Circumventricular organs (CVO) are midline small structures situated at distinct sites around the margin of the ventricular system of the brain. They have common morphological and endocrine-like characteristics that distinguish them from the rest of the nervous system.
The circumventricular organs are among the few sites in the brain which have an incomplete blood-brain barrier. This means that 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. They are sometimes classified by whether they are secretory or sensory.