Blood supply to the brainCircle of Willis

In this note, we are going to discuss the blood supply to the brain and its venous drainage.

Arterial supply

The arterial blood supply to the brain can be divided into the anterior and posterior circulation.

The anterior (or carotid) circulation is formed by two internal carotid arteries and their branches,

and the posterior (or vertebrobasilar) circulation is formed by two vertebral arteries and their branches.

Anterior (carotid) circulation

Let’s start from the anterior circulation. The internal carotid arteries branch off from the common carotid arteries,

enter the skull, make a series of curves, various bends, pass within the cavernous sinus,

Internal carotid artery passing in the cavernous sinus

and then on the basal surface of the brain, each of them is divided into two very important arteries we are interested in.

These are the anterior cerebral artery

and a direct continuation of the internal carotid – the middle cerebral artery.

In addition to this, the left and right anterior cerebral arteries are connected by the anterior communicating artery.

Distal to the cavernous sinus, each ICA gives rise to a couple of branches: the ophthalmic artery

and the anterior choroidal artery.

Posterior (vertebrobasilar) circulation

For now, let’s leave the anterior circulation for a while and move on to the posterior one. It is formed by two vertebral arteries.

They arise from the subclavian arteries and ascend along the posterior aspect of the neck, through the transverse foramina of the cervical vertebrae.

Vertebral artery - extradural and intradural segments

Then they pass through the foramen magnum

and merge to form the basilar artery, so the posterior circulation is also called vertebrobasilar.

Also, the vertebral artery gives rise to a couple of branches: these are the posterior inferior cerebellar artery

and the anterior spinal artery, which supply, respectively, the cerebellum and spinal cord.

Let’s get back to the basilar artery. It ascends superiorly in the basilar sulcus of the anterior aspect of the pons and gives rise to several branches: these are the anterior inferior cerebellar artery,

pontine arteries,

superior cerebellar artery,

and, the most important, terminal branch: the posterior cerebral artery.

Circle of Willis

The key point is that the posterior cerebral artery and the internal carotid artery are connected by the posterior communicating artery, which means that the anterior and posterior circulation are now also connected.

As a result, due to the presence of three communicating arteries (one anterior and two posterior), the arterial circle is formed on the basal surface of the brain. It is called the Circle of Willis, and its role is to provide an “emergency” blood supply to the brain if one of the four arteries is narrowed, or “blocked” for some reason.

And here (next page) is some information about the cortical branches of the anterior cerebral artery, middle cerebral artery, and posterior cerebral artery.

Venous drainage

There are two venous systems in the brain: superficial and deep.

The superficial veins collect blood, mostly from the cerebral cortex. You can see the largest of them in the picture (next page).

And the deep veins collect blood from, respectively, deeper regions of the brain. There are two types of these veins. The first is the medullary veins, which are arranged in a wedge-shaped manner.

So they originate approximately 1-2 cm below the cortical gray matter and eventually drain into the second type – the subependymal veins.

The most significant vein of the deep system is the great cerebral vein of Galen,

which is formed by the union of two internal cerebral veins.

We can also mention two basal veins of Rosenthal.

Eventually, veins of both systems drain blood into the dural venous sinuses, which we have already discussed in the video about the brain meninges.

And then venous blood from the sinuses gets into the internal jugular veins.

In addition, let’s take a look at some other functional types of veins:

Bridging veins are located in the subdural space and connect the superficial veins of the brain with the dural venous sinuses, mostly with the superior sagittal sinus.
Emissary veins connect the dural sinuses and intracranial veins with extracranial veins, meaning, veins that pass not inside the skull, but along its external surface.

Diploic veins pass within the spongy (or cancellous) tissue, called diploë, and actually drain the venous blood from the cranial bones.

Golosary