Editing Cerebral circulation

{{Short description|Brain blood supply}}
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{{Infobox artery
| Name        = Cerebral circulation
| Latin       =
| Image       = Cerebral vascular territories.jpg
| Caption     = Areas of the brain are supplied by different arteries. The major systems are divided into an anterior circulation (the [[anterior cerebral artery]] and [[middle cerebral artery]]) and a posterior circulation.
| Image2      = Gray488_blue.gif
| Caption2    = Schematic of veins and venous spaces that drain deoxygenated blood from the brain
| BranchFrom  =
| BranchTo    =
| Vein        =
| Supplies    =
}} 
'''Cerebral circulation''' is the movement of [[blood]] through a network of [[cerebral arteries]] and [[cerebral veins|veins]] supplying the [[brain]]. The rate of cerebral [[blood flow]] in an adult [[human]] is typically 750 [[milliliter]]s per [[minute]], or about 15% of [[cardiac output]]. [[artery|Arteries]] deliver [[oxidation|oxygenated]] blood, [[glucose]] and other nutrients to the brain. [[Vein]]s carry "used or spent" blood back to the [[heart]], to remove [[carbon dioxide]], [[lactic acid]], and other [[metabolism|metabolic]] products. The [[neurovascular unit]] regulates cerebral blood flow so that activated neurons can be supplied with energy in the right amount and at the right time.<ref name="Muoio" /> Because the [[brain]] would quickly suffer damage from any stoppage in blood supply, the cerebral circulatory system has safeguards including [[autoregulation]] of the [[blood vessel]]s. The failure of these safeguards may result in a [[stroke]]. The [[volume]] of blood in circulation is called the [[#Physiology|cerebral blood flow]]. Sudden intense accelerations change the [[G-force|gravitational forces]] perceived by bodies and can severely [[G-force#Human tolerance|impair cerebral circulation]] and normal functions to the point of becoming serious life-threatening conditions.

The following description is based on idealized human cerebral circulation. The pattern of circulation and its [[nomenclature]] vary between organisms.

== Anatomy ==
[[File:Cerebrovascular System.png|thumb|left|alt=An illustration of the cerebrovascular system. |Cerebrovascular system]]

=== Blood supply ===
{{multiple image
| footer = Cortical areas and their arterial blood supply
| width  = 200
| image1 = Cerebral vascular territories.jpg
| image2 = Cerebral vascular territories midline.jpg
}}
Blood supply to the brain is normally divided into anterior and posterior segments, relating to the different arteries that supply the brain. The two main pairs of arteries are the [[internal carotid artery|internal carotid arteries]] (supply the anterior brain) and [[vertebral artery|vertebral arteries]] (supplying the [[brainstem]] and posterior brain).<ref>{{cite book | vauthors = Cipolla MJ | title = The Cerebral Circulation | location = San Rafael (CA) | chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK53086/ | publisher = Morgan & Claypool Life Sciences | date = 2009 | chapter = Chapter 2: Anatomy and Ultrastructure. }}</ref> The anterior and posterior cerebral circulations are interconnected via bilateral [[Posterior communicating artery|posterior communicating arteries]]. They are part of the [[circle of Willis]], which provides backup circulation to the brain. In case one of the supply arteries is occluded, the circle of Willis provides interconnections between the anterior and the posterior cerebral circulation along the floor of the cerebral vault, providing blood to tissues that would otherwise become [[ischemic]].<ref>{{cite journal | vauthors = Chandra A, Li WA, Stone CR, Geng X, Ding Y | title = The cerebral circulation and cerebrovascular disease I: Anatomy | journal = Brain Circulation | volume = 3 | issue = 2 | pages = 45–56 | date = 2017-07-17 | pmid = 30276305 | pmc = 6126264 | doi = 10.4103/bc.bc_10_17 | doi-access = free }}</ref>

==== Anterior cerebral circulation ====
[[File:Gray514.png|thumb|The [[ophthalmic artery]] and its branches]]

