatomy & Physiology for Nurses gives a figure of 60% for the proportion of body-weight attributable to water, which approximates Guyton's 57%.
In diseased states where body water is affected, the compartment or compartments that have changed can give clues to the nature of the problem. Body water is regulated by hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s, including anti-diuretic hormone (ADH), aldosterone
Aldosterone
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...
and atrial natriuretic peptide
Atrial natriuretic peptide
Atrial natriuretic peptide , atrial natriuretic factor , atrial natriuretic hormone , or atriopeptin, is a powerful vasodilator, and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium and fat...
.
There are many methods to determine body water. One way to get a simple estimate is by calculation.
Per Netter's Atlas of Human Physiology, body water is broken down into the following compartments:
- Intracellular fluid (2/3 of body water). Per Guyton, in a body containing 40 litres of fluid, about 25 litres is intracellular, which amounts to 62.5% (5/8), close enough to the 2/3 rule of thumb. Jackson's texts states 70% of body fluid is intracellular.
- Extracellular fluidExtracellular fluidExtracellular 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...
(1/3 of body water). Per Guyton's illustration, for a 40 litre body, about 15 litres is extracellular, which amounts to 37.5% Again, this is close to the 1/3 rule of thumb cited here.- PlasmaBlood plasmaBlood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
(1/5 of extracellular fluid). Per Guyton's illustration, of the 15 litres of extracellular fluid, plasma volume averages 3 litres. This amounts to 20%, the same as per Netter's Atlas. - Interstitial fluidInterstitial fluidInterstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid...
(4/5 of extracellular fluid) - Transcellular fluidTranscellular fluidTranscellular fluid is the portion of total body water contained within epithelial lined spaces. It is the smallest component of extracellular fluid, which also includes interstitial fluid and plasma. It is often not calculated as a fraction of the extracellular fluid, but it is about 2.5% of the...
(a.k.a. "third space," normally ignored in calculations)- Contained inside organs, such as the gastrointestinal, cerebrospinalCerebrospinal fluidCerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
, peritoneal, and ocularHuman eyeThe human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
fluids.
- Contained inside organs, such as the gastrointestinal, cerebrospinal
- Plasma
Dilution and equilibration
Total body water can be determined using Flowing afterglow mass spectrometryFlowing afterglow mass spectrometry
Flowing-afterglow mass spectrometry, FA-MS, is a sensitive and quantitative mass spectrometry analytical approach that offers a route to on-line, real-time deuterium abundance measurements in water vapour in breath and above aqueous liquids, including urine and serum...
FA-MS
measurement of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
abundance in breath samples from individuals. A known dose of deuterated water (Heavy water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...
, D2O) is ingested and allowed to equilibrate within the body water. The FA-MS instrument then measures the deuterium-to-hydrogen (D:H) ratio in the exhaled breath water vapour. The total body water is then accurately measured from the increase in breath deuterium content in relation to the volume of D2O ingested.
Different substances can be used to measure different fluid compartments:
- total body water: tritiated waterTritiated waterTritiated water is a form of water where the usual hydrogen atoms are replaced with tritium. In its pure form it may be called tritium oxide or super-heavy water. Pure T2O is corrosive due to self-radiolysis. Diluted, tritiated water is mainly H2O plus some HTO . It is also used as a tracer for...
or heavy waterHeavy waterHeavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...
. - extracellular fluidExtracellular fluidExtracellular 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...
: inulinInulinInulins are a group of naturally occurring polysaccharides produced by many types of plants. They belong to a class of fibers known as fructans. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes... - blood plasmaBlood plasmaBlood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
: Evans blueEvans blueT-1824 or Evans Blue, often incorrectly rendered as Evan's Blue, is an azo dye which has a very high affinity for serum albumin. Because of this, it can be useful in physiology in estimating the proportion of body water contained in blood plasma....
Intracellular fluid may then be estimated by subtracting extracellular fluid from total body water.
Bioelectrical impedance analysis
Another method of determining total body water percentage (TBW%) is via Bioelectrical Impedance AnalysisBioelectrical impedance analysis
Bioelectrical impedance analysis is a commonly used method for estimating body composition. Since the advent of the first commercially available devices in the mid-1980s the method has become popular owing to its ease of use, portability of the equipment and its relatively low cost compared to...
(BIA). In the traditional BIA method, a person lies on a cot and spot electrodes are placed on the hands and bare feet. Electrolyte gel is applied first, and then a current of 50 kHz is introduced. This AC waveform allows the creation of a current inside the body via the very capacitive skin without causing a DC flow or burns, and limited in the ~20mA range current for safety.
BIA has emerged as a promising technique because of its simplicity, low cost, high reproducibility and noninvasiveness. BIA prediction equations can be either generalized or population-specific, allowing this method to be potentially very accurate. Selecting the appropriate equation is important to determining the quality of the results.
For clinical purposes, scientists are developing a multi-frequency BIA method that may further improve the method's ability to predict a person's hydration level. New segmental BIA equipment that uses more electrodes may lead to more precise measurements of specific parts of the body.
Fluid loss
Volume contractionVolume contraction
Volume contraction is a decrease in body fluid volume, also including any concomitant loss of osmolytes. The loss of the water component of body fluid is specifically termed dehydration.-By body fluid compartment:...
is a decrease in body fluid volume, with or without a concomitant loss of osmolyte
Osmolyte
Osmolytes are compounds affecting osmosis. They are soluble in the solution within a cell, or in the surrounding fluid, e.g. as plasma osmolytes. They play a role in maintaining cell volume and fluid balance. For example, when a cell swells due to external osmotic pressure, membrane channels open...
s. The loss of the body water component of body fluid is specifically termed dehydration
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...
.
Na+ loss approximately correlates with fluid loss from 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...
(ECF), since Na+ has a much higher concentration in ECF than intracellular fluid (ICF). In contrast, K+ has a much higher concentration in ICF than ECF, and therefore its loss rather correlates with fluid loss from ICF, since K+ loss from ECF causes the K+ in ICF to diffuse out of the cells, dragging water with it by osmosis
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
.