  A mixed suspension of red blood cells (RBCs) and microparticles flows through
a cylindrical channel with a constriction mimicking a stenosed blood vessel.
Our three-dimensional Lattice-Boltzmann simulations show that the RBCs are
depleted right ahead and after the constriction. Although the RBC mean
concentration (hematocrit) is 16.5% or 23.7%, their axial concentration profile
is very similar to that of isolated tracer particles flowing along the central
axis. Most importantly, however, we find that the stiff microparticles exhibit
the opposite behavior. Arriving on a marginated position near the channel wall,
they can pass through the constriction only if they find a suitable gap to dip
into the dense plug of RBCs occupying the channel center. This leads to a
prolonged dwell time and, as a consequence, to a pronounced increase in
microparticle concentration right in front of the constriction. For
biochemically active particles such as drug delivery agents or activated
platelets this clustering may lead to physiologically relevant hot spots of
biochemical activity.
