Cerebral Blood Supply

• Cerebral blood flow represent 15% of cardiac output

• Cerebral hemispheres and diencephalon (the posterior aspect of the forebrain containing the thalamus and hypothalamus) are supplied by branches of the internal carotid artery and vertebral arteries

• The two systems meet in the circle of Willis

Features of the Circle of Willis

• The posterior cerebral arteries are terminal branches of the basilar artery
• They supply the inferior aspects of the cerebral hemispheres and the occipital lobe

• They produce the posterior communicating arteries which meet the internal carotid artery

• The anterior communicating artery arises from and links the two anterior cerebral arteries

• The middle cerebral artery is the largest and a direct branch of the internal carotid

• It supplies the cortical areas for motor and sensory innervation to the contralateral half of the body

• Anterior cerebral arteries supply
• Medial surfaces of the hemispheres
• Superior frontal gyrus
• Area on adjacent parietal lobe
• The internal capsule is supplied by
• Anterior cerebral
• Middle cerebral
• Anterior choroidal arteries

• Note: the fibres of the optic radiations run amongst the most posterior fibres of the internal capsule and those from the frontal eye fields pass within the anterior limb

Vertebral artery

• Branch from the subclavians
• Travel through the foramen transversarium of cervical vertebrae
• Enter foramen magnum
• Travels anterior to spinal accessory nerve
• Carries postganglionic sympathetic fibres from the stellate ganglion
• Give rise to the posterior inferior cerebellar artery

Carotid arteries

• Right and left common carotids derive from
• The brachiocephalic artery
• The aortic arch
• The common carotids bifurcate at the level of C3
• There is a dilation here called the carotid sinus

• Ophthalmic

• Vidian (artery to the pterygoid canal): serves as an anastomosis with the external carotid

• Inferior hypophyseal
• Middle cerebral

External carotid branches (numbers in order of branching from bottom)

• Medial aspect
• Ascending pharyngeal (2)
• Anterior aspect
• Superior thyroid (1)
• Facial (4)
• Lingual (3)
• Posterior aspect
• Posterior auricular (6)
• Occipital (5) 
• After entering parotid (terminal branches)
• Superficial temporal (8)
• Maxillary (7)

Quick tip! Mnemonic: Some Anatomists Like Freaking Out Poor Medical Students

Carotid body chemoreceptor

• Stimulated by
• Increase in PCO2
• Decrease in PO2
• Decrease in pH
• Causes reflex peripheral vasoconstriction

Temporal artery

• Last branch of external carotid as above

• Some people have anastomoses between the internal and external carotid circulations via the temporal artery and in this case the temporal can provide significant intracranial/cerebral blood flow: hence ligation can lead to stroke

Maxillary artery (second last branch of external carotid)

• Main branches arise within the infratemporal fossa
• Middle meningeal artery
• Inferior alveolar artery: supplies mandibular gingiva
• Enters and ends in the pterygopalatine fossa
• Small branches accompanying the maxillary nerve
• Infraorbital artery supplies the maxillary incisor teeth
• Supplies muscles of mastication and the lateral wall of the nose
• Lies between the sphenomandibular ligament and mandible

Facial artery

• Travels obliquely beneath digastric and stylohyoid
• Enters a groove on the posterior surface of the submandibular gland
• Travels over the mandible and ends at the medial commissure of the eye
• Becomes the angular artery, which supplies
• Orbicularis oculi
• Lacrimal sac
• Anastomoses with the dorsal nasal branch of the ophthalmic artery