The cardiovascular system transports blood around the human body, and is made up of the heart and blood vessels.
Blood carries oxygen, nutrients and hormones to the tissues of the body, and carries waste products, such as carbon dioxide, away from the tissues.
Blood vessels can be separated into three groups; arteries, veins and capillaries. Arteries carry blood away from the heart. Veins carry blood towards the heart. Capillaries are microscopic, thin-walled blood vessels which form a vast network connecting the arteries and veins.
The walls of the blood vessels have three layers. The outer layer is called the tunica externa, and is made up of fibrous connective tissue. The middle layer is called the tunica media, and is made of smooth muscles and elastic fibres. The innermost layer is called the tunica intima, and is made of a single layer of cells called the endothelial cells. The lumen is the hollow cavity bordered by the walls of the blood vessel.