Water is a colorless and odorless liquid made up of molecules containing two atoms of hydrogen and one atom of oxygen . Water is essential for all life to exist, as it makes up more than 70 percent of most living things. While a human can survive more than a week without food, a person will die within a few days without water.
Water serves as a solvent for nutrients and delivers nutrients to cells, while it also helps the body eliminate waste products from the cells. Both the spaces between cells (intercellular spaces) and the spaces inside cells (intracellular spaces) are filled with water. Water lubricates joints and acts as shock absorbers inside the eyes and spinal cord. Amniotic fluid, which is largely water, protects the fetus from bumps and knocks.
Water also helps the body maintain a constant temperature by acting as a thermostat. When a person is too hot, whether from being in a hot environment or from intense physical activity, the body sweats. When sweat evaporates, it lowers the body temperature and restores homeostasis .
Sources of Water
About 70 percent of the earth's surface is covered with water. The amount of water in a human body depends on age, gender, body type, and level of physical activity. The bodies of infants up to about twelve months of age contain about 58 percent water; the bodies of children six to seven years of age are 62 percent water; teenage boys are about 59 percent water; and teenage girls are about 45 percent water. The body of an adult male is approximately 62 percent water, while an adult female is 51 percent water. Physically active individuals generally have more water in their bodies than those who are less physically active. Because they sweat more, active people need to replenish water more often, thus raising their water level. A trained male runner may have up to 71 percent water in his body, while a female gymnast may have 70 percent. Obese individuals, on the other hand, have a lower percentage of water in their bodies (about 48%). Morbidly
The most efficient way for the body to get water is for a person to drink water. It is recommended that an adult drink eight to ten eight-ounce glasses of water a day. Athletes and active teens should drink at least ten to twelve glasses of water daily. However, many foods and beverages contain water, which can make up part of this daily intake. Fresh fruits and vegetables, cooked vegetables, canned and frozen fruits, soups, stews, juices, and milk are all sources of water. Most fruits and vegetables contain up to 90 percent water, while meats and cheeses contain at least 50 percent. Metabolic processes in the human body generate about 2.5 liters of water daily.
Water balance refers to the balance between the amount of water consumed and the amount of water excreted. The body's water content needs to be constant for optimal functioning. Cells are bathed in interstitial fluids (fluids from between cells) that contain nutrients. These fluids also carry metabolic wastes away from the cells. Intracellular fluids facilitate chemical reactions inside the cells, and they help maintain cell structure by adhering to the cell's larger molecules, such as proteins and glycogen . Body fluids contain solutes (chemical compounds that are soluble in water), which separate into charged particles, or ions, when dissolved in water. Intracellular fluids are high in potassium and phosphate ions, while interstitial fluids are high in sodium and chloride ions. These ions help to maintain the amount of fluids both within and outside the cells. Water molecules follow the solutes moving across cell membranes from a lower to higher solute concentration to maintain homeostasis.
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 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.
Kweethai C. Neill
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.