Circumventricular organs:

structures that are not protected by the blood brain barrier. This makes them capable of interacting with the peripheral blood supply, allowing them to be involved in monitoring blood chemistry, release hormones into the bloodstream, etc. Examples of circumventricular organs include the area postrema, subfornical organ, posterior pituitary gland, and pineal gland.

Learn more:

Know your brain: Pineal gland

Know your brain: Pituitary gland