The '''anterior  cerebral circulation''' is the blood supply to the anterior portion of the brain including [[eye]]s. It is supplied by the following arteries:
* '''[[internal carotid artery|Internal carotid arteries]]''': These large arteries are the medial branches of the [[common carotid artery|common carotid arteries]] which enter the skull, as opposed to the [[external carotid artery|external carotid]] branches which supply the facial tissues; the internal carotid artery branches into the [[anterior cerebral artery]] and continues to form the [[middle cerebral artery]].<ref>{{cite web | veditors = Oiseth S, Jones L, Maza E |url=https://www.lecturio.com/concepts/carotid-arterial-system/|title=Carotid Arterial System|website=The Lecturio Medical Concept Library |access-date=2021-06-22}}</ref>
* [[Anterior cerebral artery]] (ACA)
** [[Anterior communicating artery]]: Connects both anterior cerebral arteries, within and along the floor of the cerebral vault.
* [[Middle cerebral artery]] (MCA)

==== Posterior cerebral circulation ====
[[File:Circle of Willis en.svg|thumb|The anterior and posterior circulations meet at the [[circle of Willis]], pictured here, which rests at the top of the [[brainstem]]. Inferior view.]]

The '''posterior cerebral circulation''' is the blood supply to the posterior portion of the brain, including the [[occipital lobe]]s, [[cerebellum]] and [[brainstem]].
It is supplied by the following arteries:
* '''[[vertebral artery|Vertebral arteries]]''': These smaller arteries branch from the [[subclavian artery|subclavian arteries]] which primarily supply the shoulders, lateral chest, and arms. Within the [[Human cranium|cranium]] the two vertebral arteries fuse into the [[basilar artery]].
** [[Posterior inferior cerebellar artery]] (PICA)
* [[Basilar artery]]: Supplies the [[midbrain]], [[cerebellum]], and usually branches into the [[posterior cerebral artery]]
** [[Anterior inferior cerebellar artery]] (AICA)
** [[Pontine branches]]
** [[Superior cerebellar artery]] (SCA)
* [[Posterior cerebral artery]] (PCA)
* [[Posterior communicating artery]]

===Venous drainage===
The venous drainage of the cerebrum can be separated into two subdivisions:  superficial and deep.

;The superficial system
The superficial system is composed of [[dural venous sinuses]], [[Sinus (anatomy)|sinuses]] (channels) within the [[dura mater]]. The dural sinuses are therefore located on the surface of the cerebrum.  The most prominent of these sinuses is the [[superior sagittal sinus]] which is located in the sagittal plane under the midline of the cerebral vault, posteriorly and inferiorly to the [[confluence of sinuses]], where the superficial drainage joins with the sinus that primarily drains the deep venous system.  From here, two [[transverse sinuses]] bifurcate and travel laterally and inferiorly in an S-shaped curve that forms the [[sigmoid sinuses]] which go on to form the two [[jugular vein]]s. In the neck, the [[jugular vein]]s parallel the upward course of the [[carotid arteries]] and drain blood into the [[superior vena cava]]. The veins puncture the relevant dural sinus, piercing the arachnoid and dura mater as '''bridging veins''' that drain their contents into the sinus.<ref name="Hufnagle">{{cite journal | title = Neuroanatomy, Brain Veins | date = 2022 | pmid = 31536212 | url = https://www.ncbi.nlm.nih.gov/books/NBK546605/ | website = StatPearls | publisher = StatPearls Publishing | access-date = 28 February 2023 | vauthors = Hufnagle JJ, Tadi P }}</ref>

;The deep venous system
The deep venous system is primarily composed of traditional [[veins]] inside the deep structures of the brain, which join behind the midbrain to form the [[great cerebral vein]] (vein of Galen).  This vein merges with the [[inferior sagittal sinus]] to form the [[straight sinus]] which then joins the superficial venous system mentioned above at the [[confluence of sinuses]].

