Fluid Balance
FLUID BALANCE
A large part of an individual's body weight is made up of water and chemicals (e.g., sodium, potassium, and chloride), which are called electrolytes. Although the proportions of these electrolytes are tightly regulated throughout life, as people age the relative amount of body weight made up by water changes. In the normal young adult, the capacity of the kidney to regulate fluid and electrolyte balance far exceeds the ordinary demands for conservation and excretion. Even when this capacity is substantially reduced in old age, renal function allows adequate regulation of the volume and composition of the body's fluids under most normal conditions. Inability to maintain normal fluid volumes and electrolyte concentrations is generally due to causes (defects) outside the kidney necessary for their regulation (homeostasis) rather than to insufficient kidney function.
Dehydration, by definition, means a decrease in total body water. Sodium is the primary electrolyte outside the cell (extracellular). This electrolyte has a positive charge (cation) that must be matched to a negatively charged electrolyte (anion), for example, chloride or bicarbonate, and is responsible for maintaining the state of hydration outside the cell. Dehydration may develop from a primary loss of water (insufficient intake of water, excessive sweating, vomiting or diarrhea, or an inability to concentrate the urine), in which case the concentration of sodium in the blood (serum) increases (hypernatremia). Dehydration also may occur when one loses salt (sodium chloride) with its obligated water. This can occur with excessive intestinal and urinary losses containing salt, certain hormone (adrenal) insufficiencies, and with excessive use of diuretic medications. Individuals with this condition become volume depleted, but maintain normal serum sodium concentrations (136–144 mEq/L) until blood volume becomes sufficiently depleted that antidiuretic hormone (ADH) release from the pituitary gland is stimulated. ADH is the hormone primarily responsible for reducing urine volume to conserve water, and for turning a dilute urine into a concentrated urine. If fluid is then replaced, water is retained and the concentration of sodium in the blood falls (hyponatremia).
Older persons are more prone to the development of dehydration. This dehydration is due to a combination of three factors. First, there is an inability of the kidney to conserve sodium when challenged by inadequate intake or excessive losses elsewhere. Second, there is an inability to concentrate the urine as well when similarly challenged by fluid loss. Finally and most importantly, the older person loses normal thirst. Whereas a young person becomes acutely thirsty when dehydrated and drinks to correct for the water loss, an older person often is not similarly motivated. If this becomes severe enough, especially in those with cerebrovascular disease, ingestion of a prescribed amount of water (generally 1–2 quarts) each day may become necessary to prevent recurrent dehydration. Dehydration is most likely to occur after hospital admission for acute illness, for example, infection or emergency surgery, when fluid replacement is often insufficient. Overhydration occurs when an individual retains too much salt and water. This results in edema that can be identified by applying pressure over the shin and creating an indentation that does not quickly return to normal. Usually salt and water are retained in proportionate amounts so serum sodium concentration remains normal. Olderpersons lose the ability to excrete in the urine large amounts of salt.
Older persons also are more prone to the development of low levels of sodium. This can be seen with a decreased (contracted) volume outside the cells (extracellular fluid volume) due to salt depletion followed by stimulation of ADH release to retain water and dilute the blood as described above. Sometimes it is due to dilution, when extracellular fluid volume becomes increased due to an inability to excrete water normally. This is seen with congestive heart failure, and end-stage kidney or liver failure, and is characterized by swelling of the legs (edema) and abdomen (ascites). Most commonly, it is seen when ADH continues to be secreted when the normal stimuli are no longer present. This causes retention of water that, in turn, causes dilution of sodium in the blood. The normal stimuli to secretion of ADH are either a decrease in blood (extracellular fluid) volume, or an increase in the concentration of sodium or solute (osmolality) in the blood. When high levels of ADH persist in the presence of both an increased volume and low concentrations of sodium or solute (both of which should shut off ADH), this is referred to as the syndrome of inappropriate antidiuretic hormone (SIADH). This is often due to pain, but can be due to a tumor (most commonly lung) that independently makes the hormone or something that acts like the hormone; failure of the receptors in the left side of the heart to get the message that blood volume is sufficient, for example, poor blood flow through the lungs due to tuberculosis or pneumonia; or pathology in the brain that abnormally stimulates the release of the hormone. In some cases, generally very old and frail elders, the cause remains unknown (idiopathic). Older persons secrete more ADH in response to any given increase in serum osmolality than younger persons, perhaps helping to explain the greater propensity of older persons to develop hyponatremia. Symptoms consisting of confusion and lethargy progressing to coma and seizures are caused by brain swelling. One can treat this condition with water restriction, or with drugs that interfere with the ability to concentrate the urine (furosemide, lithium, declomycin).
