StudyG Logo
Study G
Concept Breakdowns

Circle of Willis Anatomy and Aneurysm Sites

These flashcards cover the Circle of Willis — the arterial anastomotic ring at the base of the brain, and one of the most tested anatomy topics in medical school, nursing programs, and neuroscience courses. Understanding its component vessels, communicating arteries, and territory of supply is essential for interpreting stroke presentations and understanding why aneurysms form at specific junctions. These cards target the vessel arrangement, common aneurysm locations, and key clinical correlations emphasized in USMLE Step 1 preparation.

Interactive Deck

5 Cards
1
Front

What vessels form the Circle of Willis?

Click to reveal
1
Back

Anterior circulation (ICA-derived):

  • Anterior cerebral arteries (ACA) ×2
  • Anterior communicating artery (AComm) ×1
  • Internal carotid artery segments ×2

Posterior circulation (basilar-derived):

  • Posterior cerebral arteries (PCA) ×2
  • Posterior communicating arteries (PComm) ×2
  • Basilar artery (feeds into ring)
2
Front

What is the most common site of a Circle of Willis aneurysm?

Click to reveal
2
Back
  • Anterior communicating artery (AComm): ~30–35% — most common overall
  • Posterior communicating artery (PComm): ~25% — CN III palsy on rupture
  • MCA bifurcation: ~20%
  • ICA bifurcation / ophthalmic artery: ~10–15%
  • Basilar tip: ~5–10%
3
Front

What is a berry aneurysm and what causes it?

Click to reveal
3
Back

Berry aneurysm (saccular aneurysm): a thin-walled outpouching at arterial branch points, arising from:

  • Congenital defects in the tunica media (muscular layer)
  • Increased hemodynamic stress at bifurcations

Associated conditions: ADPKD, Marfan syndrome, Ehlers-Danlos syndrome, coarctation of the aorta

4
Locked

What are signs of a posterior communicating artery aneurysm rupture?

5
Locked

What stroke deficits result from anterior cerebral artery occlusion?

Master this topic effortlessly.

Study G helps you master any topic effortlessly using proven learning algorithms and smart review timing

Download Study G

Frequently Asked Questions

How many arterial segments make up the Circle of Willis?

The Circle of Willis consists of 9 arterial segments: 2 anterior cerebral arteries (ACA), 1 anterior communicating artery (AComm), 2 internal carotid artery segments, 2 posterior communicating arteries (PComm), and 2 posterior cerebral arteries (PCA). The basilar artery feeds into the ring but is not counted as part of it.

What is the difference between a berry aneurysm and an arteriovenous malformation (AVM)?

Berry aneurysm (saccular): a focal outpouching at an arterial bifurcation; rupture causes subarachnoid hemorrhage — sudden severe headache, stiff neck, and photophobia.

AVM: an abnormal tangle of vessels bypassing capillaries; typically causes seizures or intracerebral hemorrhage in younger patients.

  • Key distinction: aneurysms rupture at bifurcations; AVMs bleed from parenchymal vessels

Why do aneurysms form at arterial junctions in the Circle of Willis?

Aneurysms form at bifurcations because hemodynamic stress peaks where blood flow divides. Vessel walls at junctions have congenital weak points in the tunica media (smooth muscle layer). Over time, elevated blood pressure and turbulent flow cause progressive outpouching. Conditions like hypertension, ADPKD, and coarctation of the aorta significantly increase aneurysm risk.