The average daily requirement of water is approximately 2.5 liters. The amount needed to replace urine and other bodily losses is 1-1.5 liters. The kidney controls water loss by regulating the urine output. The normal healthy adult can survive on as little as 200 cc of water per day. This is the minimum amount needed to excrete waste products of metabolism. More is needed for people with less efficient kidneys, such as the elderly.
Other than urine, other losses of water are from the lungs and skin, average about 0.4-0.5 cc/kg/min, or 650-850 cc/day. Situations of increased water loss are:
2) Excessive exercise and sweating
3) Diarrhea and vomiting
4) Heat exposure.
Water intake is regulated by thirst. This is triggered by receptors in the brain, or the hypothalamus. This area of the brain responds to increased serum osmolality and decreased fluid volume. Water excretion by the kidneys is regulated by hormones such as anti-diuretic hormone, or ADH. ADH release is caused by
1) Increased serum osmolality,
2) Decreased blood volume
3) Decreased blood pressure
In these situations, ADH causes the kidney to decrease urine production. ADH release is decreased in certain conditions such as alcohol intake and diabetes insipidus. The kidneys can handle as much as 25 liters of fluid intake a day.