Illustration of the interior of a human blood vessel, showing a snake-like thrombus (blood clot) forming along the length of the vessel. 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 clot appears as a parcel of red cells bound by a network of fibrin threads. Clotting of blood occurs in response to injury and is a complex, multi-stage process.