Diseases & Conditions
Water accounts for about one half to two thirds of an average person's weight. Fat tissue has a lower percentage of water and women tend to have more fat, so the percentage of water in the average woman is lower (52 to 55%) than it is in the average man (60%). The percentage of water is also lower in older people and in obese people. The percentage of water is higher (70%) at birth and in early childhood. A 150-pound (68-kilogram) man has about 10 gallons (38 liters) of water in his body: 6 to 7 gallons (23 to 27 liters) inside the cells, 2 gallons (about 7 liters) in the space around the cells, and slightly less than 1 gallon (4 liters, or about 8% of the total amount of water) in the bloodstream. The body regulates the amount of water in each of these areas. Water is moved as needed to keep the amount in each area relatively constant, thus enabling the body to function normally.
Water intake must balance water loss. To maintain water balance—and to protect against dehydration, the development of kidney stones, and other medical problems—healthy adults should drink at least 1½ to 2 quarts (about 2 liters) of fluids a day. Drinking too much is usually better than drinking too little, because excreting excess water is much easier for the body than conserving water. However, when the kidneys are functioning normally, the body can handle wide variations in fluid intake.
The body obtains water primarily by absorbing it from the digestive tract. Additionally, a small amount of water is produced when the body processes (metabolizes) certain nutrients.
The body loses water primarily by excreting it in urine from the kidneys. Depending on the body's needs, the kidneys may excrete less than a pint or up to several gallons of urine a day. Additionally, about 1½ pints (a little less than a liter) of water are lost daily when water evaporates from the skin and is breathed out by the lungs. Profuse sweating—which may be caused by vigorous exercise, hot weather, or a fever—can dramatically increase the amount of water lost through evaporation. Normally, little water is lost from the digestive tract. However, prolonged vomiting or severe diarrhea can result in the loss of a gallon or more a day.
Usually, people can drink enough fluids to compensate for excess water loss. However, people who have severe vomiting or diarrhea may feel too ill to drink enough fluids to compensate for water loss, and dehydration may result. Also, confusion, restricted mobility, or loss of consciousness can prevent people from being able to drink enough fluids.
Mineral salts (electrolytes), such as sodium and potassium, are dissolved in the water in the body. Water balance and electrolyte balance ( Minerals and Electrolytes: Electrolytes ) are closely linked. The body works to keep the total amount of water and the levels of electrolytes in the bloodstream constant. For example, when the sodium level becomes too high, thirst develops, leading to an increased intake of fluids. In addition, a hormone secreted by the brain in response to thirst causes the kidneys to excrete less urine. The combined effect is an increased amount of water in the bloodstream. As a result, sodium is diluted and the balance of sodium and water is restored. When the sodium level becomes too low, the kidneys excrete more urine, which decreases the amount of water in the bloodstream, again restoring the balance.
A Careful Balancing Act
In the body, several mechanisms work together to maintain water balance. One of the most important is thirst. When the body needs water, nerve centers deep within the brain are stimulated, resulting in the sensation of thirst. The sensation becomes stronger as the body's need for water increases, motivating a person to drink the needed fluids. When the body has excess water, thirst is suppressed.
Another mechanism for maintaining water balance involves the pituitary gland (located at the base of the brain) and the kidneys. When the body is low in water, the pituitary gland secretes antidiuretic hormone (also called vasopressin) into the bloodstream. Antidiuretic hormone stimulates the kidneys to conserve water and excrete less urine. When the body has excess water, the pituitary gland secretes little antidiuretic hormone, enabling the kidneys to excrete excess water in the urine.
The body can move water from one area to another as needed. When water loss is severe, the amount of water in the bloodstream decreases, so the body moves water from inside the cells to the bloodstream until it can be replaced through increased intake of fluids. When the body has excess water, the amount of water in the bloodstream increases, so the body moves water from the bloodstream into and around the cells. In this way, blood volume and blood pressure can be kept relatively constant.
Source: The Merck Manual Home Edition