Coronary artery disease (CAD) is a disease in which a waxy substance called plaque builds up inside the coronary arteries. These arteries supply oxygen-rich blood to your heart muscle. When Plaque builds up in the arteries, the conditions is called atherosclerosis. Over time plaque can harden up rupture (break open). Hardened plaque narrows the coronary arteries and reduces the flow of oxygen-rich blood to the heart.