Brain Blood Supply

Why is brain blood supply so important?

The brain represents about 2% of the total body weight in humans, but it receives one fifth of the resting cardiac output and accounts for 15-20% of the body's blood supply. Brain cells die if the supply of blood carrying oxygen is blocked; therefore the brain has the highest priority for the blood. The human body endeavours to supply the brain with an uninterrupted flow of blood.

Blood supply to the brain is carried by two pairs of arteries: the internal carotid arteries and vertebral arteries, which anastomose at the base of the brain to form what is known as the circle of Willis. The right and left vertebral arteries form a single basilar artery at the base of the brain. This then provides the brain blood supply that feeds the circle of Willis. Refer to the diagram below, reproduced from Wikipedia. 

The carotid arteries and their branches supply the front portion of the brain; and the vertebrobasilar system supplies the rear portion of the brain. These are known as anterior and posterior circulation respectively.

What Relevance is the Brain Blood Supply?

If the supply of blood to the brain is stopped, brain cells die. Ischemic strokes are caused by a blood clot. These can occur within the brain itself, or in the arteries that make up the brain blood supply. Both situations result in a stroke as the brain is deprived of critical blood.

The area of brain blood supply is unique because as a foetus grows, the arteries grow from both the heart and the brain, meeting up in the circle of Willis. Sometimes, the anastomosis (how these arterise join into a network) is not perfect and this can make a person susceptible to problems where plaque can build up at the join. In such cases, the risk of stroke is increased.

It's not essential to understand all the parts of the brain blood supply. However, it's worthwhile to understand that this is a complex part of human anatomy.

The Circle of Willis

The Circle of Willis or the Circulus Arteriosus is an arterial polygon where the blood carried by the two internal carotid arteries and the basilar system comes together and then is redistributed by the anterior, middle, and posterior cerebral arteries. The posterior cerebral artery is connected to the internal carotid artery by the posterior communicating artery.

THe anterior communicating artery joins the anterior cerebral arteries of the two hemispheres together. The middle cerebral arteries are connected to the posterior cerebral arteries by the posterior communicating arteries. This anastomosis between arteries is responsible for developing collateral circulation. It provides a safety mechanism. If one of the major vessels becomes occluded within the circle or proximal to it, the circle will still provides the brain with continued supply of blood. Thus the circle of Willis is of great use in preventing neurological damage.

As long as this circle is successful at maintaining blood pressure at fifty percent of normal, no infarction or death of brain tissue will occur in the blocked area and no permanent effects are produced.

Smaller arteries arise from the circle of Willis and from the major cerebral arteries. They form four groups which include the anteromedial, the anterolateral, the posteromedial and the posterolateral.

Internal Carotid System

The internal carotid artery divides into two main branches called the middle cerebral artery and the anterior cerebral artery. The middle cerebral artery supplies blood to the frontoparietal somatosensory cortex. The anterior cerebral artery supplies blood to the frontal lobes and medial aspects of the parietal and occipital lobes. Before this divide, the internal carotid artery gives rise to the anterior communicating artery and the posterior communicating artery.

Vertebral Artery

The two vertebral arteries run along the medulla and fuse at the pontomedullary junction to form the midline basilar artery, also called the vertebro-basilar artery. Before forming the basilar artery, each vertebral artery gives rise to the posterior spinal artery, the anterior spinal artery, the posterior inferior cerebellar artery (PICA) and branches to the medulla.

Basilar Artery

At the ponto-midbrain junction, the basilar artery divides into the two posterior cerebral arteries. Before this divide, it gives rise to numerous paramedian, short and long circumferential penetrators and two other branches known as the anterior inferior cerebellar artery and the superior cerebellar artery.

Venous Drainage

Arteries provide the brain blood supply. The veins take the blood away, after the brain has taken nutrition from it. 

The prime course of venous drainage of the brain is through cerebral veins that empty into the dural venous sinuses and ultimately into the internal jugular vein.

Cerebral veins are divided into two groups, superficial and deep. The superficial veins usually lie on the surface of the cerebral hemispheres and empty themselves into the superior sagittal sinus. The deep veins drain internal structures and ultimately drain into the straight sinus.

Cerebral veins are thin-walled and valveless. They are also interconnected by several functional anastomoses both within a group and between the superficial and deep groups. The numerous connections between cerebral veins and dural sinuses and venous systems of the meninges, skull, scalp and nasal sinuses assist the spread of thrombus or infection between these vessels.