Blood supply to the brain can be divided into an anterior and a posterior circulation. The posterior circulation supplies the cerebellum, brainstem, occipital lobes, and inferomedial temporal lobes, and comes from the vertebral arteries. The vertebral arteries combine to form the basilar artery, which eventually divides into the posterior cerebral arteries. The posterior circulation then connects to the anterior circulation through the posterior communicating arteries. Remember that the anterior circulation comes from the internal carotid artery which divides into the anterior and middle cerebral arteries. Together, the connection between the posterior and anterior circulation form the Circle of willis, which is an anastomotic network of arteries at the base of the brain that ensure adequate blood flow even in cases where part of this circulation becomes occluded! However, there are still instances where obstruction of these arteries and their branches can disrupt blood flow to the brain, so understanding their anatomy and what parts of the brain they nourish can help us better understand the clinical manifestations and management.
When blood flow to the brain is obstructed, that causes a stroke, which can be either ischemic or hemorrhagic. Ischemic strokes can be caused by thrombi, emboli, and hypoperfusion injuries, with the latter most commonly affecting the watershed areas of the brain. Hemorrhagic strokes, on the other hand, occur when there is a bleed within the brain tissue called an intracerebral or intraparenchymal hemorrhage, or a bleed in the subarachnoid space called a subarachnoid hemorrhage. The posterior circulation of the brain is susceptible to all of these, and the clinical signs and symptoms depend on which artery is occluded.