Blood supply of the brain (arterial & venous)

🧠 Notes: Blood Supply of the Brain

🖇 I. General Principles

• The brain is supplied by two main arterial systems:
  1️⃣ Vertebrobasilar system
  2️⃣ Internal Carotid Artery (ICA) system
• The brain uses about 25% of the body’s oxygen.
• Stroke: every minute, about 1.9 million neurons are lost → irreversible damage.
• The brain has few anastomoses, so many arteries act as end arteries.
• Two types of branches for each cerebral artery:
• • Cortical (superficial): supply the cerebral cortex.
  • Central (deep): supply basal ganglia and internal capsule (high risk if blocked).

🟠 II. Arterial Supply Systems

🔹 A. Vertebrobasilar System

• Vertebral arteries (from Subclavian) join at the lower border of the pons → form the Basilar artery.
• Vertebral branches:
• • Anterior spinal artery
  • Posterior spinal arteries (2)
  • Medullary branches
  • PICA (Posterior Inferior Cerebellar Artery) → supplies posterior inferior cerebellum
• Basilar artery: runs on the anterior pons and ends at its upper border.
• • Branches: Pontine, Labyrinthine, AICA, SCA, and terminal Posterior Cerebral Arteries (PCA)

🔹 B. Internal Carotid Artery (ICA) System

• Terminal branch of Common Carotid artery.
• Main branches:
• • Ophthalmic artery
  • Anterior choroidal artery
  • Posterior communicating artery
  • Terminal branches → ACA (Anterior Cerebral Artery) & MCA (Middle Cerebral Artery)

🟣 III. Circle of Willis (Circulus Arteriosus)

• Located in the interpeduncular fossa.
• Usually formed by 9 (sometimes 10) arteries.
• Includes:
• • 2 ACA + 2 ICA + 2 PCA
  • 1 Anterior communicating + 2 Posterior communicating arteries
• ⚠️ MCA does NOT take part in the circle.
• Functions:
• • Provides anastomosis between carotid & vertebrobasilar systems
  • Equalizes pressure
  • Allows collateral circulation if blockage occurs
• Berry aneurysm: congenital arterial wall weakness, commonly at ICA bifurcation or ACA–Ant. communicating junction → rupture → subarachnoid hemorrhage (fatal)

🟠 IV. Cerebral Arteries (Terminal Branches)

🔸 1. Anterior Cerebral Artery (ACA)

• Origin: Internal Carotid
• Course: along the callosal sulcus
• Cortical supply:
• • Upper inch of lateral surface
  • Entire medial surface of frontal & parietal lobes
  • Medial orbital surface
• Central supply: Corpus striatum, internal capsule, septal region
• Callosal branch: supplies corpus callosum (except splenium)

🔸 2. Middle Cerebral Artery (MCA)

• Origin: Internal Carotid
• Supply: almost all lateral surface of the hemisphere (except upper & lower inch)

🔸 3. Posterior Cerebral Artery (PCA)

• Origin: Basilar artery
• Cortical supply:
• • Entire occipital lobe (visual cortex)
  • Lower inch of lateral surface
  • Medial surface of temporal lobe (except pole)
• Central supply: thalamus, hypothalamus, epithalamus

🟣 V. Venous Drainage

• Brain veins drain into dural venous sinuses.
• Features:
• • Thin walls, no valves → bidirectional flow
• Main groups:
• • External cerebral veins (superficial & deep)
  • Internal cerebral veins
  • Terminal veins (basal & great cerebral veins)
• Examples:
• • Superior cerebral veins → superior sagittal sinus
  • Inferior cerebral veins → inferior sagittal/transverse sinuses

🚨 VI. Key Clinical & Exam Points

1. Stroke: irreversible loss of neurons (1.9 million/min).
2. Circle of Willis: memorize 9 arteries, its role in collateral circulation, and its clinical importance.
3. Berry aneurysm: common at ICA bifurcation or ACA–Ant. comm. junction → subarachnoid hemorrhage.
4. Artery origins:
5. • ACA & MCA → from ICA
   • PCA → from Basilar artery
5. ACA distribution: upper inch of lateral surface + medial surfaces of frontal/parietal lobes.
6. Occlusion effects:
7. • PCA → vision loss
   • ACA → monoplegia/hemiplegia + urinary incontinence
7. Always mention both Cortical & Central branches when describing any cerebral artery.
8. Cerebellar arteries:
9. • PICA → from Vertebral artery
   • AICA & SCA → from Basilar artery