Disorders of Nutrition and Metabolism

Two-thirds of body weight correspond to water. A person weighing 68 kg has approximately 38 liters of water in the body. Of these, between 23 and 26 are inside the cells, 7.5 in the surrounding space and a slightly less than 4 liters (about 8 percent of total water quantity) is in the blood. This volume of water, relatively small in the blood, it is very important for the functioning of the body and must remain constant. The water outside the blood, acts as a reservoir to replace or absorb excess water from the blood, if necessary.

Water enters the body primarily by absorption from the gastrointestinal tract and leaves as the kidneys excrete urine. They can excrete several liters of urine per day, or conserve water by excreting less than 500 milliliters per day. About 750 milliliters of water are lost each day by evaporation through the skin and lungs. The intense sweating, such as during vigorous exercise or in hot climates, may substantially increase the volume lost to evaporation. Normally, little water is lost through the digestive system, but may lose an amount not less than 4 liters per day by protracted vomiting or severe diarrhea.

When water consumption offset the amount lost, the body's water is in equilibrium. To maintain this balance, healthy people with normally functioning kidneys and excessive sweat should not drink at least a quart of fluid a day.

However, it is recommended that healthy adults drink 1.5 to 2 liters a day to protect them from dehydration and the development of kidney stones.

When the brain and kidneys function properly, the body can cope with extreme changes in water consumption. Usually you drink enough to compensate for excessive water loss and thereby maintain blood volume and concentration of dissolved minerals (electrolytes) in blood. However, a person can become dehydrated if you do not drink enough water to compensate for the excessive loss, as in case of prolonged vomiting or severe diarrhea.