What is the primary blood supply to the brain in a general adult population?

Blood Supply of the Brain

Primary Arterial Supply

The brain receives its blood supply through two major arterial systems: the anterior circulation via the internal carotid arteries and the posterior circulation via the vertebral-basilar system. 1

Anterior Circulation (Carotid System)

The internal carotid arteries provide the primary blood supply to the anterior and middle portions of the cerebral hemispheres 1. This system delivers blood through the following pathway:

• Internal carotid artery (ICA) enters the skull base, traverses the petrous bone, and enters the subarachnoid space near the ophthalmic artery level 1
• The ICA gives rise to the posterior communicating artery, which connects to the posterior cerebral artery through the circle of Willis 1
• The ICA then bifurcates into two major terminal branches 1:
  • Anterior cerebral artery (ACA): supplies the medial surfaces of the frontal and parietal lobes 1
  • Middle cerebral artery (MCA): supplies the lateral surfaces of the cerebral hemispheres, including most of the frontal, parietal, and temporal lobes, and is the largest terminal branch of the ICA 1

The left and right anterior cerebral arteries connect through the anterior communicating artery, forming the anterior portion of the circle of Willis 1.

Posterior Circulation (Vertebrobasilar System)

The vertebral and basilar arteries supply the posterior brain regions 1. This system functions as follows:

• Vertebral arteries are divided into four segments (V1-V4), with the first three being extracranial 1
• Vertebral artery atherosclerosis accounts for approximately 20% of posterior circulation strokes 1
• The basilar artery gives rise to the posterior cerebral arteries, which supply the occipital lobes, inferior temporal lobes, and portions of the thalamus 1

Deep Brain Structures

The internal capsule and basal nuclei receive blood from perforating branches 2:

• Lenticulostriate arteries arising from the MCA supply the internal capsule 3
• Perforating branches from the ACA, Heubner's artery, MCA, ICA, and anterior choroidal artery supply both the internal capsule and basal nuclei 2
• MCA perforators range in number from 2 to 13 (mean 8.1) and in diameter from 80 to 1,300 micrometers 2

Circle of Willis and Collateral Circulation

The circle of Willis provides crucial collateral pathways between the anterior and posterior circulations, though a complete circle exists in fewer than 50% of individuals 1. The configuration is highly variable, with significant implications for collateral flow during arterial occlusion 1.

Key collateral pathways include 1:

• Anterior communicating artery connecting left and right ACAs
• External carotid to internal carotid collaterals via the internal maxillary and superficial temporal arteries to ophthalmic branches
• External carotid to vertebral artery collaterals via the occipital branch

Blood Flow Volumes

Normal blood flow values in healthy adults over 60 years without stenosis 4:

• Internal carotid artery: 290 mL/min per vessel
• External carotid artery: 125 mL/min per vessel
• Vertebral artery: 80 mL/min per vessel
• Total mean cerebral blood flow: 985 mL/min

Clinical Significance

The neurovascular unit comprises neurons, glia (astrocytes, microglia, oligodendrocytes), and vascular cells (endothelium, smooth-muscle cells/pericytes, adventitial cells) that are closely related developmentally, structurally, and functionally 3. Understanding this integrated system is essential for comprehending cerebrovascular pathology 5.

Common pitfall: All potential blood-supplying vessels, including the external carotid artery, can be involved in brain tissue perfusion in patients with significant stenosis, demonstrating the importance of collateral circulation 4. Failure to recognize compensatory blood flow patterns may lead to inappropriate treatment decisions 4.