Microcirculatory blood flow and transport are controlled to meet local and systemic demands for material exchange and body fluid balance. Control mechanisms act through effectors (smooth muscle cells) at many sites within the microvascular bed. Responses at different sites are not uniform, resulting in a broadly heterogeneous distribution of pressures and flows which is constantly changing. Simplified, uniform models of microvascular networks have made it possible to identify the principles governing blood circulation and blood-tissue transport. However, knowledge of how these principles are integrated at the microcirculatory level requires the variability and heterogeneity to be taken into account. Indeed, there is much reason to believe that heterogeneity is an important part of microcirculatory control.