Why is there a circular route of arteries to the brain?
1) If the vertebral veins become blocked, the brain can receive blood from the carotid veins and vice versa.
2) To act as back up if the jugular veins become obstructed.
3) If the external jugular arteries become blocked, the brain can receive blood from the internal jugular arteries.
4) If the vertebral arteries become blocked, the brain can receive blood from the carotid arteries and vice versa.