Capillary. Transmission electron micrograph (TEM) of a section through a capillary, showing a single red blood cell (erythrocyte, black) in its interior. Capillaries are the smallest diameter blood vessels (5-8 micrometres) and are just wide enough for single red blood cells to pass along them. Their very thin walls are made of endothelial cells, across which gases, nutrients, fluids and cells may pass selectively in either direction. External to the endothelium, most capillaries have another layer of cells called pericytes that may be supportive (fibroblasts) or contractile (myofibroblasts). Magnification: x11000, when printed 10 centimetres wide.