The Facts
Two thirds of the human body is made up of water. That means that if a person weighs about 154 pounds, their body contains about 12 gallons 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 of fluid a day 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.
Some dehydration is relatively mild, but it can be potentially life threatening. When the body's supply of fluids falls below a certain amount, a condition called hypovolemic shock may result.
Dehydration is a serious problem in young children and infants due to their larger skin surface area and can occur even after a few hours of vomiting or diarrhea or even profuse sweating.