The heart functions as a muscular pump, ensuring the continuous circulation of blood throughout the body. This rhythmic action is known as the cardiac cycle, a repeating sequence of relaxation and contraction that powers blood flow. A healthy heart completes this entire sequence dozens of times every minute, moving blood to the lungs for oxygenation and distributing oxygen-rich blood to the body’s tissues. The precise coordination of the heartbeat depends on an intrinsic electrical trigger that immediately precedes the physical squeezing action of the muscle.
The Heart’s Internal Electrical System
The impulse for every heartbeat originates within the cardiac conduction system. This process begins in the Sinoatrial (SA) node, often termed the heart’s natural pacemaker, which is located in the upper wall of the right atrium. The SA node spontaneously generates a regular electrical signal, setting the pace for the entire organ.
The electrical wave rapidly spreads across both the right and left atria, causing these upper chambers to contract and push blood toward the ventricles. This initial signal then converges at the Atrioventricular (AV) node, which is situated near the junction between the atria and the ventricles. The AV node introduces a brief delay of approximately 100 milliseconds to the signal’s transmission.
This pause is purposeful, ensuring that the atria have sufficient time to finish emptying their blood into the ventricles before the lower chambers begin to contract. After the delay, the signal travels rapidly down the Bundle of His, a tract of specialized fibers running through the wall separating the ventricles. The impulse then spreads out through Purkinje fibers, which distribute the electrical energy throughout the muscular walls of both ventricles, triggering their forceful contraction.
The Four Stages of the Mechanical Cardiac Cycle
The physical pumping action of the heart is divided into two major mechanical events: systole (muscle contraction and blood ejection) and diastole (muscle relaxation and chamber filling). These events are refined into four distinct phases that describe the movement of the ventricles, which are the main pumping chambers.
Ventricular Filling
This phase is the primary component of diastole. During this phase, the ventricular muscle is relaxed, and the pressure inside the ventricles is low. Blood flows passively from the atria into the ventricles, accounting for the majority of the blood volume increase.
Isovolumetric Contraction
This phase marks the beginning of systole. Once the electrical signal reaches the ventricles, the muscle fibers begin to contract, causing the pressure inside the ventricles to rise sharply. Both the atrioventricular and semilunar valves are closed during this short period. The blood volume within the chambers remains constant even as the pressure builds dramatically.
Ventricular Ejection
This is the main pumping action of systole. As the pressure in the ventricles surpasses the pressure in the major arteries (the aorta and pulmonary artery), the semilunar valves are forced open. Blood is rapidly ejected from the left ventricle into the aorta and from the right ventricle into the pulmonary artery.
Isovolumetric Relaxation
This phase initiates diastole. After the ventricles have ejected the blood, they begin to relax, causing the pressure to drop quickly. The semilunar valves snap shut to prevent backflow, and for a brief moment, all four heart valves are closed. The blood volume is fixed, but the pressure continues to fall until it drops below the atrial pressure, allowing the atrioventricular valves to open and restart the filling phase.
How Valve Movement Directs Blood Flow
The heart contains four valves that operate in coordination with the pressure changes established by the mechanical cycle, ensuring that blood moves in a single, forward direction.
- The Atrioventricular (A-V) valves (the tricuspid on the right and the mitral on the left) control flow from the atria to the ventricles.
- The Semilunar valves (the pulmonary and the aortic) regulate blood flow out of the ventricles into the major arteries.
During the filling phase of diastole, the A-V valves are open because the pressure in the relaxed ventricles is lower than the pressure in the atria. This pressure gradient allows blood to flow into the lower chambers. As the ventricles begin to contract during isovolumetric contraction, the rapidly rising internal pressure forces the A-V valves to close immediately, preventing blood from being pushed back into the atria.
The simultaneous closing of the mitral and tricuspid valves produces the first distinct sound of the heartbeat, often described as “lub.” Later, during ventricular ejection, the high ventricular pressure opens the semilunar valves, allowing blood to exit the heart. When the ventricles relax and the pressure falls below the arterial pressure, the blood attempts to flow backward, which causes the pulmonary and aortic valves to slam shut. The closing of the semilunar valves creates the second sound of the heartbeat, the “dub,” completing the rhythm.