== Maturation of cerebral blood vessels ==
The maturation of blood vessels in the brain is a [[Critical period|critical process]] that occurs postnatally.<ref name="Slaoui">{{cite journal | vauthors = Slaoui L, Gilbert A, Rancillac A, Delaunay-Piednoir B, Chagnot A, Gerard Q, Letort G, Mailly P, Robil N, Gelot A, Lefebvre M, Favier M, Dias K, Jourdren L, Federici L, Auvity S, Cisternino S, Vivien D, Cohen-Salmon M, Boulay AC | title = In mice and humans, brain microvascular contractility matures postnatally | journal = Brain Structure & Function | volume = 228 | issue = 2 | pages = 475–492 | date = March 2023 | pmid = 36380034 | doi = 10.1007/s00429-022-02592-w }}</ref> It involves the acquisition of key barrier and contractile properties essential for brain function. During the early postnatal phase, [[endothelial cell]]s (ECs) and [[vascular smooth muscle cell]]s (VSMCs) undergo significant molecular and functional changes.

Endothelial cells begin to express [[P-glycoprotein]], a crucial [[Efflux pump|efflux transporter]] that helps protect the brain by expelling harmful substances.<ref>{{cite journal | vauthors = Löscher W, Potschka H | title = Blood-brain barrier active efflux transporters: ATP-binding cassette gene family | journal = NeuroRx | volume = 2 | issue = 1 | pages = 86–98 | date = January 2005 | pmid = 15717060 | pmc = 539326 | doi = 10.1602/neurorx.2.1.86 }}</ref> This efflux capacity is progressively acquired and becomes fully functional by the postnatal period. Additionally, VSMCs, which initially populate the arterial network, start to express contractile proteins such as smooth muscle [[actin]] (SMA) and [[myosin-11]], transforming VSMCs into contractile cells capable of regulating blood vessel tone and cerebral blood flow.

The expression of Myh11 in VSMCs acts as a developmental switch, with significant upregulation occurring from birth to the age of 2 to 5 years.<ref name="Slaoui"/> This is a critical period needed for the establishment of vessel contractility and the overall functionality of the cerebral circulation.

==Physiology==
[[File:Sobo 1909 589.png|thumb|upright=1.2|[[Dural venous sinuses]] bordered by hard meninges (shown in blue) direct blood outflow from cerebral veins to the [[internal jugular vein]] at the [[base of skull]].]]

'''Cerebral blood flow''' (CBF) is the blood supply to the [[brain]] in a given period of time.<ref name="Tolias">{{cite web | vauthors = Tolias C, Sgouros S | date = 2006 | url = http://www.emedicine.com/med/topic3216.htm | title = Initial Evaluation and Management of CNS Injury | archive-url = https://web.archive.org/web/20070302120819/http://www.emedicine.com/med/topic3216.htm | archive-date=March 2, 2007 | work = Emedicine.com | access-date = January 4, 2007 }}</ref> In an adult, CBF is typically 750 millilitres per minute or 15.8 ± 5.7% of the [[cardiac output]].<ref name="Xing_2017">{{cite journal | vauthors = Xing CY, Tarumi T, Liu J, Zhang Y, Turner M, Riley J, Tinajero CD, Yuan LJ, Zhang R | title = Distribution of cardiac output to the brain across the adult lifespan | journal = Journal of Cerebral Blood Flow and Metabolism | volume = 37 | issue = 8 | pages = 2848–2856 | date = August 2017 | pmid = 27789785 | pmc = 5536794 | doi = 10.1177/0271678X16676826 }}</ref> This equates to an average [[perfusion]] of 50 to 54 millilitres of blood per 100 grams of brain tissue per minute.<ref name="Orlando Regional Healthcare">{{cite web | work = Orlando Regional Healthcare, Education and Development | date = 2004 | url = http://www.orlandoregional.org/pdf%20folder/overview%20adult%20brain%20injury.pdf | title = Overview of Adult Traumatic Brain Injuries | archive-url = https://web.archive.org/web/20080227162001/http://www.orlandoregional.org/pdf%20folder/overview%20adult%20brain%20injury.pdf | archive-date= 27 February 2008 | access-date = 16 January 2008 }}</ref><ref name="shep">{{cite web | vauthors = Shepherd S | date = 2004 | url = http://www.emedicine.com/med/topic2820.htm | title = Head Trauma | work = Emedicine.com | access-date = 4 January 2007 }}</ref><ref name="Walters">{{cite journal | vauthors = Walters FJ | date = 1998 | issue = 8 | page = 4 | url = http://www.nda.ox.ac.uk/wfsa/html/u08/u08_013.htm | title = Intracranial Pressure and Cerebral Blood Flow | archive-url =https://web.archive.org/web/20110514050024/http://www.nda.ox.ac.uk/wfsa/html/u08/u08_013.htm | archive-date = 14 May 2011 | journal = Physiology| access-date = 4 January 2007 | publisher = World Federation of Societies of Anaesthesiologists }}</ref>

