Anatomy of the blood supply to the brain

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Introduction0:00–1:01

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.

Internal carotid arteries1:01–3:00

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.
To simplify this complex course, the ICA can be divided into 4 parts: the cervical part, that extends from common carotid to the carotid canal; the petrous part, that extends from the carotid canal to the foramen lacerum; the cavernous part, meaning the part of the ICA within the cavernous sinus; and the cerebral part, after it exits from the cavernous sinus.

Cerebral part of the ICA3:00–4:19

The cerebral part of the ICA ends by bifurcating into two major terminal branches: the anterior cerebral artery and middle cerebral artery.
Now, the cerebral part of the ICA gives off several other collateral branches that supply various structures in the head - including the brain.
Anteriorly, right after it exits from the cavernous sinus, it gives off the ophthalmic artery. This artery, intuitively, enters the orbit and gives off the central retinal artery which is the main blood supply to the retina.
Posteriorly, the ICA gives off two branches: the posterior communicating artery and the anterior choroidal artery. The posterior communicating artery arises right before the terminal bifurcation of the ICA, and connects the middle cerebral and posterior cerebral artery in the Circle of Willis.
The anterior choroidal artery, also arises close to the terminal bifurcation, and courses posteriorly to enter the lateral ventricle of the brain, to end in the choroid plexus, supplying it and several other cerebral structures along its course.And now let’s use an image of the medial side of one cerebral hemisphere to look at the two terminal branches of the ICA: the anterior cerebral artery and the middle cerebral artery.

Anterior cerebral artery4:19–5:17

From its origin, the anterior cerebral artery runs anteriorly and then turns superiorly and travels posteriorly arching over the corpus callosum.
Throughout this course it supplies the medial aspect of the frontal lobe and the parietal lobe by giving off numerous small cortical branches.
These cortical branches will also arch over to the lateral aspect of the hemisphere and supply the supero-lateral part of the frontal and parietal lobes.
The two anterior cerebral arteries on each side are connected through the anterior communicating artery, forming the anterior part of the circle of willis.
Next, after branching from the internal carotid artery, the middle cerebral artery travels laterally along the lateral sulcus.

Middle cerebral artery5:17–6:29

While it traverses the lateral aspect, it gives off deep penetrating branches called central arteries which supply the deeper structures of the brain like the corpus striatum, which is made up of the caudate and lenticular nucleus.
This is why these central arteries are often referred to as lenticulostriate branches of the middle cerebral artery. After reaching the lateral surface, the middle cerebral artery provides a few branches that supply the lateral surface of the temporal lobe and part of the infero-lateral surface of the frontal and parietal lobe.
Now, both the anterior and middle cerebral arteries are often referred to as terminal arteries as there is no collateral circulation in areas supplied by them.
Thus any blockage in these arteries may prevent blood from reaching that part of the brain completely.Okay, that was a lot!

Quiz6:29–6:49

Let’s take a short break and see if you can recall the collateral and terminal branches of the internal carotid artery.Alright!
Now let’s switch gears and look at the vertebral arteries, which arise from the subclavian arteries and form the vertebro-basilar system.

Vertebral arteries6:49–7:40

The vertebral arteries ascend through the transverse foramina of the cervical vertebrae to enter the skull through the foramen magnum, and then they pierce the dura mater to get into the subarachnoid space.
Here, the vertebral arteries start their intracranial course, by moving forward, upward and medially over the medulla oblongata.
At the ponto-medullary junction, which is where the pons and medulla meet, the two vertebral arteries merge with each other and form a single basilar artery - hence the term “vertebro-basilar” system.
Now, before the two vertebral arteries merge, they give off two important branches: the anterior spinal artery and the posterior inferior cerebellar artery.

Anterior spinal artery7:40–8:17

The anterior spinal artery is formed by two separate branches of the vertebral artery, which travel inferiorly and medially and subsequently form the singular anterior spinal artery at the level of the foramen magnum.
The anterior spinal artery then courses in the anterior, median aspect of the spinal cord, supplying its rostral anterior two-thirds.
The posterior inferior cerebellar artery, or PICA, on the other hand, originates from the vertebral artery and winds posteriorly around the upper part of medulla oblongata, travels along the inferior aspect of the cerebellum and divides into medial and lateral branches.

Posterior inferior cerebellar artery8:17–9:08

The medial branch continues posteriorly, while the lateral branch supplies the inferior part of the cerebellum. Now, there is another branch called the posterior spinal artery which in one fourth of the population arises from the vertebral artery and in the other three fourths of the population from the PICA.
The posterior spinal artery mainly supplies the posterior aspect of the spinal cord.Now let’s look at the basilar artery.

Basilar artery9:08–10:01

After its origin, it ascends along the ventral surface of the pons. On its way, it gives off four paired collateral branches: the anterior inferior cerebellar arteries, or AICA; the labyrinthine arteries; the pontine arteries; and the superior cerebellar arteries.
Then, just superior to the origin of the oculomotor nerve, the basilar artery ultimately divides into a pair of terminal branches called the posterior cerebral arteries.
The left and right posterior communicating arteries originate from these posterior cerebral arteries and merge with the internal carotid arteries, thereby closing the posterior portion of the circle of willis.

Anterior inferior cerebellar arteries10:01–11:01

The anterior inferior cerebellar arteries course postero-laterally, and mainly supply the inferior aspect of the cerebellum, as well as part of the pons and middle cerebellar peduncle.
Next, the labyrinthine arteries course laterally, along with the seventh and eighth cranial nerves, to the internal acoustic meatus to enter and supply the inner ear.
Next up, the pontine arteries are numerous small branches that course laterally to supply the pons. Last, just before the bifurcation of the basilar artery, there are the superior cerebellar arteries, which course laterally and then travel around the cerebral peduncle to reach and supply the superior aspect of cerebellum, the pineal gland, part of the pons and part of the third ventricle.
Okay, that was a lot! Let’s take a short break and see if you can recall the branches of the vertebral artery!Great!

