From: sbharris@ix.netcom.com(Steven B. Harris)
Subject: Re: Circle of Willis & bloodflow in brain  (was: carotid artery 
	blocked)
Date: Thu Jan 29  4:14:41 PM PST 1998
Message-ID: <6ar61h$j4@sjx-ixn7.ix.netcom.com>
Newsgroups: sci.med

In <34D1013A.F16C1A6F@mdb.ku.dk> "Brian Bjørn" <bbjorn@mdb.ku.dk>
writes: 

>Russ Turpin wrote:
>
>> Curiously, there are people in whom one internal carotid
>> artery has become completely -- but asymptomatically! --
>> occluded.  Seemingly, in these cases, the other arteries
>> that supply the circle of Willis provide enough blood to
>> perfuse both hemispheres.  The circle of Willis is a truly
>> interesting bit of redundancy in the brain's anatomy, and
>> I am more than a little curious how it evolved.  Does
>> anyone know?
>
>And quite a nice piece of redundancy, isn't it (the brain really dislikes
>hypoxia) ! I do not know how it evolved, but the circulus Willisi is
>supplied by the two internal carotid arteries and the two vertebral
>arteries.
>
>--
>Venlig hilsen


   A little piece of trivia, however: only 1 person in 4 has a perfect
circle of Willis, and can survive on any one of the four arteries. 
Everybody else will get into trouble before that, as they have some
degree of narrowing in one place or another in the circle.

   For that one in four with the perfect circle of Willis, sometimes
it's a pretty impressive life-saver.  When I was in medical school we
were had the teaching case of a driver who presented with the complaint
that he fainted when he looked over one shoulder to back up, but not
when he looked over the other shoulder!  Otherwise he felt great. 
Turned out on angiography that his brain was getting along on the
circle of Willis and flow from just ONE vertebral artery.  The other
three major arteries to his brain were totally clogged.  When he looked
over one shoulder he kinked the good artery, and cut off blood to his
entire brain.  Wups.  Fortunately whenever he fainted, his neck of
course always straightened out again.

                                      Steve Harris, M.D.