  Blood viscosity decreases with shear stress, a property essential for an
efficient perfusion of the vascular tree. Shear-thinning is intimately related
to the dynamics and mutual interactions of red blood cells (RBCs), the major
constituents of blood. Our work explores RBCs dynamics under physiologically
relevant conditions of flow strength, outer fluid viscosity and volume
fraction. Our results contradict the current paradigm stating that RBCs should
align and elongate in the flow direction thanks to their membrane circulation
around their center of mass, reducing flow-lines disturbances. On the contrary,
we observe both experimentally and with simulations, rich morphological
transitions that relate to global blood rheology. For increasing shear
stresses, RBCs successively tumble, roll, deform into rolling stomatocytes and
finally adopt highly deformed and polylobed shapes even for semi-dilute volume
fractions analogous to microcirculatory values. Our study suggests that any
pathological change in plasma composition, RBCs cytosol viscosity or membrane
mechanical properties will impact the onset of shape transitions and should
play a central role in pathological blood rheology and flow behavior.
