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The human heart is a highly specialized muscular organ that works as the central pump of the circulatory system. Its primary role is to ensure that blood continuously flows throughout the body, delivering oxygen and nutrients to the tissues while simultaneously removing carbon dioxide and other metabolic wastes. To carry out this function effectively, the heart follows a highly coordinated sequence of contraction and relaxation events known as the cardiac cycle.

The term “cardiac cycle” refers to the complete set of events that occur during one heartbeat, beginning with the contraction of the atria, followed by the contraction of the ventricles, and ending with a period of relaxation before the next heartbeat starts. This cycle repeats itself continuously throughout life, making it one of the most vital processes in the body. The cardiac cycle not only controls the pumping of blood but also ensures that circulation is maintained in a precise, rhythmic, and energy-efficient manner.

In an average healthy adult, the heart beats around seventy-two times per minute. Each heartbeat, or cardiac cycle, lasts for approximately 0.8 seconds. Within this short span of time, the atria and ventricles undergo systole (contraction) and diastole (relaxation), while the heart valves open and close in perfect synchrony to regulate blood flow in a forward direction and to prevent backflow. The atrioventricular valves, which include the tricuspid and mitral valves, and the semilunar valves, which include the pulmonary and aortic valves, play a crucial role in maintaining this unidirectional flow.

The cardiac cycle is also closely related to the electrical activity of the heart, which originates in the sinoatrial node, often called the natural pacemaker. This electrical impulse initiates atrial contraction, spreads through the atrioventricular node and bundle branches, and ultimately stimulates ventricular contraction. Thus, the cycle is not merely mechanical but is driven and regulated by precise electrical signals that ensure its rhythm and coordination.

Phases

Atrial Systole – The First Phase of the Cardiac Cycle

The cardiac cycle begins with atrial systole, which is the contraction of the atria. This phase is relatively short, lasting about 0.1 seconds, yet it plays a crucial role in completing the filling of the ventricles before they begin their own contraction. Although the majority of ventricular filling—around seventy to eighty percent—occurs passively during ventricular diastole, atrial systole is responsible for adding the final twenty to thirty percent of blood volume, often referred to as the “atrial kick.” This ensures that the ventricles are completely filled and prepared for an efficient and forceful contraction in the next phase.

Atrial systole is initiated by the electrical impulse from the sinoatrial (SA) node, the natural pacemaker of the heart. Once the SA node generates an action potential, the electrical signal spreads rapidly across the atrial myocardium, causing both the right and left atria to contract almost simultaneously. This contraction increases atrial pressure and pushes the remaining blood into the ventricles through the atrioventricular (AV) valves. During this phase, the tricuspid valve (between the right atrium and right ventricle) and the mitral valve (between the left atrium and left ventricle) remain open, allowing free passage of blood into the ventricles.

At the same time, the semilunar valves—the aortic valve and pulmonary valve—remain tightly closed. This closure prevents any backflow of blood from the arteries into the ventricles and ensures that the ventricles focus solely on filling rather than pumping at this moment. The ventricular chambers are in a relaxed state, and their pressure is still lower than the pressure inside the atria, which favors the movement of blood downward.

Ventricular Systole – The Second Phase of the Cardiac Cycle

After atrial systole completes the filling of the ventricles, the cardiac cycle progresses into the second phase known as ventricular systole. This phase is longer than atrial systole, lasting about 0.3 seconds, and it is the period during which the ventricles contract forcefully to pump blood into the pulmonary and systemic circulations. Ventricular systole is critical for the effective distribution of blood throughout the body and is divided into two distinct stages: isovolumetric contraction and ventricular ejection.

The onset of ventricular systole is triggered by the electrical signal that originates from the atrioventricular (AV) node and spreads rapidly through the bundle of His, bundle branches, and Purkinje fibers. This electrical activation causes the ventricular myocardium to contract almost simultaneously. As contraction begins, the pressure inside the ventricles rises sharply. Once the ventricular pressure exceeds the pressure in the atria, the atrioventricular valves (tricuspid and mitral valves) close. This closure prevents the backflow of blood into the atria and produces the first heart sound (S1), commonly described as “lub.” At this moment, the semilunar valves remain closed because the ventricular pressure is not yet higher than the pressure in the pulmonary artery and aorta. Since all four valves are closed, the volume of blood inside the ventricles remains constant, and this brief interval is called isovolumetric contraction.

