Plant and animals vary tremendously in their water requirements. Intake increases with size and, interestingly enough, in a mathematical way. Also of interest is the time required for a given member of a species to imbibe a quantity of water equal to its own weight. A mouse take 5 days; a cow, 2 weeks; a camel, 3 months; a tortoise, 1 year; a human being 1 month;and a cactus, 29 years.

Regardless of the amount of water taken in, however, the body normally balances the gain with an equivalent loss. Conversely, the body balances a loss with an equivalent gain. Either way, the central idea is balance. Humans possess phenomenal balance, in a 24 hour period a person's weight may vary less than 250 g.

Intake
A person's daily intake of water is said to average about 2.5 liters. This water comes not only from fluid but also from preformed water(water trapped in food) and oxidative water (water resulting from cellular respiration). Obviously, food intake has a great bearing on the need for imbibed fluid. The kangaroo ra, a desert creature, never takes a drink as long as it lives, deriving its water solely from solid food. We could do th same if we live on cucumbers and lettuce.

Output
As note, the daily output of water balances the daily intake. The kidneys carry the heavies load, excreting up to about 1.7 liters. The other channels, though not so apparent, are no less vital. From the skin and lungs, about 0.5 liter and 0.4 liter of water are lost, respectively. About 0.2 liter leaves the body in the feces. Water escapes from the lungs as vapor. Wator lost from the skin is usually vapor( insensible perspiration), but it build up into sweat when the body becomes overheated. Perspiration is about 99% water, with traces of salt and urea. In certain diseases other constituents may appear, such as bile pigments, albumin ,and sugar.