<Li> Interstitial fluid (4 / 5 of extracellular fluid) </Li> <Li> Transcellular fluid (a.k.a. "third space," normally ignored in calculations) contained inside organs, such as the gastrointestinal, cerebrospinal, peritoneal, and ocular fluids . </Li> <P> Total body water can be determined using Flowing afterglow mass spectrometry measurement of deuterium abundance in breath samples from individuals . A known dose of deuterated water (Heavy water, D O) 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 D O ingested . </P> <P> Different substances can be used to measure different fluid compartments: </P>

Total body water is not a function of which of the following