Water and Electrolyte Balance Help

By — McGraw-Hill Professional
Updated on Aug 18, 2011

Distribution of Water in the Body

Water is the most abundant substance in the human body, ranging from 40 percent–80 percent of total body weight (BW). All metabolic reactions require water. Body water is distributed between two major compartments: the intracellular fluid compartment (within the cells, 35 percent–40 percent BW) and the extracellular fluid compartment (outside the cells, 15 percent–20 percent BW). Extracellular fluid is distributed throughout the body.

Water functions in regulating body temperature, participating in hydrolysis reations, lubricating organs, providing cellular turgidity, and maintaining body homeostasis.

Solute Concentrations

Two measures are used to describe solute concentration:

Percent solution = (grams of solute) / (100 ml of solution)

    = (grams of solute / (dl of solution)

Molarity (molar concentration): MW is the molecular weight of the solute. One mole of solute weighs MW grams, from which

Fluid balance in the extracellular compartment is maintained by regulation of fluid osmolarity. Osmolarity of a body fluid is a measure of the concentration of individual solute particles dissolved in it. The osmolarities of extracellular and intracellular fluids are normally the same.

Fluid Balance

Under normal conditions, fluid intake equals fluid output, so that the body maintains a constant volume. When water intake is greater than water output, a positive balance exists (hydration). Conversely, when output exceeds intake, a negative balance exists (dehydration). The amount of water consumed and the amount of urine formed are the two major mechanisms by which body water content is regulated.

Water is unconsciously regulated through the action of osmoreceptors located in the hypothalamus. These receptors sense the osmolality of the blood and determine whether more or less water is needed to maintain the correct osmolality. If blood is too concentrated, thirst is stimulated and we drink. ADH is released from the posterior pituitiary, which leads to conservation of body fluid in the collecting ducts of the kidneys, and decreased urine output. If blood is too dilute, thirst is suppressed and ADH release is inhibited, causing large volumes of dilute urine to be excreted.

A person may eliminate up to a liter of water over a 24-hour period without being aware of the loss. This is termed insensible loss. The loss occurs from the lungs and nonsweating skin.

When there is a loss of free water in the extracellular compartment, the fluid becomes too concentrated (increased osmolarity) and is referred to as being hypertonic. When there is a gain in free water, the fluid becomes too dilute and is referred to as being hypotonic.


Electrolytes are chemicals formed by ionic bonding that dissociate into electrically charged ions (cations and anions) when they dissolve in the body fluids. Examples of electrolytes are acids, bases, and salts. Nonelectrolytes are formed by covalent bonding. Most organic molecules are nonelectrolytes.

Practice problems for these concepts can be found at:

Water and Electrolyte Balance Practice Problems

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