Arteries of the Cerebral Arterial Circle and Nearby Structures
An anatomical guide to the cerebral arterial circle, the interconnected arterial network that ensures a redundant blood supply for the brain.
The cerebral arterial circle, also known as the Circle of Willis, is a polygon-shaped network of connected arteries at the base of the brain. It is situated within the interpeduncular fossa. This network connects the major arterial systems that deliver blood to the brain.
The Major Feeder Arteries
The blood supply to the cerebral arterial circle originates from two primary systems: the internal carotid arteries and the vertebral arteries. These paired vessels represent the anterior and posterior circulatory routes to the brain. The right and left internal carotid arteries ascend through the neck and enter the cranial cavity to form the anterior part of the circulation.
Posteriorly, the two vertebral arteries travel up through the cervical vertebrae before entering the skull. These two arteries then merge at the base of the brainstem to form a single basilar artery. The basilar artery and the internal carotid arteries are the principal conduits that channel blood into the arterial ring.
Components of the Cerebral Arterial Circle
The ring is formed by a series of arteries that connect the anterior and posterior circulations. A single anterior communicating artery links the left and right anterior cerebral arteries. These anterior cerebral arteries are major branches of the internal carotid arteries, and their initial, pre-communicating portions form part of the circle.
The circle is completed by two posterior communicating arteries. Each posterior communicating artery extends backward from an internal carotid artery to connect with a posterior cerebral artery. The posterior cerebral arteries, which are the terminal branches of the basilar artery, contribute their own pre-communicating segments to finalize the ring structure.
Key Arterial Branches and Nearby Structures
Branching from the cerebral arterial circle are several arteries that supply large territories of the brain. The most prominent of these are the middle cerebral arteries (MCAs), which are not part of the circle itself but arise from the internal carotid arteries. The MCAs supply blood to the lateral surfaces of the brain’s frontal, parietal, and temporal lobes.
The cerebral arterial circle encircles several important non-vascular structures at the base of the brain. It is located in close proximity to the optic chiasm, the X-shaped structure where the optic nerves cross over. The pituitary gland and its stalk, the infundibulum, are also situated within the center of this arterial ring.
Anatomical Function
The primary purpose of this circular arterial structure is to provide redundancy in the brain’s blood supply. It functions as a collateral pathway, connecting the four major arteries that feed the brain—the two internal carotids and the two vertebrals. This allows for the rerouting of blood flow between the anterior and posterior circulatory systems.
This design offers a protective mechanism against ischemia, which is a reduction in blood flow. Should one of the major feeder arteries become blocked, the connections within the circle can allow blood from the other arteries to compensate. This rerouting helps maintain perfusion to brain tissue, potentially mitigating damage from a stroke.