Cerebrovascular System

Blood supply to the brain can be divided into an anterior and a posterior circulation, which interconnect to form the circle of Willis. The anterior circulation is derived from the internal carotid arteries and consists mainly of the anterior and middle cerebral arteries. The posterior circulation is derived from the vertebral arteries and consists primarily of the cerebellar and posterior cerebral arteries. The primary venous drainage of the brain occurs via the internal jugular vein.

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Arterial Supply

Arterial origin

The arterial supply of the brain is derived from 2 arterial systems:

  • Internal carotid arteries (anterior circulation):
    • Bifurcates from common carotid artery at the level of C4
    • Continues into brain and becomes the middle cerebral artery
    • Anterior cerebral artery branches off middle cerebral artery.
  • Vertebrobasilar system (posterior circulation):
    • Vertebral arteries arise from the subclavian arteries.
    • Vertebral arteries join to become the basilar artery.
    • Basilar artery bifurcates to become the posterior cerebral arteries.
    • Cerebellar arteries arise from the vertebral and basilar arteries.
2 sources of blood supply to the brain

Blood supply to the brain is derived from 2 sources—the internal carotid arteries and the vertebrobasilar system.

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Cerebral blood supply

  • Anterior cerebral artery:
    • Origin: smaller terminal branch of the internal carotid artery
    • Course: occupies the longitudinal fissure and travels posteriorly, ultimately anastomosing with the posterior cerebral arteries
    • Divisions: distributes numerous anteromedial central branches
    • Supplies:
      • Medial frontal and parietal lobes
      • Anterior limb of internal capsule
      • Most of corpus callosum
  • Middle cerebral artery:
    • Origin: larger terminal branch of the internal carotid artery
    • Course: runs through lateral fissure and posterosuperiorly on insula
    • Divisions: cortical and central branches
    • Supplies:
      • Lateral frontal and parietal lobes
      • Portion of temporal lobes
      • Genu and posterior limb of internal capsule
      • Most of basal ganglia
  • Posterior cerebral artery:
    • Origin: arises as terminal branches of basilar arteries
    • Course:
      • Travels parallel to superior cerebellar artery
      • Receives the posterior communicating artery as it runs laterally
    • Supplies:
      • Occipital lobe
      • Posteromedial temporal lobes
      • Midbrain

Cerebellar blood supply

The cerebellum is supplied by branches of the vertebral and basilar arteries.

  • Superior cerebellar artery:
    • Origin: arises from distal aspect of the basilar artery
    • Supplies: 
      • Superior cerebellum
      • Pineal body
  • Anterior inferior cerebellar artery:
    • Origin: arises from proximal aspect of the basilar artery
    • Supplies:
      • Anterior inferior surface of cerebellum
      • Inferolateral aspect of the pons
  • Posterior inferior cerebellar artery:
    • Origin: arises from vertebral artery
    • Supplies:
      • Posterior inferior surface of cerebellum
      • Choroid plexus of 4th ventricle

Circle of Willis

The circle of Willis is an interconnected network of arteries within the brain. The circle of Willis represents the collateral pathways of arterial blood, and it has many anatomical variants. The circle is formed via anastomoses between the anterior and posterior arterial systems that supply blood to the brain. This duality provides a safety net if 1 of the systems fails via occlusion, trauma, or a neoplastic process. Vessels comprising the circle of Willis include:

  • Components of anterior circulation: 
    • Left and right internal carotid arteries 
    • Horizontal segments of the left and right anterior cerebral arteries
    • Single anterior communicating artery
  • Posterior circulation: 
    • Left and right posterior communicating arteries
    • Horizontal segments of the left and right posterior cerebral arteries, arising from the single basilar artery

Venous Drainage

Venous drainage of the brain occurs via the cerebral veins, which ultimately drain into the straight sinus, transverse sinus, and finally the sagittal sinus before reaching the internal jugular vein and traveling back to the heart. 

  • Venous drainage of the brain occurs via the superficial and deep venous systems:
    • The 2 systems are connected via anastomoses and drain via the dural venous sinuses and, ultimately, to the internal jugular vein.
    • Unlike all other veins of the body, cerebral veins do not have valves.
  • Superficial venous system:
    • Composed of cortical veins
    • Drains the cerebral cortex
  • Deep venous system:
    • Divisions:
      • Deep cerebral veins
      • Straight sinus
      • Transverse sinus
      • Sagittal sinuses
    • Drains the deep brain structures
  • Dural venous sinuses:
    • Drain blood from the cerebral veins, orbits, and skull 
    • Empty into the internal jugular veins

Clinical Relevance

  • Intracranial aneurysms: A cerebral, or intracranial, aneurysm is an abnormal dilation of a local area of the artery wall in the CNS. These aneurysms most often occur at junction points of the major arteries of the brain, usually around the circle of Willis. They can either compress adjacent structures or rupture and cause a hemorrhagic stroke.
  • Arterial dissections: The vertebral and carotid arteries are vital components of the blood supply to the brain. Arterial dissections occur when the integrity of the arterial wall structure fails, usually abruptly, resulting in an intramural hematoma formation and a false lumen between the tunica media and adventitial layers. This process may result in aneurysm, stenosis, or occlusion. Presentation is typically with unilateral head or neck pain and/or stroke-like symptoms.
  • Subclavian steal syndrome: occurs when narrowing/occlusion of the subclavian artery proximal to the origin of the vertebral artery causes a reversal of blood flow in the ipsilateral vertebral artery to continue perfusing the ipsilateral arm. The most common cause of subclavian steal syndrome is atherosclerosis. Symptoms are rare, but when they occur, they are usually triggered by physical exertion of the arm and subsequent hypoperfusion of the arm or brain.
  • Stroke: The 2 types of stroke are ischemic and hemorrhagic. Risk factors include hypertension, cerebral amyloid angiopathy, neoplastic diseases, and cerebral aneurysms. Strokes can be diagnosed with brain CT or MRI, and treatment is geared toward reperfusing the ischemic area or stopping the brain hemorrhage.


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