Publisher Summary This chapter discusses mechanical events underlying cardiac cycle. The mature heart is built upon a collagenous skeleton in the shape of a fibrotendinous ring (the annulus fibrosus), which is located at the atrioventricular junction. The atria and ventricles contract in sequence, resulting in a cycle of pressure and volume changes, and a thorough knowledge of the cycle is needed for the diagnosis of valvular defects. The volume of blood in a ventricle at the end of the filling phase is called the end-diastolic volume and is typically around 120 ml in an adult human. With the closure of the aortic and pulmonary valves, each ventricle once again becomes a closed chamber. Intracardiac pacing is a therapeutic application of the cardiac catheter in which a wire catheter is wedged in the ventricle and used to stimulate each heart beat from an external electrical device, thereby replacing the heart's own pacemaker.