Blocked artery with collateral vessels. Cutaway illustration of an artery that has been narrowed by atherosclerosis (yellow deposits), with the development of collateral blood vessels to restore the blood supply to the affected area. Atherosclerosis is a condition where deposits called atheromas (yellow) form on the inner walls of blood vessels. Atheromas typically consist of cholesterol, decaying cells, blood cells and fatty proteins. Narrowed blood vessels raises blood pressure, which puts strain on the heart. Atheromas blocking coronary blood vessels cause heart attacks.