Atherosclerosis is one of the main diseases responsible for the high global mortality rate involving heart and blood vessel disorders. The build-up of fatty materials in the inner wall of the human artery prevents sufficient oxygen and nutrients reaching the organs of the body. Atherosclerosis is a chronic, long term condition, which develops and progresses over time; however, the disease does not present any symptoms until an advanced stage is reached, which results in potential permanent debility and sometimes sudden death. This thesis is concerned with the progression of atherosclerosis in an artery with mild stenosis that has resulted in a 30% reduction in its diameter. To this end, data on the low wall shear stress has been correlated with the atherosclerotic prone region. In a stenosed artery, this region corresponds to the separation zone that is formed distal to the lumen reduction. Atherosclerosis is a complex phenomenon, and not only involves wall shear stress, but also cellular interactions. Previous research has shown that even in the absence of wall biological effects, the blood cell distribution is strongly influenced by the hydrodynamics of the fluid. The mechanisms of blood cell distribution and the dynamic behaviour of the blood flow were investigated by developing a physical model of the stenosed artery, and by using particles to represent the presence of the blood cells. Particle Image Velocimetry system was employed and the size of particles were the 10μm and 20μm.The flow field was characterised and the particle distribution was measured. The characteristics of steady flow in the stenosed artery at Reynolds numbers of 250 and 320 revealed the importance of fluid inertia and the shear gradient distal to stenosis. Unequal distribution of the particles modelling the blood cells was observed, as more particles occupied the recirculation zones than the high shear region and central jet. The particle migration was found to depend on the particle size, particle concentration and fluid flow rates. The results suggested that the presence of similar effects in the real human arterial system may be significant to the progression of atherosclerotic plaques. At lower Reynolds number of 130, a particle depleted layer was observed at the wall region. In physiological flow the cell free layer will prevent the transport of oxygen and nitrogen oxide (NO) to the muscle tissues. A numerical method was used to simulate the flow characteristics measured in the experiment. The numerical results revealed the importance of the hydrodynamic mechanism of particle migration. Drag and lift forces were found to affect the residence time of particles in the recirculation region. The findings of this work have suggested that for a complex geometry like a large stenosed artery at physiological flow rates, hydrodynamic forces are important in cell migration in the flow separation zone. Even without biological forces, the cells migrate to the low wall shear stress region. For computational dynamics studies, this study has demonstrated the need for higher-order modelling at the cellular level in order to establish the particle migration mechanisms.