Blood vessels are a closed system of tubular vessels that contain blood and forms a branching network to distribute blood to all parts of the organism. There are different types of blood vessels: arteries, arterioles, capillaries, venules and veins.Arteries, arterioles and veins have the same basic layered structure from the outside inwards:(i) a tough fibrous outer layer made from collagen, which resists pressure changes from outside and inside the blood vessel.(ii) a muscle layer made of smooth muscle which can contract to control the flow of blood(iii) an elastic layer which maintains the blood pressure by stretching and springing back (stretch and recoil)(iv) a thin inner lining of endothelium, made of squamous epithelial cells that are smooth to reduce friction and thin to allow a short diffusion distance.The lumen is the central cavity of the blood vessel through which blood flows.