Water accounts for about one half to two thirds of an average person’s weight. Fat tissue has a lower percentage of water than lean tissue and women tend to have more fat, so the percentage of body weight that is water in the average woman is lower (52 to 55%) than it is in the average man (60%). The percentage of body weight that is water is also lower in older people and in obese people. The percentage of body weight that is water is higher (70%) at birth and in early childhood. A 154-pound (70-kilogram) man has a little over 10.5 gallons (42 liters) of water in his body: 7 gallons (28 liters) inside the cells, 2.5 gallons (about 10.5 liters) in the space around the cells, and slightly less than 1 gallon (3.5 liters, or about 8% of the total amount of water) in the blood.
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 (about half a liter to over 10 liters) of urine a day. 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 high body temperature—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 impaired consciousness can prevent people from sensing thirst or 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 are closely linked. The body works to keep the total amount of water and the levels of electrolytes in the blood constant. For example, when the sodium level becomes too high, thirst develops, leading to an increased intake of fluids. In addition, vasopressin (also called antidiuretic hormone), a hormone secreted by the pituitary gland (a pea-sized gland at the base of the brain) in response to dehydration, causes the kidneys to excrete less water. The combined effect is an increased amount of water in the blood. 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 water, which decreases the amount of water in the blood, again restoring the balance.
In the body, several mechanisms work together to maintain water balance. These include
Thirst is one of the most important mechanisms to maintain water balance. 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.
An interaction between the pituitary gland and the kidneys provides another mechanism. When the body is low in water, the pituitary gland secretes vasopressin (also called antidiuretic hormone) into the bloodstream. Vasopressin stimulates the kidneys to conserve water and excrete less urine. When the body has excess water, the pituitary gland secretes little vasopressin, enabling the kidneys to excrete excess water in the urine.
In osmosis, water flows passively from one area or compartment of the body to another. This passive flow allows the larger volumes of fluid in the cells and the area around the cells to act as reservoirs to protect the more critical but smaller volume of fluid in the blood vessels from dehydration.