hypothalamus and thirst mechanism: Another appetitive mechanism under hypothalamic control is thirst. Drinking is regulated by plasma osmolality and extracellular fluid (ECF) volume in much the same fashion as vasopressin secretion. Water intake is increased by increased effective osmotic pressure of the plasma by decreases in ECF volume, and by psychologic and other factors. Osmolality acts via osmoreceptors, receptors that sense the osmolality of the body fluids. These osmoreceptors are located in the anterior hypothalamus. Decreases in ECF volume also stimulate thirst by a pathway independent of that mediating thirst in response to increased plasma osmolality . Thus, hemorrhage causes increased drinking even if there is no change in the osmolality of the plasma. The effect of ECF volume depletion on thirst is mediated in part via the renin–angiotensin system. Renin secretion is increased by hypovolemia and results in an increase in circulating angiotensin II. The angiotensin II acts on the subfornical organ, a specialized receptor area in the diencephalon, to stimulate the neural areas concerned with thirst. Some evidence suggests that it acts on the organum vasculosum of the lamina terminalis (OVLT) as well. These areas are highly permeable and are two of the circumventricular organs located outside the blood–brain barrier. However, drugs that block the action of angiotensin II do not completely block the thirst response to hypovolemia, and it appears that the baroreceptors in the heart and blood vessels are also involved.