[1][2][3][4] Although the term "circumventricular organs" was originally proposed in 1958 by Austrian anatomist Helmut O. Hofer concerning structures around the brain ventricular system,[5] the penetration of blood-borne dyes into small specific CVO regions was discovered in the early 20th century.

[6] The permeable CVOs enabling rapid neurohumoral exchange include the subfornical organ (SFO), the area postrema (AP), the vascular organ of lamina terminalis (VOLT — also known as the organum vasculosum of the lamina terminalis (OVLT)), the median eminence, the pituitary neural lobe, and the pineal gland.

[10][11] Highly permeable capillaries allow the CVOs to act as an alternative route for peptides and hormones in the neural tissue to sample from and secrete to circulating blood.

[7][11] These organs are responsible for secreting hormones and glycoproteins into the peripheral blood using feedback from both the brain environment and external stimuli.

Although the choroid plexus also has permeable capillaries, it does not contain neural tissue; rather, its primary role is to produce cerebrospinal fluid (CSF), and therefore is typically not classified as a CVO.

[17] There is also evidence that the area postrema is the site at which angiotensin stimulates glucose metabolism, presumed efferent neural activity, blood pressure control, and thirst.

[20][21] The area postrema also has integrative capacities that enable it to send major and minor efferents to sections of the brain involved in the autonomic control of cardiovascular and respiratory activities.

Conversely, the vascular organ of the lamina terminalis maintains efferent projections to the stria medullaris and basal ganglia.

[14] As a major player in the maintenance of the mammalian body fluid homeostasis, the VOLT features the primary neurons responsible for osmosensory balance.

As previously mentioned, the vascular organ of lamina terminalis features neurons responsible for the homeostatic conservation of osmolarity.

[14] Finally, VOLT neurons have been observed to respond to temperature changes indicating that the organum vasculosum of the lamina terminalis is subject to different climates.

Protruding into the third ventricle of the brain, the highly vascularized SFO can be divided into 3–4 anatomical zones, especially by its capillary density and structure.

[25][27] The SFO has many efferent projections, shown to broadcast efferent projections to regions involved in cardiovascular regulation including the lateral hypothalamus with fibers terminating in the supraoptic (SON) and paraventricular (PVN) nuclei, and the anteroventral 3rd ventricle (AV3V) with fibers terminating in the VOLT and the median preoptic area.

[28] While the afferent projections of the SFO are considered less important than the various efferent connections, it is still notable that the subfornical organ receives synaptic input from the zona incerta and arcuate nucleus.

[29] Study of subfornical organ anatomy is still ongoing but evidence has demonstrated slow blood transit time which may facilitate the sensory capability of SFO, enabling increased contact time for blood-borne signals to penetrate its permeable capillaries and influence regulation of blood pressure and body fluids.

[14] The subfornical organ is active in many bodily processes including, but not limited to, osmoregulation,[27][29] cardiovascular regulation,[27] Both hyper- and hypotonic stimuli facilitated an osmotic response.

[14] Additional research has demonstrated that the subfornical organ may be an important intermediary though which leptin acts to maintain blood pressure within normal physiological limits via descending autonomic pathways associated with cardiovascular control.

[28] Additionally, it is assumed that the SFO is the lone forebrain structure capable of constant monitoring of circulating concentrations of glucose.

[30] While the function of the subcommissural organ remains under investigation,[1] it may be part of the mechanism of aldosterone secretion and CSF detoxification, along with osmoregulation.

This finding implies that the subcommissural organ and its associated Reissner's fiber are integral parts of fluid electrolyte balance and water homeostasis.

[1] The anterior pituitary contains non-neural secretory cells derived from oral ectoderm which are indirectly controlled by "releasing hormones" from the median eminence of the hypothalamus, through the hypophyseal portal circulation.

The intermediate lobe (also called pars intermedia) synthesizes and secretes a hormone stimulating melanocytes under neural control by the hypothalamus.

This seal can be attributed to the tight junctions observed between tanycytes and functions to restrict the travel of molecules between the median eminence and the third ventricle.

The most commonly used classification for this gland takes into account its location relative to the diencephalon and the third ventricle of the brain, as well as its size and shape.

Its average weight is 100–180 mg.[40] The pineal gland consists of a central core made up of small lobes and a cortex that possesses a diffuse distribution of neurons.