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. 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 60% of the body mass is water. This value is 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.