Inversion of hematocrit partition at microfluidic bifurcations

Results are encouraging

Partitioning of red blood cells (RBCs) at the level of bifurcations in the microcirculatory system affects many physiological functions yet it remains poorly understood. We address this problem by using T-shaped microfluidic bifurcations as a model. Our computer simulations and in vitro experiments reveal that the hematocrit (ϕ0) partition depends strongly on RBC deformability, as long as ϕ0b20% (within the normal range in microcirculation), and can even lead to complete deprivation of RBCs in a child branch. Furthermore, we discover a deviation from the Zweifach–Fung effect which states that the child branch with lower flow rate recruits less RBCs than the higher flow rate child branch. At small enough ϕ0, we get the inverse scenario, and the hematocrit in the lower flow rate child branch is even higher than in the parent vessel.We explain this result by an intricate up-stream RBC organization and we highlight the extreme dependence of RBC transport on geometrical and cellmechanical properties. These parameters can lead to unexpected behaviors with consequences on the microcirculatory function and oxygen delivery in healthy and pathological conditions.

Analysis and further testing

As a result of the interplay between the Zweifach–Fung effect and the Fåhræus effect the hematocrit in microcirculation can reach values as low as 10–20% compared to the average hematocrit in human body (45%). At such a low hematocrit, our simulations and in vitro microfluidic experiments have revealed that RBC partition at the level of bifurcations depends strongly on the viscosity contrast between the viscosities of the RBC hemoglobin and the suspending fluid. In the extreme hemodilution, our results exhibit a newly reported phenomenon:

The low flow rate branch may receive higher hematocrit than the high flow rate branch in opposition to the known Zweifach–Fung effect. This phenomenon is observed under moderate confinement and is the result of a peculiar structuring of the cell suspension. Under stronger confinement, other strong discrepancy with Pries et al.’s empirical law was highlighted, with a strong asymmetry in the partitioning (enhanced Zweifach–Fung effect). Our findings suggest that the viscosity contrast must be taken into consideration and carefully analyzed in order to have a firm understanding of RBC distribution in microcirculation. This physiological parameter increases with aging as well as with some pathologies.

The results of our present work provide a valuable background needed to pinpoint the various RBC properties that govern hematocrit partition, and thus oxygen delivery in the microcirculation in general.

Microvascular Research 105 (2016) 40–46

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