Why is brain blood supply so important?
The brain represents about 2% of the total body weight in humans, but it receives one fifth of the resting cardiac output and accounts for 15-20% of the body's blood supply. Brain cells die if the supply of blood carrying oxygen is blocked; therefore the brain has the highest priority for the blood. The human body endeavours to supply the brain with an uninterrupted flow of blood.
Blood supply to the brain is carried by two pairs of arteries: the internal carotid arteries and vertebral arteries, which anastomose at the base of the brain to form what is known as the circle of Willis. The right and left vertebral arteries form a single basilar artery at the base of the brain. This then provides the brain blood supply that feeds the circle of Willis.
The carotid arteries and their branches supply the front portion of the brain; and the vertebrobasilar system supplies the rear portion of the brain. These are known as anterior and posterior circulation respectively.
What Relevance is the Brain Blood Supply?
If the supply of blood to the brain is stopped, brain cells die. Ischemic strokes are caused by a blood clot. These can occur with in the brain itself, or in the arteries that make up the brain blood supply. Both situations result in a stroke as the brain is deprived of critical blood.The area of brain blood supply is unique because as a foetus grows, the arteries grow from both theheart and the brain, meeting up in the circle of Willis. Sometimes, the anastomosis (how these arterise join into a network) is not perfect and this can make a person susceptible to problems where plaque can build up at the join. In such cases, the risk of stroke is increased.It's not essential to understand all the parts of the brain blood supply. However, it's worthwhile to understand that this is a complex part of human anatomy.
The Circle of Willis is a vital formation of arteries at the base of thebrain which supplies all thought processes with the necessary fuel.
There is a grouping of arteries near the base of the brain which is called the Arterial Circle of Willis. It is named after a very influential English physician named Thomas Willis, who discovered it and then published his findings in his 1664 work, a seminal peace on the inner workings of the brain entitled, Cerebri anatomi (from the Latin for “Anatomy of the Brain”).
The Circle of Willis
The Circle of Willis or the Circulus Arteriosus is an arterial polygon where the blood carried by the two internal carotid arteries and
the basilar system comes together and then is redistributed by the anterior, middle, and posteriorcerebral arteries. The posterior cerebral artery is connected to the internal carotid artery by the posterior communicating artery.
The anterior communicating artery joins the anterior cerebral arteries of the two hemispheres together. The middle cerebral arteries are connected to the posterior cerebral arteries by the posterior communicating arteries. This anastomosis between arteries is responsible for developing collateral circulation. It provides a safety mechanism. If one of the major vessels becomes occluded within the
Circle or proximal to it, the circle will still provides the brain with continued supply of blood. Thus the circle of Willis is of great use in preventing neurological damage. As long as this circle is successful at maintaining blood pressure at fifty
Percent of normal, no infarction or death of brain tissue will occur in the blocked area and no permanent effects are produced.Smaller arteries arise from the circle of Willis and from the major cerebral arteries. They form four groups which include the anteromedial, the anterolateral, the posteromedial and the posterolateral.
Internal Carotid System
The internal carotid artery divides into two main branches called the middle cerebral artery and the anterior cerebral artery. The middle cerebral artery supplies blood to the frontoparietal somatosensory cortex.
The anterior cerebral artery supplies blood to the frontal lobes and medial aspects of the parietal and occipital lobes. Before this divide, the internal carotid artery gives rise to the anterior communicating artery and the posterior communicating artery.
The two vertebral arteries run along the medulla and fuse at the pontomedullary junction to form the midline basilar artery, also called the vertebro-basilar artery.
Before forming the basilar artery, each vertebral artery gives rise to the posterior spinal artery, the anterior spinal artery, the posterior inferior cerebellar artery (PICA) and branches to the medulla.
At the ponto-midbrain junction, the basilar artery divides into the two posterior cerebral arteries. Before this divide, it gives rise to numerous paramedian, short and long circumferential penetrators and two other branches known as the anterior inferior cerebellar artery and the superior cerebellar artery.
Venous Drainage Arteries provide the brain blood supply. The veins take the blood away,after the brain has taken nutrition from it.The prime course of venous drainage of the brain is through cerebral veins that empty into the dural venous sinuses and ultimately into the internal jugular vein.
