Arterial ring is surrounded by CSF. It connects left internal carotid artery (ICA), right ICA, and vertebrobasilar circulation via communicating arteries (anterior communicating artery (ACOM) and 2 posterior communicating arteries (PCOM)) and left and right posterior cerebral arteries (PCA). For overall pathway of blood flow, basilar artery branches at pons to form left and right PCAs. ICA gives rise to a PCOM at anterior perforated substance that runs back through interpeduncular cistern and joins ipsilateral PCA. Each ICA branches into an ACA. ACAs are then connected by ACOM in chiasmatic cistern. Circle of Willis functions as a compensatory system when there is arterial blockage. It equalizes blood flow between the two hemispheres.