Cerebral blood supply
Also known as: blood supply to the brain, cerebral circulation
The cerebral blood supply is the arterial network that perfuses the brain, fed by the paired internal carotid arteries anteriorly and the vertebral arteries posteriorly. These systems join at the circle of Willis, which distributes blood through the anterior, middle, and posterior cerebral arteries.
Blood reaches the brain through two arterial systems. The anterior circulation arises from the internal carotid arteries, which give off the anterior and middle cerebral arteries. The posterior circulation arises from the vertebral arteries, which fuse to form the basilar artery and ultimately the posterior cerebral arteries. The two systems are connected at the base of the brain by the circle of Willis, formed by the anterior communicating artery joining the two anterior cerebral arteries and the paired posterior communicating arteries linking each internal carotid to a posterior cerebral artery.
Each major vessel supplies a predictable territory, which is what makes stroke localization possible. The anterior cerebral artery serves the medial surface of the frontal and parietal lobes, including the paracentral lobule — hence contralateral weakness and sensory loss that is worse in the leg than the arm. The middle cerebral artery supplies the lateral cortex, producing contralateral face and arm deficits, and, when the dominant hemisphere is involved, aphasia. The posterior cerebral artery supplies the occipital lobe and inferior temporal lobe, so occlusion causes contralateral homonymous hemianopia, often with macular sparing.
The brainstem and cerebellum are perfused by branches of the vertebrobasilar system, including the posterior inferior, anterior inferior, and superior cerebellar arteries. Deep structures such as the basal ganglia and internal capsule are fed by small penetrating lenticulostriate branches of the middle cerebral artery, the vessels most vulnerable to chronic hypertension and the usual site of lacunar infarcts and hypertensive hemorrhage. Watershed zones between adjacent territories are the first regions to infarct during systemic hypotension.
Cerebral vascular anatomy is high-yield on USMLE Step 1. Vignettes present a focal neurologic deficit and ask which artery is occluded, or describe a ruptured aneurysm at a circle of Willis junction — the anterior communicating artery for the most common site, the posterior communicating artery for a third nerve palsy with a blown pupil.
Key takeaways
- The brain is supplied by the internal carotid arteries anteriorly and the vertebral-basilar system posteriorly.
- The circle of Willis connects both systems and provides collateral flow when one vessel narrows.
- Anterior cerebral artery strokes affect the leg more than the arm; middle cerebral artery strokes affect the face and arm and may cause aphasia.
- Posterior cerebral artery occlusion causes contralateral homonymous hemianopia with macular sparing.
- Lenticulostriate branches supply deep structures and are the classic site of lacunar infarcts in chronic hypertension.