The brain can compensate inadequate blood flow in a single artery by the collateral system. This system relies on the efficient connection between the carotid and vertebral arteries through the circle of Willis and, to a lesser extent, the major arteries supplying the cerebral hemispheres. However, variations in the circle of Willis, caliber of collateral vessels, and acquired arterial lesions such as atherosclerosis can disrupt this compensatory mechanism, increasing the risk of brain ischemia resulting from artery blockage.