Body water
This water makes up a significant fraction of the human body, both by weight and by volume.
Ensuring the right amount of body water is part of fluid balance, an aspect of homeostasis.
However, this varied substantially by age, sex, and adiposity (amount of fat in body composition).
[6] Water is also contained inside organs, in gastrointestinal, cerebrospinal, peritoneal, and ocular fluids.
[7][8] In Netter's Atlas of Human Physiology (2002), body water is broken down into the following compartments:[6] An individual’s total body water can be determined using flowing-afterglow mass spectrometry (FA-MS) to measure the abundance of deuterium in breath samples.
Then, the FA-MS instrument measures the ratio D:H of deuterium to hydrogen in the water vapour in exhaled breath.
The total body water is then accurately measured from the increase in breath deuterium content in relation to the volume of D2O ingested.
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.
[12] 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.
New segmental BIA equipment that uses more electrodes may lead to more precise measurements of specific parts of the body.
The resultant value is the approximate volume of free water required to correct a hypernatremic state.
[18] Anthropometric TBW equations do not generalize well beyond healthy adult subjects outside their measured population, and it has been recommended to use a more accurate method such as BIA for most clinical settings.
[19] Water in the animal body performs a number of functions: as a solvent for transportation of nutrients; as a medium for excretion; a means for heat control; as a lubricant for joints; and for shock absorption.
The older the vertebrate animal, the higher its relative bone mass and the lower its body water content.
