The Cardiac Cycle and its Five Core Phases
The cardiac cycle is the complete sequence of mechanical and electrical events that occurs during one single heartbeat, ensuring the systematic and continuous pumping of blood throughout the body. It begins at the onset of one heartbeat and concludes at the beginning of the next. The entire cycle, which takes approximately 0.8 seconds in a resting adult, is an exquisitely coordinated process of alternating contraction (systole) and relaxation (diastole) of the heart’s four chambers—the two atria and the two ventricles. While the atria and ventricles each undergo their own cycles of systole and diastole, the clinical and physiological focus is primarily on the ventricular cycle, as the ventricles are the powerful chambers responsible for ejecting blood into the systemic and pulmonary circulations. The proper timing and pressure regulation within this cycle are governed by the heart’s electrical conduction system and the opening and closing of four key valves: the mitral, tricuspid (atrioventricular), aortic, and pulmonary (semilunar) valves. The efficient progression through these phases is essential for maintaining cardiac output and delivering oxygen and nutrients to all peripheral tissues.
The Two Fundamental Phases: Systole and Diastole
The cardiac cycle is broadly divided into two major periods: ventricular systole and ventricular diastole. Systole represents the period of ventricular contraction and blood ejection. During systole, the ventricles build pressure and forcefully pump blood out into the aorta and the pulmonary artery. Diastole, conversely, represents the period of ventricular relaxation, allowing the heart chambers to passively refill with blood returning from the circulatory system. Diastole is significantly longer than systole in a normal heart rhythm, which allows adequate time for the ventricles to fill completely before the next contraction. This is a critical distinction, as the efficiency of the entire cycle—and thus the health of the circulation—depends on both the strength of systole and the duration and completeness of diastole. For a more detailed understanding, the complete cycle is typically broken down into a series of distinct sub-phases. The following five-phase model focuses on the critical mechanical stages of the ventricle, integrating pressure and volume changes with electrical activity and heart sounds to provide a clear and comprehensive framework for the mechanics of the heart pump.
Phase 1: Isovolumic Contraction (Early Systole)
The first phase of the ventricular cycle is the isovolumic contraction. This stage marks the very beginning of ventricular systole and immediately follows the electrical depolarization of the ventricles, which is represented by the QRS complex on the electrocardiogram (ECG). As the ventricular muscle fibers begin to contract, the pressure inside the ventricles rises rapidly. Crucially, the semilunar valves (aortic and pulmonary) remain closed because the ventricular pressure is still lower than the diastolic pressure in the great arteries. Simultaneously, the rising ventricular pressure quickly exceeds the pressure in the relaxing atria, forcing the atrioventricular (AV) valves—the mitral and tricuspid valves—to snap shut. This abrupt closure of the AV valves marks the true onset of systole and generates the first heart sound (S1). Since all four heart valves are briefly closed at this moment, the volume of blood within the ventricles remains constant (isovolumic), even as the pressure within the chambers dramatically increases. This build-up of tension is essential for overcoming the high resistance in the aorta and pulmonary artery, preparing the ventricles for the forceful ejection of blood.
Phase 2: Ventricular Ejection (Late Systole)
The second phase, ventricular ejection, begins the instant the pressure inside the contracting left ventricle surpasses the pressure in the aorta (around 80 mmHg in the left side) and the pressure in the right ventricle exceeds the pressure in the pulmonary artery (around 10 mmHg in the right side). This pressure gradient forces the aortic and pulmonary semilunar valves to open. Blood is then forcefully and rapidly ejected from the ventricles into the respective arteries. Ejection is often subdivided into a rapid phase, where the majority of the stroke volume is expelled, followed by a reduced ejection phase, where the rate of flow slows down as the ventricles begin to repolarize. Even as blood is ejected, the ventricular pressure continues to rise to a peak before beginning its decline as ventricular relaxation starts. This phase continues until the ventricular repolarization (represented by the T wave on the ECG) is complete, and the ventricular muscle tension falls to a level where it can no longer sustain pressure greater than that in the great arteries. The total volume of blood ejected is the stroke volume, and the volume remaining in the ventricles at the end of this phase is the end-systolic volume (ESV).
Phase 3: Isovolumic Relaxation (Early Diastole)
The third phase, isovolumic relaxation, is the first mechanical event of ventricular diastole. Once ventricular pressure falls below the high pressure remaining in the aorta and pulmonary artery, the momentum of the blood reverses, causing a brief back-flow that forces the semilunar valves to close abruptly. The closure of the aortic and pulmonary valves generates the second heart sound (S2), which physiologically marks the end of systole and the beginning of diastole. For a short period, typically lasting about 80 milliseconds, all four heart valves are closed. The ventricular muscle is now relaxing, and the pressure within the chambers plummets without any change in the contained blood volume. This process, known as lusitropy, involves the rapid re-sequestration of calcium ions and is characterized by a significant and rapid decrease in intraventricular pressure. This sharp drop in pressure is necessary to create the large pressure gradient between the atria and the ventricles that will soon allow for the next phase of rapid filling.
Phase 4: Rapid Ventricular Filling (Mid Diastole)
The fourth phase, rapid ventricular filling, begins when the rapidly falling pressure within the relaxing ventricles finally drops below the pressure in the passively filling atria. This pressure reversal causes the atrioventricular (mitral and tricuspid) valves to spring open. The stored potential energy in the stretched atrial and venous walls, combined with a “suction” effect created by the rapid relaxation of the ventricular muscle, results in an initial, unimpeded rush of blood—accounting for the majority of the total ventricular filling (approximately 70% to 80% of the final volume). This is a fast, passive process that occurs very early in diastole and does not require atrial contraction. It is during this rapid filling period that a third heart sound (S3) can sometimes be heard, particularly in cases of heart failure or in healthy, young individuals, due to the turbulent flow and vibration caused by the sudden deceleration of blood against the stiffening ventricular walls.
Phase 5: Diastasis and Atrial Systole (Late Diastole)
The final phase combines the remaining stages of ventricular filling to complete the cycle and prepare for the next beat. Following the rapid filling phase, the pressure gradient between the atria and ventricles dissipates, and the flow of blood into the ventricles slows down considerably. This prolonged period of slow, passive ventricular filling is called diastasis. It continues until the sinoatrial (SA) node fires, initiating the next electrical sequence. The resulting electrical impulse spreads across the atria, causing them to contract—this event is atrial systole, often referred to as the “atrial kick,” and is represented by the P wave on the ECG. Atrial systole is the only active part of the ventricular filling process, forcefully pushing the remaining 20% to 30% of blood into the already partially filled ventricles. This final injection of blood maximizes the end-diastolic volume (EDV) or preload, which is the volume of blood in the ventricles immediately before the next contraction. The entire cardiac cycle is completed when the atrial contraction ends and the electrical impulse reaches the ventricles, triggering the QRS complex and the start of the next isovolumic contraction phase, thereby ensuring the continuous, rhythmic operation of the heart as the central pump of the circulatory system.