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Fluid can enter the body as preformed water, ingested food and drink, and to a lesser extent as metabolic water, which is produced as a biproduct of airobic respiration (cellular respiration) and dehydration synthesis. A constant supply is needed to replenish the fluids lost through normal psychological activities, such as respiration, sweating and urination. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and dessert animals, but it provides only a small fraction of a human's necessary intake, In the nromal resting state input of water ingested fluids is approximately 1200 ml/day, from ingested foods 1000 ml/dsy and from aerobic respiration 300 ml/day, totaling 2500 ml/ day.

Body water homeostasis is regulated mainly through ingested fluids which in turn depends on thirst. An insufficiency of water results in a increased osmolarityin the extracellular fluid. This is sensed by osmoreceptors which trigger the sensation of thirst. Thirst can to some degree be voluntarily resisted as during fluid restriction.

An osmoreceptor is a sensory receptor primarily found in th hypothalmus of most homeothermic organisms, that detect changes in osmotic pressure, osmoreceptors can be found in several structures, including the organum vasculosum of the lamina terminalis and the subfornical organ. They contribute to fluid balance in the body.

When the osmotic pressure of blood changes (it is more or less dilute) water diffusion into and out of the osmoreceptor cell changes. They expand when the blood plasma is more dilute and contract with higher concentration. This causes an afferent neural signal to be sent to the hypothalamus which increases or decreases vasopressin (ADH) secretion from the posterior pituitary to return blood concentration.

Reference

Regulation of Water Intake. (n.d). Retrieved from
https://www.boundless,com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/body-fluids-and-acid-base-balance-26/water-balance-245/regulation-of-water-intake-1203-8545/