The continuous blood supply to the brain is the most importance because of its high metabolic demands for oxygen and glucose. It is highly sensitive to hypoxia (inadequate oxygen) and hypoglycaemia (subnormal concentration of glucose in the blood). The consciousness is lost within 10 seconds of cessation of blood flow, and if the state continues, an irreversible brain damage starts to occur at about 4 minutes and is completed within 10 minutes. The brain is one of the most metabolically active organs of the body as it depends on aerobic metabolism of glucose.
Although the brain constitutes only 2% (1/50) of the total body weight, it receives 20% (1/5) of the total cardiac output and consumes 20% of the total oxygen used by the body.
The cerebrovascular diseases (thrombosis, embolism and haemorrhage) are the third most common cause of death and the neurological signs depend on the site of lesion. Therefore an adequate knowledge of the blood supply of the brain is essential for proper diagnosis and treatment of these diseases.
The brain is supplied by the paired internal carotid and vertebral arteries via an extensive system of branches. The two vertebral arteries unite at the lower border of the pons to form the basilar artery which ascends in the midline on the ventral surface of the pons and at its upper border terminates by dividing into right and left posterior cerebral arteries. Each internal carotid artery ends in the region of anterior perforated substance by dividing into a larger middle cerebral artery, and a smaller anterior cerebral artery. The major arteries supplying the cerebrum (i.e. branches of basilar and internal carotid arteries) get interconnected to one another at the base of the brain to form a six-sided polygon of arteries called circulus arteriosus or circle of Willis The circle of Willis is formed around the interpeduncular fossa and lies in the interpeduncular subarachnoid cistern. It contributes most of the arterial blood supply to the brain.
Normally there is little or no mixing of blood streams: (a) of two vertebral arteries in the basilar artery, (b) of two anterior cerebral arteries in the anterior communicating artery, and (c) of internal carotid and posterior cerebral arteries in the posterior communicating artery. Therefore, right half of the brain is supplied by right vertebral and right internal carotid arteries and left half of the brain is supplied by left vertebral and left internal carotid arteries.
The vertebral artery, a branch of subclavian artery, ascends on the foramina transversaria of upper six cervical vertebrae. On reaching the base of skull, it winds backwards and medially around the lateral mass of the atlas and pierces posterior atlanto-occipital membrane, to enter the posterior cranial fossa through the foramen magnum where it runs on the anterolateral aspect of the medulla. Here the two vertebral arteries converge, and unite at the lower border of the pons to form the basilar artery.
Basilar artery is formed by the union of two vertebral arteries at the lower border of the pons. It ascends in the basilar sulcus on the ventral aspect of the pons in the cisterna pontis and terminates at the upper border of the pons by dividing into right and left posterior cerebral arteries.
The internal carotid artery, a terminal branch of the common carotid artery, traverses the carotid canal in the base of the skull and enters the middle cranial fossa beside the dorsum sellae of the sphenoid bone. Here it first runs forwards along the floor and medial wall of the cavernous sinus and then turns upwards on the medial side of the anterior clinoid process. At this point the artery pierces the dural roof of the cavernous sinus and also the arachnoid mater to enter the subarachnoid space. Now it first runs backwards and then upwards to come to lie lateral to the optic chiasma just underneath the anterior perforated substance of the brain, where it terminates by dividing into two branches, a larger middle cerebral artery and a smaller anterior cerebral artery.
Anterior cerebral artery is a smaller terminal branch of the internal carotid artery. It runs forwards and medially above the optic nerve to the commencement of the median longitudinal cerebral fissure, where it comes very close to its fellow of the opposite side and gets joined with it by a short transverse anterior communicating artery. The anterior cerebral artery then curves around the genu of corpus callosum. The branches given off just distal to the anterior communicating artery supply the medial part of the orbital surface of the frontal lobe.
The artery continues along the upper surface of the corpus collosum as the pericallosal artery and gives a large branch, the callosomarginal artery which runs in the cingulate sulcus. Near the splenium of corpus callosum, the artery ends by anastomosing with the branches of the posterior cerebral artery.
Middle cerebral artery is the larger terminal branch of the internal carotid artery. It appears to be the direct continuation of the internal carotid artery and carries about 30% of the carotid blood flow.
The veins of the brain drain into the intracranial dural venous sinuses, which eventually opens into the internal jugular veins of the neck. The veins emerge from the brain, travers the subarachnoid space, pierce the arachnoid mater and meningeal layer of dura mater to drain into the venous sinuses.
The characteristic features of venous drainage of the brain are: The venous return in the brain does not follow the arterial pattern. The veins of the brain are extremely thin-walled due to absence of muscular tissue in their walls. The veins of the brain possess no valves. The veins of the brain run mainly in the subarachnoid space. The cerebral veins, generally enter obliquely into the dural venous sinuses against the flow of blood in the sinuses to avoid their possible collapse following an increased intracranial pressure as they are thin walled. The veins of the brain comprise, cerebral veins, cerebellar veins, and veins of the brainstem.
The cerebral veins are divided into external (superficial) and internal (deep) cerebral veins which drain the external surfaces and the internal regions of the cerebral hemisphere respectively.
The external cerebral veins drain the surface (cortex) of the hemisphere and are divided into three groups: (a) superior, (b) middle, and (c) inferior. Superior cerebral veins are about 8 to 12 in number and drain the upper parts of the superolateral and medial surfaces of the cerebral hemisphere. They ascend upwards, pierce the arachnoid mater and traverse the subdural space to enter the superior sagittal sinus. The anterior veins open at right angle while the posterior open obliquely against the flow of blood stream in the superior sagittal sinus (embryologically it is due to backward growth of the rapidly increasing cerebrum), thereby preventing their collapse by increased CSF pressure.
Middle cerebral veins are four in number, two on each side: (a) superficial middle cerebral vein, and (b) deep middle cerebral vein. The superficial middle cerebral vein lies superficially in the lateral sulcus. Anteriorly it runs forwards to drain into the cavernous sinus while posteriorly it communicates with the superior sagittal sinus via superior anastomotic vein (of Trolard) and with the transverse sinus via inferior anastomotic vein (of Labbe). The deep middle cerebral vein lies deep in the lateral sulcus on the insula along with middle cerebral artery. It runs downwards and forwards and joins the anterior cerebral vein to form the basal vein. Inferior cerebral veins are many in number but smaller in size. They drain the inferior surface and lower parts of medial and superolateral surfaces of the cerebral hemisphere into nearby intracranial dural venous sinuses, viz. transverse sinus, etc. Anterior Cerebral Vein accompanies the anterior cerebral artery around the corpus callosum and drains the parts of medial surface which cannot be drained into the superior and inferior sagittal sinuses.