Cerebral Blood Supply and Vascular Territories

Anterior Circulation

The American Heart Association notes that the internal carotid arteries provide the primary blood supply to the anterior and middle portions of the cerebral hemispheres 1, 2
The internal carotid artery enters the skull base, traverses the petrous bone, and enters the subarachnoid space near the ophthalmic artery level, giving rise to the posterior communicating artery, which connects to the posterior cerebral artery through the circle of Willis 1, 3
The internal carotid artery then bifurcates into two major terminal branches: the anterior cerebral artery (ACA) and middle cerebral artery (MCA) 1, 3
The anterior cerebral artery 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, 2
The middle cerebral artery supplies the lateral surfaces of the cerebral hemispheres, including most of the frontal, parietal, and temporal lobes 1
The middle cerebral artery is the largest terminal branch of the internal carotid artery 1

The vertebral and basilar arteries supply the posterior brain regions 4, 5
The vertebral arteries are divided into four segments (V1-V4), with the first three being extracranial 4, 5
Vertebral artery atherosclerosis accounts for approximately 20% of posterior circulation strokes 4, 5
The basilar artery gives rise to the posterior cerebral arteries, which supply the occipital lobes, inferior temporal lobes, and portions of the thalamus 1, 4

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, 2
The anterior communicating artery connects the left and right anterior cerebral arteries 1, 3
External carotid to internal carotid collaterals via the internal maxillary and superficial temporal arteries to ophthalmic branches 1, 3
External carotid to vertebral artery collaterals via the occipital branch 1, 3
The configuration of the circle of Willis is highly variable, with significant implications for collateral flow during arterial occlusion 1, 2

The border zones between the anterior cerebral artery (ACA) and middle cerebral artery (MCA), between the MCA and posterior cerebral artery (PCA), and between the anterior and posterior circulations constitute watershed territories that are especially susceptible to injury during systemic hypoperfusion states. 6

Circulation, 2011

Stroke, 2008