Illustration showing the interior of an artery blocked by atherosclerosis. Atherosclerosis is narrowing (stenosis) of an artery due to fatty deposits (yellow) on its inner walls. Here, the deposit has grown to completely block the artery. The deposits are made of atheroma, a substance comprising cholesterol, decaying cells, blood cells and fatty proteins. Narrowing of blood vessels raises blood pressure, which puts strain on the heart. Atherosclerosis of coronary arteries is the main cause of heart attacks.