Capillary and blood cells. Transmission electron micrograph (TEM) of a section through a capillary showing red blood cells (red), a single white cell and platelets within 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. Here, a pericyte can be seen surrounding the capillary. Pericytes are contractile cells that wrap around the endothelial cells of capillaries and venules. Magnification: x2000 when printed 10 centimetres wide.