The SA node is the pacemaker of the heart which is stimulated by two nerves to control the heartbeat. It is located in the top of the right atrium. The AV node receives signals from the SA node and delays firing by a tenth of a second to allow the atria to contract before the ventricles. The impulse then travels down the ventricles through specialized fibers to ensure the ventricles contract from bottom to top. Coronary heart disease occurs when arteries become hardened and narrowed due to plaque buildup, restricting blood flow. Risk factors include older age, male sex, family history, high blood pressure, lack of exercise, smoking, and high cholesterol.