Microvascular networks play a critical role in sustaining life by delivering oxygen and other nutrients to the tissue and removing carbon dioxide and waste. The distribution of blood in microvascular networks depends on the topology and geometry of the network, as well as the history of blood flow through the network. In this presentation I will discuss the important rheological properties of blood and examine a model in which blood flow in microvascular networks is governed by state-dependent delay equations. I will show that, even in the absence of active biological control or feedback, this nonlinear dynamical system gives rise to hysteresis and bistability as well as spontaneous oscillations of blood hematocrit and flow velocity.