What percentage of brain blood supply is provided by the internal carotid artery?

• A. 50%
• B. 30%
• C. 70%
• D. 90%

Answer: (C)

The brain gets its blood from two main arterial systems: the carotid system (anterior circulation) and the vertebrobasilar system (posterior circulation). The internal carotid arteries feed the anterior circulation, giving rise to the anterior and middle cerebral arteries that perfuse large areas of the cerebral hemispheres. The vertebral arteries form the basilar artery, which supplies the posterior circulation to the brainstem, occipital lobes, and cerebellum.

Taken together, the carotid (internal carotids) supply about seven-tenths of the brain’s total blood flow, with the vertebrobasilar system contributing the remaining roughly 30%. This proportion can vary between individuals, but 70% is the classic estimate. The Circle of Willis provides collateral routes that can help redistribute flow if one pathway is compromised.