Coronary artery disease (CAD) occurs when the coronary arteries of the heart become damaged or diseased. The coronary arteries are the main blood vessels of the heart that provide it with oxygen, blood and essential nutrients. When cholesterol, calcium, or other substances from the blood build up over a period of time, plaque can develop on the walls of the coronary arteries. This causes the arteries to stiffen and become narrow, which limits the blood flow to the heart.