From the perspective of blood flow, blood has some unusual properties: it is a suspension of blood cells of which the red blood cells are most numerous and are both deformable (at moderate and high flow rates) and will aggregate under conditions of slow flow. Also, the cellular volume concentration is high (about 40-45%). These features cause blood to have variable viscosity, dependent on flow conditions, and cause both red blood cell sedimentation and syneresis effects under slow flow conditions (which can lead to rheological artifacts). These effects also cause unusual flow phenomena when blood flows in systems of small diameter vessels (especially for diameters of about 500 μm or less). These phenomena are seen in non-uniform cell distributions in vessel cross sections, a cell-poor layer of mostly blood plasma at vessels walls, non-proportionate cellular distribution during blood flow through vascular bifurcations, which leads to a very wide distribution of vessel cellular concentrations (from zero to systemic values) in the smaller vessels of the microcirculation, etc. All these phenomena are discussed in this book, as well as the difficulties presented by in vivo microvessels having non-ideal geometries.