Biorheology - Volume 34, issue 1

Authors: Weston, M. W. | Tarbell, J. M.

Article Type: Research Article

Abstract: Since atherosclerotic lesions tend to be localized at bends and branching points, knowledge of wall shear rate patterns in models of these geometries may help elucidate the mechanism of atherogenesis. This study uses the photo chromic method of flow visualization to determine both the mean and amplitude of the wall shear rate waveform in straight and curved elastic arterial models to demonstrate the effects of curvature, elasticity, and the phase angle between the flow and pressure waveforms (impedance phase angle). Under sinusoidal flow conditions characteristic of large arteries, the mean shear rate at the inner wall of the curved tube …is reduced 40–56% from its steady flow value, depending on the phase angle. Wall shear rate amplitudes in the curved tube are significantly reduced by wall motion (36–55% of the Womersley amplitude for a straight rigid tube). The shear rate amplitude at the outer wall decreases 30% as the phase angle is reduced from -20° to -66°, while the shear rate amplitude at the inner wall increases 45%. As a result, the oscillatory nature of flow at the outer wall decreases with decreasing negative phase angle, but flow at the inner wall becomes much more oscillatory. At large negative phase angles, characteristic of hypertension or vasoactive agents, the shear rate at the inner wall has a small mean and cycles through positive and negative values; the shear rate at the outer wall remains positive throughout the flow cycle. Thus, the impedance phase angle could affect atherogenesis along the inner wall if temporal and directional changes in wall shear rate play a role. Show more

Keywords: Wall shear rate, phase angle, curvature, elastic, photo chromic

DOI: 10.3233/BIR-1997-34101

Citation: Biorheology, vol. 34, no. 1, pp. 1-17, 1997

Authors: Vlastos, G. | Lerche, D. | Koch, B.

Article Type: Research Article

Abstract: To understand the pulsatility of human blood flow in vivo, it is necessary to separately investigate (1) steady shear and oscillatory flow, and (2) the superposition of steady shear flow on oscillatory flow performed under in vitro conditions. In this study a variable steady shear rate was superimposed in parallel on oscillatory shear at a constant frequency (0.5 Hz) for human blood (45% hematocrit), and an aqueous polyacrylamide polymer solution (AP 30E, concentration 300 ppm). The effect of superposition of the above two shear flows on the viscoelasticity of blood was more pronounced for the elastic (η ”) than for …the viscous (η ’) component of viscoelasticity. With increasing superimposed shear rate, both η ’ and η ” decreased, especially at the low shear region. This behavior can be explained by the viscoelastic properties of blood and the phenomena of blood aggregation and disaggregation. Quantitatively, the dependence of the viscous component of complex viscosity on superimposed shear for both blood and polymer solution is described by a modified Carreau equation. The elastic component of complex viscosity decreased exponentially with increasing superimposed shear, and it is described by an exponential model. Show more

Keywords: Blood, superposition, viscoelasticity, polyacrylamide, blood-like model fluid

DOI: 10.3233/BIR-1997-34102

Citation: Biorheology, vol. 34, no. 1, pp. 19-36, 1997

Authors: Chapman, G.B. | Cokelet, G.R

Article Type: Research Article

Abstract: Computational fluid dynamics (CFD) and large scale model experiments were used to analyze the hemodynamic impact of leukocytes adherent to the wall of post-capillary venules. Using a large scale model and, with the aid of a finite element package, solving the Navier Stokes equations for low Reynolds number flow in a cylinder past an adherent sphere, we have developed a dimensionless correlation which permits the estimation of the pressure drop across an adherent leukocyte in an in vivo vessel. This relationship is: f · Re = exp [ 2.877 + 4.630 ( d D …) 4 ] where f is the Fanning friction factor, Re is the Reynolds number and d/D is the leukocyte to vessel diameter ratio. The friction factor is proportional to the pressure drop across the leukocyte, and does not significantly increase until d/D is greater than 0.5, and then increases rapidly with increasing d/D. Computations indicate that the length of the disturbed flow region generated by an adherent leukocyte increases with decreasing vessel size. The average wall stress in the disturbed flow region remains constant, and equal to the wall stress in the undisturbed region for d/D less than approximately 0.5. For d/D greater than 0.5, the average wall stress in the disturbed flow region increases rapidly with increasing d/D. There is an even larger increase, up to five times greater than the average disturbed stress, in the peak wall stress in the disturbed flow region. This indicates that significant wall stress gradients can be generated by an adherent leukocyte in post-capillary size vessels. Show more

