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.

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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

It should be noted that the macula densa in the kidney senses blood osmolality too. It adjusts renin secretion to modulate osmolalitiy. Renin is used to convert angiotensinogen (which is always present, produced in the liver) to angiotensin I, which is in turn converted into angiotensin II by Angiotensin Converting Enzyme (ACE) (present in the capillaries and lungs). Angiotensin II exerts systemwide affects, triggering aldosterone release from the adrenal cortex, direct vasoconstriction, and thirst behaviors originating in the hypothalamus.

See also

• hypothalamus
• vasopressin
• baroreceptors
• osmolality