Brain capillary. Transmission electron micrograph (TEM) of a section through a capillary from the brain, showing a single platelet and 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. Magnification: x4000 when printed 10 centimetres high.