The cell-free layer (CFL) in simulated microvascular networks

As blood flows in small vessels, a plasma layer forms near the vessel walls that is free of red blood cells (RBCs). This cell-free layer (CFL) plays important hemorheological and biophysical roles. Blood vessels are typically tortuous and they sequentially bifurcate in to smaller vessels or merge to form larger vessels. Because of this geometric complexity, the CFL in vivo is 3D and asymmetric, unlike in fully developed flow in straight tubes or channels. Using a high-fidelity model of cellular-scale blood flow in physiologically realistic microvascular networks, we provide the first simulation-based analysis of the 3D CFL, including hydrodynamic origins of the observed behavior. We show that the CFL significantly varies over the vascular networks. Along the vessel lengths, such variations are predominantly non-monotonic. We show that vessel tortuosity causes the CFL to become more asymmetric along the length, and identify a curvature-induced cross-flow migration of the RBCs as the underlying mechanism. The vascular bifurcations and mergers are also seen to change the CFL profiles, and in the majority of them the CFL becomes more asymmetric. For most mergers, the dominant mechanism by which such changes occur is identified as the geometric focusing.