Cerebral veins are divided into two groups, superficial and deep. The superficial veins usually lie on the surface of the cerebral hemispheres and empty themselves into the superior sagittal sinus. The deep veins drain internal structures and ultimately drain into the straight sinus.
Cerebral veins are thin-walled and valveless. They
are also interconnected by several functional anastomoses both within a group and between the superficial and deep groups. The numerous connections between cerebral veins and dural sinuses and venous systems of the meninges, skull, scalp and nasal sinuses assist the spread of thrombus or infection between these vessels.
The Veins of the Brain
The veins of the brain possess no valves, and their walls, owing to the absence of muscular tissue, are extremely thin. They pierce the arachnoid membrane and the inner or meningeal layer of the dura mater, and open into the cranial venous sinuses. They may be divided into two sets, cerebral and cerebellar.
The cerebral veins (vv. cerebri) are divisible into external and internal groups according as they drain the outer surfaces or the inner parts of the hemispheres.
The external veins are the superior, inferior, and middle cerebral.
The Superior Cerebral Veins (vv. cerebri superiores), eight to twelve in number, drain the superior, lateral, and medial surfaces of the hemispheres, and are mainly lodged in the sulci between the gyri, but some run across the gyri. They open into the superior sagittal sinus; the anterior veins runs nearly at right angles to the sinus; the posterior and larger veins are directed obliquely forward and open into the sinus in a direction more or less opposed to the current of the blood contained within it.
The Middle Cerebral Vein (v. cerebri media; superficial Sylvian vein) begins on the lateral surface of the hemisphere, and, running along the lateral cerebral fissure, ends in the cavernous or the sphenoparietal sinus. It is connected (a) with the superior sagittal sinus by the great anastomotic vein of Trolard, which opens into one of the superior cerebral veins; (b) with the transverse sinus by the posterior anastomotic vein of Labbé, which courses over the temporal lobe.
The Inferior Cerebral Veins (vv. cerebri inferiores), of small size, drain the under surfaces of the hemispheres. Those on the orbital surface of the frontal lobe join the superior cerebral veins, and through these open into the superior sagittal sinus; those of the temporal lobe anastomose with the middle cerebral and basal veins, and join the cavernous, sphenoparietal, and superior petrosal sinuses.
The basal vein is formed at the anterior perforated substance by the union of (a) a small anterior cerebral vein which accompanies the anterior cerebral artery, (b) the deep middle cerebral vein (deep Sylvian vein), which receives tributaries from the insula and neighboring gyri, and runs in the lower part of the lateral cerebral fissure, and (c) the inferior striate veins, which leave the corpus striatum through the anterior perforated substance. The basal vein passes backward around the cerebral peduncle, and ends in the internal cerebral vein (vein of Galen); it receives tributaries from the interpeduncular fossa, the inferior horn of the lateral ventricle, the hippocampal gyrus, and the mid-brain.
The Internal Cerebral Veins (vv. cerebri internæ; veins of Galen; deep cerebral veins) drain the deep parts of the hemisphere and are two in number; each is formed near the interventricular foramen by the union of the terminal and choroid veins. They run backward parallel with one another, between the layers of the tela chorioidea of the third ventricle, and beneath the splenium of the corpus callosum, where they unite to form a short trunk, the great cerebral vein; just before their union each receives the corresponding basal vein.
The terminal vein (v. terminalis; vena corporis striati) commences in the groove between the corpus striatum and thalamus, receives numerous veins from both of these parts, and unites behind the crus fornicis with the choroid vein, to form one of the internal cerebral veins. The choroid vein runs along the whole length of the choroid plexus, and receives veins from the hippocampus, the fornix, and the corpus callosum.
The Great Cerebral Vein (v. cerebri magna [Galeni]; great vein of Galen) (Fig. 565), formed by the union of the two internal cerebral veins, is a short median trunk which curves backward and upward around the splenium of the corpus callosum and ends in the anterior extremity of the straight sinus.
The cerebellar veins are placed on the surface of the cerebellum, and are disposed in two sets, superior and inferior. The superior cerebellar veins (vv. cerebelli superiores) pass partly forward and medialward, across the superior vermis, to end in the straight sinus and the internal cerebral veins, partly lateralward to the transverse and superior petrosal sinuses. The inferior cerebellar veins (vv. cerebelli inferiores) of large size, end in the transverse, superior petrosal, and occipital sinuses.