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). They contribute to fluid balance in the body.

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 vasopressin (ADH) secretion from the posterior pituitary to return blood concentration to normal.

Macula densa
The macula densa region of the kidney's juxtaglomerular apparatus is another modulator of blood osmolality. The macula densa responds to changes in osmotic pressure through changes in the rate of chloride anion flow through the nephron. Increased Cl- flow stimulates tubuloglomerular autoregulation, a signal (thought to be regulated by adenosine) sent to the nearby juxtaglomerular cells of the afferent arteriole, causing the JG cells to release the protease renin into circulation. Renin cleaves the zymogen angiotensinogen, always present in plasma as a result of constitutive production in the liver, into a second inactive form, angiotensin I, which is then converted to its active form, angiotensin II, by angiotensin converting enzyme (ACE), which is widely-distributed in the small vessels of the body, but particularly concentrated in the pulmonary capillaries of the lungs. Angiotensin II exerts systemwide effects, triggering aldosterone release from the adrenal cortex, direct vasoconstriction, and thirst behaviors originating in the hypothalamus.