The pulmonary capillary networks (PCNs) embody organ-specific microvasculatures, where blood vessels form dense meshes that maximize the surface area available for gas exchange in the lungs. With characteristic capillary lengths and diameters similar to the size of red blood cells (RBCs), seminal descriptions coined the term “sheet flow” nearly half a century ago to differentiate PCNs from the usual notion of Poiseuille flow in long straight tubes. Here, we revisit in true-scale experiments the original “sheet flow” model and devise for the first time biomimetic microfluidic platforms of organ-specific PCN structures perfused with RBC suspensions at near-physiological hematocrit levels. By implementing RBC tracking velocimetry, our measurements reveal a wide range of heterogonous RBC pathways that coexist synchronously within the PCN; a phenomenon that persists across the broad range of pressure drops and capillary segment sizes investigated. Interestingly, in spite of the intrinsic complexity of the PCN structure and the heterogeneity in RBC dynamics observed at the microscale, the macroscale bulk flow rate versus pressure drop relationship retains its linearity, where the hydrodynamic resistance of the PCN is to a first order captured by the characteristic capillary segment size. To the best of our knowledge, our in vitro efforts constitute a first, yet significant, step in exploring systematically the transport dynamics of blood in morphologically inspired capillary networks.

Objective: The deformation ability of erythrocytes is critical for their function and affects their mobility in the circulation. To circulate and travel through narrow vessels, erythrocytes must continuously alter their shape by modifying their cytoskeleton, which controls their elasticity and stability. Erythrocytes lack nuclei and other major organelles, and thus most of the known signaling cascades are thought to be inactive in these cells. As the noncanonical Wnt pathway affects cytoskeleton dynamics by posttranslational modifications, we hypothesized that this signaling pathway may affect erythrocytes. Approach and Results: We demonstrate that components of the noncanonical Wnt pathway are expressed in erythrocytes and that incubation of erythrocytes with Wnt ligands prolonged their survival both ex vivo, under storage conditions, and in posttransfusion recipient mice. We show that Wnt ligands modulate the erythrocyte cytoskeleton, enhancing its flexibility and strength. Importantly, we show that the noncanonical Wnt-5A ligand is secreted into the plasma and that monocytes and lymphocytes are a possible source of the Wnt effectors in the blood circulation. Conclusions: These findings provide evidence for intracellular signaling activity in enucleated cells and signal transduction in the blood circulation and thus open new and exciting avenues for studying the function of signaling pathways in the bloodstream.

Microfluidic-based assays have become increasingly popular to explore microcirculation in vitro. In these experiments, blood is resuspended to a desired haematocrit level in a buffer solution, where frequent choices for preparing RBC suspensions comprise notably Dextran and physiological buffer. Yet, the rational for selecting one buffer versus another is often ill-defined and lacks detailed quantification, including ensuing changes in RBC flow characteristics. Here, we revisit RBC suspensions in microflows and attempt to quantify systematically some of the differences emanating between buffers. We measure bulk flow rate (Q) of RBC suspensions, using PBS- and Dextran-40, as a function of the applied pressure drop (ΔP) for two hematocrits (∼0% and 23%). Two distinct microfluidic designs of varying dimensions are employed: a straight channel larger than and a network array similar to the size of individual RBCs. Using the resulting pressure-flow curves, we extract the equivalent hydrodynamic resistances and estimate the relative viscosities. These efforts are a first step in rigorously quantifying the influence of the 'background' buffer on RBC flows within microfluidic devices and thereby underline the importance of purposefully selecting buffer suspensions for microfluidic in vitro assays.

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