THE NEUROLOGICAL SELF
The circle of Willis also allows blood to flow across the midline of the brain if an artery on one side is occluded. The circle of Willisthereby serves a safety valve function for the brain, allowing collateral circulation (or flow of blood through an alternate route) to take place if the flow is reduced to one area.
Circle of Willis functions. The Circle of Willis is basically an arrangement of interconnected vascular channels that ensure that the (oxygenated) blood flow to the brain continues unimpeded, in case any of the principal suppliers are obstructed by injury, physical pressure or disease.