Fluid regulation is a complex intrinsic internal bodily activity associated with the nervous system.
The concentration of fluids and their solutes are extremely important, as fluids are responsible for nourishment of body tissues. Thirst is a neurological response to decreased fluid. There are two types of thirst: osmotic and hypovolemic. Hypovolemic thirst normally results from severe physical trauma, when copious amounts of blood have been lost and drinking in further fluids is necessary to physically rebalance the level of fluid.
Baroreceptors are located in the major arteries of the body - the aorta, the jugulars, and carotids, and they measure pressure. When they are not stretched, blood pressure is too low, and the baroreceptors signal the juxtoglomerlular apparatus (JGA) of the kidney to produce renin, which stimulates the production of angiotensis precursor, which is converted into angiotensin I and angiotensin II.
Angiotensin I is a potent vasoconstrictor.
Angiotensin II activates the subventricular nucleus of the hypothalamus.
The hypothalamus is also continuous with the CSF-filled 3rd ventricle. Specialized cells with fluid-gated ion channels in the superoptic and paraventricular areas of the hypothalamus are activated, and they subsequently activate the downstream OVLT to trigger the production of ADH. ADH minimizes water loss through the kidneys.