To understand aneurysms, it is helpful to understand the circulatory system of the brain. The heart pumps oxygen- and nutrient-laden blood to the brain, face, and scalp via two major sets of vessels: the internal carotid arteries and the vertebral arteries. The jugular and other veins bring blood out of the brain.
The carotid arteries run along the front of the neck – one on the left and one on the right. They are what you feel when you take your pulse just under your jaw. The carotid arteries split into external and internal arteries near the top of the neck.
The external carotid arteries supply blood to the face and scalp. The internal carotid arteries supply blood to the front (anterior) three-fifths of cerebrum, except for parts of the temporal and occipital lobes.
The vertebral arteries travel along the spinal column and cannot be felt from the outside. They join to form a single basilar artery (hence the name vertebrobasilar arteries) near the brain stem at the base of the skull. The arteries supply blood to the posterior two-fifths of the cerebrum, part of the cerebellum, and the brain stem.
Because the brain relies on only two sets of major arteries for its blood supply, it is very important that these arteries are healthy. These arteries that conduct blood to the brain — the internal-carotid and vertebral arteries — connect through the Circle of Willis, which loops around the brainstem at the base of the brain. From this circle, other arteries — the anterior cerebral artery (ACA), the middle cerebral artery (MCA), and the posterior cerebral artery (PCA) — arise and travel to all parts of the brain. Brain aneurysms tend to occur at the junctions between the arteries that make up the Circle of Willis.