Blood Supply of the Brain

Keywords

vertebral basilar system, internal carotid, circle of Willis, stroke, autoregulation, blood brain barrier, circumventricular organs, superior sagittal sinus, internal jugular veins

Chapter Outline
The Internal Carotid Arteries and Vertebral Arteries Supply the Brain, 32
- The Internal Carotid Arteries Supply Most of the Cerebrum, 32
- The Vertebral-Basilar System Supplies the Brainstem and Parts of the Cerebrum and Spinal Cord, 32
- The Circle of Willis Interconnects the Internal Carotid and Vertebral-Basilar Systems, 33
- Blood Flow to the CNS Is Closely Controlled, 34
- Imaging Techniques Allow Arteries and Veins to Be Visualized, 34
- Strokes Result From Disruption of the Vascular Supply, 34
A System of Barriers Partially Separates the Nervous System From the Rest of the Body, 35
Superficial and Deep Veins Drain the Brain, 35

The central nervous system (CNS) is tremendously active metabolically—relative to its weight, it uses much more than its share of the available oxygen and glucose. Corresponding to this metabolic activity, it has an abundant and closely regulated arterial supply and a large venous drainage system. Also, for its proper functioning, the CNS depends on carefully controlled extracellular ion concentrations. Part of the basis for this control is a system of diffusion barriers, of which cerebral blood vessels are a major part.

The Internal Carotid Arteries and Vertebral Arteries Supply the Brain

Two interconnected arterial systems provide the blood supply to the brain ( Fig. 6.1 ). The internal carotid system of each side supplies the ipsilateral cerebral hemisphere, except for the medial surface of the occipital lobe and the medial and inferior surfaces of the temporal lobe. The vertebral-basilar system supplies those parts of the occipital and temporal lobes, as well as the brainstem and cerebellum. The supply of the diencephalon is shared by the two systems, with the vertebral/basilar system supplying most of the thalamus and the internal carotid system supplying most of the hypothalamus.

The Internal Carotid Arteries Supply Most of the Cerebrum

Key Concept

Small perforating arteries supply deep cerebral structures.

The internal carotid artery has two major terminal branches, the anterior (ACA) and middle cerebral arteries (MCA). These are two of the three arteries that supply the cerebral cortex. On their way to their cortical areas of supply, both give rise to lots of small perforating or ganglionic arteries that supply most of the hypothalamus, basal ganglia, and internal capsule. (A comparable set from the vertebral-basilar system supplies the thalamus and brainstem.) The anterior and middle cerebral arteries have large terminal areas of distribution that are suggested, to some extent, by the name of each artery ( Fig. 6.2 ). The capillaries at the ends of the MCA and ACA, as well as the posterior cerebral artery (PCA), can form connections called anastomoses in which there may be some overlap at the very distal ends of the arterial territories (termed border areas) to ensure blood supply. However, these border areas are vulnerable to a lack of blood supply in situations of severe hypotension or anoxia. Damage to the border areas is often referred to as “watershed” infarcts.

The Vertebral-Basilar System Supplies the Brainstem and Parts of the Cerebrum and Spinal Cord

The two vertebral arteries fuse near the pontomedullary junction to form the single basilar artery , which ascends to the midbrain and terminates by bifurcating into the two PCAs. As they move along the brainstem, the vertebral and basilar arteries give off a series of branches; each has a name suggesting its terminal area of distribution (e.g., the superior cerebellar artery supplies the superior surface of the cerebellum and lateral parts of the midbrain). Along the way, each of these arteries provides perforating branches that supply the lateral part of the brainstem it passes over on its way to the cerebellum. So knowing the levels at which these vertebral-basilar branches emerge ( Fig. 6.3 ) provides a first approximation of the supply of the brainstem (e.g., the anterior inferior cerebellar artery supplies the anterior-inferior parts of the cerebellum and also the lateral parts of the pons, while the posterior inferior cerebellar artery supplies the corresponding area of the cerebellum and the lateral parts of the medulla). The vertebral and basilar arteries also give off small penetrating arteries that feed the midline structures of the brainstem.

The Circle of Willis Interconnects the Internal Carotid and Vertebral-Basilar Systems

The internal carotid and vertebral-basilar systems are interconnected by a posterior communicating artery on each side. In addition, the two anterior cerebral arteries are interconnected by the anterior communicating artery . These communicating arteries complete the arterial circle of Willis ( Fig. 6.4 ). The communicating arteries are ordinarily fairly small and not capable of carrying much blood. However, in cases of slowly developing occlusions within the circle or in arteries leading into the circle, they can enlarge and provide a major alternative pathway for blood flow.

Aneurysms often occur at these branch sites within the circle of Willis. These small blood-filled outpocketings of the arterial walls are termed berry aneurysms. Common areas include the branch sites of the basilar towards the PCAs and the anterior communicating artery; however, any branch site is vulnerable to a berry aneurysm. Eruption of the berry aneurysm will result in bleeding into the subarachnoid space, causing a severe headache and immediate repair. Unfortunately, there is a high rate of mortality and/or disability associated with a ruptured berry aneurysm.

The numerous perforating or ganglionic arteries that leave the major arteries at the base of the brain—many from within the circle of Willis, some from outside it—have an importance out of proportion to their size, because they supply deep structures where parts of multiple systems are packed together—places like the brainstem, diencephalon, and internal capsule. For example, one of these major perforating branches that originate off the MCAs on both sides, close to the base of the brain, is the lenticulostriate artery. Each of these small arteries that comes off the large MCAs is vulnerable to both high blood pressure hemorrhage and/or ischemic lacunar (small, <15 mm in size) infarcts within the basal ganglia and internal capsule. Damage to the internal capsule, for example, can disable major inputs to and outputs from a large expanse of cerebral cortex.