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Dr. Yasser Mokhtar, M.D.

Introduction

The brain, though representing 2% of the total body weight, it receives one fifth of the resting cardiac output. This blood supply is carried by the two internal carotid arteries (ICA) and the two vertebral arteries that anastomose at the base of the brain to form the circle of Willis.

Carotid arteries and their branches (referred to as the anterior circulation) supply the anterior portion of the brain while the vertebrobasilar system (referred to as posterior circulation) supplies the posterior portion of the brain.

Anatomy of the cerebral circulation

The brain receives its blood supply from the heart by way of the aortic arch that gives rise to the brachiocephalic (innominate) artery, left common carotid artery (CCA) and the left subclavian artery (Sheldon 1981).

A. Carotid Arterial System

1. Common Carotid artery (CCA):

The left CCA arises from the aortic arch while the right arises from the bifurcation of the innominate artery.

2. External carotid artery (ECA):

It starts at the CCA bifurcation. Its branches supply the jaw, face, neck and meninges. The bulk of the meningeal circulation is supplied by the middle meningeal artery, the most important branch of the maxillary artery which is one of the two terminal branches of the ECA (the other terminal branch is the superficial temporal artery). These two terminal branches in addition to the occipital artery can serve as collateral channels for blood supply to the brain in instances of obstruction of the ICA. The ascending pharyngeal artery can serve as a source of blood in instances of occlusion of the ICA (Lasjaunias and Moret 1976).

3. Internal carotid artery (ICA)

It starts at the carotid sinus at bifurcation of CCA at the level of the upper border of the thyroid cartilage at the level of the fourth cervical vertebra. It ascends just behind and lateral to the hypopharynx where it can be palpated (Hollinshead 1982). It passes up the neck without any branches to the base of the skull where it enters the carotid canal of the petrous bone. It then runs through the cavernous sinus in an S-shaped curve (the carotid siphon), then it pierces the dura (beginning its subarachnoid course) and exits just medial to the anterior clinoid process and then ascends to bifurcate into anterior cerebral artery and the larger middle cerebral artery.  


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