Blood vessels. Coloured scanning electron micrograph (SEM) of a freeze-fractured sample of tissue containing blood vessels (red). The fracture has passed through each blood vessel lumen. The thin-walled vessel (centre) is a vein. At top left is a venule (a small vein) and next to it a thicker-walled blood vessel known as an arteriole, a small artery. Veins transport blood back to the heart, while arteries distribute blood from the heart around the body. The blood vessels walls consist of endothelial cells, with connective tissue surrounding the blood vessels. Magnification: x200 when printed at 10 centimetres across.