Atherosclerosis is a disease where the inside of the arteries are blocked by plaque. Plaque builds up when cholesterol, white blood cells, calcium and other substances accumulate on an artery wall. When plaque builds up in an artery, it narrows the artery and reduces blood flow. In time, the plaque could break off, causing a blood clot. If the clot is large enough, it can block blood flow through a coronary artery and cause a heart attack.