Osmoreceptor

An osmoreceptor is a sensory receptor primarily found in the hypothalamus of most homeothermic organisms that detects changes in osmotic pressure. Osmoreceptors can be found in several structures, including the organum vasculosum of the lamina terminalis (OVLT) and the subfornical organ (SFO).

Mechanism in humans
Osmoreceptors, as the name suggests, sense change in osmotic pressure. When the osmotic pressure of blood changes (i.e. it is more or less dilute), water diffusion into and out of the osmoreceptor cells changes. That is, 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 vassopressin (ADH) secretion from the posterior pituitary to return blood concentration to normal.

It should be noted that the macula densa in the kidney senses blood osmorality too. It adjusts renin secretion to optimise osmoralitiy. Renin is converted to angiotensin I by angiotensinogen and this is in turn converted into angiotensin II by angiotensin converting enzyme (in the lungs), which acts, in high concentrations to decrease diuresis.