Anatomy of the blood supply to the brain

The human brain is one of the most important and sophisticated organs of the human body. In fact, every minute, it receives about 15% of the total blood pumped by the heart to our entire body! Cerebral circulation is a complex circulatory system, formed by the two internal carotid arteries, the two vertebral arteries, and their branches. The terminal branches of both the internal carotid arteries and the vertebral arteries lie in the subarachnoid space, which is a space between two meningeal layers called the arachnoid mater and pia mater. Anatomoses between these two arteries and their branches give rise to the Circle of Willis, which is a system of vessels at the base of the brain that helps to ensure adequate blood flow to this vital organ.

Let’s start off with the internal carotid arteries, or ICAs, which are the terminal branches of the common carotid arteries, and form the anterior part of the cerebral vascular system. The ICAs ascend on both sides of the neck to reach the base of the skull, where they enter a passageway in the petrous part of the temporal bone, called the carotid canal. Within the carotid canal, the ICA is close to venous plexuses as well as the carotid plexuses of sympathetic nerves. Within the canal, each ICA turns 90 degrees anteromedially, then another 90 degrees superiorly to exit the carotid canal and enter the cranial cavity.

Inside the cranial cavity, the ICA runs through the cavernous sinus, which is, in fact, a dural venous sinus. So the ICA, an artery, actually runs through a sinus filled with venous blood! Within the cavernous sinus, the ICA travels alongside the abducens nerve, and lies in proximity to the oculomotor nerve, the trochlear nerve and the ophthalmic and maxillary divisions of the trigeminal nerve. Then, the ICA emerges from the cavernous sinus and divides into the anterior cerebral artery, the middle cerebral artery and several smaller branches.