Abstract
The flow behavior of blood is strongly affected by red blood cell (RBC) properties, such as the viscosity ratio C between cytosol and suspending medium, which can significantly be altered in several pathologies (e.g. sickle-cell disease, malaria). The main objective of this study is to understand the effect of C on macroscopic blood flow properties such as flow resistance in microvessels, and to link it to the deformation and dynamics of single RBCs. We employ mesoscopic hydrodynamic simulations to investigate flow properties of RBC suspensions with different cytosol viscosities for various flow conditions in cylindrical microchannels. Starting from a dispersed cell configuration which approximates RBC dispersion at vessel bifurcations in the microvasculature, we find that the flow convergence and development of RBC-free layer (RBC-FL) depend only weakly on C, and require a convergence length in the range of 25D-50D, where D is channel diameter. In vessels with , the final resistance of developed flow is nearly the same for C=5 and C=1, while for , the flow resistance for C=5 is about 10% larger than for C=1. The similarities and differences in flow resistance can be explained by viscosity-dependent RBC-FL thicknesses, which are associated with the viscosity-dependent dynamics of single RBCs. The weak effect on the flow resistance and RBC-FL explains why RBCs can contain a high concentration of hemoglobin for efficient oxygen delivery, without a pronounced increase in the flow resistance. Furthermore, our results suggest that significant alterations in microvascular flow in various pathologies are likely not due to mere changes in cytosolic viscosity.
Highlights
Blood is a multi-component suspension which consists of plasma (≈ 55%) and cells
Note that the velocity of individual cells depends on their location within the tube and can fluctuate in time even after the average flow velocity has reached steady state. vT is inversely proportional to the flow resistance, and can be used to obtain the relative suspension viscosity ηrel = v/vT, which compares the volumetric flow rate of red blood cells (RBCs) suspension with that of external fluid for the same pressure gradient
Ηrel quantifies an increase in the flow resistance due to the presence of RBCs
Summary
Blood is a multi-component suspension which consists of plasma (≈ 55%) and cells (red blood cells ≈ 45%, white blood cells and platelets < 1%). Two representative examples of the flow behavior of RBC suspension in microvessels or glass capillaries are the Fahraeus [19] and Fahraeus-Lindqvist [20, 21] effects The former effect concerns RBC volumetric flux (or the so-called discharge hematocrit Hd), which appears to be larger than the tube (bulk) hematocrit Ht in vessels with a diameter D in the range of 7 − 200 μm. The thickness of RBC-FL is directly associated with blood flow resistance [24,25,26,27] and plays a crucial role in the adhesion of leukocytes, platelets, and drug-delivery carriers to vessel walls [6, 15]
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