Cerebral circulation, the supply of blood to the brain
Understanding how the brain is supplied with blood is important because a significant number of neurological disorders that result in hospital admissions are due to problems with cerebral vascular disease. In some hospitals, nearly half the admissions due to neurologic disorders relate in some form to problems with cerebral circulation.
Insufficient supply of blood to the brain can cause fainting (syncope) or a more severe loss of consciousness. A continuous supply of highly oxygenated blood is critical to brain tissue function and a decrease in pressure or oxygenation (percentage of oxygen content) can cause tissue damage within minutes. Depending on a number of other physiological factors (e.g., temperature, etc.), brain damage or death may occur within two to 10 minutes of severe oxygen deprivation. Although there can be exceptions—especially when the body is exposed to cold temperatures—in general, after two minutes of oxygen deprivation, the rate of brain damage increases quickly with time.
Arterial supply of oxygenated blood
Four major arteries and their branches supply the brain with blood. The four arteries are composed of two internal carotid arteries (left and right) and two vertebral arteries that ultimately join on the underside (inferior surface) of the brain to form the arterial circle of Willis, or the circulus arteriosus.
The vertebral arteries actually join to form a basilar artery. It is this basilar artery that joins with the two internal carotid arteries and their branches to form the circle of Willis. Each vertebral artery arises from the first part of the subclavian artery and initially passes into the skull via holes (foramina) in the upper cervical vertebrae and the foramen magnum. Branches of the vertebral artery include the anterior and posterior spinal arteries, the meningeal branches, the posterior inferior cerebellar artery, and the medullary arteries that supply the medulla oblongata.
The basilar artery branches into the anterior inferior cerebellar artery, the superior cerebellar artery, the posterior cerebral artery, the potine arteries (that enter the pons), and the labyrinthine artery that supplies the internal ear.
The internal carotids arise from the common carotid arteries and pass into the skull via the carotid canal in the temporal bone. The internal carotid artery divides into the middle and anterior cerebral arteries. Ultimate branches of the internal carotid arteries include the ophthalmic artery that supplies the optic nerve and other structures associated with the eye and ethmoid and frontal sinuses. The internal carotid artery gives rise to a posterior communicating artery just before its final splitting or bifurcation. The posterior communicating artery joins the posterior cerebral artery to form part of the circle of Willis. Just before it divides (bifurcates), the internal carotid artery also gives rise to the choroidal artery (also supplies the eye, optic nerve, and surrounding structures). The internal carotid artery bifurcates into a smaller anterior cerebral artery and a larger middle cerebral artery.
The anterior cerebral artery joins the other anterior cerebral artery from the opposite side to form the anterior communicating artery. The cortical branches supply blood to the cerebral cortex.
Cortical branches of the middle cerebral artery and the posterior cervical artery supply blood to their respective hemispheres of the brain.
The circle of Willis is composed of the right and left internal carotid arteries joined by the anterior communicating artery. The basilar artery (formed by the fusion of the vertebral arteries) divides into left and right posterior cerebral arteries that are connected (anastomsed) to the corresponding left or right internal carotid artery via the respective left or right posterior communicating artery. A number of arteries that supply the brain originates at the circle of Willis, including the anterior cerebral arteries that originate from the anterior communicating artery.
In the embryo, the components of the circle of Willis develop from the embryonic dorsal aortae and the embryonic intersegmental arteries.
The circle of Willis provides multiple paths for oxygenated blood to supply the brain if any of the principal suppliers of oxygenated blood (i.e., the vertebral and internal carotid arteries) are constricted by physical pressure, occluded by disease, or interrupted by injury. This redundancy of blood supply is generally termed collateral circulation.
Arteries supply blood to specific areas of the brain. However, more than one arterial branch may support a region. For example, the cerebellum is supplied by the anterior inferior cerebellar artery, the superior cerebellar artery, and the posterior inferior cerebellar arteries.
Venous return of deoxygenated blood from the brain
Veins of the cerebral circulatory system are valve-less and have very thin walls. The veins pass through the subarachnoid space, through the arachnoid matter, the dura, and ultimately pool to form the cranial venous sinus.
There are external cerebral veins and internal cerebral veins. As with arteries, specific areas of the brain are drained by specific veins. For example, the cerebellum is drained of deoxygenated blood by veins that ultimately form the great cerebral vein.
