Overview

Systole is the phase of the cardiac cycle during which the heart contracts and propels blood outward. It follows diastole, the relaxation and filling period, and primarily involves the ventricles — the large pumping chambers of the heart. During systole the muscular walls shorten and thicken, forcing blood through the arterial valves toward the lungs and the rest of the body, including the systemic circulation.

Mechanical phases

Ventricular systole can be subdivided into recognizable mechanical stages that describe how pressure and volume change within the heart:

  • Isovolumetric contraction: the ventricles build pressure with all valves briefly closed.
  • Ejection phase: once pressure exceeds arterial pressure, semilunar valves open and blood is expelled.
  • Transition to diastole: ventricular pressure falls, semilunar valves close, and the cycle moves back to filling.

Electrical control

The timing of systole is controlled by the heart's electrical conduction system. An impulse originating in the sinoatrial node triggers atrial contraction and then travels to the atrioventricular node and the specialized conduction pathways, coordinating ventricular contraction. The organized wave of depolarization is recorded as the QRS complex on an electrocardiogram and precedes mechanical systole.

Clinical importance and measurements

Systole determines the systolic blood pressure value commonly reported in routine measurements and used to assess cardiovascular status. The first heart sound (S1) is produced by closure of the atrioventricular valves at the start of ventricular systole. Abnormalities during systole — such as reduced ejection, valve insufficiency, or high outflow resistance — are central to many heart conditions and are evaluated with tools like auscultation, echocardiography, and blood pressure measurement.

History, terminology and distinctions

The word "systole" derives from a Greek root meaning "contraction". In clinical and physiological descriptions it is useful to distinguish atrial systole (a smaller contraction that tops up ventricular filling) from ventricular systole (the main pumping event). Systolic function is typically considered alongside diastolic function to give a complete picture of cardiac performance.

Normal and abnormal patterns of systole affect exercise capacity, organ perfusion and symptoms such as shortness of breath. Understanding the phases and control of systole helps interpret diagnostic tests and guides treatments for hypertension, valve disease and heart failure. For additional anatomical or clinical background, see resources on the lungs and cardiopulmonary circulation.