Brain
Blood Supply to Brain
The brain has an anterior and a posterior blood circulation. The anterior circulation is supplied by the paired internal carotid arteries and the posterior circulation is supplied by the basilar artery (which itself is formed from the convergence of the two vertebral arteries).
The anterior and posterior circulations are joined by the anterior and posterior communicating arteries, which form an anastomosis known as the circle of Willis – named after Thomas Willis (1621-1673), an English physician. This anastomosis theoretically allows continuation of blood flow should one vessel be occluded.
The Circle of Willis is located in the interpenduncular cistern at the base of the brain and surrounds the optic chiasm, pituitary stalk and mamillary bodies.
The 3 main arteries arising from the circle of Willis are the anterior, middle and posterior cerebral arteries. The anterior and middle cerebral arteries arise from the internal carotid arteries and the posterior cerebral artery arises from the vertebrobasilar system.
The anastomosis between the anterior and posterior cerebral circulations is formed by the anterior and posterior communicating arteries.
Cerebral Aneurysm
Cerebral aneurysms are saccular out-pouchings of the cerebral arteries usually involving branches of the circle of Willis.
Approximately 85% of cerebral aneurysms develop in the anterior part of the Circle of Willis, and involve the internal carotid arteries and the major branches that supply the anterior and middle sections of the brain.
The most common sites include:
- Anterior communicating artery (30-35%)
- Bifurcation of the internal carotid and posterior communicating artery (30-35%)
- Bifurcation of the middle cerebral artery (20%)
- Bifurcation of the basilar artery, and the remaining posterior circulation arteries (10%)
Territories Supplied by Major Cerebral vessels
The deep penetrating branches of these cerebral arteries are functional end arteries, although there are some anastomoses between distal branches. The boundary zones between the areas supplied by individual arteries (watershed zones) are the areas at most risk from ischaemia during periods of systemic hypotension.
Ischaemic damage in these areas leads to watershed infarcts.
Cerebral Venous Drainage
The cerebral venous system consist of:
The superficial system – the sagittal sinuses and cortical veins. These drain superficial surfaces of both cerebral hemispheres
The deep system – the lateral sinus, straight sinus and sigmoid sinus and draining deeper cortical veins
Both systems drain into the internal jugular veins.