Circle of Willis (ACA, MCA, PCA)
Circulus arteriosus cerebri
CardiovascularBlood vesselsHead
- What it is
- A ring of arteries on the base of the brain around the optic chiasm and pituitary stalk, linking the two internal carotids (via the anterior communicating artery and the anterior cerebral arteries) with the basilar system (via the posterior communicating and posterior cerebral arteries). Stubs of the anterior, middle and posterior cerebral arteries leave it.
- Function
- Provides alternative routes so that blood can still reach all parts of the brain if one feeding artery narrows; ACA supplies the medial frontal and parietal lobes, MCA most of the lateral cortex, PCA the occipital lobe.
- Size
- Ring ~3 × 2.5 cm; component arteries 1–4 mm diameter.
- Clinical note
- A complete, symmetric circle is present in fewer than half of people; berry aneurysms at its junctions (especially the anterior communicating artery) cause most subarachnoid haemorrhages.