  Many of the intriguing properties of blood originate from its cellular
nature. Bulk effects, such as viscosity, depend on the local shear rates and on
the size of the vessels. While empirical descriptions of bulk rheology are
available for decades, their validity is limited to the experimental conditions
they were observed under. These are typically artificial scenarios (e.g.,
perfectly straight glass tube or in pure shear with no gradients). Such
conditions make experimental measurements simpler, however, they do not exist
in real systems (i.e., in a real human circulatory system). Therefore, as we
strive to increase our understanding on the cardiovascular system and improve
the accuracy of our computational predictions, we need to incorporate a more
comprehensive description of the cellular nature of blood. This, however,
presents several computational challenges that can only be addressed by high
performance computing. In this chapter we describe HemoCell , an open-source
high performance cellular blood flow simulation, which implements validated
mechanical models for red blood cells and is capable of reproducing the
emergent transport characteristics of such a complex cellular system. We
discuss the accuracy, the range of validity, and demonstrate applications on a
series of human diseases.
