In medicine, body water is all of the water content of the human body. A significant fraction of the human body is water.

Guyton's Textbook of Medical Physiology states that "the total amount of water in a man of average weight (70 kilograms) is approximately 40 liters, 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%.

Skin also contains much water. The human body is about 60% water in adult males and 55% in adult females.

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 to determine body water. One way to get a simple estimate is by calculation.

Per Netter's Atlas of Human Physiology, body water is broken down into the following compartments:

• Intracellular fluid (2/3 of body water). Per Guyton, in a body containing 40 liters of fluid, about 25 liters is intracellular, which amounts to 62.5% (5/8), close enough to the 2/3 rule of thumb. Jackson's texts states 70% of body fluid is intracellular.
• Extracellular fluid (1/3 of body water). Per Guyton's illustration, for a 40 litre body, about 15 litres is extracellular, which amounts to 37.5% Again, this is close to the 1/3 rule of thumb cited here.
• • Plasma (1/5 of extracellular fluid). Per Guyton's illustration, of the 15 litres of extracellular fluid, plasma volume averages 3 litres. This amounts to 20%, the same as per Netter's Atlas.
  • Interstitial fluid (4/5 of extracellular fluid)
  • Transcellular fluid (a.k.a. "third space," normally ignored in calculations)
  • • Contained inside organs, such as the gastrointestinal, cerebrospinal, peritoneal, and ocular fluids.

Measurement of body water

Dilution and equilibration

Total body water can be determined using Flowing afterglow mass spectrometry [http://www.fa-ms.com 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.

Different substances can be used to measure different fluid compartments:

• total body water: tritiated water or heavy water.
• extracellular fluid: inulin
• blood plasma: Evans blue

Intracellular fluid may then be estimated by subtracting extracellular fluid from total body water.

Bioelectrical impedance analysis

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 current of 50 kHz is introduced. 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.

Fluid loss

Volume contraction is a decrease in body fluid volume, with or without a concomitant loss of osmolytes. The loss of the body water component of body fluid is specifically termed dehydration.

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.

osmosis , transfer of a liquid solvent through a semipermeable membrane that does not allow dissolved solids (solutes) to pass. Osmosis refers only to transfer of solvent; transfer of solute is called dialysis . In either case the direction of transfer is from the area of higher concentration of the material transferred to the area of lower concentration. This spontaneous migration of a material from a region of higher concentration to a region of lower concentration is called diffusion . Principles of Osmosis Osmosis will occur if a vessel is separated into two compartments by a semipermeable membrane, both compartments are filled to the same level with a solvent, and solute is added to one side. The level of the liquid on the side containing the solute will rise as the solvent flows from the side of its higher concentration to the side of lower concentration. If an external pressure is exerted on the side containing the solute, the transfer of solvent can be stopped and even reversed (reverse osmosis). Two solutions separated by a semipermeable membrane are said to be isotonic if no osmosis occurs. If osmosis occurs, transfer of solvent is from the hypotonic solution to the hypertonic solution, which has the higher osmotic pressure. The minimum pressure necessary to stop solvent transfer is called the osmotic pressure. Since the osmotic pressure is related to the concentration of solute particles, there is a mathematical relationship between osmotic pressure, freezing-point depression, and boiling-point elevation. Properties such as osmotic pressure, freezing point, and boiling point, which depend on the number of particles present rather than on their size or chemical nature, are called colligative properties. For dilute solutions the mathematical relationship between the osmotic pressure, temperature, and concentration of solute is much like the relation between pressure, temperature, and volume in an ideal gas (see gas laws ). A number of theories explaining osmotic pressure by analogy to gases have been devised, but most have been discarded in favor of thermodynamic interpretations using such concepts as the entropy of dilution. Biological Importance of Osmosis Osmosis and dialysis are of prime importance in living organisms, where they influence the distribution of nutrients and the release of metabolic waste products. Living cells of both plants and animals are enclosed by a semipermeable membrane called the cell membrane, which regulates the flow of liquids and of dissolved solids and gases into and out of the cell. The membrane forms a selective barrier between the cell and its environment; not all substances can pass through the membrane with equal facility. Without this selectivity, the substances necessary to the life of the cell would diffuse uniformly into the cell's surroundings, and toxic materials from the surroundings would enter the cell. If blood cells (or other cells) are placed in contact with an isotonic solution, they will neither shrink nor swell. If the solution is hypertonic, the cells will lose water and shrink (plasmolyze). If the solution is hypotonic (or if pure solvent is used) the cells will swell; the osmotic pressure that is developed may even be great enough to rupture the cell membrane. Saltwater from the ocean is hypertonic to the cells of the human body; the drinking of ocean water dehydrates body tissues instead of quenching thirst. In plants osmosis is at least partially responsible for the absorption of soil water by root hairs and for the elevation of the liquid to the leaves of the plant. However, plants wilt when watered with saltwater or treated with too much fertilizer, since the soil around their roots then becomes hypertonic.