Circumventricular organs

Circumventricular organs are so named because they are positioned at distinct sites around the margin of the ventricular system of the brain. They are among the few sites in the brain which have an incomplete blood-brain barrier. As a result, neurons located in circumventricular organs can directly sense the concentrations of various compounds, particularly peptide hormones, in the bloodstream, without the need for specialized transport systems which move those compounds across the blood-brain barrier. A useful mnemonic device for remembering this aspect of their function, though not the source of the name, is that they allow factors to 'circumvent' the blood-brain barrier. These organs secrete or are sites of action of a variety of different hormones, neurotransmitters and cytokines.

Examples
Circumventricular organs include the following:
 * Pineal gland - secretes melatonin and is associated with circadian rhythms
 * Subfornical organ- regulates body fluids
 * Organum vasculosum of the lamina terminalis- detects peptides
 * Choroid plexus
 * Area Postrema- the vomiting centre of the brain (makes sense that the brain can detect noxious substances in the blood and stimulate vomiting in order to rid the body of the substances)
 * Median eminence- regulates the anterior pituitary through the release of neurohormones
 * Subcommissural organ
 * Posterior pituitary (neurohypophysis)- to detect levels of oxytocin and ADH in the blood