The circumventricular organs are neuroendocrine anatomical structures localized around the ventricles of the brain. They are characterized by a rich vasculature and fenestrated (permeable) capillaries. As such, with the exception of the subcomissural organ, they are also characterized by the absence of the blood-brain barrier.
Circumventricular organs can be classified either as sensitive or secretory. The former act as chemoreceptors, monitoring hormone levels in the cerebrospinal fluid and blood, and the latter allow secretion of hormones in the peripheral blood.