Your brain is awake and working hard all day, every day, even when you’re sleeping! So it makes sense that it needs a lot of oxygen and energy, which is why it is well supplied from several major arteries. The circulation of the brain can ultimately be divided into the anterior and posterior circulation, and understanding their anatomy can help us understand the clinical consequences and management of various issues that can arise! So let’s delve into the anterior circulation of the brain!
Remember that the anterior circulation supplies the anterior portion of the brain, and comes from the internal carotid arteries which divide into the anterior and middle cerebral arteries. The anterior circulation then connects to the posterior circulation through the posterior communicating arteries. The posterior circulation comes from the vertebral arteries, which combine to form the basilar artery. Together, the connection between the anterior and posterior circulation form the circle of Willis, which is an anastomotic network of arteries at the base of the brain which ensure adequate blood flow to the brain, even in cases where part of this circulation becomes occluded! However, there are still instances where obstruction of these arteries and their branches disrupts blood flow to the brain, causing a stroke, which can lead to irreversible neuronal damage.
Now, a stroke can be classified as either ischemic or hemorrhagic. Ischemic strokes are much more common, and they happen because of an acute blockage of one of the blood vessels supplying the brain. Ischemic strokes can be thrombotic, embolic or hypoxic. A thrombotic stroke occurs when there’s a blood clot in the artery, formed directly at the site of infarction, which typically occurs because of a ruptured atherosclerotic plaque. An embolic stroke, on the other hand, is where an embolus from another part of the body travels to the site of infarction to cause obstruction. For example, with atrial fibrillation, a blood clot can form in the heart, where it then travels through the circulation to eventually obstruct brain vessels. Then there are hypoxic strokes, where there is not a direct blockage of a vessel but systemic hypoperfusion or hypoxemia of the brain. This can cause inadequate oxygenation of the brain, especially in watershed areas which are supplied by the terminal branches of two large vessels, and are therefore more prone to hypoperfusion injuries.