Arteris and veins have walls made of three layers but the middle layer of smooth muscle is thicker in arteries than in veins. Since the walls of veins are thinner, they can expand to a greater extent. Veins have blood forced through them by skeletal muscles contracting. Veins have valves and arteries do not. Blood pressure is higher in arteries than in veins.
Platelets are repsosnsible for blood clotting.
Examples of these are the macrophages and neutrophils. The other major type of white blood cell is the antibody producing cell. These are lymphocytes. Atnibodies tag and mark foreing substances for destruction by the immune system.