Blood Flow and Red Blood Cell Deformation in Nonuniform Capillaries: Effects of the Endothelial Surface Layer

ABSTRACT Objective: A theoretical model is used to examine the mechanics of red blood cell (RBC) motion in nonuniform capillaries. The model includes effects of the endothelial surface layer (ESL), which is a layer of macromolecules adjacent to the endothelium and which impedes plasma flow. Methods: The motion of an RBC traversing a capillary with diameter varying sinusoidally between 5.4 µm and 7.4 µm is simulated numerically. The ESL is assumed to be 0.7‐µm wide and deformable. Axisymmetric RBC shapes are assumed. Lubrication theory is used to analyze the motion of plasma around the RBC and through the ESL. Results: In a nonuniform capillary with no ESL, moving RBCs undergo large transient deformations and predicted flow resistance is substantially higher than in a uniform capillary with the same mean diameter. The presence of a deformable ESL reduces the transient fluid shear stresses and deformations experienced by RBCs traversing a nonuniform capillary. With an ESL, the increase in flow resistance resulting from nonuniformity is less than twofold versus three‐ to fourfold with no ESL in vessel geometries with the same ESL‐free luminal region. Conclusions: The presence of the ESL reduces the impact of capillary irregularity on flow resistance and may protect RBCs traversing irregular capillaries from damage due to large, rapidly fluctuating external stresses.