Abstract-The suggestion was made in the 1870s that mechanical irritation of the arterial wall is a cause of atherosclerosis, because the changes were chiefly found at points "exposed to the full stress and impact of the blood." The mechanical damage theory persisted until well into the 20th century when, with interest increasing in multidisciplinary research, two fluid mechanical proposals were advanced for the patchy distribution of the lesions. One advocated high-and the other low-wall shear. Arterial wall shear stress levels appeared, however, insufficiently high to damage the endothelium. In contrast, examination of cadaver human arteries, combined with flow studies in models and casts of arteries, implied that the lesions occurred preferentially in regions expected to experience low-wall shear; a mechanism, involving arterial wall lipid metabolism and shear-dependent blood-wall mass transport, was suggested to account for that distribution. These proposals helped stimulate extensive investigation of arterial fluid mechanics/mass transport and vascular biology/pathology, revealing mechanisms that may explain the now widely confirmed preferred occurrence of atherosclerosis in low wall shear regions in adult human beings. Fluid Mechanics and PathologyPolymaths, including Leonhard Euler (1707-1783) and Thomas Young (1773-1829), studied wave propagation in arteries, but they do not appear to have investigated the arterial flow field. Interest in atherosclerosis and the mechanisms that underlay its patchy distribution in arteries increased from the later 19th century onward. However, in contrast to the polymath approach, there was then a tendency toward specialization in engineering and biology/pathology/ medicine, a development unfavorable to research in the essentially multidisciplinary field of arterial fluid dynamics. The specialization largely persisted until the middle of the 20th century, when interest increased in multidisciplinary research. Therefore, it was against a background of specialization that much of the research of the 19th century and first half of the 20th century on blood flow and atherosclerosis was undertaken.It is important to appreciate that wall shear stress and wall shear rate are characteristics of the local flow field. The former is the force applied parallel, or tangential, to a boundary, and the latter is the gradient of velocity normal to the boundary. The wall shear stress is the product of the wall shear rate and the viscosity of the fluid. In citing relevant studies, we comment, where indicated, on instances of incomplete understanding of the fluid mechanics.Several workers suggested that flow separation might occur in the arterial system at, for example, sites of curvature, branching or expansion of the cross-section. Suggested links between flow separation and the development of atherosclerosis included mechanical damage to the wall, 1,2 platelet deposition, 3 and fibrin deposition. 4 Flow separation and static zones may occur in arteries and investigation of the issue conti...

We have studied numerically steady and unsteady flow in a straight and a helically stented common carotid artery, in order to model porcine experimental results that show reduced intimal hyperplasia (IH) in the helical case. The combination of flow pulsatility and threedimensionality generates a sweeping motion of the Dean vortices, which overall reduced extremes of both oxygen flux to the vessel wall and wall shear stress (WSS). Since IH and atherosclerosis affect preferentially low WSS regions, these findings imply that vessel threedimensionality and flow pulsatility can play important protective roles in respect of these diseases. The amplitude and frequency of the velocity waveform are important parameters of the system. Increase in amplitude increases WSS and oxygen flux to the vessel wall. Increase in frequency has a small effect; it increases WSS but has no effect on the oxygen flux to the vessel wall.

Arterial geometry is commonly non-planar and associated with swirling blood flow. In this study, we examine the effect of arterial three-dimensionality on the distribution of wall shear stress (WSS) and the mass transfer of oxygen from the blood to the vessel wall in a U-bend, by modelling the blood vessels as either cylindrical or helical conduits. The results show that under physiological flow conditions, three-dimensionality can reduce both the range and extent of low WSS regions and substantially increase oxygen flux through the walls. The Sherwood number and WSS distributions between the three-dimensional helical model and a human coronary artery show remarkable qualitative agreement, implying that coronary arteries may potentially be described with a relatively simple idealized three-dimensional model, characterized by a small number of well-defined geometric parameters. The flow pattern downstream of a planar bend results in separation of the Sh number and WSS effects, a finding that implies means of investigating them individually.

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