Chapter 26: Brain Vascular Supply & Venous Drainage

The arterial supply is provided by two major sources: the internal carotid arteries (forming the anterior circulation) and the vertebral arteries (forming the vertebrobasilar or posterior circulation), which together converge to create the crucial anastomosis at the base of the brain known as the circulus arteriosus cerebri or Circle of Willis. The internal carotid artery follows a long course segmented into cervical, petrous, cavernous, and intracranial parts, famously forming the 'carotid siphon,' and gives rise to the anterior and middle cerebral arteries (ACA and MCA), which supply the majority of the forebrain, including large areas of the motor and somatosensory cortices. Meanwhile, the vertebral arteries merge to form the basilar artery, which branches into the posterior inferior cerebellar, anterior inferior cerebellar, and superior cerebellar arteries, and terminates as the paired posterior cerebral arteries (PCA), supplying the brainstem, cerebellum, and occipital lobes. Deep structures like the internal capsule, basal ganglia, and thalamus are supplied by specialized central (perforating or ganglionic) arteries, notably the lateral striate (lenticulostriate) arteries, which are highly susceptible to stroke, and areas at the edges of major arterial territories are vulnerable to hypoperfusion, creating watershed infarcts. Clinical pathology related to this system includes ischaemic strokes, intracranial haemorrhage, and localized defects like aneurysms, which frequently occur at the Circle of Willis junctions—such as posterior communicating artery aneurysms known to compress the oculomotor nerve—as well as developmental abnormalities like arteriovenous malformations (AVMs). The venous drainage system is valveless, composed of deep veins (e.g., internal cerebral vein and basal vein of Rosenthal) and superficial veins (including the superior and inferior anastomotic veins), all draining into the dural venous sinuses, with obstruction of this outflow leading to serious pathology like intracranial venosinus thrombosis and resulting venous infarction.