An osmoreceptor is a sensory receptor

Sensory receptor

In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

 primarily found in the hypothalamus

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

 of most homeotherm

Homeotherm

Homeothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This temperature is often, though not necessarily, higher than the immediate environment...

ic organisms that detects changes in osmotic pressure

Osmotic pressure

Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....

. Osmoreceptors can be found in several structures, including the organum vasculosum of the lamina terminalis

Organum vasculosum of the lamina terminalis

The organum vasculosum of the lamina terminalis is one of the circumventricular organs of the brain. Other circumventricular organs are the subfornical organ and the area postrema in the brainstem.-AV3V region:...

 (OVLT) and the subfornical organ

Subfornical organ

The subfornical organ, situated on the ventral surface of the fornix, at the interventricular foramina , is one of the circumventricular organs of the brain.-Relations with other circumventricular organs:...

 (SFO). They contribute to fluid balance

Fluid balance

Fluid balance is the concept of human homeostasis that the amount of fluid lost from the body is equal to the amount of fluid taken in. Euvolemia is the state of normal body fluid volume. Water is necessary for all life on Earth...

 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

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

, which increases or decreases vasopressin

Vasopressin

Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...

 (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

Renin

Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

 into circulation. Renin cleaves the zymogen

Zymogen

A zymogen is an inactive enzyme precursor. A zymogen requires a biochemical change for it to become an active enzyme. The biochemical change usually occurs in a lysosome where a specific part of the precursor enzyme is cleaved in order to activate it...

 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

Aldosterone

Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

 release from the adrenal cortex

Adrenal cortex

Situated along the perimeter of the adrenal gland, the adrenal cortex mediates the stress response through the production of mineralocorticoids and glucocorticoids, including aldosterone and cortisol respectively. It is also a secondary site of androgen synthesis.-Layers:Notably, the reticularis in...

, direct vasoconstriction

Vasoconstriction

Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...

, and thirst

Thirst

Thirst is the craving for fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids and/or an increase in the concentration of certain osmolites, such as salt...

 behaviors originating in the hypothalamus

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

.