One of the most important is the thirst mechanism. Nerve centers deep in the brain are stimulated when the body needs more water, resulting in the sensation of thirst. The sensation becomes stronger as the body's need for water increases, motivating a person to drink and replace the needed water. Another mechanism for controlling the amount of water in the body involves the pituitary gland at the base of the brain. When the body is low in water, the pituitary gland secretes a substance into the bloodstream called antidiuretic hormone.
Antidiuretic hormone stimulates the kidneys to retain as much water as possible.
When the body has insufficient water, the kidneys conserve water, while water automatically moves from the large reservoir in the cells to the bloodstream to maintain blood volume and blood pressure until the water can be replaced by increased intake. When the body has excess water, thirst is suppressed and the pituitary gland produces very little antidiuretic hormone, allowing the kidneys to excrete the excess water in the urine. Water enters the body primarily by absorption from the gastrointestinal tract. Water leaves the body primarily as urine excreted from the kidneys. The kidneys can excrete as much as several gallons of urine a day or can conserve water by excreting less than a pint a day.