Circumventricular organs (CVO) are so named because they are positioned at distinct sites around the margin of the ventricular system

Ventricular system

The ventricular system is a set of structures containing cerebrospinal fluid in the brain. It is continuous with the central canal of the spinal cord.-Components:The system comprises four ventricles:* right and left lateral ventricles* third ventricle...

 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

Mnemonic

A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

 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 hormone

Hormone

A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

s, neurotransmitter

Neurotransmitter

Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

s and cytokine

Cytokine

Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

s.

They are sometimes classified by whether they are secretory or sensory.

Examples

Circumventricular organs include the following:

Name	Secretory/sensory	Ventricle	Function
Subfornical organ Subfornical organ The subfornical organ, situated on the ventral surface of the fornix, at the interventricular foramina , is one of the circumventricular organs of the brain.-Relations with other circumventricular organs:...	sensory	third	fluid regulation
Organum vasculosum of the lamina terminalis Organum vasculosum of the lamina terminalis The organum vasculosum of the lamina terminalis is one of the circumventricular organs of the brain. Other circumventricular organs are the subfornical organ and the area postrema in the brainstem.-AV3V region:...	sensory and secretory	third	detects peptide Peptide Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond... s, fluid regulation
Median eminence Median eminence The median eminence is part of the inferior boundary for the hypothalamus part of the human brain. A small swelling on the tuber cinereum posterior to the infundibulum - atop the pituitary stalk - the median eminence lies in the area roughly bounded on its posterolateral region by the cerebral...	secretory	third	regulates the anterior pituitary Anterior pituitary A major organ of the endocrine system, the anterior pituitary, also called the adenohypophysis, is the glandular, anterior lobe of the pituitary gland...  through the release of neurohormone Neurohormone A neurohormone is any hormone produced and released by neurons.Examples include:*Thyrotropin-releasing hormone *Gonadotropin-releasing hormone *Adrenocorticotropin-releasing hormone*Oxytocin*Antidiuretic hormone *Epinephrine... s
Posterior pituitary Posterior pituitary The posterior pituitary comprises the posterior lobe of the pituitary gland and is part of the endocrine system. Despite its name, the posterior pituitary gland is not a gland, per se; rather, it is largely a collection of axonal projections from the hypothalamus that terminate behind the anterior...  (neurohypophysis)	secretory	third	store and secretes the hormones oxytocin Oxytocin Oxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...  and ADH Vasopressin Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...  into the blood, but does not synthesize either hormone
Subcommissural organ Subcommissural organ The subcommissural organ is a gland in the brain. It is a circumventricular organ consisting of ependymal cells which secrete somatostatin.It is named for its location relative to the posterior commissure.It is near the aqueduct of Sylvius....	secretory	third	secretes certain proteins into the Cerebrospinal fluid Cerebrospinal fluid Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord... , its specific function is as yet unknown.
Pineal gland Pineal gland The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...	sensory and secretory	third	stimulated by darkness to secrete melatonin Melatonin Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...  and is associated with circadian rhythms
Area postrema Area postrema The area postrema is a medullary structure in the brain that controls vomiting. Its privileged location in the brain also allows the area postrema to play a vital role in the control of autonomic functions by the central nervous system.-Anatomy:...	sensory	fourth	the vomiting centre of the brain (can detect noxious substances in the blood and stimulate vomiting Vomiting Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...  in order to rid the body of these toxic chemicals)

The choroid plexus

Choroid plexus

The choroid plexus is a structure in the ventricles of the brain where cerebrospinal fluid is produced...

is also sometimes considered a circumventricular organ.