A Simple ‘‘Streak Length Method’’ for Quantifying and Characterizing Red Blood Cell Velocity Profiles and Blood Flow in Rat Skeletal Muscle Arterioles BARAA K. AL-KHAZRAJI,* NICOLE M. NOVIELLI,* DANIEL GOLDMAN,* ,  , PHILIP J. MEDEIROS,* AND DWAYNE N. JACKSON* ,  * Department of Medical Biophysics, University of Western Ontario, London, Ontario, Canada;   Biomedical Engineering Program, University of Western Ontario, London, Ontario, Canada Address for correspondence: Dwayne N. Jackson: Department of Medical Biophysics and Biomedical Engineering Program, Room 404 Medical Sciences Building, Schulich School of Medicine & Dentistry, The University of Western Ontario, London, ON, Canada N6A 5C1. E-mail: dwayne.jackson@schulich.uwo.ca Received 26 September 2011; accepted 22 January 2012. ABSTRACT Objectives: To develop a valid experimental method for quantifying blood flow in continuously branching skeletal muscle arterioles, and to derive an empirical relationship between velocity ratio (V Max ⁄ V Mean ) and arteriolar diameter. Methods: We evaluated arteriolar trees using IVVM of rat gluteus maximus muscle and developed a method to acquire single fluorescent-labeled RBC velocities across arteriolar lumens to create velocity profiles. These data were used to calculate the blood flow for 37 vessel segments (diameters: 21–115 lm). Results: Mass balance at arteriolar bifurcations had 0.6 ± 3.2% error. Velocity ratios ranged from 1.35 to 1.98 and were positively correlated with diameter (p < 0.0001), and V RBC profiles were blunted with decreasing diameter. Conclusions: We present a means for quantifying blood flow in continuously branching skeletal muscle arterioles. Further, we provide an equation for calculating velocity ratios based on arteriolar diameter, which may be used by others for blood flow calculations. Key words: microvascular blood flow, intravital video microscopy, red blood cell velocity profiles, skeletal muscle arterioles, hemodynamics Abbreviations used: A RBC, coronal cross-sectional area of center of red blood cell; A Vessel, cross-sectional area of vessel lumen; FITC, fluorescein isothiocyanate; fps, frames per second; GM, gluteus maximus; Hct, hematocrit; IVVM, intravital video microscopy; PSS, physiological salt solution; _ Q, total blood flow; _ Q CFL , flow through cell-free layer; _ Q RBC , flow through red blood cell column; RBC, red blood cell; R C , radius from the center of vessel to the edge of red blood cell column; R S , radius from the center of vessel to the centroid of streak on edge of red blood cell column; R Wall , radius from center of vessel to internal vessel wall; V Center , red blood cell velocity at the center of vessel; V Max , maximum red blood cell velocity; V Mean , mean red blood cell flow velocity; V Ratio , velocity ratio (V Max ⁄ V Mean ); V RBC , red blood cell velocity; V Rc , red blood cell velocity at R C; V Rs , red blood cell velocity at R S . Please cite this paper as: Al-Khazraji BK, Novielli NM, Goldman D, Medeiros PJ, Jackson DN. A simple ‘‘Streak Length Method’’ for quantifying and charac- terizing red blood cell velocity profiles and blood flow in rat skeletal muscle arterioles. Microcirculation 19: 327–335, 2012. INTRODUCTION The function of skeletal muscle, which is essential for locomotion and shares many basic features with other met- abolically active tissues, relies on microvascular resistance networks that are directly involved in blood flow and pressure regulation [18]. The unique structure of the microvascular units within skeletal muscle are arranged to maximize diffusion and exchange. Arteriolar resistance is modulated by intrinsic systems (e.g., myogenic control and endothelium derived factors) and extrinsic systems (e.g., sympathetic nerves) that tightly match blood flow (oxygen delivery) to metabolic demand. Thus, for decades, physiol- ogists have been interested in the characteristics of skeletal muscle blood flow, with an emphasis on measuring and understanding red blood cell velocity (V RBC ). Blood flow through a vessel segment is generally calcu- lated as a product of mean red blood cell flow velocity (V Mean ) and vessel cross-sectional area (A Vessel ). Centerline V RBC (V Center ) is commonly measured using Doppler veloc- imetry [10,27] or the dual-slit ⁄ sensor technique [12,21] and is converted to V Mean through use of the Baker and Wayland velocity ratio conversion factor (V Ratio ; an index of profile bluntness and transition layer) of 1.6 [2]. Use of DOI:10.1111/j.1549-8719.2012.00165.x Original Article ª 2012 John Wiley & Sons Ltd 327 The Official Journal of the Microcirculatory Society, Inc. and the British Microcirculation Society