Schematic illustrating normal oxygen delivery to tissues through a healthy microvasculature (top). Slow, evenly distributed blood flow facilitates effective oxygen transfer. In this scenario, tissue OEF mirrors the oxygen difference between the artery and vein. Schematic illustrating impaired oxygen delivery in regions with vascular pruning and inhomogenous vascular transit time (bottom). Fast flow through low-resistance channels leads to high nonnutritive perfusion and ineffective oxygen unloading. The tissues are hypoxic (high OEF), even though the oxygen difference between the artery and the vein is small. Figure provided by Adam Bush.