What is the blood supply to the brain?

Blood supply to the brain:

The brain gets its blood supply by two main artery system

1. Carotid artery system or anterior artery system (Main blood supplier for the brain), and
2. Vertebrobasilar system or posterior system

Some points

• The brain is provided with 15% of the total cardiac output
• All the major arteries of the brain are in the subarachnoid space
• Branches of cerebral arteries anastomose with one another on the surface of the brain. Inside of brain substance, there is no further anastomosis.

Vertebrobasilar system or posterior system

The course of vertebral or vertebrobasilar artery system or posterior circulation:

On each side, each vertebral artery arises from the first part of the subclavian artery, goes up through the vertebral foramina of the cervical spine, enter into the cranial cavity through the foramen magnum, pierce the Dura and arachnoid matter and ascend until the pontomedullary junction on the anterior aspect where they meet with the opposite-sided vertebral artery and form the basilar artery. Then the basilar artery ascends along the anterior midline of the pons and midbrain. Finally, it divides into two posterior cerebral arteries just above the level of the 3rd nerve nucleus.

Branches of the vertebrobasilar artery system:

• Meningeal artery
• Contributing branches to the anterior spinal artery (ASA): Form from the vertebral artery in front of the medulla. Descend along the anterior median fissure of the spinal cord.
• Posterior spinal arteries (PSA) arise either from a vertebral artery or PICA. 
• Posterior inferior cerebellar artery (PICA): The largest branch of the vertebral artery. Provide blood supply to the lateral part of the medulla, undersurface of the cerebellum and the choroid plexus of the fourth ventricle.
• Medullary arteries: Arise from the vertebral artery and give blood supply to the medulla.
• Anterior inferior cerebellar artery (AICA): It comes from a vertebral artery. It may also come from the basilar artery. Besides the undersurface of the cerebellum, it also supplies the lateral part of the pons.
• Labyrinthine artery: Arise from the basilar artery and supplies the internal ear. It may arise from AICA.
• Pontine arteries: Arise from the basilar artery and provide blood to the pons.
• Superior cerebellar artery: From the basilar artery, below the level of 3rd never nucleus. It supplies the superior surface of the cerebellum. It also supplies the pons, midbrain, pineal gland and the superior medullary velum.
• Posterior cerebral artery: Two terminal branches of the basilar artery.
• Cortical branches: Supply the inferolateral and medial surface of the temporal lobe (except temporal pole) and the lateral and medial surface of the occipital lobe. The posterior cerebral artery supplies the visual cortex and the occipital pole.
• A choroidal branch: It supplies the choroid plexus of lateral and third ventricles.

Carotid system/anterior circulation:

The course of the internal carotid artery: 

Internal carotid artery (on each side) originates as one of the bifurcating branches of the common carotid artery. It ascends and enters into the carotid canal of the temporal bone at the base of the cranium. It exits through the foramen lacerum and enters into the middle cranial fossa where it moves forward through the cavernous sinus. Then medial to the anterior clinoid process of the sphenoid bone, it goes up, pierces the dura and the arachnoid matter and reaches the subarachnoid space. At the medial end of the lateral cerebral sulcus is gives off the two terminal branches.

Branches of the internal carotid artery:

• Ophthalmic artery: Among other branches, it gives rise to the central retinal artery. Sudden blindness may happen due to occlusion of the central retinal artery due to emboli in the carotid system.
• Post communicating artery: It makes the connection between anterior and posterior circulation by making anastomosis with posterior cerebral artery. It runs posteriorly above the oculomotor nerve.
• The choroidal artery: Supply the choroid plexus of the lateral ventricle. Also supply the crus cerebi, the lateral geniculate body, optic tract and the internal capsule.
• The anterior communicating artery:  It acts as a connection between the right and left the carotid system. And the most commonplace for Berry aneurysm.
  
• Anterior cerebral artery: One of the terminal branches. It goes forward and medially above the optic nerve and enters the longitudinal fissure of the cerebrum. It turns back over the corpus callosum and finally anastomoses with the posterior cerebral artery.
• Cortical branches: Supply all the medial surface of the cerebral cortex as far back as the parieto-occipital sulcus. They also supply a strip of cortex about 2.5 cm wide on the adjoining lateral surface. So that anterior cerebral artery supplies the leg area of the precentral gyrus. The frontal pole is supplied by the anterior cerebral artery.
• Central branches: A group of central branches helps to supply parts of lentiform and caudate nuclei and the internal capsule.
• Middle cerebral artery: The largest branch of the internal carotid artery and runs laterally along the lateral cerebral sulcus.
• Cortical branches: Supply the whole lateral surface of cerebral hemisphere except a narrow strip supplied by anterior cerebral artery, the occipital pole and the inferolateral surface of the hemisphere which are supplied by the posterior cerebral artery. Middle cerebral artery supplies the all motor area except the leg area. The temporal pole is supplied by Middle cerebral artery.
• Central branches: Also known as lenticulostriate branch and supply the lentiform and caudate nuclei and the internal capsule

Blood supply to the specific areas of the brain:

• The corpus striatum and the internal capsule: Supplied by the medial and lateral striate central branches of the middle cerebral artery. Partly, these structures get supply from the central branches of the anterior cerebral artery.
• The thalamus: Supplied by the branches of the posterior communicating artery, basilar artery and posterior cerebral arteries.
• The midbrain: Supplied by the branches of the basilar artery, posterior cerebral and superior cerebellar arteries.
• The pons: Supplied by the basilar artery, AICA, PICA and superior cerebral arteries.
• The medulla oblongata: Supplied by the vertebral, the basilar, anterior spinal artery, posterior spinal artery and PICA
• The cerebellum: By superior cerebellar, AICA and PICA