Organum vasculosum of lamina terminalis

The organum vasculosum of the lamina terminalis (OVLT or supraoptic crest) is one of the circumventricular organs of the brain. Other circumventricular organs are the subfornical organ (SFO) and the area postrema in the brainstem.

AV3V region
The OVLT and the SFO are both strongly interconnected with the median preoptic nucleus of the hypothalamus (the nucleus medianus), and together these three structures comprise the "anterioventral third ventricle (AV3V) region" -- the region anterior and ventral to the third ventricle. The AV3V region is very important in the regulation of fluid and electrolyte balance by controlling thirst, sodium excretion, blood volume regulation, and vasopressin secretion.

Function
The OVLT lacks a blood brain barrier, and so neurons in this region can respond to osmotic pressure factors that are present in the systemic circulation.

Some neurons in the OVLT are osmoreceptors, being sensitive to the osmotic pressure of the blood. These neurons project to the supraoptic nucleus and paraventricular nucleus to regulate the activity of vasopressin-secreting neurons. In a situation of lowered blood volume, the secretion of renin by the kidneys and its subsequent conversion to angiotensin II stimulate the OVLT and SFO to complete a positive feedback loop. These neurons also project to the nucleus medianus (also called the medial preoptic nucleus) which is involved in controlling thirst.