Two-thirds of the human body is made up of water. That means that if a person weighs about 154 pounds (70 kilograms), their body contains about 12.5 gallons (47 liters) of water. Almost 70% of this water is inside the body's cells, 20% is in the space surrounding cells, and slightly less than 10% is in the bloodstream. The water in the human body is essential to keeping it healthy.
When the amount of water you intake matches the water you excrete, the body's water supply will be balanced. If you are healthy and do not sweat excessively, you should drink at least 2 to 3 quarts (2 to 3 liters) of fluid a day (about 8 glasses of water) to maintain your water balance and protect against the development of kidney stones.
Dehydration results when there is a deficiency in the body's water supply. If the brain and kidneys are functioning properly, the body will be able to manage minor changes in water intake. It's usually possible to drink enough water to make up for any water loss. However, it may be difficult to drink enough water if you are vomiting, have severe diarrhea, are exposed to excessive heat, or have a fever.
Dehydration is common among seniors, infants, and children. Seniors sense thirst more slowly so they may not recognize that they are becoming dehydrated and not drink enough fluids. Infants and young children lose more fluid during diarrhea or vomiting than older children and adults.
Some dehydration is relatively mild; however, severe loss of the body's supply of fluids can be potentially life-threatening. When the body's supply of fluids falls below a certain amount, a condition called hypovolemic shock may result.
Some of the conditions that can result in extreme fluid loss are:
Gastroenteritis: If the gastrointestinal tract becomes infected or inflamed by a virus or bacteria that results in vomiting or severe diarrhea, dehydration may develop. Gastroenteritis usually does not last for more than 36 hours.
Cholera: Consuming water or foods that have been contaminated by human wastes infected with the cholera bacterium will cause vomiting and severe diarrhea and can lead to dehydration, hypovolemic shock, and, in some cases, death.
Excessive use of certain medications: Diuretics, also known as "water pills," stimulate the body to increase the rate and volume of urination and the loss of electrolytes (dissolved mineral salts such as sodium, potassium, and chloride) in the urine. Although problems are uncommon when diuretics are used and monitored properly, overuse combined with a low-salt diet may cause dehydration.
Bulimia: Bulimia nervosa is an eating disorder that involves uncontrolled or compulsive binge eating. Most often, the condition also involves purging through self-induced vomiting, laxatives, enemas, diuretics, or excessive exercising. All types of purging can result in dehydration.
Addisonian crisis: The adrenal gland, a gland located above the kidneys, produces steroid hormones that are involved in keeping the body's water in balance. Although it's rare, the adrenal gland can fail and cause a disease called Addison's disease, which can result in excessive loss of fluid and electrolytes and lead to dehydration.