ABSTRACT
The heart must contract then relax in each cardiac cycle. These phases of the cardiac cycle are systole (ventricular contraction) and diastole (ventricular relaxation). Systole and diastole each have several different subphases, such as isovolumetric contraction (in the case of systole) and isovolumetric relaxation (in the case of diastole). The heart sounds that one can hear via a stethoscope or simply by placing an ear against someone's chest are known as the first (S1, 'lub') and second (S2, 'dup') heart sounds, and are caused by closure of heart valves.