Potassium is the primary cation inside the cell with only 2 percent of total body potassium found outside the cell. Because there is a steep concentration gradient for potassium between inside and outside the cell, the serum concentration (normally between 3.5 and 5.0 mEq/L) is not always an accurate measure of potassium in the body. However, the serum potassium concentration usually determines whether or not there are going to be problems with a deficit or excess of this electrolyte in the body. Low serum potassium concentrations (hypokalemia) can be due to inadequate intake, losses from the intestine (vomiting, diarrhea), or losses from the kidney (renal and adrenal causes). The most common cause is diuretic therapy, but also common in the elderly is excessive use of enemas and purgatives, often overlooked unless the individual is specifically questioned about this. Muscular weakness and pain (cramps) can be early symptoms. Potassium replacement is available in a variety of powder, liquid, and pill forms, but requires a physician's prescription to avoid potentially lethal complications from overdosage.
A high serum potassium concentration (hyperkalemia) can be caused by excessive intake, release of potassium from cells due to tissue breakdown (catabolism), inability of the kidney to excrete potassium, or often a combination of factors. Older persons are much more likely to develop hyperkalemia than younger persons for several reasons. First, they are more likely to have impaired kidney function limiting their ability to excrete potassium in the urine. Second, they secrete less of the adrenal hormone aldosterone into the circulation. This is the hormone that aids secretion of potassium into the urine. A number of medications commonly used by elders (potassium-sparing diuretics; beta-adrenergic blocking agents; nonsteroidal anti-inflammatory agents, or NSAIDs; and angiotensin converting enzyme (ACE) inhibitors) also impair the ability of the kidney to excrete potassium. The symptoms of hyperkalemia are very subtle (anxiety, restlessness, apprehension, weakness), and may precede potentially lethal cardiac arrythymias only briefly, making it important to check serum potassium concentrations periodically if risk is present, for example, during potassium replacement therapy.
Other important electrolytes in the blood and body include calcium and magnesium, both of which can cause symptoms when present in deficient or excessive amounts.
Robert D. Lindeman
See also Disease Presentation; Kidney, Aging.
BIBLIOGRAPHY
Lindeman, R. D. "Renal and Electrolyte Disorders." In Practice of Geriatrics, 3d ed. Edited by E. H. Duthie, Jr. and P. R. Katz. Philadelphia: W. B. Saunders Co., 1998. Pages 546–561.
Zawada, E. T., Jr. "Disorders of Water and Electrolyte Balance." In The Merck Manual of Geriatrics, 3d ed. Edited by M. H. Beersand and R. Berkow. Whitehouse Station, N.J.: Merck Research Laboratories, 2000. Pages 561–571.
Fluid Balance
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 —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.
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.
Resources
BOOKS
Shaw, Patricia, ed. Fluids & Electrolytes Made Incredibly Easy! Springhouse, PA: Springhouse Publishing Co.,1997.
Speakman, Elizabeth and Weldy, Norma Jean. Body Fluids and Electrolytes, 8th ed. London, UK: Mosby Incorporated, 2001.
Workman, M. Linda. Introduction to Fluids, Electrolytes and Acid-Base Balance. London, UK: W. B. Saunders Co.,2001.
PERIODICALS
Beck, L. H. "The aging kidney. Defending a delicate balance of fluid and electrolytes." Geriatrics 55, no. 4 (2000): 26-28, 31-32.
Gennari, F. J. "Hypokalemia." New England Journal of Medicine 339 (1998): 451-458.
Sawka, M. N. and S. J. Montain. "Fluid and electrolyte supplementation for exercise heat stress." American Journal of Clinical Nutrition 72, Suppl. 2 (2000): 564S-572S.
OTHER
Nr-Space, et al. Fluids & Electrolytes CD-ROM Delmar Publishers, 2001.
Crystal Heather Kaczkowski, MSc.
Fluid Balance
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 outside 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.
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 diseases such 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 drinks such 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 over-hydration 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 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.
KEY TERMS
Antidiuretic hormone— A hormone that encourages the kidney 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.
Resources
BOOKS
Shaw, Patricia, ed. Fluids & Electrolytes Made Incredibly Easy! Springhouse, PA: Springhouse Publishing Co., 1997.
Speakman, Elizabeth and Weldy, Norma Jean. Body Fluids and Electrolytes, 8th ed. London: Mosby Incorporated, 2001.
Workman, M. Linda Introduction to Fluids, Electrolytes and Acid-Base Balance. London: W. B. Saunders Co., 2001.
PERIODICALS
Beck, L. H. "The aging kidney. Defending a delicate balance of fluid and electrolytes." Geriatrics 55, no. 4 (2000): 26-28, 31-32.
Gennari, F. J. "Hypokalemia." New England Journal of Medicine 339 (1998): 451-458.
Sawka, M. N. and S. J. Montain. "Fluid and electrolyte supplementation for exercise heat stress." American Journal of Clinical Nutrition 72, Suppl. 2 (2000): 564S-572S.
OTHER
Nr-Space, et al. Fluids & Electrolytes CD-ROM Delmar Publishers, 2001.