Body water
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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 males about 60% 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.
Of this water, two-thirds of it is in the cells, and is called intracellular water. The other third is extracellular water.
The extracellular water is made up of blood plasma, which is one-fifth of extracellular water, and extracellular fluid constitutes the other four-fifths.
Water exists in other places, (such as in the eye) but this can be ignored as being of insignificant quantity in quick calculation.
Measurement of body water
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.