The brain requires a constant flow of blood to provide glucose and oxygen that it needs to function properly and to remove any wastes. Four arteries supply the circulation the brain needs, the left and right internal carotid arteries, and the left and right vertebral arteries. The internal carotid arteries supply blood mainly to the anterior part of the brain, while the vertebral arteries supply blood to the posterior parts of the brain. All four arteries meet at an area in the brain known as the circle of Willis.

The circle of Willis supplies blood to all the brain even if one of these arteries becomes blocked. The circle of Willis is a ring-shaped arterial meeting place that has smaller arterioles and capillaries branching from this main ring of arteries. These arterioles and capillaries are able to carry blood to individual regions of the brain through the pia mater (the innermost of three meninges covering the brain; a fine vascular membrane that envelops the brain and the spinal cord under the arachnoid and the dura mater). The external carotid artery supplies blood to the face and scalp.

Anatomy of the circle of WiIlis:

• Internal right carotid artery

• Internal left carotid artery

• Left vertebral artery

• Right vertebral artery

According to the Neuroscience 2nd edition web publication featured at the National Institute of Health, the brain receives blood from two sources: the internal carotid artery and the vertebral arteries.

The internal carotid arteries, which arise from a point in the neck where the where the common carotid arteries bifurcate, then branch to form two major cerebral arteries, the anterior and the middle cerebral arteries.

The right and left vertebral arteries come together at a level of the pons on the ventral surface of the brainstem to form the midline basilar artery. The basilar artery joins the blood supply from the internal carotids in an arterial ring at the base of the brain (in the vicinity of the hypothalamus and the cerebral peduncles) called the circle of Willis.

The posterior cerebral arteries arise at this merger and two small bridging arteries, the anterior and posterior communicating arteries can be seen. This merger of the two major sources of arterial blood flow to the brain is what improves the chances of the brain receiving the vital blood flow necessary to carry out all its’ functions in the event one of these arteries becomes blocked or occluded.