Circle of Willis

1. Circle of Willis
by Luiz Roberto Meier
16/07/2014
The motivation to write about the Circle of Willis (COW) occurred from week 4, module 2, where
the title “Plan B” drew my attention to the beauty of the body to attempt to survive the misfortune
of an ischemia and other problems of vascular nature.
In the medical literature, it is also called Loop of Willis, Willis Polygon and cerebral arterial circle.
named after Thomas Willis (1621–1675), an important and famous English physician.
Before continuing, picture 1 will help to remember the types of strokes:
Picture 1
Introduction and Abbreviations
Blood vessels supplying the human brain consist of two independent vascular systems:
Internal carotid arteries (ICA) and vertebral arteries (VA).
In the base of the brain both systems of vessels are connected bilaterally by posterior
communicating arteries (PCoA) and left-to-right side by anterior communicating artery (ACoA)
forming the só called Circle of Willis (COW).
Anterior and middle cerebral arteries are formed from internal carotid arteries. Vertebral arteries
join to form the basilar artery (BA), which is soon divided into two posterior cerebral arteries.
The main branches of the COW are segments of the anterior cerebral arteries (A2), the middle
cerebral arteries (M2) and the posterior cerebral arteries
(P2).

2. See Picture 2a and Picture 2b
Importance
The Circle of Willis is thought to have a major role in collateral circulation in the
brain, both during physiological neck movements partially occluding the vessels supplying
blood to intracranial regions and in pathological situations, for example in the case of
obliterate atheromatosis* in carotid arteries or in the vertebra-basilar system.
A deposit or degenerative accumulation of lipid-containing plaques on the innermost layer of the
wall of an artery.
Picture 2a
Picture 2b
Main Functions
1) Physiological neck movements (see above)
2) Obliterate atheromatosis (see above)
3) Redundancies (more pathways for Cerebral Perfusion Pressure (CPP))
4) If one part of the circle becomes blocked or narrowed (stenosed) or one of the arteries
supplying the circle is blocked or narrowed, blood flow from the other blood vessels can
often preserve the cerebral perfusion well enough to avoid the symptoms of ischemia.
Problems
In subclavian steal syndrome, blood is "stolen" from the Circle of Willis to preserve blood flow to
the upper limb. Subclavian steal syndrome results from a proximal stenosis (narrowing) of the
subclavian artery, an artery supplied by the aorta which is also the same blood vessel that eventually
feeds the Circle of Willis via the vertebral artery.
Subclavian steal syndrome refers to steno-occlusive disease of the proximal subclavian artery with

3. retrograde flow in ipsilateral vertebral artery and associated cerebral ischaemic symptoms. (source:
radiopaedia.org)
Circle of Willis with the most common locations of ruptured aneurysms:
Variation
Importantly, the Circle of Willis is a model whose shape varies from individual to individual.
Considerable anatomic variation exists in the Circle of Willis. Based on a study of 1413 brains, the
classic anatomy of the circle is only seen in 34.5% of cases. In one common variation the proximal
part of the posterior cerebral artery is narrow and its ipsilateral posterior communicating artery is
large, so the internal carotid artery supplies the posterior cerebrum. In another variation the anterior
communicating artery is a large vessel, such that a single internal carotid supplies both anterior
cerebral arteries.
Variations were also found to be associated with migraine and mentally ill patients. The anomalies
encountered include hypoplasia of the component vessels, duplications/triplications of anterior
communicating arteries, triplicate anterior cerebral arteries, embryonic origin of posterior cerebral
and the absent posterior communicating arteries. Hypoplasia and absent vessels were more
prevalent in posterior circle, while accessory vessels were common in anterior portion of the circle.
The neurosurgical importance lies during the exposure of the region for different purposes.

4. Conclusion
The Circle of Willis plays a key role in many chemical and surgical treatments. With the lack of
ingenious "Plan B" probably there would be no time for a medical intervention and treatment. I
learned about brain anatomy and physiology. Dr. Mason introduced me to the COW and this really
interested me. I used the introduction from week 4 to understand the research that I did.
References and pictures credits:
http://www.wikipedia.org
http://www.eol.org
http://www.radiopaedia.org
http://www.answers.com
Images taken from Google Images (public domain only filter)
to my beloved father Walter, in memoriam.