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Brain Circulation

The brain derives its arterial supply from the paired carotid and vertebral arteries. Every minute, about 600-700 ml of blood flow through the carotid arteries and their branches while about 100-200 ml flow through the vertebral-basilar system.

The carotid and vertebral arteries begin extracranially, and course through the neck and base of the skull to reach the cranial cavity. The internal carotid arteries and their branches supply the anterior 2/3 of the cerebral hemispheres, including its deep white matter and the basal ganglia. The vertebral arteries and basilar artery, with their branches, supply the remaining posterior and medial regions of the hemispheres, most of the diencephalon, the brainstem, cerebellum, and cervical spinal cord. 

The carotid and vertebral-basilar circulations are anatomically interconnected with each other, and with their counterparts in the opposite hemisphere, through the circle of Willis. However, these connections usually cannot carry enough blood flow to maintain adequate cerebral circulation if either a carotid or a vertebral arterery is suddenly blocked. This is because in most cases the connecting vessels have small diameters, and pressure differences between the two circulations are usually too small to drive much blood flow through them.