Blocked artery. Cutaway computer artwork showing the interior of an artery affected by atherosclerosis. Atherosclerosis is narrowing of an artery (stenosis) due to fatty deposits (beige) on its inner walls. 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 is the main cause of heart attacks. Here, the clot has completed blocked the vessel and red blood cells (red) can be seen building up behind it.