Two-thirds of the human body is made up of water. That means that if a person weighs about 70 kilograms (154 pounds), their body contains about 47 litres 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 consume at least 2 to 3 litres of fluid a day (about 8 glasses of water) to maintain your water balance and protect against complications, like 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 a regular day's 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 in turn may not drink enough fluids. Infants and young children lose proportionately more fluid from 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.