Fluid Balance

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

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

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

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

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

Overhydrationn 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.

Role in human health

For a normal adult, a daily intake between 0.74-0.84 US quarts (700-800 ml) is required to meet water losses and maintain fluid balance. To protect against dehydration and developing kidney stones, a greater water consumption between 1.5-2 US quarts/day (1.4-2 L/day) is advised. Water losses occur through evaporation in expired air and through the skin. Sweat losses are usually minimal but can be significant in warmer climates or with accompanying fever.

The body can accommodate extreme changes in water intake when the brain and kidneys are functioning normally. It is usually possible for a person to consume enough water to maintain blood volume and electrolyte balance in the blood. However, if a person is unable to consume enough water to equal excessive water loss, dehydration may result.

Common diseases and disorders

Dehydration

Dehydration is a deficit of body water that results when the output of water exceeds intake. Dehydration stimulates the thirst mechanism, instigating water consumption. Sweating and the output of urine both decrease. If water intake continues to fall short of water loss, dehydration worsens.

Causes of dehydration may include:

• vomiting
• diarrhea
• diuretics
• excessive heat
• excessive sweating
• fever
• decreased water intake

Dehydration induces water to move from the reservoir inside cells into the blood. If dehydration progresses, body tissues begin to dry out and the cells start to shrivel and malfunction. The most susceptible cells to dehydration are the brain cells. Mental confusion, one of the most common signs of severe dehydration, may result and can lead to coma. Dehydration can occur when excessive water is lost with such diseases as diabetes mellitus, diabetes insipidus, and Addison's disease.

Dehydration is often accompanied by a deficiency of electrolytes, sodium and potassium in particular. Water does not move as rapidly from the reservoir inside of the cells into the blood when electrolyte concentration is decreased. Blood pressure can decline due to a lower volume of water circulating in the bloodstream. A drop in blood pressure can cause lightheadedness, or a feeling of impending blackout, especially upon standing (orthostatic hypotension). Continued fluid and electrolyte imbalance may further reduce blood pressure, causing shock and damage to many internal organs including the brain, kidneys, and liver.

TREATMENT OF DEHYDRATION. Consumption of plain water is usually sufficient for mild dehydration, although when both water and electrolyte losses have occurred after vigorous exercise, electrolytes must be replaced; sodium and potassium in particular. Adding a little salt to drinking water or consuming such drinks as Gatorade during or following exercise can replace lost fluids. Individuals with heart or kidney problems should consult a physician regarding the replacement of fluids after exercise.

Sodium chloride may be administered intravenously by medical personnel if blood pressure decreases enough to induce shock or risk of shock. The underlying cause of dehydration must be addressed along with treatment to replace fluids. For example, if the cause of dehydration is due to diarrhea, then drugs may be given to alleviate diarrhea as well. Following treatment of the cause, individuals may be monitored to ensure that oral fluid intake is adequate to maintain fluid hydration.

Overhydration

Overhydration is an excess of body water that results when water intake exceeds output. Drinking large amounts of water does not typically lead to overhydration if the kidneys, heart, and pituitary gland are functioning properly. An adult would have to drink more than 2 US gallons per day (7.6 L per day) to exceed the body's ability to excrete water. Excessive body water causes electrolytes in the blood, including sodium, to become overly diluted. Overhydration occurs in individuals whose kidneys do not function normally, primarily in kidney, heart, or liver disease. People with these conditions may have to limit their water and dietary salt intake. Similar to dehydration, the brain is the most sensitive organ to overhydration. The brain cells can adapt to increased fluid volume when overhydration increases slowly; however, when it occurs rapidly, mental confusion, seizures, and coma can result.

Overhydration can occur alone or in conjunction with excess blood volume. Distinguishing between the two conditions may be quite complicated. Overhydration induces water accumulation within and around the cells but does not typically show symptoms of fluid accumulation. On the other hand, with excess blood volume, there is an accumulation of sodium and the body cannot transfer water into the reservoir within cells. Conditions such as heart failure and liver cirrhosis may induce volume overload, whereby fluid accumulates around cells in the abdomen, chest, and lower legs.

TREATMENT OF OVERHYDRATION. Treatment of overhydration depends somewhat upon the cause, although whatever the underlying condition, fluids must be limited. Drinking less than about 1 US quart (1 L) of fluid a day usually improves overhydration over several days. Fluids should only be limited at a physician's request. A diuretic may be prescribed to increase water output by the kidneys. Diuretics are particularly beneficial for treatment of excess blood volume where overhydration is accompanied by excess blood volume.

Crystal Heather Kaczkowski, MSc.