Distributions of Wall Shear Stress in Venular Convergences of Mouse Cremaster Muscle

OBJECTIVE: Wall shear stress regulates a variety of vascular functions and can be affected by variations in blood viscosity and wall shear rate independently. Therefore, the distribution of wall shear stress was characterized in converging flow regions of postcapillary venules to identify the relative contributions of shear rate and viscosity to wall shear rate and to determine whether venular branching directly affects the local shear environment. METHODS: Blood viscosity was evaluated as a function of vessel diameter and hematocrit, determined from fluorescent red blood cell (RBC) flux. Wall shear rate was measured directly from velocity profiles acquired using fluorescent RBC tracking or was estimated assuming Poiseuille flow conditions. Wall shear stress was calculated for measurements made across regions of converging flow. RESULTS: Measurements of wall shear stress showed no significant variation with location across the convergence (p < 0.9). Measures of viscosity showed little variation, for local measures of hematocrit (2.9 +/- 0.1 cp; coefficient of variation [CV]: 12%) or systemic hematocrit (3.4 +/- 0.1 centipoise [cp]; CV: 14%). Measures of wall shear rate showed greater variability for both direct measurements (61.2 +/- 8.7s; CV: 92%) and estimates assuming Poiseuille flow (122.7 +/- 15.7 s; CV: 83%). CONCLUSIONS: Variations in wall shear stress are independent of location through a venular convergence in contrast to arteriolar bifurcations. In addition, wall shear rate is significantly more variable than viscosity and, therefore, primarily accountable for wall shear stress variations in postcapillary venules.