[Water-electrolyte and acid-base changes. IV. Water metabolism and water balance]

Two interrelated physiologic feedback systems, thirst and antidiuretic hormone, serve to regulate water balance, and thereby body fluid osmolality. These two systems share at least one common receptor component: a device for sensing body fluid tonicity, the hypothalamic osmoreceptor. The output of the osmoregulatory system is subserved by two distinct effector components: oral water ingestion and renal water excretion. Acting in concert, these two processes serve to maintain body fluid osmolality. Under normal circumstances body fluid osmolality is carefully maintained within precisely defined limits. For body fluid osmolality to remain constant, the amount of water lost each day (by insensible, gastrointestinal, and renal routes), must be replenished by the ingestion of water and, to a lesser extent, by the production of "metabolic" water from the catabolism of foodstuffs.