Ruptured blood vessel. Coloured scanning electron micrograph (SEM) showing stacks (rouleaux) of red blood cells exposed inside a ruptured venule. A venule is a very small blood vessel in the microcirculation that allows deoxygenated blood to return from the capillary beds to the larger blood vessels (veins). Red blood cells are the most abundant cell in the blood. They have no nucleus and are about 7 micrometers across. Magnification: x3500 when printed at 10 centimetres wide.