The heart muscle needs oxygen that is carried by the blood through the coronary arteries in order to work properly.  CHD occurs when the blood flow to the heart decreases in the coronary arteries through a process called atherosclerosis.  This process occurs when the arteries hardened and narrow due to a build up of a fatty substance called plague inside the walls of the arteries.  The plague is made up of cholesterol, fatty compounds, calcium, and blood clotting material (fibrin).  The plague can breakup in the arteries which leads to a blood clot.  The breakup is covered by the blood clot within the artery and then becomes hard.  Through each stage of buildup and hardening, the arteries continue to narrow reducing the flow of oxygen-rich blood to the heart.