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Question 9

In several sentences, explain why the circle of Willis does NOT protect the brain against SUDDEN blockage of the internal carotid artery. 

Answer to Question 9

The circle of Willis is often incomplete, or the connections between the major arteries are made by such slender communicating vessels that they simply cannot carry enough blood to supply the large amount of brain tissue that would be rendered ischemic by sudden carotid occlusion.
If, on the other hand, carotid occlusion has occurred slowly and progressively, it is possible that these tiny arteries will expand in diameter so that they can carry much more blood from the opposite carotid or from the posterior cerebrals, perhaps enough to protect the brain. Furthermore, other anastomoses may also develop - for example between the internal and external carotid via the ophthalmic artery - that provide additional ways for large amounts of blood to reach the jeopardized carotid territory.
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