Cerebral Vascular Anatomy

The brain is roughly 2% of body weight yet consumes about 15–20% of cardiac output and a fifth of the body's oxygen, with almost no capacity to store fuel. That voracious, unforgiving metabolism is supported by an arterial architecture that is at once elegant and clinically decisive: two paired inflow systems, a ring of communicating vessels at the base of the brain, and a fan of deep penetrators that brook no collateral. Learn the plumbing once and the angiogram, the stroke syndrome, and the favorite haunts of berry aneurysms all fall into place.

The Dual Blood Supply

The brain is fed by two paired arterial systems that meet and anastomose at the base of the brain in the circle of Willis:

• Anterior circulation — the paired internal carotid arteries (ICAs), which supply most of the cerebral hemispheres (frontal, parietal, and lateral temporal lobes) along with the deep gray nuclei and internal capsule.
• Posterior circulation — the paired vertebral arteries, which unite at the pontomedullary junction to form the single midline basilar artery. Together this vertebrobasilar system supplies the brainstem, cerebellum, thalamus, and the occipital and inferomedial temporal lobes.

The two circulations are not isolated. The circle of Willis stitches them together so that, in principle, a single patent vessel can perfuse the entire brain — the anatomic basis of collateral flow and a recurring theme in stroke.

Anterior Circulation: The Internal Carotid Artery

The ICA enters the skull through the carotid canal, traverses the cavernous sinus, and emerges to give off its branches before bifurcating into its two terminal vessels. Its key branches, in order, are:

• Ophthalmic artery — the first major intradural branch, supplying the retina and orbit. Embolic occlusion produces amaurosis fugax (transient monocular blindness), a hallmark warning sign of ipsilateral carotid disease.
• Posterior communicating artery (PCom) — projects backward to join the posterior cerebral artery, linking anterior and posterior circulations.
• Anterior choroidal artery — a small but important vessel supplying the optic tract, the posterior limb of the internal capsule, and the medial temporal lobe; its occlusion can produce a striking triad of contralateral hemiparesis, hemisensory loss, and homonymous hemianopia.
• Terminal bifurcation — the ICA ends by dividing into the anterior cerebral artery (ACA) and the middle cerebral artery (MCA).

The two ACAs are connected across the midline by the short anterior communicating artery (ACom), completing the anterior half of the circle. The MCA is the direct continuation of the ICA — both its caliber and its trajectory make it the single most common destination for emboli arising in the heart or carotid bifurcation.

Posterior Circulation: Vertebrobasilar System

Each vertebral artery arises from the subclavian artery and ascends through the transverse foramina of the upper cervical vertebrae before entering the skull through the foramen magnum. The major vessels of the posterior circulation are:

• PICA (posterior inferior cerebellar artery) — the largest branch of the vertebral artery, supplying the lateral medulla and the inferior cerebellum. Its territory is the seat of the lateral medullary (Wallenberg) syndrome.
• Basilar artery — formed by the union of the two vertebral arteries; it runs along the ventral pons and gives perforators to the brainstem.
• AICA (anterior inferior cerebellar artery) — branches from the lower basilar to supply the lateral pons and, via the labyrinthine artery, the inner ear.
• SCA (superior cerebellar artery) — branches from the upper basilar to supply the superior cerebellum.
• Posterior cerebral arteries (PCAs) — the two terminal branches of the basilar, supplying the occipital lobes and inferomedial temporal lobes.

The posterior communicating arteries (PCom) connect each ICA to the ipsilateral PCA, closing the posterior half of the ring and completing the circle of Willis.

The Circle of Willis as Collateral Network

The circle of Willis is the great anastomotic safety net of the cerebral circulation. The anterior communicating artery permits cross-flow between the two anterior circulations, and the posterior communicating arteries bridge the anterior and posterior systems. When one feeding vessel is gradually occluded — a slowly stenosing carotid, say — these communicating channels can recruit flow from the contralateral or posterior supply and avert infarction.

• The functional segments are named by their relationship to the communicating arteries: the A1 segment of the ACA lies before the ACom; the P1 segment of the PCA lies before the PCom.
• A complete, symmetric circle — with all communicating segments of adequate caliber — is present in only a minority of people. Most individuals harbor at least one hypoplastic or absent segment.
• This anatomic variation is clinically load-bearing: it explains why an identical vessel occlusion can be silent in one patient (robust collaterals) and catastrophic in another (an incomplete circle that cannot reroute flow).

