If there are no problems in either system, the pressure of the streams will be equal and they will not mix. However, if there is a blockage in one of them blood will flow from the intact artery to the damaged one, preventing a cerebral vascular accident. As long as the Circle of Willis can maintain blood pressure at fifty percent of normal, no infarction or death of tissue will occur in an area where a blockage exists. If collateral circulation is good, no permanent effects may result from a blockage. In such cases, a transient ischemic attack may occur, meaning that parts of the brain are temporarily deprived of oxygen.
Some people lack one of the communicating arteries that form the Circle of Willis.
In this case, if a blockage develops, collateral circulation will be impeded and the collateral blood supply will be compromised, causing brain damage to occur. There are some watershed areas in the brain located at the ends of the vascular systems. Problems with blood supply are particularly likely to occur here, especially in those who have hardening of the arteries. Blockages in the water shed areas can cause transcortical aphasia . Abnormally low blood pressure can cause brain damage.