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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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