Body water

Measurement

Dilution and equilibration

An individual’s total body water can be determined using flowing-afterglow mass spectrometry (FA-MS) to measure the abundance of deuterium in breath samples. A known dose of deuterated water (heavy water, D₂O) is ingested and allowed to equilibrate within the body water. Then, the FA-MS instrument measures the ratio D:H of deuterium to hydrogen in the water vapour in exhaled breath. The total body water is then accurately measured from the increase in breath deuterium content in relation to the volume of D₂O ingested.

The water in individual compartments can be measured with different substances:

• 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 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 is limited in the ≈ 20 mA current range 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.

Functions

Water in the animal body performs a number of functions, acting as a:

• solvent for transportation of nutrients

• medium for excretion

• means for heat control

• lubricant for joints

• vector for shock absorption

Changes

The usual way of adding water to a body is by drinking. Water also enters the body with foods, especially those rich in water, such as plants, raw meat, and fish. About 10% of human adult water intake comes as a by-product of metabolism.

The amount of this water that is retained in animals is affected by several factors. Water amounts vary with the age of the animal. The older the vertebrate animal, the higher its relative bone mass and the lower its body water content.

In diseased states, where body water is affected, the fluid compartment or compartments that have changed can give clues to the nature of the problem, or problems. Body water is regulated by hormones, including antidiuretic hormone, aldosterone and atrial natriuretic peptide.

Loss of water

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.

Sodium loss approximately correlates with fluid loss from extracellular fluid, since sodium has a much higher concentration in extracellular fluid (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.

Calculation

In humans, total body water can be estimated based on the premorbid (or ideal) body weight and correction factor.

$${\displaystyle TBW=weight\ast C}$$

C is a coefficient for the expected percentage of weight made up of free water. For adult, non-elderly males, C = 0.6. For adult elderly males, malnourished males, or females, C = 0.5. For adult elderly or malnourished females, C = 0.45. A total body water deficit (TBWD) can then be approximated by the following formula:

$${\displaystyle TBWD=TBW\ast (1-\frac{[N{a}^{+}]t}{[N{a}^{+}]m})}$$

where [Na⁺]‍t = target sodium concentration (usually 140 mEq/L) and [Na⁺]‍m = measured sodium concentration.

The resultant value is the approximate volume of free water required to correct a hypernatremic state. In practice, the value rarely approximates the actual amount of free water required to correct a deficit due to insensible losses, urinary output, and differences in water distribution among patients.

Total water may also be estimated by use of anthropometric equations:

$${\displaystyle TB{W}_{\text{male}}\text{ (L)}=2.447-0.09516\times \text{age (years)}+0.1074\times \text{height (cm)}+0.3362\times \text{weight (kg)}}$$ $${\displaystyle TB{W}_{\text{female}}\text{ (L)}=-2.097+0.1069\times \text{height (cm)}+0.2466\times \text{weight (kg)}}$$

The Watson equations above have been found to give reasonable estimates in most cases, as a diverse set of subject data was used. Other equations have been given for specific populations, such as Americans, Tunisians, and Cameroonians. Anthropometric TBW equations do not generalize well beyond healthy adult subjects outside their measured population, and it has been recommended to use a more accurate method such as BIA for most clinical settings.