Blood Supply

The brain is extremely vulnerable to even a momentary reduction in or loss of blood flow, and an uninterrupted supply of well-oxygenated blood is critical. A complex system of arteries and veins convey blood to and from the brain, and interruption of the vascular system may result in stroke, one of the most common and devastating neurologic disorders.

The arterial supply of the brain comes entirely from four major vessels that originate in the neck (Fig. 4). The right and left common carotid arteries arise from the right subclavian artery and ascending aorta, respectively, and within a few centimeters each bifurcates into an external branch, which supplies extra-cranial structures, and an internal carotid artery (ICA), which irrigates major portions of the brain. The paired vertebral arteries are somewhat smaller and arise from the subclavian arteries; these ascend in parallel to a level just below the pons, where they join to form the single basilar artery.

The two internal carotid arteries and basilar artery then contribute to a vascular structure at the base of the brain called the circle of Willis. This is a continuous vascular loop comprising paired posterior cerebral arteries (PCAs) that bifurcate from the top of the basilar artery and go on to supply posterior parts of the brain, paired posterior communicating arteries (PCoAs) that connect the PCAs with the ICAs, paired anterior cerebral arteries (ACAs) that arise from the ICAs and go on to irrigate anterior regions of the brain, and a single anterior communicating artery (ACoA) that joins the two ACAs. Another important artery arising from the circle of Willis is the middle cerebral artery (MCA), one of which ascends to each hemisphere and nourishes the lateral aspects of the brain. Clinically, the MCA, ACA, and PCA are the most important cerebral arteries in that the majority of strokes occur in these distributions and lead to major neurologic deficits because of the large areas of the cerebrum that may be damaged (Fig. 5).

The venous drainage of the brain is accomplished by a complex system of superficial and deep veins, all of which are richly anastomosed with each other. The superficial veins drain into the superior sagittal sinus

Figure 4 The blood supply of the brain, originating from four major arteries that arise from the aorta. Reprinted with permission from Nolte, J. (1999). The Human Brain, 4th ed., p. 145. Mosby, St. Louis.

and the deep veins into the straight sinus. The final destinations of venous blood from the brain are the paired internal jugular veins in the neck, by which the blood is conveyed ultimately to the heart. Disorders of the venous system are far less common than those affecting the arterial system of the brain.

A final point regarding the blood supply of the brain is the existence of a blood-brain barrier. This term refers to an anatomic-physiologic complex at the level of the capillary endothelium that prevents the entry of many substances into the brain from the blood stream. The blood-brain barrier offers clear advantages, as in the case of barring the intrusion of blood-borne pathogens, but it also carries the liability that antibiotics and other drugs may have difficulty reaching the sites in the brain where they are needed.

Figure 5 Distributions of the middle cerebral, anterior cerebral, and posterior cerebral arteries on the lateral surface of the brain. Reprinted with permission from Nolte, J. (1999). The Human Brain, 4th ed., p. 122. Mosby, St. Louis.

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