Keywords: Leukocyte, computational fluid dynamics, flow resistance, wall stress

DOI: 10.3233/BIR-1997-34103

Citation: Biorheology, vol. 34, no. 1, pp. 37-56, 1997

Authors: Konstantopoulos, K. | Chow, T.W. | Turner, N.A. | Hellums, J.D. | Moake, J.L.

Article Type: Research Article

Abstract: Shear stress-induced platelet aggregation requires von Willebrand factor (vWF), platelet glycoprotein (GP) Ib, GPlIb-lIla, Ca2+ , and adenosine diphosphate (ADP). Recent reports using vWF labeled with either 125 I or fluorescein isothiocyanate (FITC) have demonstrated that in shear-fields, vWF binds to both GPlb and GPIIb-IIIa. The sequence of the vWF binding to the two platelet receptors has not been precisely determined in these reports. In this study, a flow cytometry technique using a primary anti-vWF antibody and a secondary FITC IgG antibody was used to measure shear stress-induced vWF binding to platelets. Washed normal platelets suspended at 50,000/µl with purified …large vWF multimers were exposed to laminar shear stresses of 15 to 120 dynes/cm2 for 30 sec. At this low platelet count, little or no aggregation occurred in the shear fields. A significant increase in post-shear vWF-positive platelets was consistently observed. Experiments with platelets from normal and severe von Willebrand’s disease (vWD) (which lack plasma and platelet α -granule vWF) demonstrated that exogenous vWF predominately contributed to the platelet-vWF binding. Blockade of platelet GPlb with the monoclonal anti-GPlb antibody, 6Dl, completely inhibited shear stress-induced platelet-vWF attachment. In contrast, blockade of GPIIb-IIIa with monoclonal anti-GPIIb-IIIa antibodies, 10E5 or c7E3, or with the GPIIb-IIIa-blocking tetrapeptide, RGDS, had little or no inhibitory effect on platelet-vWF binding. These data demonstrate that the binding of vWF to GPIb is likely to be the initial shear-induced platelet-ligand binding event. Show more

Keywords: von Willebrand factor, glycoprotein (GP) Ib, GPIIb-IIla, shear stress

DOI: 10.3233/BIR-1997-34104

Citation: Biorheology, vol. 34, no. 1, pp. 57-71, 1997

Authors: Mantzavinos, D. | Bailey, A.I. | Rampling, M.W.

Article Type: Research Article

Abstract: Freezing whole blood in bulk usually results in severe cellular destruction through the action of ice crystals and osmotic effects in the freezing liquid. The potential of flash freezing blood aerosols onto a liquid nitrogen surface as a means of inhibiting cellular damage was studied in this work. Three commercial spraying devices were employed to spray-freeze either whole blood or concentrated erythrocyte suspensions, using hydroxyethyl starch (HES) as a cryoprotectant. The integrity and viability of the processed cells were assessed by measuring gross rheological properties and the extent of hemolysis. Cells were found to be susceptible to the very high …shear stresses imposed by some of the spraying devices. Bulk freezing of blood, even in the presence of the cryoprotectant, resulted in complete cellular destruction. Whereas flash freezing was capable of substantially reducing the level of hemolysis to 12.6% and preserving the cellular deform ability. Show more

Keywords: Blood, freezing, aerosol, hydroxyethyl starch

DOI: 10.3233/BIR-1997-34105

Citation: Biorheology, vol. 34, no. 1, pp. 73-83, 1997