Fluid Balance Health Article

Definition

When water intake equals water loss, the body is in fluid balance. When water loss is greater than intake, or vice versa, a fluid imbalance may result.

Description

Total body water content averages 60% of body weight in young normal-weight men and is slightly lower in women at approximately 55%. Total body water can be significantly lower in obese individuals and the elderly. Approximately two-thirds of the body water is located inside the cells (intracellular) while the remaining is out-side the cells (extracellular). About three-quarters of the extracellular fluid is present in the interstitial space and connective tissues surrounding cells, while the remainder is intravascular. Approximately 8% of body water is in the bloodstream. This fairly small volume of water in the bloodstream must be kept relatively constant because it is critical for proper body function.

KEY TERMS

Antidiuretic hormone—A hormone that encourages the kidneys to retain water when body stores are low.

Dehydration—A deficit of body water that results when the output of water exceeds intake.

Diuretic—An agent or drug that eliminates excessive water in the body by increasing the flow of urine.

Electrolyte—A substance that dissociates into electrically charged atoms (ions) when dissolved in water.

Homeostasis—An organism's regulation of body processes to maintain internal equilibrium in temperature and fluid content.

Overhydration—An excess of body water that results when water intake exceeds output.

Function

The kidney is responsible for maintaining fluid balance through the elimination of waste products and excess water. Water is primarily absorbed through the gastrointestinal tract and excreted by the kidneys as urine. Water intake can vary widely on a daily basis, influenced by such factors as access to water, thirst, habit, and cultural factors. The variation in water volume ingested is dependent on the ability of kidneys to dilute and concentrate the urine as needed. There is a reservoir of water outside of the bloodstream that can replace or absorb excess water in the blood when necessary.

The body also works to maintain water balance through mechanisms such as the thirst sensation. When the body requires more water, nerve centers in the hypothalamus of the brain are stimulated to encourage a person to drink in order to replenish the water stores. The pituitary gland in the brain is also involved in maintaining fluid balance through secretion of antidiuretic hormone (ADH) into the blood. This hormone encourages the kidney to retain water when body stores are low. During water conservation by the kidneys, water is transferred from a large reservoir in the cells into the blood in order to maintain blood pressure and blood volume until water intake is increased. The thirst mechanism is suppressed when the body has excess water, small amounts of ADH are secreted, and through function of the kidneys, excess water is excreted in the urine.