External cerebral veins include veins from the lateral surface of the cerebral hemispheres that join to form the superficial middle cerebral vein.
Nourishing brain tissue
The cerebral arteries provide blood to the brain, but a sufficient arterial blood pressure is required to provide an adequate supply of blood to all brain tissue. Unlike the general body blood pressure, the cerebral blood pressure and cerebral blood flow remain relatively constant, a feat of regulation made possible by rapid changes in the resistance to blood flow within cerebral vessels. Resistance is lowered, principally through changes in the diameter of the blood vessels, as the cerebral arterial pressure lowers, and resistance increases as the incoming arterial pressure increases.
A complex series of nerves, including a branch of the glossopharyngeal nerve (the sinus nerve), relate small changes in the size of the carotid sinus (a dilation or enlargement of the internal carotid artery) such that if arterial pressure increases and causes the sinus to swell, the nervous impulses transmit signals to areas of the brain that inhibit the heart rate.
An oxygenated blood supply is critical to brain function
An adequate blood supply is critical to brain function and healthy neural tissue. Physiological studies utilizing radioisotopes and other traceable markers establish that the majority of the blood originally passing through the left vertebral and left internal carotid arteries normally supply the left side of the brain, with a similar situation found on the right with the right vertebral and right internal carotid arteries. Accordingly, the left half of the brain receives its blood supply from the left internal carotid and left vertebral artery. The right half of the brain receives its blood supply from the right internal carotid and right vertebral artery.
The two independent blood supplies do not normally mix or crossover except for a small amount in the posterior communicating artery (and in some cases, the arterial circle of Willis).
Compensating mechanisms
However, if there is some obstruction of blood flow (cerebral ischemia), there is a compensating mechanism. The two left and right supplies of blood normally do not mix in the posterior communicating artery because they are at roughly equal pressures. Even after the two vertebral arteries join to form the basilar artery prior to joining the arterial circle of Willis, the bloodstreams from the two vertebral arteries remain largely separated as though there were a partition in the channel.
If there is an obstruction on one side that reduces the flow of blood, the pressures of the two sides do not remain equal and so blood from the unaffected side (at a relatively higher pressure) is able to crossover and help nourish tissue on the occluded side of the brain.
The arterial circle of Willis can also permit crossover flow when the pressures are altered by an obstruction or constriction in an internal carotid or vertebral artery.
In addition to crossover flow, the size of the communicating arteries and the arteries branching from the circle of Willis is able to change in response to increased blood flow that accompanies occlusion or interruption of blood supply to another component of the circle.
Accordingly, oxygenated blood from either vertebral artery or either internal carotid may be able to supply vital oxygen to either cerebral hemisphere.
Vascular disorders
The disorders that result from an inadequate supply of blood to the brain depend largely on which artery is occluded (blocked) and the extent of the occlusion.
There are three general types of disorders that can result in inadequate blood flow to the brain. Although there are pressure-compensating mechanisms in the cerebral circulation, heart disease and diseases that affect blood pressure in the body can also influence cerebral blood pressure. Sometimes people get lightheaded or dizzy when they stand up suddenly after sitting for long periods. The dizziness is often due to postural hypotension, an inadequate supply of blood to the brain due to a lowered cerebral arterial blood pressure initially caused by an obstruction to the return of venous blood to the heart. Shock can also cause a lowering of cerebral blood pressure.
Disorders or diseases that result in the blockage of arteries can certainly have a drastic impact on the quality of cerebral circulation. A clot (thrombus) that often originates in plaque lining the carotid or vertebral arteries can directly obstruct blood flow in the cerebral circulation. Cerebral aneurysms, small but weakening dilations of the cerebral blood vessels, can rupture, trauma can cause hemorrhage, and a number of other disorders can directly impair blood flow.
Lastly, diseases that affect the blood vessels themselves, especially the arterial walls, can result in vascular insufficiency that can result in loss of consciousness, paralysis, or death.
BOOKS
Bear, M., et al. Neuroscience: Exploring the Brain. Baltimore: Williams & Wilkins, 1996.
Goetz, C. G., et al. Textbook of Clinical Neurology. Philadelphia: W. B. Saunders Company, 1999.
