Chapter 17: Blood Supply

 Chris Rorden University of South Carolina Norman J. Arnold School of Public Health Department of Communication Sciences and Disorders University of South Carolina

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Role of Blood Supply

 Nutrition to parenchyma (functional parts of organ = brain cells).

– Brain is completely dependent on oxygen supply - No O2 reserves.

 Irreversible damage in 4-6 minutes if no oxygen  Brain requires 20% of O2 for 2% of body weight. – Brain requires glucose for energy  No ability to use fat   Remove carbon dioxide and waste-products from cells Cerebral perfusion – > 60 ml/100gr min-1 in Gray matter – 40 < X < 60 in White matter – 750mL blood pumped per minute and circulated blood returned for reoxygenation

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Vascular Network

 Network of arteries and veins – Arteries carry blood away from the heart – Arteries divide into smaller vessels called: arterioles – Arterioles divide further into capillaries  Vascular Network – Veins carry blood toward the heart – Smallest level are venules that are connected to capillaries – Venules Carry blood to sinuses on cortical surface

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Local blood flow

Arteries Arterioles Capillaries Venules Sinuses

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Cerebrovascular Supply

 Two Systems – Carotid System – Vertebral Basilar System  Meet in Circle of Willis

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Vertebral / Basilar Arteries

 Brain stem – Basilar Artery – Vertebral Arteries

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Vertebral Basilar System

 Two Vertebral Arteries enter skull through Foramen Magnum  Join to form the Basilar Artery and then Circle of Willis  Numerous small branches  Supplies the brainstem and cerebellum Cerebellum: •Superior Cerebellar Artery •Anterior Inferior Cerebellar Artery •Posterior Inferior Cerebellar Artery

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Carotid Arteries

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Carotid System

 Common Carotid Artery  Divides into two branches – External Branch  Supply blood to facial muscles, forehead and oral, nasal and orbital cavities – Internal Branch (enters the skull through the carotid foramen)  Anterior choroidal artery (supplies optic tract, posterior limb of internal capsule, branches to midbrain,and lateral geniculate nucleus).  Ophthalmic artery (Supplies blood to the eyeball and ocular muscles) – Major source of blood to the brain – After joining Circle of Willis becomes anterior cerebral and middle cerebral arteries

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Circle of Willis

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Circle of Willis

 Wreath-shaped circle of Willis located at ventral surface of brain  Connects Carotid and Vertebral Basilar Systems – Middle and Anterior Cerebral Arteries – Anterior Communicating Artery – Posterior Cerebral Artery – Posterior Communicating Artery

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Cortical Blood Supply

– ACA: Medial Frontal Perfusion – MCA: Lateral Perfusion – PCA: Posterior medial Perfusion, cerebellum – Watershed Areas: overlap between major arteries Major Arteries  Carotid  Anterior Cerebral  Middle Cerebral  Posterior Cerebral

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Two Types of Arteries

 Cortical (Circumferential) Branches – Supply External Brain Structures  Central (Penetrating) Branches – Small – Penetrate ventral surface to supply internal brain structures

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Anterior Cerebral Artery

 Supplies Orbital and Medial Surfaces of Frontal and Parietal Lobes  Interruption causes – Cortical Arteries – Paralysis of legs and feet – Difficulty in prefrontal lobe functions of cognitive thinking, judgment, motor initiation and self monitoring

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Middle Cerebral Artery

 Direct continuation of carotid artery  Cortical Branches – Temporal – Parietal – Frontal

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MCA: Cortical Branches

 Supplies blood to entire lateral surface of brain – Somatosensory Cortex – Motor Cortex – Broca's Area – Heschl’s Gyrus – Wernicke’s Area  Therefore, symptoms include – Aphasia – Motor deficits – Neglect (right hemisphere)

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MCA – Penetrating Arteries

 Supplies basal ganglia and diencephalon  Interruption causes – Contralateral hemiplegia – Impaired sensory systems – Touch – Pain and temperature

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Central Penetrating Arteries

 Arise from cortical arteries or from Circle of Willis  Penetrate inferior surface of brain  Can form channels to facilitate blood supply  Central (Penetrating) Arteries  Supply – Thalamus – Hypothalamus – Basal Ganglia – Internal capsule – Choroid Plexus

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Posterior Cerebral Artery

 Supplies blood to anterior and inferior temporal lobes, uncus, inferior temporal gyri, inferior and medial occipital lobe  Watersheds with middle cerebral artery  Interruption causes – Homonymous hemianopsia – Possible total blindness – Cerebellar symptoms

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Collateral Circulation

 Redundant arteries provide alternative supply when primary supply is lost  Small, normally closed arteries open up after occlusion, connecting two larger arteries or different parts of the same artery.

 Dependent on location and severity of blockage – Better collateral circulation if blockage is near main trunk – Better if blockage occurs gradually

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Vascular Pathologies

 Occlusive (Something is blocked causing ischemia). 80% of strokes are ischemic – Embolism: object from another part of the body which travels through artery until it gets stuck.

– Thrombosis: object originating within a blood vessel: local buildup of fatty substances usually at a bifurcation of artery, these can rupture leading to catastrophic blockage  Hemorrhagic (Bleed). 20% of strokes are due to arteries rupturing.

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Risk factor for ischemic stroke

 Atherosclerosis from reduced lumen due to lipids, calcium fatty particles etc. (‘Athera’ = porridge in Greek)  A form of Arteriosclerosis: a general term describing any hardening (and loss of elasticity) of arteries.

 Leads to stenosis (narrowing) of the artery, gradually leading to insufficient blood supply  Plaques can rupture, creating a thrombus in blood supply

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Vascular Pathologies - Bleeds

 Hemorrhagic  Bleeding from ruptured vessels  Types – Intracerebral: More common in thalamus and basal ganglia – Subdural – Aneurysm Aneurysm

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Haemorrhages

  20% of strokes are bleeds Typically, due to ruptured aneurysm – An aneurysm is a sac-like protrusion of an artery caused by a weakened area within the vessel wall. – Introspectively, the worst headache of your life.

– http://www.microvent.com/ – Surgery to clip aneurysm can save patients life.

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Arteriovenous Malformations

 AVMs can cause hemorrhagic strokes  Tangled web of arteries and veins present since birth  ~3 percent of all AVMs hemorrhage

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Venous Sinus System

 Blood returns to lungs for oxygen  Drain into Sinus System  Dural Sinuses – Superior Sagittal Sinus – Inferior Sagittal Sinus – Straight Sinus – Transverse Sinus – Cavernous Sinus – Petrosal Sinuses

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Notes

 Regulation of Cerebral Blood Flow – Closed system under constant pressure – Controlled by elasticity of blood vessels, and metabolic needs – Aging can cause less elasticity responsiveness and eveness of flow  Blood-Brain Barrier – Tight system does not allow direct contact of all brain tissues with blood – Specialized system to extract needed molecules – May impede medicine entering the brain

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