Cerebrovascular Anatomy and Hemodynamics

Arterial System

The brain of essentially all mammalian species is supplied with blood from several major sources, that is, the internal and external carotid, vertebral, and spinal anterior arteries. However, the relative importance of these channels in any species is unclear. Although the internal carotid artery leads directly to the brain, in some species this vessel is unimportant, and it may be the external carotid that carries the major proportion of blood reaching the brain. In humans, the anterior three-fifths of the cerebrum, except for parts of the occipital and temporal lobes, are supplied by the carotid arteries. The posterior two-fifths of the cerebrum, the cerebellum, and brain stem are supplied by the vertebral–basilar system. The carotid and vertebral arteries unite at the base of the brain to form the circle of Willis ( Fig. 1.1 ). This vascular ring then gives rise to three pairs of arteries, the anterior, middle, and posterior cerebral arteries, which cover the external surface of the corresponding regions of the cerebral cortex. These arteries divide into progressively smaller arteries, penetrating brain tissue and supplying blood to specific regions. Branches of the vertebral and basilar arteries form the blood supply for the cerebellum and brain stem. While there is some variability among individuals, the internal carotids and the vertebral–basilar system generally contribute equally to the circle of Willis. Even though the internal carotid and basilar arteries converge (forming the circle of Willis), blood from the two tributaries normally does not mix completely because blood pressure in each arterial tributary is almost equal. Angiography and/or dye injections indicate that blood from the various tributaries is ultimately distributed to relatively specific and delineated brain regions. Under normal conditions, vertebral–basilar arterial blood is mainly distributed to tissues in the posterior fossa while the internal carotids supply the remainder of the brain. In addition, there is relatively little bilateral crossing, again due to the similarity in blood pressure. Normally the circle of Willis functions primarily as an anterior–posterior shunt than as a side-to-side shunt. However, under pathological conditions, especially those that involve focal obstructions in arterial feeders to the circle, the balance of pressures may be altered and the circle of Willis can then serve either as an anterior–posterior or as a side-to-side shunt.

In addition, there are a number of arterial anastomotic vessels on each side of the head between the intracranial and extracranial circulations. These include: (1) a connection between the vertebral and occipital arteries; (2) a communication between the ascending pharyngeal and internal carotid arteries; (3) the middle meningeal artery branching off from the internal maxillary artery and connecting with the internal carotid artery, (4) the anastomotic artery between the internal maxillary and internal carotid arteries, (5) pathways between the external and internal ophthalmic arteries, (6) anastomosis between the external and internal ethmoidal arteries, (7) collaterals between the vertebral and the omocervical arteries, and (8) connections between the spinal anterior and vertebral arteries. In some species the external carotid system branches into a complicated network of arteries, the rete mirable, prior to its entrance to the circle of Willis. This rete system has been proposed to be involved in a heat-exchange countercurrent mechanism, which acts to lower the temperature of the blood entering the brain.

As the major arteries leave the circle of Willis they reduce their diameter to become arterioles and pial vessels. Pial arteries then plunge at a 90 degree angle into brain parenchyma. There is much evidence that there is a close relationship between pial vessels and the leptomeninges. These vessels, as they enter the parenchyma, are invested with a leptomeningeal sheath and are surrounded by a cerebrospinal fluid (CSF)–containing space. It should be mentioned that most studies of cerebral vessels using methods of staining and light microscopy have not shown differences between brain vessels and vessels in other organs.

Venous System

While the brain’s arterial system is complicated, the configuration of the brain’s venous system is even more complex and provides many opportunities for mixing of blood draining various brain regions ( Fig. 1.2 ). Blood is drained from the brain via two primary sets of veins: the external group and the deep or internal group. These drain into the dural sinuses and then the internal jugular veins. The external venous system is divided into the superior, middle, inferior, and occipital cerebral veins, which drain the outer portion of the cerebral hemispheres. The superior cerebral vein drains the cortex and underlying white matter above the corpus callosum. Several veins on each side merge to form three large trunks, which enter the superior sagittal sinus or straight sinus. The most prominent superior cerebral vein is the great anastomotic vein of Trolard connecting the superior sagittal sinus with the Sylvian vein. The internal cerebral or deep veins include a variety of small transcerebral veins draining the bulk of white matter from the anterior and middle group of the brain. This system eventually drains through the great vein of Galen and the straight sinus. The cerebellum is drained primarily by two sets of veins. The inferior cerebellar veins are larger and end in the transverse superior petrosal and occipital sinuses. The superior cerebellar veins are smaller and empty in part into transverse and superior petrosal sinuses and in part into the great vein of Galen and the straight sinus. The brain stem is drained by veins terminating in the inferior and transverse petrosal sinuses. Veins from all parts of the brain drain into many sinuses situated between two layers of dura, that is, superior sagittal sinus, inferior sagittal sinus, occipital sinus, superior petrosal sinus, cavernous sinus, and transverse sinus. Extensive intervenous collateral anastomoses exist between the two main venous draining systems and with the extracranial venous draining system.