Biorheology - Volume 39, issue 3-4

Article Type: Other

Citation: Biorheology, vol. 39, no. 3-4, pp. 289-290, 2002

Authors: David, T. | Walker, P.G.

Article Type: Research Article

Abstract: Adherent platelets are an important part of both thrombus formation and in certain stages of atherogenesis. Platelets can be activated by potent chemicals released from adherent platelets and adhere far more readily than unactivated ones. An analytical and numerical model is presented utilising high Peclet number for the activation and adhesion of platelets in shear flows. The model uses a similarity transformation, which characterises the relationship between convective, diffusive transport and the bulk platelet activating reaction mechanism. A first order surface reaction mechanism is used to model platelet adhesion at the wall (cell) surface. The reduced Damköhler number, ℳ, characterises …the importance of the bulk reaction and includes both convective and diffusive terms. For a high rate of blood flow (ℳ→0) the activation of platelets can effectively be terminated. In contrast, for (ℳ→∞) an inner layer of activated platelets exists with an infinitesimally thin reaction sheet separating activated and non‐activated platelets. This characterisation by the Damköhler number highlights results found clinically, in that thrombus forms in areas of low shear (high ℳ) and in some cases an increased blood flow (low ℳ) can inhibit the activation of platelets completely. The model shows the critical balance that exists between convection, diffusion and reaction. Show more

Keywords: Mathematical model, thrombus formation, wall shear stress

Citation: Biorheology, vol. 39, no. 3-4, pp. 293-298, 2002

Authors: Davies, Peter F. | Polacek, Denise C. | Shi, Congzhu | Helmke, Brian P.

Article Type: Research Article

Abstract: The completion of the Human Genome Project and ongoing sequencing of mouse, rat and other genomes has led to an explosion of genetics‐related technologies that are finding their way into all areas of biological research; the field of biorheology is no exception. Here we outline how two disparate modern molecular techniques, microarray analyses of gene expression and real‐time spatial imaging of living cell structures, are being utilized in studies of endothelial mechanotransduction associated with controlled shear stress in vitro and haemodynamics in vivo. We emphasize the value of such techniques as components of an integrated understanding of vascular rheology. In …mechanotransduction, a systems approach is recommended that encompasses fluid dynamics, cell biomechanics, live cell imaging, and the biochemical, cell biology and molecular biology methods that now encompass genomics. Microarrays are a useful and powerful tool for such integration by identifying simultaneous changes in the expression of many genes associated with interconnecting mechanoresponsive cellular pathways. Show more

Citation: Biorheology, vol. 39, no. 3-4, pp. 299-306, 2002

Authors: Griffith, Tudor M.

Article Type: Research Article

Abstract: Local haemodynamic forces acting on the endothelium modulate vascular tone through mechanisms that normalize intimal shear stress. This flow‐dependent diameter response contributes to the optimization of circulatory function and is mediated via shear stress‐induced release of NO, vasodilator prostanoids and a putative endothelium‐derived hyperpolarizing factor or EDHF. There is growing evidence that NO/prostanoid independent relaxations involve direct heterocellular signalling between endothelial and smooth muscle cells via gap junctions.

Keywords: Shear stress, flow, NO synthase, EDHF, connexin

Citation: Biorheology, vol. 39, no. 3-4, pp. 307-318, 2002

Authors: Lehoux, Stéphanie | Tronc, François | Tedgui, Alain

Article Type: Research Article

Abstract: Chronic changes in wall shear stress lead to vascular remodeling, characterized by increased vascular wall diameter and thickness, to restore wall shear stress values to baseline. Release of nitric oxide from endothelial cells exposed to excessive shear is a fundamental step in the remodeling process, and potentially triggers a cascade of events, including growth factor induction and matrix metalloproteinase activation, that together contribute to restructuralization of the vessel wall. Understanding these processes could help explain how changes in blood vessel wall structure occur in the context of atherosclerosis or aortic aneurisms.

Keywords: Shear stress, nitric oxide, growth factors, matrix metalloproteinases

Citation: Biorheology, vol. 39, no. 3-4, pp. 319-324, 2002

Authors: Truskey, George A. | Barber, Kevin M. | Rinker, Kristina D.

Article Type: Research Article

Abstract: Adhesion of monocytes to arterial endothelium may contribute to the asymmetric distribution of atherosclerotic lesions. Possible mechanisms for adhesion in the relatively high shear stress environment found in arteries include greater monocyte deformation and/or more frequent penetration of microvilli through steric and charge barriers. In vivo, secondary flows generate forces acting normal to the endothelial cell surface. These forces may cause compression of the microvilli or enable cells to overcome steric or electrostatic barriers, increasing adhesion. To investigate this, we examined monocyte adhesion to activated endothelium in recirculating flow. Adhesion was characterized by short arrests in a narrow region on …either side of the reattachment line. The median arrest time was longer than that observed at comparable shear stresses in a linear shear flow. The lifetimes of adhesion were analyzed using a model for multiple bond formation. For cells adhering near the reattachment line, the bond number per cell was greater than the value found for similar shear stresses under shear flow. Thus, multiple bond formation arising from greater normal forces in recirculating flow permits monocytes to adhere at higher shear stresses. Show more

