What vessels supply blood to different areas of the brain?

Blood Supply to the Brain: Vascular Territories

The brain receives blood from two major arterial systems: the internal carotid arteries (anterior circulation) supplying the anterior two-thirds of the cerebral hemispheres, and the vertebrobasilar system (posterior circulation) supplying the posterior brain, brainstem, and cerebellum. 1

Anterior Circulation (Internal Carotid System)

The internal carotid arteries provide the primary blood supply to the anterior and middle portions of the cerebral hemispheres 1:

Internal Carotid Artery Course and Branches

• The internal carotid artery enters the skull base, traverses the petrous bone, and enters the subarachnoid space near the ophthalmic artery level 1
• The artery gives rise to the posterior communicating artery, which connects to the posterior cerebral artery through the circle of Willis 1
• The internal carotid artery then bifurcates into two major terminal branches: the anterior cerebral artery (ACA) and middle cerebral artery (MCA) 1

Anterior Cerebral Artery Territory

• Supplies the medial surfaces of the frontal and parietal lobes 1
• The left and right anterior cerebral arteries connect through the anterior communicating artery, forming the anterior portion of the circle of Willis 1

Middle Cerebral Artery Territory

• Supplies the lateral surfaces of the cerebral hemispheres, including most of the frontal, parietal, and temporal lobes 1
• This is the largest terminal branch of the internal carotid artery 1

Posterior Circulation (Vertebrobasilar System)

The vertebral and basilar arteries supply the posterior brain regions 1:

Vertebral Artery Anatomy

The vertebral arteries are divided into four segments (V1-V4), with the first three being extracranial 1:

• V1 segment: Extends from the vertebral artery origin (typically from the subclavian artery) to the transverse foramina at C6 1
• V2 segment: Travels through the transverse foramina from C6 to C2, with branches supplying vertebrae and musculature 1
• V3 segment: Exits the C2 transverse foramen, courses around C1, and enters the foramen magnum 1
• V4 segment (intracranial): Extends from dural entry to the vertebrobasilar junction, giving off the anterior and posterior spinal arteries, posterior meningeal artery, medullary branches, and posterior inferior cerebellar artery (PICA) 1

Basilar Artery and Its Branches

• The left and right vertebral arteries join to form the basilar artery 1
• The basilar artery gives rise to the posterior cerebral arteries, which supply the occipital lobes, inferior temporal lobes, and portions of the thalamus 1
• Vertebral artery atherosclerosis accounts for approximately 20% of posterior circulation strokes 1

Circle of Willis: Critical Collateral Network

The circle of Willis provides crucial collateral pathways between the anterior and posterior circulations, though a complete circle exists in fewer than 50% of individuals 1:

Key Collateral Pathways

• Posterior communicating arteries connect the internal carotid arteries to the posterior cerebral arteries 1
• Anterior communicating artery connects the left and right anterior cerebral arteries 1
• External carotid to internal carotid collaterals via the internal maxillary and superficial temporal arteries to ophthalmic branches 1
• External carotid to vertebral artery collaterals via the occipital branch 1

Clinical Pitfalls and Anatomical Variations

Common Anatomical Variants

• Hypoplasia of the posterior communicating artery is common 2
• Agenesis of the anterior communicating artery occurs in some individuals 2
• Fenestration or duplication of the basilar artery represents persistent embryological variants 2
• Persistent trigeminal artery (carotid-vertebrobasilar anastomosis) may persist from embryological development 2

Important Considerations

• The configuration of the circle of Willis is highly variable, with significant implications for collateral flow during arterial occlusion 1
• The total cross-sectional area of inflow vessels (51.43 mm²) is larger than major outflow vessels (37.76 mm²), providing a mechanism for pressure regulation 3
• Vertebral artery blood flow can be compromised by extrinsic compression from bony structures, atherosclerosis, or sustained end-range cervical rotation 4