Illustration of the interior of a human blood vessel, showing the various cells that are suspended in blood plasma. The large saucer-like discs are red blood cells (erythrocytes) that function in the transport of oxygen from the lungs to the tissues (and waste carbon dioxide in the opposite direction). Mature red cells have no nucleus & cannot divide to form new cells; they are synthesized & released by the bone marrow. The small, less numerous cells are platelets, which are involved in the repair of minor damage to the endothelial cells that line the interior of all blood vessels.