The blood supply of the brain

 
A. Arteries of the brain : 2 internal carotid arteries & 2 vertebral arteries

	Internal carotid arteries	Vertebral arteries
Course	1. Begins at the bifurcation of the common carotid artery (There it possess a local dilatation – Carotid sinus) 2. Ascends the neck & perforates the base of the skull by passing through the carotid canal of the temporal bone 3. Then it runs horizontally forward through the cavernous sinus & emerges on the medial side of the anterior clinoid process by perforating the dura mater 4. Then it enters the subarachnoid space by piercing the arachnoid mater & turns posteriorly to the region of the medial end of the lateral cerebral sulcus There it divides into anterior & middle cerebral arteries A branch of the 1st part of the subclavian artery	1.Ascends the neck by passing through the foramina in the transverse processes of the upper 6 cervical vertebrae 2.Enters the skullthrough foramen magnum & enter the subarachnoid space 3. Then it passes upwards, forward & medially on the medulla 4.At the lower border of the pons, it joins the vessels of the opposite side to form the basilar artery
Branchesof the cranial part	Ophthalmic artery Posterior communicating artery Choroidal artery Anterior cerebral artery (ACA) The smaller terminal branch Joined to the ACA of the opposite side by the anterior communicating artery Supply the ‘leg area’ of the precentral gyrus Middle cerebral artery (MCA) The largest branch of the ICA Supply all the motor areas except ‘leg area’	Meningeal branches Posterior spinal artery Anterior spinal artery Posterior inferior cerebellar artery (Largest branch of the vertebral artery) Medullary arteries

Basilar artery
1) It’s formed by the union of 2 vertebral arteries
2) Ascends in a groove on the anterior surface of the pons
3) At the upper border of the pons, it’s divided into 2 posterior cerebral arteries
4) Branches
1. Pontine arteries
2. Labyrinthine arteries
3. Anterior inferior cerebellar arteries
4. Superior cerebellar artery
5. Posterior cerebral artery : Supplies the visual cortex
The circle of Willis

1) It lies in the inter peduncular fossa at the base of the brain
2) It’s formed by the anastomosis between the 2 internal carotid & vertebral arteries
3) It allows blood that enters by either internal carotid or vertebral arteries to be distributed to any part of both cerebral hemispheres
4) Cortical & central branches arise from the COW & supply the brain substance
Areas supplied by the cerebral arteries
Arteries to specific brain areas
Arterial supply of the deep cerebral structures from the middle cerebral artery

1)      Corpus striatum & internal capsule	Medial & lateral striate central branches the middle cerebral artery Central branches of the anterior cerebral artery
2)      Thalamus	Posterior communicating  artery Posterior cerebral artery Basilar artery
3)      Midbrain	Posterior cerebral artery Superior cerebellar artery Basilar artery
4)      Pons	Anterior, inferior & superior cerebellar arteries Basilar artery
5)      Medulla	Vertebral artery Anterior & posterior spinal artery Posterior inferior cerebellar artery Basilar artery
6)      Cerebellum	Superior cerebellar artery Anterior inferior cerebellar artery Posterior inferior cerebellar artery

A. Veins of the brain
1) No muscular tissue in their thin walls
2) Don’t possess any valves
3) Lie in the subarachnoid space
4) Drain into cranial venous sinuses
Veins of specific brain areas

1)      Midbrain	Drain to basal or great cerebral veins
2)      Pons	Drain to basal vein, cerebellar vein & neighboring venous sinuses
3)      Medulla oblongata	Drain to spinal veins & neighboring venous sinuses
4)      Cerebellum	Drain to great cerebral vein

Cerebral circulation
1) Cerebral blood flow is measured by the intra carotid injection or inhalation of radioactive Krypton or Xenon
2) Normal cerebral blood flow : 50 – 60ml per 100g of brain per minute
3) Right half of the brain is supplied by the vertebral & internal carotid arteries on the right side and vice versa
4) The streams of the vertebral & internal carotid arteries come together in the posterior communicating artery (At that point pressure is equal on both sides & don’t mix)
5) If one of the arteries is occluded, blood moves forward or backward to compensate the reduction
6) If the vertebral or internal carotid artery is occluded on one side, blood can flow across the midline (Via anterior communicating artery)
7) Streams of 2 vertebral arteries remain separate within the basilar artery & don’t mix
8.) Cerebral blood flow remain constant despite changes in general blood pressure (Compensatory lowering of cerebral vascular resistance in low blood pressure & vice versa)
Brain capillaries
1) Greater in the grey matter than in white matter
2) Blood brain barrier isolates the brain from the rest of the body
3) BBB is formed by the tight junctions that exist between the endothelial cells in the capillary beds
Nerves of the cerebral arteries
1) Cerebral arteries are supplied by sympathetic post ganglionic fibers (derived from the superior cervical sympathetic ganglion) & cause vasoconstriction
2) Under normal circumstances, the local blood flow is mainly controlled by CO₂, H⁺ & O₂ levels
3) Local increase in CO₂ & H⁺ level (Viewing an object will increase O₂ & glucose consumption in the visual cortex of the occipital lobe) or decrease in O₂ tension will result in local increase in blood flow (Vasodilatation)
Subdural vs Extradural Hemorrhage

Subdural hemorrhage	Extradural hemorrhage