The ratio index of cerebral blood flow/cardiac output (CCRI) decreases by 1.3% per decade, even though cardiac output remains unchanged.<ref name="Xing_2017"/> Across the adult lifespan, women have a higher CCRI than men.<ref name="Xing_2017"/> CBF is inversely associated with [[body mass index]].<ref name="Xing_2017"/>

CBF is tightly regulated to meet the brain's [[metabolism|metabolic]] demands.<ref name="Orlando Regional Healthcare" /><ref name="sgo">{{cite web | vauthors = Singh J, Stock A | date = 2006 | url = http://www.emedicine.com/ped/topic929.htm | title = Head Trauma | work = Emedicine.com | access-date = 4 January 2007 }}</ref> Too much blood (a clinical condition of a normal homeostatic response of [[hyperemia]])<ref name="Muoio">{{cite journal | vauthors = Muoio V, Persson PB, Sendeski MM | title = The neurovascular unit - concept review | journal = Acta Physiologica | volume = 210 | issue = 4 | pages = 790–798 | date = April 2014 | pmid = 24629161 | doi = 10.1111/apha.12250 | s2cid = 25274791 | doi-access = free }}</ref> can raise [[intracranial pressure]] (ICP), which can compress and damage delicate brain tissue. Too little blood flow ([[ischemia]]) results if blood flow to the brain is below 18 to 20 ml per 100 g per minute, and tissue death occurs if flow dips below 8 to 10 ml per 100 g per minute. In brain tissue, a [[biochemical cascade]] known as the [[ischemic cascade]] is triggered when the tissue becomes ischemic, potentially resulting in damage to and the death of [[neuron|brain cells]]. Medical professionals must take steps to maintain proper CBF in patients who have conditions like [[Shock (circulatory)|shock]], [[stroke]], [[cerebral edema]], and [[traumatic brain injury]].

Cerebral blood flow is determined by a number of factors, such as [[viscosity]] of blood, how dilated [[blood vessel]]s are, and the net pressure of the flow of blood into the brain, known as [[cerebral perfusion pressure]], which is determined by the body's [[blood pressure]]. Cerebral perfusion pressure (CPP) is defined as the mean arterial pressure (MAP) minus the intracranial pressure (ICP). In normal individuals, it should be above 50&nbsp;mm Hg. Intracranial pressure should not be above 15&nbsp;mm Hg (ICP of 20&nbsp;mm Hg is considered as intracranial hypertension).<ref>{{cite book| vauthors = Mattle H, Mumenthaler M, Taub E |title=Fundamentals of Neurology|publisher=Thieme|isbn=978-3-13-136452-4|pages=129|date=2016-12-14}}</ref> Cerebral blood vessels are able to change the flow of blood through them by altering their diameters in a process called [[cerebral autoregulation]]; they constrict when systemic blood pressure is raised and dilate when it is lowered.<ref name="Kandel">{{cite book | vauthors = Kandel ER, Schwartz JH, Jessell TM | date =  2000 | title = Principles of Neural Science | edition = 4th | publisher = McGraw-Hill | location = New York | page = 1305 }}</ref> Arterioles also constrict and dilate in response to different chemical concentrations. For example, they dilate in response to higher levels of [[carbon dioxide]] in the blood and constrict in response to lower levels of carbon dioxide.<ref name="Kandel"/>