As the ventricular contraction continues, the pressure within the ventricles rises further. When the pressure in the right ventricle exceeds the pressure in the pulmonary artery, the pulmonary valve opens, and when the pressure in the left ventricle surpasses that in the aorta, the aortic valve opens. This marks the beginning of the ventricular ejection phase. During this stage, blood is forcefully expelled from both ventricles into the great arteries. The ejection occurs in two parts: a rapid ejection period, during which the majority of the stroke volume is discharged quickly, followed by a slower ejection period as the ventricular contraction starts to weaken and the pressure begins to fall.

Ventricular ejection continues until the pressure inside the ventricles drops below the pressure in the arteries. At this point, the semilunar valves close suddenly to prevent the backflow of blood into the heart. The closure of these valves produces the second heart sound (S2), known as “dup.” This sound marks the end of ventricular systole and the beginning of the next major phase, ventricular diastole.

Ventricular Diastole – The Third Phase of the Cardiac Cycle

Once the ventricles have completed their powerful contraction and ejected blood into the aorta and pulmonary artery, they enter a state of relaxation known as ventricular diastole. This phase is the longest portion of the cardiac cycle, lasting about 0.5 seconds when the heart rate is around seventy-two beats per minute. Ventricular diastole is crucial because it allows the heart chambers to refill with blood, preparing them for the next cycle of contraction and ensuring that circulation remains continuous and efficient.

Ventricular diastole begins immediately after the closure of the semilunar valves. When the pressure inside the ventricles falls below the pressure in the great arteries, the aortic and pulmonary valves shut firmly to prevent any backward flow of blood. The sudden closure of these valves generates the second heart sound (S2), described as “dup.” At this point, the ventricles are relaxing, but all four valves—the atrioventricular and the semilunar valves—are closed. Because the volume of blood inside the ventricles does not change during this short interval, this stage is referred to as isovolumetric relaxation. It lasts only a brief moment, but it is important for reducing the intraventricular pressure to a level that permits the next stage of filling.

As the ventricles continue to relax, their internal pressure drops below the pressure in the atria. This pressure difference causes the atrioventricular valves (tricuspid and mitral) to open, marking the beginning of the ventricular filling phase. Blood then flows passively from the atria into the ventricles. Ventricular filling itself occurs in three parts. The first is the rapid filling phase, during which blood rushes quickly into the ventricles as soon as the valves open. This is followed by a slower filling phase, often referred to as diastasis, in which the flow of blood becomes more gradual as the pressure difference between atria and ventricles narrows. Finally, the filling phase is completed by atrial systole in the next cardiac cycle, which provides the final volume of blood necessary to fully stretch the ventricles before contraction begins again.

Ventricular diastole is not merely a passive resting stage but an active part of the cycle that determines how effectively the ventricles are prepared for their next contraction. Adequate filling during this phase is essential for maintaining an optimal stroke volume and cardiac output.

Therefore, ventricular diastole, lasting for half of the cardiac cycle, represents the vital recovery and filling period of the heart. It ensures that the ventricles receive an adequate blood supply from the atria, stores the potential energy needed for the next systolic contraction, and maintains the balance between circulation and cardiac workload. In combination with atrial systole and ventricular systole, it completes the rhythmic cycle of the heartbeat, allowing the heart to function as a perfectly timed pump throughout life.

Heart Sounds in the Cardiac Cycle

In a healthy adult, two primary sounds, designated as the first heart sound (S1) and the second heart sound (S2), are normally audible.

The first heart sound (S1), commonly described as the “lub,” marks the beginning of ventricular systole. It occurs when the atrioventricular valves—the tricuspid valve on the right side and the mitral valve on the left side—close suddenly as the pressure inside the ventricles rises above atrial pressure. This abrupt closure prevents the backflow of blood into the atria and generates vibrations that resonate through the cardiac structures and chest wall. S1 is relatively long and low-pitched compared to the second sound and is best heard over the apex of the heart with the diaphragm of the stethoscope. Clinically, its intensity can vary depending on factors such as the position of the atrioventricular valves at the onset of systole, the rate of ventricular pressure rise, and the thickness of the chest wall.

The second heart sound (S2), described as the “dup,” signals the onset of ventricular diastole. It is produced by the sudden closure of the semilunar valves—the aortic valve on the left side and the pulmonary valve on the right side—when ventricular pressure falls below the pressure in the great arteries. The closure of these valves prevents blood from flowing back into the ventricles and creates sharp vibrations that are shorter and higher-pitched than those of S1. S2 is best heard over the base of the heart, particularly in the aortic and pulmonary areas. Importantly, S2 is composed of two components: the aortic closure sound (A2) and the pulmonary closure sound (P2). In normal individuals, these may be heard separately during inspiration due to a physiological delay in pulmonary valve closure, a phenomenon called physiological splitting of S2.