Fluorescein angiography is a remarkable in vivo technique, because it is able to resolve the minute capillaries in the neighborhood of the foveal avascular zone which measure only μm in diameter (top). This is half the diameter of red blood cells. Because of their small diameter, the capillaries in the vicinity of the foveal avascular zone cannot be perfused with whole blood, since the red blood cells have to change their shape in order to pass, thus requiring much more space (bottom).
