Circumventricular organs

4 areas in or near the brain stem which are outside the blood-brain barrier

1. Posterior pituitary
   * Including the nearby median eminence of the hypothalamus
2. Area postrema (AP)
3. Organum vasculosum of the lamina terminalis (OVLT)
4. Subfornical organ (SFO)

Posterior pituitary

ADH and oxytocin are released from posterior pituitary

Area postrema

(Chemoreceptor trigger zone)

On the floor of the 4th ventricle

Chemoreceptor trigger zone that initiates vomiting in response to chemical changes in plasma

OVLT

(Osmoreceptor)

• In supraoptic crest
• Angiotensin II may also act here to increase thirst (in addition to actions on SFO)
• Location of osmoreceptor
  --> Controls ADH secretion

SFO

(Thirst centre)

Angiotensin II acts here to increase thirst

Other notes

• Pineal gland and anterior pituitary have fenestrated capillaries and are also outside the BBB.
  --> But classified as endocrine gland and not part of brain.