Circumventricular organs (CVOs) are positioned at distinct sites around the margin of the ventricular system of the brain. They are among the few sites in the brain that 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 that 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.

CVOs allow for linkage between the central nervous system (CNS) and peripheral blood flow, and are an integral part of neuroendocrine function. 

CVO

$$View of the third and fourth ventricles. The CVOs border these ventricles.

This diagram of the third and fourth ventricles delineates the superior sagittal sinus, choroid plexus, interventricular foramen, cerebral aqueduct, lateral aperture, median aperture, central canal, right lateral ventricle, meningeal dura mater, subarachnoid space, and arachnoid granulation.

Sensory Circumventricular Organs

The sensory organs are able to sense plasma molecules and pass that information into other regions of the brain. Therefore, they provide direct information to the autonomic nervous system from the systemic circulation. These organs include:

Area postrema: Site of the chemoreceptor trigger zone for vomiting, sends major and minor efferents to sections of the brain involved in the autonomic control of cardiovascular and respiratory activities.

Subfornical organ: Active in osmoregulation, cardiovascular regulation, and energy homeostasis.

Vascular organ of lamina terminalis: Responsible for the homeostatic conservation of osmolarity.

Secretory Circumventricular Organs

The secretory organs are responsible for secreting hormones and glycoproteins into the peripheral vascular system using feedback from both the brain environment and external stimuli. These include:

Subcommissural organ (SCO): Secretion of the glycoprotein SCO-spondin. 

Posterior pituitary: Stores and releases oxytocin and vasopressin, also known as antidiuretic hormone, produced in the hypothalamus.

Pineal gland: The main function is the secretion of melatonin.

Median eminence: Allows for the transport of neurohormones between the CSF and the peripheral blood supply.