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Chapter 17: Blood Supply

Chris RordenUniversity of South CarolinaNorman J. Arnold School of Public HealthDepartment of Communication Sciences and DisordersUniversity 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 WillisNumerous small branchesSupplies the brainstem and cerebellumCerebellum:

•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 CerebralPosterior 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 arteryCortical 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

HemorrhagicBleeding from ruptured vesselsTypes

– 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 SystemDural 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