Body water
A significant fraction of the human body is water. This body water is distributed in different compartments in the body. Lean muscle tissue contains about 75% water. Blood contains 83% water, body fat contains 25% water and bone has 22% water.
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 hormones, including anti-diuretic hormone (ADH), aldosterone and atrial natriuretic peptide.
There are many methods that can be used to determine body water. One way to get a simple estimate is by calculation.

# Calculation of body water
In individuals of normal weight, water is abundant in most parts of the body, except in adipose tissue (fat). These calculations are for adults of average build, and are inappropriate for obese or overly muscular people. These proportions are very simplified and use round numbers for quick calculation.
In men about 69% of the body mass is water. This value is about 55% in women due to a higher proportion of body fat. This is the total body water.
Body water is broken down into the following compartments:
- Intracellular fluid (2/3 of Body Water)
- Extracellular fluid (1/3 of Body Water)
Plasma (1/4 of Extracellular fluid)
Interstitial fluid (3/4 of Extracellular fluid)
Transcellular fluid (normally ignored in calculations)
Contained inside organs, such as the gastrointestinal, cerebrospinal, and ocular fluids.
- Plasma (1/4 of Extracellular fluid)
- Interstitial fluid (3/4 of Extracellular fluid)
- Transcellular fluid (normally ignored in calculations)
Contained inside organs, such as the gastrointestinal, cerebrospinal, and ocular fluids.
- Contained inside organs, such as the gastrointestinal, cerebrospinal, and ocular fluids.

# Measurement of body water

## Dilution and equilibration
Total body water can be determined using Flowing afterglow mass spectrometry  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:
- total body water: tritiated water or deuterium
- extracellular fluid: inulin
- blood plasma: Evans blue

## Bioelectrical impedance analysis
Another method of determining total body water percentage (TBW%) is via Bioelectrical Impedance Analysis (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. 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.
Ingesting pure water has been proven to cause unrepeatable and erroneous results with the BIA system.  This is according to RJL Systems 

# Conditions associated with abnormal body water
- Renal failure
- Obesity
- Third Spacing