 The microcirculation is composed of a continuum of small vessels including small arterioles, capillaries, and post-capillary venules. The microcirculation represents an actively-adjusting vascular circuit that matches blood flow and O2 delivery to local tissue oxygen demands. While the physiologic mechanisms that match O2 delivery to local requirements are incompletely understood, endothelium-derived nitric oxide NO clearly plays an important role. Interestingly, recent work has revealed that in addition to transporting O2 and carbon dioxide CO2, the RBC also controls local NO concentrations and thus may also play a surprisingly important role in regulating blood flow in the microcirculation. 