Editing Human brain (section)

===Blood supply===
{{Main|Cerebral circulation}}
[[File:Circle of Willis en.svg|thumb|upright|Two circulations joining at the circle of Willis (inferior view)]]
[[File:Gray769-en.svg|thumb|Diagram showing features of cerebral [[meninges|outer membranes]] and supply of blood vessels]]
<!-- Arteries -->The [[internal carotid arteries]] supply [[Blood#Oxygen transport|oxygenated blood]] to the front of the brain and the [[vertebral arteries]] supply blood to the back of the brain.{{sfn|Gray's Anatomy|2008|p=247}} These two circulations [[anastomosis|join]] in the [[circle of Willis]], a ring of connected arteries that lies in the [[interpeduncular cistern]] between the midbrain and pons.{{sfn|Gray's Anatomy|2008|pp=251-2}}

The internal carotid arteries are branches of the [[common carotid arteries]]. They enter the [[cranium]] through the [[carotid canal]], travel through the [[cavernous sinus]] and enter the [[subarachnoid space]].{{sfn|Gray's Anatomy|2008|p=250}} They then enter the [[circle of Willis]], with two branches, the [[anterior cerebral arteries]] emerging. These branches travel forward and then upward along the [[longitudinal fissure]], and supply the front and midline parts of the brain.{{sfn|Gray's Anatomy|2008|p=248}} One or more small [[anterior communicating artery|anterior communicating arteries]] join the two anterior cerebral arteries shortly after they emerge as branches.{{sfn|Gray's Anatomy|2008|p=248}} The internal carotid arteries continue forward as the [[middle cerebral arteries]]. They travel sideways along the [[sphenoid bone]] of the [[orbit (anatomy)|eye socket]], then upwards through the [[insula cortex]], where final branches arise. The middle cerebral arteries send branches along their length.{{sfn|Gray's Anatomy|2008|p=250}}

The vertebral arteries emerge as branches of the left and right [[subclavian arteries]]. They travel upward through [[Vertebra#Cervical vertebrae|transverse foramina]] which are spaces in the [[cervical vertebrae]]. Each side enters the cranial cavity through the foramen magnum along the corresponding side of the medulla.{{sfn|Gray's Anatomy|2008|p=250}} They give off [[Posterior inferior cerebellar artery|one of the three cerebellar branches]]. The vertebral arteries join in front of the middle part of the medulla to form the larger [[basilar artery]], which sends multiple branches to supply the medulla and pons, and the two other [[Anterior inferior cerebellar artery|anterior]] and [[Superior cerebellar artery|superior cerebellar branches]].{{sfn|Gray's Anatomy|2008|p=251}} Finally, the basilar artery divides into two [[posterior cerebral arteries]]. These travel outwards, around the superior cerebellar peduncles, and along the top of the cerebellar tentorium, where it sends branches to supply the temporal and occipital lobes.{{sfn|Gray's Anatomy|2008|p=251}} Each posterior cerebral artery sends a small [[posterior communicating artery]] to join with the internal carotid arteries.

====Blood drainage====
<!--Veins-->
[[Cerebral veins]] drain [[Blood#Oxygen transport|deoxygenated blood]] from the brain. The brain has two main networks of [[vein]]s: an exterior or [[Superior cerebral veins|superficial network]], on the surface of the cerebrum that has three branches, and an [[Internal cerebral veins|interior network]]. These two networks communicate via [[anastomosis|anastomosing]] (joining) veins.{{sfn|Gray's Anatomy|2008|pp=254-6}} The veins of the brain drain into larger cavities of the [[dural venous sinuses]] usually situated between the dura mater and the covering of the skull.{{sfn|Elsevier's|2007|pp=311–4}} Blood from the cerebellum and midbrain drains into the [[great cerebral vein]]. Blood from the medulla and pons of the brainstem have a variable pattern of drainage, either into the [[spinal veins]] or into adjacent cerebral veins.{{sfn|Gray's Anatomy|2008|pp=254-6}}

The blood in the [[Anatomical terms of location#deep|deep]] part of the brain drains, through a [[venous plexus]] into the [[cavernous sinus]] at the front, and the [[superior petrosal sinus|superior]] and [[inferior petrosal sinus]]es at the sides, and the [[inferior sagittal sinus]] at the back.{{sfn|Elsevier's|2007|pp=311–4}} Blood drains from the outer brain into the large [[superior sagittal sinus]], which rests in the midline on top of the brain. Blood from here joins with blood from the [[straight sinus]] at the [[confluence of sinuses]].{{sfn|Elsevier's|2007|pp=311–4}}

Blood from here drains into the left and right [[transverse sinus]]es.{{sfn|Elsevier's|2007|pp=311–4}} These then drain into the [[sigmoid sinus]]es, which receive blood from the cavernous sinus and superior and inferior petrosal sinuses. The sigmoid drains into the large [[internal jugular vein]]s.{{sfn|Elsevier's|2007|pp=311–4}}{{sfn|Gray's Anatomy|2008|pp=254-6}}

====The blood–brain barrier====
The larger arteries throughout the brain supply blood to smaller [[capillaries]]. These smallest of [[blood vessel]]s in the brain, are lined with cells joined by [[tight junction]]s and so fluids do not seep in or leak out to the same degree as they do in other capillaries; this creates the [[blood–brain barrier]].{{sfn|Guyton & Hall|2011|pp=748–749}} [[Pericyte]]s play a major role in the formation of the tight junctions.<ref name="Daneman">{{cite journal |last1=Daneman |first1=R. |last2=Zhou |first2=L. |last3=Kebede |first3=A.A. |last4=Barres |first4=B.A. |title=Pericytes are required for blood-brain barrier integrity during embryogenesis |journal=Nature |date=November 25, 2010 |volume=468 |issue=7323 |pages=562–6 |pmid=20944625 |doi=10.1038/nature09513 |pmc=3241506|bibcode=2010Natur.468..562D }}</ref> The barrier is less permeable to larger molecules, but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (including [[anaesthetic]]s and alcohol).{{sfn|Guyton & Hall|2011|pp=748–749}} The blood-brain barrier is not present in the [[circumventricular organs]]{{Em dash}}which are structures in the brain that may need to respond to changes in body fluids{{Em dash}}such as the [[pineal gland]], [[area postrema]], and some areas of the [[hypothalamus]].{{sfn|Guyton & Hall|2011|pp=748–749}} There is a similar [[Choroid plexus#Function|blood–cerebrospinal fluid barrier]], which serves the same purpose as the blood–brain barrier, but facilitates the transport of different substances into the brain due to the distinct structural characteristics between the two barrier systems.{{sfn|Guyton & Hall|2011|pp=748–749}}<ref name="BCSF">{{cite book |last1=Laterra |first1=J. |last2=Keep |first2=R. |last3=Betz |first3=L.A. |title=Basic neurochemistry: molecular, cellular and medical aspects |date=1999 |publisher=Lippincott-Raven |location=Philadelphia |edition=6th |section-url=https://www.ncbi.nlm.nih.gov/books/NBK27998/ |section=Blood–cerebrospinal fluid barrier |display-authors=etal}}</ref>