Blood in microcirculation behaves as a dense ssupension of heterogeneous cells. The primary constituent of blood, namely, the erythrocytes or red blood cells are extrnemely deformable cells. Deformability of erythrocytes is key to many hemodynamic phenomena under normal and disease conditions. A significant progress has been made in recent years in developing computer simulations that can address flow of dense suspension of red blood cells. However, many of these simulations considered blood flow in simple geometry, for example, in straight tubes of uniform circular cross section. In contrast, the architecture of a microvascular network is very complex with continually bifurcating, merging and tortuous vessels. Thus, the numerical modeling must consider the network architecture of the blood vessels in order to be able to address many real physiological issues. We have developed an immersed-boundary-based method that can consider flow of deformable blood cells in dense suspension through physiologically realistic and complex vascular network. Typical netwroks considered in our simulations contain up to nearly 50 vesels and almost 50 bifurcations and mergers, more than 500 flowing cells, with vessel diameters ranging from 6 to 25 microns, lengths from 10 to 75 microns, and hematocrits from 5 to 30% in various vessels. These simulations will allow us to study fundamental mechanisms underlying heterogeneous distribution of blood cells in vascular networks, and the role of vascular architecture on cardiovascular disease genesis and progression, and distribution of drug particulates.