Arteriole. Coloured transmission electron micrograph (TEM) of a section through red blood cells in a arteriole (small artery) in human skin. Red blood cells (erythrocytes) are seen stacked together in rouleau formation due to their convex shape. They are passing through the lumen of the arteriole. Walls of the arteriole are made up of a single layer of endothelial and muscle cells. Up to 40 percent of the blood consists of red blood cells that contain haemoglobin which transports oxygen around the body. Each day a red blood cell travels about 15 kilometers through the body's blood vessels. Magnification: x2500 when printed at 10cm.