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Arthur Guyton 's Textbook of Medical Physiology states that "the total amount of water in a man of average weight (70 kilograms) is approximately 40 litres, averaging 57 percent of his total body weight. In a newborn infant, this may be as high as 75 percent of the body weight, but it progressively decreases from birth to old age, most of the decrease occurring during the first 10 years of life. Also, obesity decreases the percentage of water in the body, sometimes to as low as 45 percent". These figures are statistical averages, so are illustrative, and like all biostatistics, will vary with things like type of population, age and number of people sampled, and methodology. So there is not, and cannot be, a figure that is exactly the same for all people, for this or any other physiological measure. For example, Jackson's (1985) Anatomy & Physiology for Nurses gives a figure of 60% for the proportion of body-weight attributable to water, which approximates Guyton's 57%.
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.
Per Netter's Atlas of Human Physiology, body water is broken down into the following compartments:
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, 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:
Intracellular fluid may then be estimated by subtracting extracellular fluid from total body water.
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 weak current of frequency 50 kHz is introduced. This AC waveform allows the creation of a current inside the body via the very capacitive skin without causing a DC flow or burns, and limited in the ~20mA range current for safety.
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.
Na+ loss approximately correlates with fluid loss from extracellular fluid (ECF), since Na+ has a much higher concentration in ECF than intracellular fluid (ICF). In contrast, K+ has a much higher concentration in ICF than ECF, and therefore its loss rather correlates with fluid loss from ICF, since K+ loss from ECF causes the K+ in ICF to diffuse out of the cells, dragging water with it by osmosis.