Cardiovascular diseases are the first cause of death in industrialized countries. It is now widely established that blood fluid dynamics and its interaction with the vascular wall play an essential role in the development and evolution of vascular pathologies. Mathematical modeling and numerical simulation are precious tools which may help to understand biomechanical phenomena in both physiological and pathological states and can lead to the development of new therapies or to assist physicians in clinical decision making and treatment. This paper reports on the simulation of blood flow and its interaction with the vascular wall using an aorta model. Then, to understand the complex flow patterns in pathological states, the simulation of a cerebral aneurysm is executed.