systole

Meaning: Systole is contraction; diastole is relaxation and filling. Atrial systole tops off ventricular filling. In ventricular systole, pressure in the ventricles rises above atrial pressure, so the AV valves (tricuspid and mitral) close, producing the first heart sound, S1. All valves are briefly closed while pressure builds (isovolumetric contraction). When ventricular pressure exceeds pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected. When the ventricles relax and pressure falls, the semilunar valves close (S2), marking the start of diastole. The QRS complex on an ECG comes just before ventricular systole.

Equation: SV = EDV − ESV, where SV is stroke volume, EDV is end-diastolic volume (volume before ejection) and ESV is end-systolic volume (volume left after ejection). All are volumes in m³, though mL is used in practice.

On the MCAT: Expect a pressure-versus-time or pressure-volume graph of the left heart, where you identify which valves are open or closed at a given point. Blood pressure is reported as systolic over diastolic, and systolic pressure is the peak arterial pressure during ventricular ejection.

Common confusion: S1 comes from the AV valves closing, not from valves opening. Systole is also not the filling phase. Most ventricular filling happens passively during diastole.