Editing Homeostasis (section)

===Fluid balance===
{{Main |Osmoregulation |Thirst}}
The [[body water|total amount of water]] in the body needs to be kept in balance. [[Fluid balance]] involves keeping the fluid volume stabilized, and also keeping the levels of [[electrolyte]]s in the extracellular fluid stable. Fluid balance is maintained by the process of [[osmoregulation]] and by behavior. [[Osmotic pressure]] is detected by [[osmoreceptor]]s in the [[median preoptic nucleus]] in the [[hypothalamus]]. Measurement of the plasma [[osmolality]] to give an indication of the water content of the body, relies on the fact that water losses from the body, (through [[Transepidermal water loss|unavoidable water loss through the skin]] which is not entirely waterproof and therefore always slightly moist, [[Breathing#Upper airways|water vapor in the exhaled air]], [[Perspiration|sweating]], [[vomiting]], normal [[feces]] and especially  [[diarrhea]]) are all [[hypotonic]], meaning that they are less salty than the body fluids (compare, for instance, the taste of saliva with that of tears. The latter has almost the same salt content as the extracellular fluid, whereas the former is hypotonic with respect to the plasma. Saliva does not taste salty, whereas tears are decidedly salty). Nearly all normal and abnormal losses of [[body water]] therefore cause the extracellular fluid to become [[Tonicity#Hypertonic solution|hypertonic]]. Conversely, excessive fluid intake dilutes the extracellular fluid causing the hypothalamus to register [[hypotonic hyponatremia]] conditions.

When the [[hypothalamus]] detects a hypertonic extracellular environment, it causes the secretion of an antidiuretic hormone (ADH) called [[vasopressin]] which acts on the effector organ, which in this case is the [[kidney]]. The effect of vasopressin on the kidney tubules is to reabsorb water from the [[distal convoluted tubule]]s and [[Collecting duct system|collecting ducts]], thus preventing aggravation of the water loss via the urine. The hypothalamus simultaneously stimulates the nearby [[Thirst|thirst center]] causing an almost irresistible (if the hypertonicity is severe enough) urge to drink water. The cessation of urine flow prevents the [[hypovolemia]] and [[Tonicity#Hypertonic solution|hypertonicity]] from getting worse; the drinking of water corrects the defect.

Hypo-osmolality results in very low plasma ADH levels. This results in the inhibition of water reabsorption from the kidney tubules, causing high volumes of very dilute urine to be excreted, thus getting rid of the excess water in the body.

Urinary water loss, when the body water homeostat is intact, is a ''compensatory'' water loss, ''correcting'' any water excess in the body. However, since the kidneys cannot generate water, the thirst reflex is the all-important second effector mechanism of the body water homeostat, ''correcting'' any water deficit in the body.