Cerebral arterial circle

The cerebral arterial circle (a.k.a. the circle of Willis) is a critical anastomotic ring of arteries situated within the subarachnoid space at the base of the brain. It interconnects the anterior (carotid) circulation with the posterior (vertebrobasilar) circulation supplying the brainstem and the occipital lobes, thus providing collateral blood flow to protect the brain against ischemia.

Inflow Systems

• Anterior circulation (carotid system) of the brain arises from the internal carotid artery (ICA) that enters the skull via the carotid canal to supply the anterior cerebral territories.

• Posterior circulation (a.k.a. the vertebrobasilar circulation) is driven by paired vertebral arteries that arise from the subclavian arteries. They enter the skull through the foramen magnum, and unite at the pontomedullary junction to form the midline basilar artery.

Vessels forming the circle of Willis

The anastomotic ring is completed by the following interconnected vessels:

• Anterior cerebral arteries (ACAs): Terminal branches of the ICAs that travel anteriorly and medially between the hemispheres to supply the medial brain surface.

• Anterior communicating artery (AComA): A short vascular bridge connecting the left and right ACAs across the midline.

• Internal carotid arteries (terminal segments): Enter the cranial vault and divide into anterior and middle cerebral arteries.

• Posterior communicating arteries (PComA): Bilateral communicating channels bridging each ICA (anterior circulation) to the ipsilateral posterior cerebral artery (posterior circulation).

• Posterior cerebral arteries (PCAs): Form the terminal bifurcation of the basilar artery at the upper pons to supply the inferior temporal surface, occipital lobes, and visual cortex.

---Note

The Middle Cerebral Artery (MCA) arises directly from the ICA and travels laterally through the lateral sulcus to perfuse the lateral hemispheric surface; although it is a terminal ICA branch, it is not part of the circle itself).

Clinical correlates

• Subarachnoid hemorrhage: Because the Circle of Willis resides inside the subarachnoid space, the rupture of saccular berry aneurysms (frequently at arterial junctions such as the communicating arteries) causes subarachnoid haemorrhage.

• Collateral Perfusion: Intercommunication via the anterior and posterior communicating arteries ensures that acute occlusion or stenosis of a single carotid or vertebral inflow vessel can be compensated for by collateral blood flow from the contralateral or opposing circulation.