Capillary and red blood cell. Coloured transmission electron micrograph (TEM) of a longitudinal section through a capillary showing red blood cells (red) almost filling the lumen (inner space) of the capillary. Capillaries are the smallest blood vessels in the body, only a few micrometres across, just large enough for the passage of red blood cells. The capillary lining consists of endothelial cells (purple). A basement membrane surrounds the capillary. Magnification: x1000 when printed 10 centimetres wide.