Abstract
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.
Highlights
The past decades have witnessed tremendous efforts to quantify red blood cell (RBC) flows in the microvasculature.[1–12] Both in vivo[13–16] and in vitro[17–19] studies have been critical in elucidating how microcirculation is highly dynamic[2,3,20,21] with phenomena that include the Fahraeus effect,[1,2] the Fahraeus-Lindqvist effect,[3] plasma skimming,[5] cell screening,[6] and the pathway effect[8,12] amongst other
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 pulmonary capillary networks (PCNs) from the usual notion of Poiseuille flow in long straight tubes
We revisit in truescale 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
Summary
The past decades have witnessed tremendous efforts to quantify red blood cell (RBC) flows in the microvasculature.[1–12] Both in vivo[13–16] and in vitro[17–19] studies have been critical in elucidating how microcirculation is highly dynamic[2,3,20,21] with phenomena that include the Fahraeus effect,[1,2] the Fahraeus-Lindqvist effect,[3] plasma skimming,[5] cell screening,[6] and the pathway effect[8,12] amongst other Such transport dynamics are driven by morphology[4,12] and size[3,22] of the microvasculature, as well as local blood viscosity properties due to phase separation,[8,12] apparent hematocrit[22,23] (Hct), and changes in mechanical properties of RBCs under diseased conditions.. Since individual capillary segments are short and exhibit lengths and diameters of nearly the same size[47] ($3–10 lm), seminal descriptions[48] coined the term "sheet flow" to capture the assembly of such capillaries into quasi-2D networks constructed by the voids formed between regularly positioned posts that fill the available inter-septal alveolar wall This specific morphology distinguishes PCNs from the usual notion of Poiseuille flow in long cylindrical tubes. To the best of our knowledge, there are currently no experiments quantifying physiologically realistic RBC flows in the organ-specific morphologies of PCNs
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