Describe the circulation of the human brain.
The two brain hemispheres are supplied from four arteries, two internal carotid arteries and two
vertebral arteries. The internal carotid and vertebral systems anastomose, or join with each other, at the
base of the brain, forming the circle of Willis (which is actually polygonal in shape). Posteriorly, the two
vertebral arteries fuse to form a single basilar artery. Normally little mixing of blood from opposing
streams occurs in the communicating arteries of the circle of Willis because the pressures of the two
streams are equal. Should either vertebral or carotid system be occluded, redistribution (clinically, termed
cross-filling) of blood occurs through the circle of Willis.
The arterial supply of the cerebral cortex is provided by the three cerebral arteries (anterior, middle,
and posterior) branching off the circle of Willis. The former two arise from the internal carotid system, and
the posterior cerebral artery is the terminal branch of the basilar artery. The branches of these three
arteries anastomose across the borders of their respective territories and on the surface of the brain, but
very sparsely. The middle cerebral artery is the largest and most direct branch of the internal carotid artery.
It feeds most of the lateral aspect of the cerebral cortex and gives off central branches to supply the
internal capsule, which contains the motor and sensory neural fibers supplying most of the opposite side
of the body. Venous drainage from the brain occurs by way of the deep veins and rural sinuses, emptying
mainly into the internal jugular veins.
The brain of a normal adult typically maintains a blood flow of 50-55 mL/100 g/min, corresponding
to a volumetric blood flow rate of 800 to lOOOmL/min, and can adjust to both acute and chronic
physiological changes such as increases or decreases in blood pressure to ensure a relatively constant
flow.
The two brain hemispheres are supplied from four arteries, two internal carotid arteries and two
vertebral arteries. The internal carotid and vertebral systems anastomose, or join with each other, at the
base of the brain, forming the circle of Willis (which is actually polygonal in shape). Posteriorly, the two
vertebral arteries fuse to form a single basilar artery. Normally little mixing of blood from opposing
streams occurs in the communicating arteries of the circle of Willis because the pressures of the two
streams are equal. Should either vertebral or carotid system be occluded, redistribution (clinically, termed
cross-filling) of blood occurs through the circle of Willis.
The arterial supply of the cerebral cortex is provided by the three cerebral arteries (anterior, middle,
and posterior) branching off the circle of Willis. The former two arise from the internal carotid system, and
the posterior cerebral artery is the terminal branch of the basilar artery. The branches of these three
arteries anastomose across the borders of their respective territories and on the surface of the brain, but
very sparsely. The middle cerebral artery is the largest and most direct branch of the internal carotid artery.
It feeds most of the lateral aspect of the cerebral cortex and gives off central branches to supply the
internal capsule, which contains the motor and sensory neural fibers supplying most of the opposite side
of the body. Venous drainage from the brain occurs by way of the deep veins and rural sinuses, emptying
mainly into the internal jugular veins.
The brain of a normal adult typically maintains a blood flow of 50-55 mL/100 g/min, corresponding
to a volumetric blood flow rate of 800 to lOOOmL/min, and can adjust to both acute and chronic
physiological changes such as increases or decreases in blood pressure to ensure a relatively constant
flow.
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