Quiz11:01–11:16

Circle of willis11:16–12:44

Now let’s put all this information together and look at the circle of Willis, named after the English physician Thomas Willis.
See, the anterior communicating artery connects the two anterior cerebral arteries, while the posterior communicating arteries connect the posterior cerebral arteries with the internal carotid arteries on each side.
This forms the circle of Willis in the subarachnoid space at the base of the brain, right in front of the midbrain. This circle helps create a collateral arterial circulation, so complete or partial blockage of any one of the cerebral arteries can be compensated by other arteries.
In this way, cerebral perfusion is more likely to be maintained which helps to prevent ischemic and hypoxic brain injury from occurring.The circle of willis is closely related to some important surrounding structures.
For example, the anterior communicating artery travels just anterior to the optic chiasm, while the posterior communicating artery courses just medial to the third cranial nerve on each side.
Thus any abnormal dilatation or aneurysm in a particular artery may lead to compression of a related nerve or other associated structure, causing specific neurological deficits.On a quick side note, even though the major cerebral arteries participate in the formation of the circle of Willis, each of them predominantly supplies one part of the cerebral cortex.

Cortical arterial supply12:44–13:22

The anterior cerebral arteries supply the anteromedial surface of the cerebral cortex. The middle cerebral arteries are located laterally, supplying the majority of the lateral surface of the cerebral cortex.
Finally, the posterior cerebral arteries supply both the posterior and inferior surfaces of the cerebral cortex. An important concept to remember when considering vasculature supply and infarcts is the cortical homunculus.

Cortical homunculus13:22–14:49

The homunculus is a kind of neurological map that depicts the neurons in a particular area of the cortex that process a particular body region, as well as the density of neurons that are dedicated to that region of the body.
The motor homunculus is located in the frontal lobe, while the sensory homunculus is just caudal to it, in the parietal lobe.
The cortical homunculus can be represented as a body lying on top of the brain, where each body part lies on top of its corresponding brain area.
The toes are represented first on the medial aspect of the cerebral hemisphere. Then moving superiorly and then laterally, more superior parts of the body are progressively represented, until we see the face most lateral.
The motor and sensory homunculi on each side of the brain correspond to motor and sensory functions on the opposite side of the body.
The medial area of the homunculus, representing the lower body, is supplied by the anterior cerebral artery, while the lateral area representing the upper body and face is supplied by the middle cerebral artery.Now, as we move away from the origin of the anterior, middle and posterior cerebral arteries and more towards the areas of cerebral cortex that border the cortical branches of these arteries, blood supply by these cerebral arteries gradually decreases.

Watershed zone14:49–16:08

As a result, these border areas are more prone to ischemic injuries. In these cases, the pattern of injury is called a watershed infarct or watershed stroke, where healthy tissue continues to extract what it needs from the blood flowing by, leaving little or no oxygen and nutrients for the tissue furthest away and as a result, the tissues that are the furthest downstream are affected the most.
The “furthest downstream” tissues are the watershed areas, which are at the border of blood supply from two separate groups of cerebral arteries.There are three main watershed areas, where watershed infarcts typically occur: between the anterior cerebral and the middle cerebral arteries; between the posterior cerebral and the middle cerebral arteries; and between the superficial and deep branches of the middle cerebral arteries.Finally, let’s briefly look at the venous drainage of the brain, which starts as a series of small superficial veins.

Venous drainage16:08–17:04

These veins drain into dural venous sinuses, which are venous channels located between the two layers of dura mater: the periosteal layer and meningeal layer.
The superior sagittal sinus runs along the longitudinal fissure, where it absorbs cerebrospinal fluid from the meninges.
It drains to the confluence of sinuses, along with the straight sinus and occipital sinuses, which then empties into the transverse sinuses.
The transverse sinuses drain to the sigmoid sinuses, which then drain to the internal jugular veins, ultimately returning to the heart.Alright, as a quick recap.

Review17:04–19:30

The anterior part of the cerebral circulatory system is created by the internal carotid arteries. The ICA has 4 parts: the cervical part, the petrous part, the cavernous part, and the cerebral part.
Collateral branches of the internal carotid artery include the ophthalmic artery, the posterior communicating artery, and the anterior choroidal artery.
The terminal branches of the ICA are the anterior and middle cerebral arteries. The anterior cerebral arteries are connected via the anterior communicating artery.
The posterior aspect of the cerebral circulatory system arises from the vertebral arteries which merge to form the basilar artery.
Before they join, they give off the anterior spinal artery and the posterior inferior cerebellar artery. The basilar artery gives off four paired collateral branches: the anterior inferior cerebellar arteries, or AICA; the labyrinthine arteries; the pontine arteries; and the superior cerebellar arteries.
Ultimately, the basilar artery divides into a pair of terminal branches called the posterior cerebral arteries, which in turn give off the left and right posterior communicating arteries.
The circle of willis is formed by the anterior cerebral arteries, that are connected by the anterior communicating artery, and the posterior cerebral arteries and the internal carotid arteries, which are connected by the posterior communicating arteries.
The three main watershed areas of the brain are between the anterior cerebral and the middle cerebral arteries; between the posterior cerebral and the middle cerebral arteries; and between the superficial and deep branches of the middle cerebral arteries.
Blood returns from the brain to the systemic circulation through a series of venous channels called dural venous sinuses that ultimately drain into the internal jugular veins.