Introduction

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. A 150-pound man has about 10 gallons of water in his body: 6 to 7 gallons inside the cells, 2 gallons in the space around the cells, and slightly less than 1 gallon (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 1 ½ to 2 quarts of fluids a day. Drinking too much is 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. About 1½ pints 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, a person can drink enough fluids to compensate for excess water loss. However, a person may be unable to drink enough fluids to compensate for water loss caused by prolonged vomiting or severe diarrhea, and dehydration may result. Also, confusion, restricted mobility, or loss of consciousness can prevent a person 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 (see Section 12, Chapter 155) 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.

See the sidebar A Careful Balancing Act.