Vascular Anatomy and Blood Supply

The brain is supplied by two arterial systems — the anterior circulation from the internal carotid arteries and the posterior circulation from the vertebral and basilar arteries — joined at the base of the brain by the circle of Willis. This arrangement distributes blood across the hemispheres, brainstem and deep structures and provides routes of collateral flow.

Definition

The cerebral blood supply is the arterial and venous network that perfuses the brain, comprising the anterior (carotid) and posterior (vertebrobasilar) arterial systems, their anastomotic ring (the circle of Willis), the perforating branches, and the dural venous sinuses that drain the brain.

Scope

This topic covers the major cerebral arteries and their territories, the circle of Willis and its anastomotic role, the deep perforating vessels, and the venous drainage of the brain. It presents this vasculature as reference anatomy relevant to localisation and imaging, not as clinical guidance.

Core questions

• Which arteries supply the brain, and what territories does each serve?
• What is the circle of Willis and why does it matter?
• How is venous blood drained from the brain?

Key concepts

• Anterior (carotid) circulation
• Posterior (vertebrobasilar) circulation
• Circle of Willis and collateral flow
• Arterial territories (ACA, MCA, PCA)
• Deep perforating arteries
• Watershed (border-zone) regions
• Dural venous sinuses and cerebral veins

Mechanisms

The internal carotid arteries supply most of the cerebral hemispheres through the anterior and middle cerebral arteries, while the vertebral arteries fuse into the basilar artery to supply the brainstem, cerebellum and posterior cerebrum via the posterior cerebral arteries. At the brain's base these systems connect through communicating arteries to form the circle of Willis, an anastomotic ring that can provide collateral flow when a feeding vessel is compromised. Small deep perforating branches supply structures such as the thalamus and basal ganglia, and because these territories have limited collateral overlap, occlusion produces characteristic regional patterns (Schmahmann, 2003); chronic small-vessel changes affect deep and periventricular tissue and are well characterised on imaging (Wardlaw et al., 2013). Venous blood drains through cortical and deep veins into the dural sinuses and onward to the internal jugular veins.

Clinical relevance

Knowing the arterial territories and venous drainage underpins the localisation of vascular findings and the interpretation of brain imaging. This entry is reference anatomy on the blood supply; it does not provide diagnostic criteria, risk thresholds or treatment advice.

History

The anastomotic ring at the base of the brain was described by Thomas Willis in the seventeenth century and still bears his name. Classical anatomy then mapped the arterial territories and venous sinuses, and modern imaging refined the understanding of deep perforator territories (Schmahmann, 2003) and of chronic small-vessel involvement of brain tissue (Wardlaw et al., 2013).

Debates

How variable is the circle of Willis between individuals?

A complete, symmetric circle of Willis is the textbook form, but anatomical variants and incomplete segments are common, which affects how much collateral protection any given person's anatomy provides.

Key figures

• Thomas Willis
• Jeremy Schmahmann
• Joanna Wardlaw

Seminal works

• schmahmann-2003
• wardlaw-2013

Frequently asked questions

What are the two main arterial systems supplying the brain?

The anterior circulation arises from the internal carotid arteries and the posterior circulation from the vertebral and basilar arteries; the two are joined at the base of the brain by the circle of Willis.

Why is the circle of Willis important?

It is an anastomotic ring that connects the anterior and posterior circulations, providing potential routes of collateral blood flow if a feeding artery is narrowed or blocked.