Mass Transfer And Atherosclerosis In The Human Arterial Circulation

In our modern society, with the improvement of living standard, the morbility and mortality of cardiovascular diseases are getting higher and higher. Cardiovascular diseases have become the leading cause of deaths in the developing and developed countries. Among them, atherosclerosis accounts for the majority of these deaths. Although atherosclerosis is a chronic and multi-factored disease, both clinical and postmortem studies in humans indicate that atherosclerotic lesions do not develop randomly, and not everywhere within the circulation, but are localized at certain selected sites in the arterial tree. Areas of arterial branching, stenoses and curved segments have the greatest predilection for this disease. This phenomenon is called "localization of atherosclerosis". Many theories and hypothesis have been proposed to explain the mechanism of atherogenesis, such as injure respond theory, monoclonal theory and inflammation theory. But all of them cannot sufficiently explain the localization phenomenon of atherosclerosis.In recent years, Deng and Karino et al hypothesized that concentration polarization (a very well-known mass transfer phenomenon in engineering) of atherogenic lipids, such as low density lipoproteins (LDL), may occur in the arterial system with the lipids increasing in concentration from the bulk value towards the interface. The lipids concentration at the blood/wall interface may vary according to its location within the circulation, being much higher in regions of disturbed flow with flow recirculation and stagnation and with low wall shear rates. These local variations in the luminal surface lipid concentration may contribute to the localization of atherosclerosis.Although more and more evidence indicates the involvement of hemodynamic factors in the genesis and development of atherosclerosis, the correlations between circulation hemodynamics and mass transport of atherogenic materials between blood flow and arterial wall are still the focus of the controversy. In this thesis, to clarify the correlation between hemodynamics, mass transport and atherosclerosis, both theoretical and experimental studies have been carried out as follows: (1) An ex vitro experimental study on concentration polarization of macromolecules (LDL) at a canine carotid arterial stenosis; (2) Theoretical and experimental study on the effects of recirculation flows on mass transfer from the arterial wall to flowing blood; (3) Numerical simulation of oxygen transfer in human carotid artery bifurcations by finite element method. The results from these studies indicate that: (1) Concentration polarization of atherogenic lipids can indeed occur in the human arterial system; (2) The luminal surface concentration of lipids is enhanced significantly in regions of disturbed flow. The fluid layer with highly concentrated lipids in the area of flow separation point may be responsible for the formation and development of atherosclerotic stenosis; (3) Filtration flow across the arterial wall has apparent impact on the luminal surface concentration of lipids. The concentration increases almost linearly with increasing filtration rate; (4) Mass transfer rate from the arterial wall to flowing blood is significantly higher in regions of disturbed flow with a local maximum around the reattachment point and it is independent of wall shear rates; (5) Flow disturbance may enhance the transport of biochemicals and macromolecules synthesized within the blood vessel wall from the blood vessel wall to flowing blood. (6) The depression of oxygen supply to the arterial wall at the entry of the carotid sinus may lead to an atherogenic response of the arterial wall and contribute to the development of atherosclerotic stenosis.The findings of the present study may not only help better understand the role of hemodynamic factors in atherogenesis, but may have very practical applications in the design of arterial prostheses and the design of vascular reconstructive surgeries.