Editing Blood–brain barrier (section)

== Function ==
{{See also|Neuroimmune system}}
The blood–brain barrier acts effectively to protect brain tissue from circulating [[pathogen]]s and other potentially toxic substances.<ref name="Blood-brain barrier dysfunction in">{{cite journal | vauthors = Abdullahi W, Tripathi D, Ronaldson PT | title = Blood-brain barrier dysfunction in ischemic stroke: targeting tight junctions and transporters for vascular protection | journal = American Journal of Physiology. Cell Physiology | volume = 315 | issue = 3 | pages = C343–C356 | date = September 2018 | pmid = 29949404 | pmc = 6171039 | doi = 10.1152/ajpcell.00095.2018 }}</ref> Accordingly, [[blood-borne disease|blood-borne infections]] of the brain are rare.<ref name=daneman/> [[Infection]]s of the brain that do occur are often difficult to treat. [[Antibodies]] are too large to cross the blood–brain barrier, and only certain [[antibiotics]] are able to pass.<ref name=Raza>{{cite journal | vauthors = Raza MW, Shad A, Pedler SJ, Karamat KA | title = Penetration and activity of antibiotics in brain abscess | journal = Journal of the College of Physicians and Surgeons--Pakistan | volume = 15 | issue = 3 | pages = 165–167 | date = March 2005 | pmid = 15808097 | url = https://www.researchgate.net/publication/7928383 }}</ref> In some cases, a drug has to be administered directly into the cerebrospinal fluid where it can enter the brain by crossing the [[blood-cerebrospinal fluid barrier]].<ref>{{cite journal | vauthors = Pardridge WM | title = Drug transport in brain via the cerebrospinal fluid | journal = Fluids and Barriers of the CNS | volume = 8 | issue = 1 | pages = 7 | date = January 2011 | pmid = 21349155 | pmc = 3042981 | doi = 10.1186/2045-8118-8-7 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Chen Y, Imai H, Ito A, Saito N | title = Novel modified method for injection into the cerebrospinal fluid via the cerebellomedullary cistern in mice | journal = Acta Neurobiologiae Experimentalis | volume = 73 | issue = 2 | pages = 304–311 | year = 2013 | doi = 10.55782/ane-2013-1938 | pmid = 23823990 | doi-access = free }}</ref>

===Circumventricular organs===
{{Main |Circumventricular organs}}
[[Circumventricular organs]] (CVOs) are individual structures located adjacent to the [[fourth ventricle]] or [[third ventricle]] in the brain, and are characterized by dense capillary beds with [[vascular permeability|permeable]] endothelial cells unlike those of the blood–brain barrier.<ref name="peering">{{cite journal | vauthors = Gross PM, Weindl A | title = Peering through the windows of the brain | journal = Journal of Cerebral Blood Flow and Metabolism | volume = 7 | issue = 6 | pages = 663–72 | date = December 1987 | pmid = 2891718 | doi = 10.1038/jcbfm.1987.120 | doi-access = free }}</ref><ref name="gross1">{{cite book | vauthors = Gross PM | title = Circumventricular Organs and Brain Fluid Environment - Molecular and Functional Aspects | chapter = Chapter 31: Circumventricular organ capillaries | series = Progress in Brain Research | volume = 91 | pages = 219–33 | year = 1992 | pmid = 1410407 | doi = 10.1016/S0079-6123(08)62338-9 | isbn = 9780444814197 }}</ref> Included among CVOs having highly permeable capillaries are the [[area postrema]], [[subfornical organ]], [[vascular organ of the lamina terminalis]], [[median eminence]], [[pineal gland]], and three lobes of the  [[pituitary gland]].<ref name=peering/><ref name="miyata">{{cite journal | vauthors = Miyata S | title = New aspects in fenestrated capillary and tissue dynamics in the sensory circumventricular organs of adult brains | journal = Frontiers in Neuroscience | volume = 9 | pages = 390 | year = 2015 | pmid = 26578857 | pmc = 4621430 | doi = 10.3389/fnins.2015.00390 | doi-access = free }}</ref>

Permeable capillaries of the sensory CVOs (area postrema, subfornical organ, vascular organ of the lamina terminalis) enable rapid detection of circulating signals in systemic blood, while those of the secretory CVOs (median eminence, pineal gland, pituitary lobes) facilitate transport of brain-derived signals into the circulating blood.<ref name=peering/><ref name=gross1/> Consequently, the CVO permeable capillaries are the point of bidirectional blood–brain communication for [[neuroendocrine]] function.<ref name=peering/><ref name=miyata/><ref name="Design">{{cite journal | vauthors = Rodríguez EM, Blázquez JL, Guerra M | title = The design of barriers in the hypothalamus allows the median eminence and the arcuate nucleus to enjoy private milieus: the former opens to the portal blood and the latter to the cerebrospinal fluid | journal = Peptides | volume = 31 | issue = 4 | pages = 757–76 | date = April 2010 | pmid = 20093161 | doi = 10.1016/j.peptides.2010.01.003 | s2cid = 44760261 | url = https://www.researchgate.net/publication/41111270 }}</ref>

===Specialized permeable zones===
The border zones between brain tissue "behind" the blood–brain barrier and zones "open" to blood signals in certain CVOs contain specialized hybrid capillaries that are leakier than typical brain capillaries, but not as permeable as CVO capillaries. Such zones exist at the border of the area postrema—[[nucleus tractus solitarii]] (NTS),<ref name="nts">{{cite journal | vauthors = Gross PM, Wall KM, Pang JJ, Shaver SW, Wainman DS | title = Microvascular specializations promoting rapid interstitial solute dispersion in nucleus tractus solitarius | journal = The American Journal of Physiology | volume = 259 | issue = 6 Pt 2 | pages = R1131-8 | date = December 1990 | pmid = 2260724 | doi = 10.1152/ajpregu.1990.259.6.R1131 }}</ref> and median eminence—[[hypothalamus|hypothalamic]] [[arcuate nucleus]].<ref name=Design/><ref name="ctr">{{cite journal | vauthors = Shaver SW, Pang JJ, Wainman DS, Wall KM, Gross PM | s2cid = 27789146 | title = Morphology and function of capillary networks in subregions of the rat tuber cinereum | journal = Cell and Tissue Research | volume = 267 | issue = 3 | pages = 437–48 | date = March 1992 | pmid = 1571958 | doi = 10.1007/BF00319366 }}</ref> These zones appear to function as rapid transit regions for brain structures involved in diverse neural circuits—like the NTS and arcuate nucleus—to receive blood signals which are then transmitted into neural output.<ref name=Design/><ref name=nts/> The permeable capillary zone shared between the median eminence and hypothalamic arcuate nucleus is augmented by wide pericapillary spaces, facilitating bidirectional flow of solutes between the two structures, and indicating that the median eminence is not only a secretory organ, but may also be a sensory organ.<ref name=Design/><ref name=ctr/>