Blood Supply of the Brain

Arterial Supply Introduction - Brain's Blood Highways

• Brain's blood supply originates from two primary pairs of arteries:
  • Internal Carotid Arteries (ICAs): Form the anterior circulation. Supply ~2/3 of cerebral hemispheres.
  • Vertebral Arteries (VAs): Unite to form Basilar Artery (posterior circulation). Supply brainstem, cerebellum, occipital lobes.
• These systems anastomose at the base, forming the Circle of Willis - a critical collateral pathway.
• High metabolic activity:

  ⭐ Despite being only 2% of body weight, the brain consumes ~20% of the body's oxygen and 25% of its glucose.

Internal Carotid System - Anterior Circulation Powerhouse

• Originates from common carotid artery; enters skull via carotid canal.
• Key Branches (OPAMs):
  • Ophthalmic Artery: Supplies eye, orbit.
  • Posterior Communicating Artery (PCoA): Connects ICA to posterior circulation (PCA).
  • Anterior Choroidal Artery: Supplies optic tract, choroid plexus, parts of internal capsule, thalamus, hippocampus.
  • Anterior Cerebral Artery (ACA): Supplies medial aspects of frontal and parietal lobes, corpus callosum.
  • Middle Cerebral Artery (MCA): Largest branch; lateral aspects of cerebral hemispheres.

⭐ MCA is the artery most commonly affected in stroke; supplies motor and sensory cortices (upper limb, face), speech areas (Broca's, Wernicke's).

Vertebrobasilar System - Posterior Circulation Lifeline

• Origin: VAs (from subclavian a.).
• Course: VAs ascend C6-C1 foramina, enter foramen magnum.
• Formation: VAs unite → BA (pontomedullary jct.).
• BA: On pons, bifurcates → PCAs.
• Branches & Supply:
  • VA:
    • PICA: Lat. medulla, inf. cerebellum.
    • ASA: Ant. spinal cord, med. medulla.
  • BA:
    • AICA: Inf. cerebellum, pons.
    • Labyrinthine a.: Inner ear (often from AICA).
    • Pontine aa.: Pons.
    • SCA: Sup. cerebellum, midbrain.
    • PCA: Occipital, med. temporal lobes, thalamus.

⭐ Occlusion of PICA (branch of vertebral artery) leads to Wallenberg syndrome (lateral medullary syndrome).

Circle of Willis - Cerebral Traffic Circle

Arterial anastomosis at the brain's base, crucial for collateral flow between anterior (ICA) & posterior (vertebrobasilar) circulations.

• Key Arteries forming the circle:
  • Anterior Cerebral A. (ACA)
  • Anterior Communicating A. (AComA)
  • Internal Carotid A. (ICA - supraclinoid segment)
  • Posterior Cerebral A. (PCA)
  • Posterior Communicating A. (PComA)
• Significance: Provides alternative blood routes if a primary artery is blocked, reducing stroke impact.

⭐ Berry aneurysms are most common at branch points, especially at the AComA-ACA junction.

Venous Drainage - Brain's Exit Routes

• Superficial Cortical Veins: Drain cortex → Superior Sagittal Sinus.
• Deep Cerebral Veins: Drain deep structures (e.g., basal ganglia) → Great Cerebral Vein (of Galen).
• Dural Venous Sinuses: Channels in dura.
  • Key sinuses: Superior Sagittal, Transverse, Sigmoid, Cavernous.
  • Pathway: Confluence → Transverse → Sigmoid Sinus → Internal Jugular Vein (IJV).
• Cavernous Sinus: Receives ophthalmic veins; prone to thrombosis.

  ⭐ Cavernous sinus thrombosis: ophthalmoplegia (CN III, IV, VI) & facial sensory loss (V1, V2). A

High‑Yield Points - ⚡ Biggest Takeaways

• Circle of Willis: Critical anastomosis between ICA and Vertebrobasilar systems.
• MCA stroke: Most common; contralateral face/arm weakness/sensory loss, aphasia (dominant) or neglect (non-dominant).
• ACA stroke: Supplies medial cortex; contralateral leg/foot weakness/sensory loss.
• PCA stroke: Supplies occipital lobe, thalamus; contralateral homonymous hemianopia, often with macular sparing.
• Lenticulostriate arteries: From MCA, supply basal ganglia/internal capsule; risk of hypertensive lacunar strokes.
• Watershed infarcts: At border zones between territories, from systemic hypoperfusion.