Four chambers: right atrium, right ventricle, left atrium, left ventricle. Pumps blood into two circuits: pulmonary, systemic. Four valves: open and close in response to pressure change in the chambers, keeps blood flowing in one direction. Atrioventricular (av) valves close when ventricles contract: right av valve/tricuspid valve, left av valve/bicuspid valve/mitral valve. Semilunar valves open when ventricles contract: pulmonary semilunar valve, aortic semilunar valve. When one set of valves is open, the other set is closed. Lubb dupp: lubb av valves close, dupp semilunar valves close. Why: ventricles have a thicker myocardium because they have to push blood out of the heart. Why: the left ventricle has the thickest myocardium because it has to pump blood through the entire body. Right and left coronary arteries branch off of the aorta. Blood empties back into heart through coronary sinus. Once excited, similar mechanism of contraction in both skeletal and cardiac muscle. Interconnected by intercalated discs: thickenings of the sarcolemma containing.