(460d) Understanding Red Blood Cell Migration in Small Arterioles

The cross-flow movement of red blood cells, i.e. migration, in pressure-driven flow has been extensively studied in capillaries (~10 microns) and large arterioles (30~50 microns). In capillaries, red blood cells are in a nearly single-file flow. In large arterioles, red blood cells form a cell-free layer and a concentration peak at the centerline. The transition between these two types of distributions in small (precapillary) arterioles remains underexplored. A bilayer distribution (without a center concentration maximum) has been mentioned in previous studies, and the loss of a center concentration peak cannot easily be explained with our existing knowledge of red blood cell migration. With emerging interest in small arterioles for drug delivery purposes, we focus this talk on red blood cell migration under such confinement and identify the governing mechanism of cross-flow transport.

To examine the distribution of red blood cells in flow through channels or tubes of cross sectional breadth of 20 microns, we use a novel and parallelized immersed boundary simulation technique. We present the simulated center-of-mass and shape distributions of red blood cells at various hematocrits, viscosity ratios and capillary numbers. We interpret our results on a theoretical basis and highlight the importance of hydrodynamic fluctuations in the center region. This center region matches the size of a red blood cell and has been found in previous research to be of negligible importance in wider channels. In small channels, however, the center region occupies the majority of the cell-laden region and thus takes on greater importance. Hydrodynamic fluctuations in this region balances deformability-induced red blood cell lift even though the average shear rate in this region is small. Analyzing this balance allows insight into the mechanism by which red blood cells can form a bilayer distribution in smaller arterioles. We have also discovered that increasing the viscosity ratio (relative to surrounding plasma) can substantially slow the lift velocity of the cells and this in turn directly influences the evolution of the cell-free-layer. Thus, we also examine the effect of viscosity contrast on the transient development and steady-state value of the RBC concentration profile in small arterioles.