Biorheology - Volume 35, issue 6

Authors: Mestel, A.J. | Mokady, A.J. | Parker, K.H. | Winlove, C.P.

Article Type: Research Article

Abstract: The polyelectrolyte layer coating mammalian cells, known as the glycocalyx, may be important in communicating flow information to the cell. In this paper, the layer is modelled as a semi‐infinite, doubly periodic array of parallel charged cylinders. The electric potential and ion distributions surrounding such an array are found using the linearised Poisson–Boltzmann equation and an iterative domain decomposition technique. Similar methods are used to calculate Stokes flows, driven either by a shear at infinity or by an electric field, parallel or transverse to the cylinders. The resulting electric streaming currents due to flow over endothelial cells, and the electrophoretic …mobilities of red blood cells are deduced as functions of polymer concentration and electrolyte molarity. It is shown that only the top portion of the layer is important in these effects. Show more

Keywords: Red cell , endothelial cell, electrophoretic mobility, streaning current, glycocalyx, polyelectrolyte theory, Poisson–Boltzmann

Citation: Biorheology, vol. 35, no. 6, pp. 365-381, 1998

Authors: Ehrlich, S. | Wolff, N. | Schneiderman, R. | Maroudas, A. | Parker, K.H. | Winlove, C.P.

Article Type: Research Article

Abstract: We used equilibrium dialysis to measure the osmotic pressure of chondroitin sulphate (CS) solutions as a function of their concentration and fixed charge density (FCD) and the ionic strength and composition of the solution. Osmotic pressure varied nonlinearly with the concentration of chondroitin sulphate and in 0.15 M NaCl at FCDs typical of uncompressed cartilage (approximately 0.4 mmol/g extrafibrillar H_{2} O) was \sim 3 atmospheres. Osmotic pressure fell by 60% as solution ionic strength increased up to about 1 M, but remained relatively constant at higher ionic strengths. The ratio of Ca^{2+} to Na^{+} …in the medium was a minor determinant of osmotic pressure. The data are compared with a theoretical model of the electrostatic contribution to osmotic pressure calculated from the Poisson–Boltzmann equation using a rod‐in‐cell model for CS. The effective radius of the polyelectrolyte rod is taken as a free parameter. The model qualitatively reproduces the non‐linear concentration dependence, but underestimates the osmotic pressure by an amount that is independent of ionic strength. This difference, presumably arising from oncotic and entropic effects, is approximately 1/3 of the total osmotic pressure at physiological polymer concentrations and ionic strength. Show more

Keywords: Polyelectrolyte theory, Poisson–Boltzmann, ion distribution, cartilage mechanics

Citation: Biorheology, vol. 35, no. 6, pp. 383-397, 1998

Authors: Wells, Sarah M. | Adamson, S. Lee | Langille, B. Lowell | Lee, J. Michael

Article Type: Research Article

Abstract: We have used hydrothermal isometric tension (HIT) techniques in a sheep model to assess collagen crosslink stability and its contribution to the mechanical properties of the ovine thoracic aorta during perinatal and postnatal development. Aortic tissue was studied from fetal sheep, lambs, and adult sheep. Strips of tissue were loaded under isometric tension and heated to a 90^{\circ} C isotherm which was sustained for 3 hours. The decrease in load at this temperature is associated with collagen peptide bond hydrolysis and chain slippage, and the rate of this decrease is an inverse indicator of collagen crosslinking. The half‐time of …load decay (t_{1/2} ) was computed before and after tissue was treated with NaBH_{4} which stabilizes immature, reducible crosslinks. We observed a two‐fold increase in t_{1/2} of untreated tissue from the lamb to the adult, indicating that aortic collagen crosslinking increased during postnatal development. Furthermore, the t_{1/2} of NaBH_{4} ‐stabilized lamb tissue was similar to that of the untreated adult tissue, suggesting that much of the immature crosslinking in the lamb is stabilized during postnatal development. These observations suggest (a) increased crosslinking occurs during postnatal development and (b) that this increase is largely due to a conversion of immature crosslinks into their mature, heat stable form. Show more

Keywords: Collagen, crosslinking, development, hydrothermal isometric tension tests, denaturation

Citation: Biorheology, vol. 35, no. 6, pp. 399-414, 1998

Authors: Cheer, A.Y. | Dwyer, H.A. | Barakat, A.I. | Sy, E. | Bice, M.

Article Type: Research Article

Abstract: Arterial hemodynamic forces may play a role in the localization of early atherosclerotic lesions. We have been developing numerical techniques based on overset or “Chimera” type formulations to solve the Navier–Stokes equations in complex geometries simulating arterial bifurcations. This paper presents three‐dimensional steady flow computations in a model of the rabbit aorto‐celiac bifurcation. The computational methods were validated by comparing the numerical results to previously‐obtained flow visualization data. Once validated, the numerical algorithms were used to investigate the sensitivity of the computed flow field and resulting wall shear stress distribution to various geometric and hemodynamic parameters. The results demonstrated that …a decrease in the extent of aortic taper downstream of the celiac artery induced looping fluid motion along the lateral walls of the aorta and shifted the peak wall shear stress from downstream of the celiac artery to upstream. Increasing the flow Reynolds number led to a sharp increase in spatial gradients of wall shear stress. The flow field was highly sensitive to the flow division ratio, i.e., the fraction of total flow rate that enters the celiac artery, with larger values of this ratio leading to the occurrence of flow separation along the dorsal wall of the aorta. Finally, skewness of the inlet velocity profile had a profound impact on the wall shear stress distribution near the celiac artery. While not physiological due to the assumption of steady flow, these results provide valuable insight into the fluid physics at geometries simulating arterial bifurcations. Show more

Keywords: Hemodynamics, computational fluid dynamics, aorta, atherosclerosis, shear stress

Citation: Biorheology, vol. 35, no. 6, pp. 415-435, 1998

Authors: Helmke, Brian P. | Sugihara‐Seki, Masako | Skalak, Richard | Schmid‐Schönbein, Geert W.

Article Type: Research Article

Abstract: In spite of the relatively small number of leukocytes in the circulation, they have a significant influence on the perfusion of such organs as skeletal muscle or kidney. However, the underlying mechanisms are incompletely understood. In the current study a combined in vivo and computational approach is presented in which the interaction of individual freely flowing leukocytes with erythrocytes and its effect on apparent blood viscosity are explored. The skeletal muscle microcirculation was perfused with different cell suspensions with and without leukocytes or erythrocytes. We examined a three‐dimensional numerical model of low Reynolds number flow in a capillary with a …train of erythrocytes (small spheres) in off‐axis positions and single larger leukocytes in axisymmetric positions. The results indicate that in order to match the slower axial velocity of leukocytes in capillaries, erythrocytes need to position themselves into an off‐axis position in the capillary. In such off‐axis positions at constant mean capillary velocity, erythrocyte axial velocity matches on average the axial velocity of the leukocytes, but the apparent viscosity is elevated, in agreement with the whole organ perfusion observations. Thus, leukocytes influence the whole organ resistance in skeletal muscle to a significant degree only in the presence of erythrocytes. Show more

Keywords: Skeletal muscle, rat, capillaries, hematocrit, leukocyte count

Citation: Biorheology, vol. 35, no. 6, pp. 437-448, 1998