Vessel
Circle of Willis
A variable arterial network with collateral potential
High-yield
The standard ring includes ACA A1 segments, ACom, terminal ICAs, PCom arteries and PCA P1 segments. The MCA branches away from this ring and is not one of its connecting sides.
Overview
This basal subarachnoid arterial ring links the left and right carotid circulations with the vertebrobasilar system. Communicating segments can redirect flow when pressure differs between its inputs. Vessel size and configuration vary considerably, and an anatomically visible connection does not guarantee enough collateral flow to prevent ischemia.
Reference facts
- Origin
- Connections between the paired internal carotid terminations and the proximal PCAs supplied from the basilar termination.
- Course
- Lies around the optic-chiasmatic, infundibular and interpeduncular regions at the brain's base.
- Branches
- Major cerebral arteries continue from the ring, and small perforators arise from its component vessels to supply nearby deep structures.
- Supplies
- Provides possible cross-circulation between right and left sides and between carotid and vertebrobasilar systems; flow capacity varies.
Clinical correlation
Saccular aneurysms often develop near major intracranial arterial junctions, including ACom and the ICA–PCom region; the MCA bifurcation is another important site outside the ring. Rupture can release blood into the subarachnoid space. Symptoms and collateral protection depend on the individual anatomy.
Frequently asked questions
Where does the circle of Willis originate?
Connections between the paired internal carotid terminations and the proximal PCAs supplied from the basilar termination.
What does the circle of Willis supply?
Provides possible cross-circulation between right and left sides and between carotid and vertebrobasilar systems; flow capacity varies.
What are the branches of the circle of Willis?
Major cerebral arteries continue from the ring, and small perforators arise from its component vessels to supply nearby deep structures.
What is the course of the circle of Willis?
Lies around the optic-chiasmatic, infundibular and interpeduncular regions at the brain's base.