Circle of Willis
The Circle of Willis is a circle
A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

 of arteries
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....

 that supply blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 to the brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. It is named after Thomas Willis
Thomas Willis
Thomas Willis was an English doctor who played an important part in the history of anatomy, neurology and psychiatry. He was a founding member of the Royal Society.-Life:...

 (1621–1675), an English
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...

The Circle of Willis is composed of the following arteries:
  • Anterior cerebral artery (left and right)
  • Anterior communicating artery
    Anterior communicating artery
    In human anatomy, the anterior communicating artery is a blood vessel of the brain that connects the left and right anterior cerebral arteries....

  • Internal carotid artery
    Internal carotid artery
    In human anatomy, the internal carotid arteries are two major arteries, one on each side of the head and neck. They arise from the common carotid arteries where these bifurcate into the internal and external carotid artery, and they supply the brain....

     (left and right)
  • Posterior cerebral artery
    Posterior cerebral artery
    -External links: - Posterior Cerebral Artery Stroke* at* at State University of New York Upstate Medical University* at* at

     (left and right)
  • Posterior communicating artery
    Posterior communicating artery
    In human anatomy, the posterior communicating artery is one of a pair of right-sided and left-sided blood vessels in the circle of Willis. It connects the three cerebral arteries of the same side. Anteriorly, it connects to the internal carotid artery prior the terminal bifurcation of the ICA...

     (left and right)

The basilar artery
Basilar artery
In human anatomy, the basilar artery is one of the arteries that supplies the brain with oxygen-rich blood.The two vertebral arteries and the basilar artery are sometimes together called the vertebrobasilar system, which supplies blood to the posterior part of circle of Willis and anastomoses with...

 and middle cerebral arteries, supplying the brain, are also considered part of the circle.
The arrangement of the brain's arteries into the Circle of Willis creates redundancies or collaterals
Collateral circulation
Collateral circulation is when an area of tissue or an organ has a number of different pathways for blood to reach it. This is often as a result of anastamoses - branches formed between adjacent blood vessels....

 in the cerebral circulation.