  The flow behavior of blood in microvessels is directly associated with tissue
perfusion and oxygen delivery. Current efforts on modeling blood flow have
primarily focused on the flow properties of blood with red blood cells (RBCs)
having a viscosity ratio $C$ of unity between the cytosol and suspending
medium, while under physiological conditions the cytosol viscosity is about
five times larger than the plasma viscosity (i.e., $C\approx 5$). The
importance of $C$ for the behavior of single RBCs in fluid flow has already
been demonstrated, while the effect of $C$ on blood flow has only been sparsely
studied. We employ mesoscopic hydrodynamic simulations to perform a systematic
investigation of flow properties of RBC suspensions with different cytosol
viscosities for various flow conditions in cylindrical microchannels. Our main
aim is to link macroscopic flow properties such as flow resistance to single
cell deformation and dynamics as a function of $C$. Starting from a dispersed
cell configuration, we find that the flow convergence and the development of a
RBC-free layer (RBC-FL) depend only weakly on $C$, and require a convergence
length in the range of $25D-50D$, where $D$ is the channel diameter. The flow
resistance for $C=5$ is nearly the same as that for $C=1$, which is facilitated
by a slightly larger RBC-FL thickness for $C=5$. This effect is due to the
suppression of membrane motion and dynamic shape deformations by a more viscous
cytosol for $C=5$, resulting in a more compact cellular core of the flow in
comparison to $C=1$. The weak effect of cytosol viscosity on the flow
resistance and RBC-FL explains why cells can have a high concentration of
hemoglobin for efficient oxygen delivery, without a pronounced increase in the
flow resistance.
