  The supply of oxygen and nutrients and the disposal of metabolic waste in the
organs depend strongly on how blood, especially red blood cells, flow through
the microvascular network. Macromolecular plasma proteins such as fibrinogen
cause red blood cells to form large aggregates, called rouleaux, which are
usually assumed to be disaggregated in the circulation due to the shear forces
present in bulk flow. This leads to the assumption that rouleaux formation is
only relevant in the venule network and in arterioles at low shear rates or
stasis. Thanks to an excellent agreement between combined experimental and
numerical approaches, we show that despite the large shear rates present in
microcapillaries, the presence of either fibrinogen or the synthetic polymer
dextran leads to an enhanced formation of robust clusters of red blood cells,
even at haematocrits as low as 1%. Robust aggregates are shown to exist in
microcapillaries even for fibrinogen concentrations within the healthy
physiological range. These persistent aggregates should strongly affect cell
distribution and blood perfusion in the microvasculature, with putative
implications for blood disorders even within apparently asymptomatic subjects.
