The cardiac cycle represents the sequence of mechanical and electrical events that take place during a single heartbeat. This coordinated process involves alternating periods of relaxation and contraction, ensuring the heart chambers fill with blood and then forcefully eject it into the circulatory system. The entire cycle, which lasts approximately 0.8 seconds at a resting heart rate of 75 beats per minute, is divided into two major phases: relaxation and filling (diastole), and contraction and ejection (systole). This rhythmic activity maintains constant blood circulation and delivers oxygen and nutrients to all tissues.
The Electrical Trigger: Initiating the Heartbeat
The mechanical pumping action of the heart is timed by an electrical signal that travels through specialized muscle cells. The process begins with the Sinoatrial (SA) node, a cluster of cells in the upper right atrium called the heart’s natural pacemaker. These cells spontaneously generate an electrical impulse that determines the heart rate, typically setting the rhythm between 60 and 100 beats per minute at rest.
This signal spreads rapidly across the walls of both atria, causing them to contract and push blood into the ventricles. The impulse then reaches the Atrioventricular (AV) node, which delays the electrical signal for a fraction of a second. This brief pause ensures the atria have time to fully empty their blood into the ventricles before the lower chambers begin their contraction. The signal subsequently travels down a network of fibers, spreading the impulse throughout the ventricular muscle walls to coordinate the contraction of the ventricles.
Diastole: The Heart’s Filling Phase
Diastole is the period of relaxation when the ventricles expand and fill with blood, preparing for the next contraction. This phase begins immediately after ventricular contraction as the pressure in the ventricles rapidly decreases. The drop in ventricular pressure below that of the aorta and pulmonary artery causes the semilunar valves to snap shut, preventing the backflow of blood. The initial part of diastole is called isovolumetric relaxation, a brief moment where the ventricles are relaxing but all four heart valves are closed, so the blood volume remains unchanged.
As relaxation continues, the ventricular pressure eventually falls below the pressure in the atria. This pressure gradient causes the Atrioventricular (AV) valves—the mitral and tricuspid valves—to open, marking the start of ventricular filling. The majority of ventricular filling occurs passively during the rapid filling phase, followed by a period of slower filling. The filling phase is completed by the final contraction of the atria, which pushes the last 25% of blood into the ventricles. The total volume of blood contained in the ventricles at the end of this phase is known as the End-Diastolic Volume (EDV).
Systole: The Heart’s Ejection Phase
Systole is the phase when the ventricles contract to eject blood into the major arteries. This phase begins immediately after the End-Diastolic Volume is reached and the electrical signal has spread through the ventricular walls. The first event is a rapid rise in ventricular pressure as the muscle fibers begin to shorten. As the ventricular pressure exceeds that of the relaxed atria, the mitral and tricuspid valves are forced closed, preventing blood from flowing back into the upper chambers.
For a short time, all heart valves are closed, and the ventricles contract without changing their blood volume; this is the isovolumetric contraction phase. During this moment, the ventricular pressure builds dramatically in preparation for ejection. When the pressure inside the left ventricle surpasses the pressure in the aorta, and the right ventricle’s pressure exceeds that in the pulmonary artery, the semilunar valves open. This allows blood to be ejected from the ventricles into the systemic and pulmonary circulations, respectively.
The peak pressure generated by the left ventricle can reach approximately 120 mmHg in a healthy adult. The ejection phase concludes when the ventricular muscle begins to relax and the pressure drops below that of the major arteries, causing the semilunar valves to close again. The volume of blood remaining in the ventricles at the end of this contraction is termed the End-Systolic Volume (ESV).
Cardiac Output and Heart Sounds
The efficiency of the cardiac cycle is measured by the Cardiac Output, which is the total volume of blood pumped by one ventricle per minute. This measurement is calculated by multiplying the Heart Rate (beats per minute) by the Stroke Volume (volume of blood pumped per beat). Stroke Volume is the difference between the End-Diastolic Volume and the End-Systolic Volume, representing the net volume of blood ejected during systole. For a healthy adult at rest, the cardiac output averages around five liters per minute.
The mechanical events of the cycle also produce distinct, audible Heart Sounds as the valves close. The first heart sound, S1 or the “lub,” is produced by the simultaneous closure of the mitral and tricuspid valves at the beginning of systole. The second heart sound, S2 or the “dub,” is caused by the closure of the aortic and pulmonary semilunar valves at the end of systole and the start of diastole. These sounds are the auditory manifestation of the pressure-driven valve actions that govern the one-way flow of blood through the heart.