Ventricular systole is the dynamic period of the cardiac cycle when the ventricles, the heart’s lower chambers, contract forcefully to eject blood into the body’s major arteries. This muscular contraction drives blood circulation, ensuring oxygen and nutrients are delivered to all tissues. Systole is the active pumping phase, which contrasts with diastole, the relaxation phase where the ventricles refill with blood. Ventricular systole ensures that blood from the left ventricle is sent to the aorta for systemic circulation, while blood from the right ventricle is propelled into the pulmonary artery toward the lungs. This event is a precisely timed sequence of electrical triggers, pressure changes, and valve movements.
The Electrical Signal That Triggers Contraction
The mechanical action of ventricular systole is immediately preceded by depolarization, an electrical event. This impulse travels rapidly through the heart’s specialized conduction system, causing the muscle cells of the ventricles to activate almost simultaneously. This depolarization is represented on an electrocardiogram (ECG) as the QRS complex, a distinct spike that signals the start of the contraction sequence.
The QRS complex acts as the electrical trigger, spreading the signal from the atrioventricular node down through the bundle branches and Purkinje fibers. This coordinated electrical spread ensures the ventricular fibers contract in a unified, powerful squeeze. The brief delay between the electrical signal and the onset of muscle tension allows the ventricles to finish filling before the mechanical contraction begins. This electrical-to-mechanical coupling is the foundation for the heart’s pumping efficiency.
Isovolumetric Contraction and Pressure Dynamics
The first mechanical phase of ventricular systole is isovolumetric contraction, a short period where the ventricles begin to contract while all four heart valves are closed. As the ventricular muscle fibers shorten, the pressure inside the chambers rises sharply. This rapid pressure increase exceeds the pressure in the relaxed atria, forcing the mitral and tricuspid valves (AV valves) to snap shut.
The AV valves close to prevent the backflow of blood into the atria, creating a closed system. With both the AV valves and the semilunar valves closed, the volume of blood within the ventricles remains constant, hence the term “isovolumetric.” This brief period is dedicated to building the pressure needed to overcome the higher pressure in the great arteries.
Ventricular Ejection and Blood Flow
Once the pressure inside the left ventricle surpasses the pressure in the aorta, and the right ventricle’s pressure exceeds that of the pulmonary artery, the semilunar valves are forced open. This opening marks the beginning of the ventricular ejection phase, where blood is powerfully thrust out of the heart. This phase begins with a rapid ejection, where approximately 70% of the blood is expelled due to the high muscular force.
As the ventricle continues to contract, the rate of ejection slows down, entering the reduced ejection phase. The total volume of blood ejected by one ventricle in a single beat is called the stroke volume. The ejection fraction is the percentage of blood in the ventricle at the end of filling that is expelled. As the ventricles empty and their pressure drops below the pressure in the arteries, the blood begins to flow backward momentarily. This backflow causes the semilunar valves to abruptly close, which signals the end of ventricular systole.
The Audible Markers of Systole (Heart Sounds S1 and S2)
The mechanical events of systole generate distinct sounds that a doctor can hear using a stethoscope. The first heart sound, known as S1 or the “lub,” occurs at the beginning of ventricular systole. This sound is generated by the sudden and forceful closure of the mitral and tricuspid (AV) valves during the isovolumetric contraction phase. The closure prevents blood from being pushed back into the atria, and the reverberation of the blood against the closed valves creates the audible sound.
The second heart sound, S2 or the “dub,” defines the end of ventricular systole and the beginning of the relaxation phase. This sound is caused by the closure of the aortic and pulmonary semilunar valves. The snapping shut of these valves occurs when the pressure in the emptying ventricles falls below the pressure in the major arteries. The period between the S1 and S2 sounds represents the entire duration of ventricular systole.