Water Health Article

Water Intake Regulation

When the body has lost a lot of water, the concentration of solutes in the blood becomes too high. The solutes attract water from the salivary glands, making the mouth dry and causing a person to feel thirsty. The sense of thirst is a craving for water or other fluids. When water loss is slow, a person may have time to feel thirsty enough to replenish the water loss. In cases where the water loss is excessive and acute, however, and replenishment is not adequate, a state of dehydration can occur. Dehydration is a state in which the body has lost so much water that normal physiologic functions cannot take place, resulting in symptoms such as fainting and nausea.

Heat, intense physical activity (profuse sweating), diarrhea, vomiting, and excessive urination can all cause excessive fluid loss. A runner can sweat off six cups of fluid in an hour. Mild dehydration occurs with a loss of 5 percent or less of a person's bodily fluids, moderate dehydration is a loss of 5 to 10 percent of a person's bodily fluids, and severe dehydration is a loss of 10 to 15 percent of fluids. Severe dehydration can cause death. Some clinical signs of dehydration include dry skin, less frequent urination, fatigue, light-headedness, dark-colored urine, dry mouth, and lack of skin elasticity. Often, increased fluid intake and replacement of lost electrolytes are sufficient oral rehydration therapy for mild dehydration. However, the cause of dehydration has to be addressed for further improvement. In cases of severe dehydration, it may be necessary to hospitalize the person and restore fluid balance through intravenous fluid replacement.

Water Excretion Regulation

The brain and kidneys regulate the amount of water excreted by the body. When the blood volume is low, the concentration of solutes in the blood is high. The brain responds to this situation by stimulating the pituitary gland to release an antidiuretic hormone (ADH), which signals the kidneys to reabsorb and recirculate water. When the individual needs more water, the kidneys will excrete less and even reabsorb some.

When excessive fluid loss occurs, the blood volume will fall, as will blood pressure. The kidneys respond by secreting an enzyme called rennin. Rennin activates the blood protein angiotensinogen to convert to angiotensin, which causes the blood vessels to constrict and blood pressure to rise. Angiotensin also activates the adrenal glands to release a hormone called aldosterone. Aldosterone causes the kidneys to retain sodium and water. When the body needs water, less is excreted and more is retained.

Water Intoxication

Water intoxication occurs when there is too much fluid in the body. Excess fluid may collect in bodily tissue, particularly in the feet and legs, a condition called edema. Excess consumption of fluids, as well as kidney disorders that reduce urine output, may contribute to water intoxication. The symptoms of water intoxication are confusion, convulsions, and, in extreme cases, death.

SEE ALSO DEHYDRATION; DIARRHEA; NUTRIENTS; ORAL REHYDRATION THERAPY.

Kweethai C. Neill

Bibliography

Whitney, Eleanor N., and Rolfes, Sharon R. (2002). Understanding Nutriton, 9th edition. Belmont, CA: Wadsworth/Thomson Learning.

Shils Maurice, E., and Young, Vernon R. (1988). Modern Nutrition in Health and Disease, 7th edition. Philadelphia, PA: Lea and Febinger.