The cerebrovascular system comprises the vessels that transport blood to and from the brain. The brain's arterial supply is provided by a pair of internal carotid arteries and a pair of vertebral arteries, the latter of which unite to form the basilar artery. The anterior cerebral artery, a branch of the internal carotid artery, perfuses the anteromedial cerebral cortex; the middle cerebral artery, also a branch of the internal carotid artery, perfuses the lateral cerebral cortex; and the posterior cerebral artery, a branch of the basilar artery, perfuses the medial and lateral portions of the posterior cerebral cortex. The internal carotid arteries, the anterior cerebral arteries, and the posterior cerebral arteries anastomose through the anterior and posterior communicating arteries to form the circle of Willis, a vascular circuit surrounding the optic chiasm and pituitary stalk. The circle of Willis provides an alternative channel for blood flow in case of vascular occlusion and equalizes blood flow between the cerebral hemispheres. The cerebral hemispheres are drained by superficial cerebral veins (superior cerebral veins, middle cerebral veins, inferior cerebral veins) and deep cerebral veins (great cerebral vein, basal vein), which empty into the dural venous sinuses. Brain perfusion is regulated by the partial pressure of carbon dioxide (PaCO2). The interruption of perfusion due to occlusion or hemorrhage of the cerebral vessels results in a stroke, which manifests with focal neurologic deficits in the body parts controlled by the affected brain territory.
Internal carotid arteries (ICA) 
- A terminal branch of the
- Neck: lies within the and enters the cranium through the
- Cranium: lies on the roof of the , in close proximity to
- Only has intracranial branches
- Caroticotympanic artery (the first intracranial branch of the internal carotid artery that enters the middle ear cavity and anastomoses with the inferior tympanic artery)
- Artery of the pterygoid canal (Vidian artery)
- Meningohypophyseal trunk (posterior trunk)
- Inferolateral trunk
- Superior pituitary artery
- anastomosis in Circle of Willis :
- Terminal branches
- Arise from the
- Neck: The vertebral arteries pass through the foramina in the transverse processes of the cervical vertebrae of C1–C6 (C7 transverse foramina transmit only veins) and enter the cranium through the foramen magnum.
- Cranium: The right and left vertebral arteries unite at the midline of the anterior surface of the pons to form the basilar artery.
- Definition: a vascular circuit formed by the anastomoses between branches of the internal carotid arteries (anterior circulation) and vertebral arteries (posterior circulation) around the optic chiasm and pituitary stalk 
- Consists of paired:
- Two anastomoses
- The circuit provides alternative channels to bypass a potential site of vascular occlusion.
- Equalizes arterial flow to both cerebral hemispheres
Most saccular cerebral aneurysms, also known as berry aneurysms, occur in the anterior circulation of the brain, usually at the junction of the anterior cerebral artery and the anterior communicating artery in the circle of Willis. They are the most common cause of nontraumatic subarachnoid hemorrhage.
|Overview of cerebral arterial territories |
|Artery||Arterial territory||Main branches||Features of stroke|
Branches of the
Branch of the basilar artery
Superficial cerebral veins 
| Superficial veins |
Drain the white matter
|Draining venous sinus|
Superior cerebral veins
|Superior anastomotic vein||Superiorsagittal sinus|
|Middle cerebral veins||Inferior anastomotic vein||Cavernous sinus|
|Inferior cerebral veins||Cavernous and transverse venous sinuses|
Deep cerebral veins 
Deep cerebral veins drain the cerebral medulla and drain into the straight sinus.
- Medullary veins: drain the gray matter
- Subependymal veins: receive blood from the medullary veins
- Basal vein (vein of Rosenthal): paired paramedian veins that receive blood from the temporal lobe and drain into the great cerebral vein
- Great cerebral vein (vein of Galen): receives blood from the deep veins
Dural venous sinuses 
- The dural venous sinuses drain blood from cerebral veins and CSF from the arachnoid granulations into the internal jugular vein.
- The sinuses are located intracranially between the two layers of dura mater (endosteal layer and meningeal layer).
|Superior sagittal sinus|
|Inferior sagittal sinus|
|Occipital sinus|| |
|Confluence of sinuses|
Basilar venous plexus
- Cerebral perfusion is modulated by the partial pressure of carbon dioxide (pCO2). 
- Therapeutic hyperventilation reduces pCO2 → decreased cerebral blood flow → lower intracranial pressure (e.g., used when patients with acute are unresponsive to other treatments)
- Partial pressure of oxygen (pO2) only modulates cerebral perfusion in severe hypoxic conditions
- Cerebral perfusion pressure (CPP) = mean arterial pressure (MAP) - intracranial pressure (ICP)