For example, assuming a person with an arterial partial pressure of carbon dioxide ([[PaCO2]]) of 40 mmHg (normal range of 38–42 mmHg)<ref name="PaCO2 Normal Range">{{cite web | url = https://www.nlm.nih.gov/medlineplus/ency/article/003855.htm | vauthors = Hadjiliadis D, Zieve D, Ogilvie I | title = Blood Gases | work = Medline Plus | date = 6 June 2015 }}</ref> and a CBF of 50 ml per 100g per min. If the PaCO2 dips to 30 mmHg, this represents a 10 mmHg decrease from the initial value of PaCO2. Consequently, the CBF decreases by 1ml per 100g per min for each 1mmHg decrease in PaCO2, resulting in a new CBF of 40ml per 100g of brain tissue per minute. In fact, for each 1 mmHg increase or decrease in PaCO2, between the range of 20–60 mmHg, there is a corresponding CBF change in the same direction of approximately 1–2 ml/100g/min, or 2–5% of the CBF value.<ref name="Giardino_2007">{{cite journal | vauthors = Giardino ND, Friedman SD, Dager SR | title = Anxiety, respiration, and cerebral blood flow: implications for functional brain imaging | journal = Comprehensive Psychiatry | volume = 48 | issue = 2 | pages = 103–112 | date = 2007 | pmid = 17292699 | pmc = 1820771 | doi = 10.1016/j.comppsych.2006.11.001 }}</ref> This is why small alterations in respiration pattern can cause significant changes in global CBF, specially through PaCO2 variations.<ref name="Giardino_2007" />

CBF is equal to the [[cerebral perfusion pressure]] (CPP) divided by the cerebrovascular resistance (CVR):<ref>{{cite web | work = Anaesthesia UK | date = 2007 | url = http://www.anaesthesiauk.com/article.aspx?articleid=100754 | title = Cerebral Blood Flow (CBF) | archive-url =https://web.archive.org/web/20100918034552/http://www.anaesthesiauk.com/article.aspx?articleid=100754 | archive-date = 18 September 2010 | url-status = usurped | access-date = 16 October 2007 }}</ref>

:CBF = CPP / CVR

Control of CBF is considered in terms of the factors affecting CPP and the factors affecting CVR. CVR is controlled by four major mechanisms:
# [[Metabolic]] control (or 'metabolic autoregulation')
# Pressure [[autoregulation]]
# Chemical control (by arterial [[PCO2|pCO<sub>2</sub>]] and pO<sub>2</sub>)
# [[Neural]] control

===Role of intracranial pressure===

Increased [[intracranial pressure]] (ICP) causes decreased blood perfusion of [[brain cell]]s by mainly two mechanisms:
* Increased ICP constitutes an increased [[interstitial hydrostatic pressure]] that, in turn, causes a decreased [[Starling equation|driving force for capillary filtration]] from intracerebral blood vessels.
* Increased ICP compresses cerebral arteries, causing increased cerebrovascular resistance (CVR).

===Cerebral perfusion pressure===
{{Main article|Cerebral perfusion pressure}}
Cerebral perfusion pressure is the net [[pressure]] gradient causing [[cerebral blood flow]] to the brain (brain [[perfusion]]).  It must be maintained within narrow limits; too little pressure could cause brain tissue to become [[ischemic]] (having inadequate blood flow), and too much could raise [[intracranial pressure]].

=== Imaging ===
[[Arterial spin labeling]] (ASL), [[phase contrast magnetic resonance imaging]] (PC-MRI), and [[positron emission tomography]] (PET) are [[neuroimaging]] techniques that can be used to measure CBF. ASL and PET can also be used to measure regional CBF (rCBF) within a specific brain region.
rCBF at one location can be measured over time by [[Molecular diffusion|thermal diffusion]]<ref name="pmid10930012">{{cite journal | vauthors = Vajkoczy P, Roth H, Horn P, Lucke T, Thomé C, Hubner U, Martin GT, Zappletal C, Klar E, Schilling L, Schmiedek P | title = Continuous monitoring of regional cerebral blood flow: experimental and clinical validation of a novel thermal diffusion microprobe | journal = Journal of Neurosurgery | volume = 93 | issue = 2 | pages = 265–274 | date = August 2000 | pmid = 10930012 | doi = 10.3171/jns.2000.93.2.0265 }}</ref>

== References ==
{{Reflist}}

== External links ==
* [http://www.hemodynamic.com Computer Model of the Cerebral Circulation for Training and Education]

{{Arteries of head and neck}}
{{VeinsHeadNeck}}

[[Category:Neurology]]
[[Category:Cardiology]]