Perforating (Penetrating) Arteries

The large surface vessels send tiny perforating arteries straight down into the deep gray and white matter. These are functional end-arteries with little or no collateral — when one occludes (classically from lipohyalinosis of chronic hypertension and diabetes), the result is a small, sharply demarcated deep infarct, the lacune.

• Lenticulostriate arteries — perforators from the proximal MCA (M1) that supply the basal ganglia and the internal capsule. Occlusion here produces the classic lacunar syndromes (e.g., pure motor hemiparesis from the posterior limb).
• Thalamoperforators — arising from the PCA (P1) and PCom, supplying the thalamus and rostral midbrain.
• Recurrent artery of Heubner — a prominent perforator from the proximal ACA (typically near the A1–A2 junction) supplying the head of the caudate and the anterior limb of the internal capsule.
• Pontine perforators — paramedian branches of the basilar supplying the basis pontis.

Because these vessels feed deep structures and not the cortex, their infarcts produce the defining lacunar pattern: hemiparesis or hemisensory loss without aphasia, neglect, or a visual field cut.

Watershed (Borderzone) Zones

The territories of the major arteries meet at watershed (borderzone) regions where perfusion pressure is intrinsically lowest. These zones are the first to suffer when global flow drops — during profound hypotension, cardiac arrest, or critical proximal stenosis.

• Cortical (external) borderzones lie at the junctions between the ACA–MCA and MCA–PCA territories. The ACA–MCA borderzone serves the proximal arm and shoulder, producing the striking "man in a barrel" picture of bilateral proximal arm weakness with relatively preserved hand and leg strength.
• Internal borderzones lie in the deep white matter between the superficial pial branches and the deep penetrators, and are a marker of hemodynamic compromise from large-artery stenosis.

A borderzone or bilateral infarct pattern should prompt a search for global hypoperfusion or a flow-limiting proximal lesion rather than a single embolic source.

Berry Aneurysms: Where the Circle Fails

Saccular (berry) aneurysms cluster at the branch points of the circle of Willis, where hemodynamic shear is greatest and the arterial media is congenitally thinnest. They are the leading cause of nontraumatic subarachnoid hemorrhage. The common sites, in rough order of frequency, are:

• Anterior communicating artery (ACom) complex — the single most common location.
• Posterior communicating artery at its junction with the ICA (the PCom–ICA aneurysm) — classically presents with a painful, pupil-involving third nerve palsy from local compression.
• MCA bifurcation in the Sylvian fissure.

Most ruptured aneurysms therefore sit on or near the anterior circulation. Aneurysms arising on the posterior circulation (e.g., the basilar tip) are less common but carry their own surgical and natural-history considerations.

Venous Drainage

Cerebral venous return is organized into a superficial and a deep system, both emptying ultimately into the dural venous sinuses and then the internal jugular veins.

• Superficial system — cortical veins drain the convexities into the superior sagittal sinus.
• Deep system — the internal cerebral veins join to form the great vein of Galen, which empties into the straight sinus, draining the deep gray matter and periventricular white matter.
• Common outflow — the superior sagittal and straight sinuses meet at the confluence of sinuses, which drains laterally through the transverse → sigmoid sinuses into the internal jugular veins.

Dural venous sinus thrombosis (cerebral venous thrombosis) is a key entity to recognize: it produces headache, raised intracranial pressure, seizures, and infarcts that do not respect arterial territories and are often hemorrhagic — a pattern that should always raise the question of a clotted sinus rather than an arterial occlusion.

Quick Reference: Artery, Origin & Territory

Pearl. Two structural facts drive most of clinical cerebrovascular neurology. First, the surface arteries form a communicating ring (collateral is possible), whereas the deep perforators are end-arteries (no collateral) — which is why cortical strokes vary so much with anatomy while lacunes are stereotyped. Second, aneurysms and the richest collaterals share the same real estate: the branch points of the circle of Willis are simultaneously the brain's best safety net and its most dangerous weak spots.