It is lined by a layer of cells which lies directly above the muscle layer.
This takes the form of fatty streaks consisting of deposits of oxidised LDL cholesterol and macrophages.
Major accumulation of cholesterol in macrophages causes them to swell. They are joined by muscle cells and connective tissue cells and form a sort of pillow (plaque) that constricts the artery.
If plaque ruptures, this triggers blood clotting. Blood platelets and fibrin strands are deposited at the site and form a blood clot (thrombus) which may completely obstruct the artery. The possible consequences: heart attack, stroke, blockage of a leg artery.