The Transport of Low Density Lipoprotein in the human carotid model

The arterial wall is a heterogeneous structure consist of several layers which strongly differ in their thickness and in their biological and physical properties. The layers constituting the wall are the endothelium, the intima, the internal elastic lamina or lamella, IEL, the media and the adventitia.
The endothelium is a type of ephitelium composed of a single layer of smooth, thin cells that lines the heart, blood vessels, lymphatics and serous cavities. It forms a continuous lining on blood contacting a surfaces in the vascular system, providing the principal barrier against the entry of cholesterol and blood cells into the wall and inhibiting platelet adherence to the vessel walls.
Endothelial cells create chemicals and control the transport of the mass into and out of the wall.
The sub-endothelial layer is an extra-cellular matrix of randomly distributed fibres, mainly collagenous bundles and proteoglycans, that are defined as glycoproteins which have a very high polysaccharide content. This layer is surrounded by the IEL, which is composed by elastic fibres.
Under normal physiological loading, the fibres form an approximately circular band. Together with the sub-endothelial layer it helps the wall to withstand haemodynamic stresses. Outside the IEL there is the media, which is made of smooth muscle cells and it is primary regulator of vessel diameter. The outer layer is the adventitia, which is a complex structure that merges into the surrounding tissue. It tethers arteries in place and it carries the nutrients to and wastes away form the smooth muscle cells in the media. Moreover, it provides resistance to overextension and rupture. Sometimes, what we call intima is denoted with sub-endothelial layer. In this case, the term intima is used for the group of the endothelium, sub-endothelial layer and IEL.
The behavior and the interaction of these layers are regulated by a complex set of chemical and mechanical phenomena. There is evidence that these mechanisms depend also on fluid dynamics and mass transport phenomena in the blood stream and in the wall. The role of fluid dynamics and mass transport processes in the physiological and patho-physiological functions of the vascular systems are of great interest. It is accepted that an improved understanding of vascular mass transport phenomena and the influence of fluid dynamics have a great impact on the accumulation of macromolecules, such as low density lipoprotein (LDL), or other atherogens in the arterial wall.
Atherosclerosis tends to be localized in zones of artery bifurcations and bends where the shearing forces imposed by the flowing blood are disturbed compared with the straight tube patterns.
It has been observed that LDL accumulation in the intima occurring at zones of low and oscillating wall shear stress, such as in flow separation regions, is associated with the tendency to intimal thickening. The goals of vascular mass transport studies are to correlate mass transfer in anatomic geometries with the localization of atherosclerotic lesions and to determine the influence of disturbed flow patterns on the local concentrations distribution of substances in the blood stream and in the vessel wall layers. Vascular mass transport analysis requires the development of appropriate mathematical and numerical models. Because of the extreme complexity, the biological problem can be cast with difficulty into a formal physical framework, and simplifications with respect to the biological situation are unavoidable.