Keywords: Atherosclerosis, adhesion, endothelium, shear stress, monocyte, kinetics

Citation: Biorheology, vol. 39, no. 3-4, pp. 325-329, 2002

Authors: Wada, Shigeo | Karino, Takeshi

Article Type: Research Article

Abstract: To find out whether concentration polarization of low‐density lipoprotein (LDL) occurs at the surface of a vascular endothelium or not, transport of LDL in flowing blood to an water‐permeable endothelium was studied theoretically by means of CFD. Calculations were carried out for an endothelium exposed to a Couette flow by assuming that the surface geometry of the endothelium could be expressed by a cosine function. Two typical cases were considered for the permeability of endothelium to water; one was uniform permeability everywhere in the endothelium, and the other was uneven permeability which was augmented at the intercellular junction. It was …found that, in both cases, the surface concentration of LDL increased in going distally from the entrance, taking locally high and low values at the valleys and hills of the endothelium, respectively, and the variation was larger in the case of endothelium with uneven permeability. These results clearly showed that concentration polarization of LDL which might affect the uptake of LDL by the arterial wall certainly occurs at the surface of the endothelium even if the flow is disturbed microscopically by the uneven surface of the endothelium. Show more

Keywords: Endothelium, concentration polarization, low‐density lipoproteins, blood flow, mass transport, computational analysis

Citation: Biorheology, vol. 39, no. 3-4, pp. 331-336, 2002

Authors: Watkins, N.V. | Caro, C.G. | Wang, W.

Article Type: Research Article

Abstract: Flow induced shear stress influences vascular cellular biology and pathophysiology in numerous ways. Previous in vitro studies on interactions between flow and endothelial cells using parallel‐plate flow chambers involve two‐dimensional flows, whereas flows in larger vessels are commonly three‐dimensional. We have constructed a parallel plate flow chamber with a backward facing step aligned oblique to the axis of the chamber. Flow visualisation by steady injection of ink through a hypodermic tube reveals swirling flow in the recirculation region downstream of the step. At given angles of the step, θ (to the axis of the chamber), the pitch of the swirl …and the width of the separation region, as measured in the direction perpendicular to the step, increase with the Reynolds number (Re). On the other hand, at given values of Re, reduction of θ results in increases in the swirl pitch but decreases in the width of the separation zone. Furthermore, clearance time of ink from the separation region is shorter with an oblique step than a perpendicular one at given Re. Computer simulation confirms the 3D swirling flow created by the oblique step and provides detailed distribution of wall shear stresses in the flow chamber. Show more

Keywords: Wall shear stress, vascular endothelial cells, flow chamber, swirling flow

Citation: Biorheology, vol. 39, no. 3-4, pp. 337-342, 2002

Authors: Blyth, M.G. | Mestel, A.J. | Zabielski, L.

Article Type: Research Article

Abstract: The macrocirculation is modelled by incompressible Newtonian flow through a rigid network of pipes for which possible simplifications are discussed. The common assumptions of two‐dimensionality or axisymmetry can be generalised to helical symmetry, and in the first part of the paper, the three‐dimensionality of arterial bends is considered by varying the curvature and torsion of a section of a helical pipe. The torsion is found to impart a preferential twist to the cross‐sectional flow. This loss of symmetry ensures that flow separation is less severe for a helical bend than for a toroidal bend. The effects of variations in body …size are examined using allometric scaling laws. In the second part of the paper, the approach to “fully developed” Dean or Womersley flow is considered in an attempt to quantify the regions of validity of idealised models. A perturbation approach, akin to hydrodynamic stability theory, is used. It is argued that often potential flows are more suitable for describing the rapid interactions between geometry and pulsatility rather than the eventual fully developed state so that, for example, the first 100° of the aortic arch may be considered irrotational. Helical potential flows are found to develop faster than the corresponding toroidal flows, but slower than those in a straight pipe. The presence of vorticity in the core also retards the development of symmetric flows. It is concluded that while idealised flows can occur at some points in the body, in general experimental observation is needed to justify their use. Particular caution is recommended when interpreting calculations with Poiseuille input. Show more

Citation: Biorheology, vol. 39, no. 3-4, pp. 345-350, 2002

Authors: Kumar, B.V. Rathish | Yamaguchi, T. | Liu, H. | Himeno, R.

Article Type: Research Article

Abstract: The aim of this study is to examine the interaction between two mild atherosclerotic proliferations spaced apart by a distance S by analyzing their influence on flow structure, pressure drop and stress field in an arterial vessel under pulsatile flow conditions. This has been achieved numerically by employing a time accurate, cell centered finite volume method in solving the Navier–Stokes equations governing the 3D unsteady flow dynamics in a conceptual model of an multiply constricted arterial vessel. In comparison to the pressure drop across a single stenosis, nearly a 50% increase in the late systolic and early diastolic pressure drops …has been observed across the two mild constrictions when they are spaced within a distance of S≤4. When S≤4, more than a 25% reduction in the peak systolic wall shear stress (WSS) on the downstream constriction is noted. Show more

Citation: Biorheology, vol. 39, no. 3-4, pp. 351-357, 2002