| This study investigates fluid mechanics and mass transport processes that occur on length scales similar to blood flow through the arteriolar microcirculation. In this regime, the size of suspended particles (∼6 mum) is significant compared to vessel diameters (20--100 mum), necessitating the consideration of discrete particle effects, namely the formation of heterogeneous particle concentration profiles caused by shear-induced particle migrations away from solid boundaries, and enhanced transport due to micro-mixing caused by particle-particle interactions. These phenomenon are studied with the specific case of oxygen transport by hemoglobin-based oxygen carriers (HBOCs) in mind.;The arteriolar microcirculation is the first location in the body where significant O2 transport occurs from the blood to the surrounding tissue. The amount of O2 delivered is sensitive to the thickness of a red blood cell (RBC) depleted layer that forms near vessel walls due to hydrodynamic interactions. Acellular hemoglobin (Hb) is generally associated with increased O2 as compared to whole blood, mainly because of the extracellular location of Hb. In the first part of this study, in vitro models of O2 transport from Hb solutions flowing through arteriolar-sized, gas permeable tubes are simulated, taking into account the difference in radial Hb distribution. These models quantify O2 transport as a function of HBOC design parameters, such as O2 affinity (p50), Hb concentration ([Hb]), and molecular radius (r A). Over a wide range of lumped extraluminal resistance conditions, it is found that an acellular Hb solution must significantly reduce its p50 (to 5--15 mmHg from 30 mmHg), in order to deliver O2 similar to RBC suspensions.;The models are also used as a tool to interpret in vivo transport experiments. Mixtures of RBCs and commercially-developed HBOCs are modeled and compared to transfusion experiments with HBOCs in a hemodiluted hamster model [62]. In vivo, a large-scale increase in O 2 extraction for a high p50 HBOC is seen. This can be interpreted through use of the model as a large-scale decrease in extraluminal O2 transport resistance, indicating increased in vivo arteriolar O2 consumption.;The second part of this study is an experimental investigation of a low-Reynolds number, pressure-driven suspension of rigid spheres. Measurements are recorded near the tube entrance, so particle concentration profiles are not fully developed. The ratios of tube radius (R) to sphere radius ( a) are small (R/a = 5.5, 13) compared to previous studies [38, 18, 29]. Axial velocity profiles (u) and axial (u') and radial (v') velocity fluctuations are calculated for the suspended spheres. A slow moving particle monolayer is observed near the tube wall for each R/a. The effective tube radius by one sphere diameter, an effect that elevates the center-line flow velocities for smaller R/a. The magnitude of the particle diffusivity (Dp ∼ (v' · v')Deltat), calculated from the data, is consistent with the scaling arguments that give Dp ∼ g&d2; a2, where g&d2; is the local shear rate. This scaling breaks down in the vicinity of the tube wall, where Dp is suppressed starting a distance one sphere diameter from the wall. Applied to in vivo arteriolar length scales, the micromixing due to a radially varying particle movements appear to be significant enough to speed the lateral diffusion of hemoglobin-sized molecules and enhance O2 transport up to 30%. |
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