The cardiac cycle is the sequence of mechanical and electrical events that occur in the heart during one complete heartbeat, beginning with the initiation of an impulse in the sinoatrial (SA) node and ending just before the next heartbeat begins. It includes alternating phases of contraction (systole) and relaxation (diastole) of the atria and ventricles, ensuring the efficient pumping of blood throughout the body. In a healthy adult at rest, the cardiac cycle lasts about 0.8 seconds when the heart rate is approximately 75 beats per minute. The cycle begins with atrial systole. The SA node, the natural pacemaker of the heart, generates an electrical impulse that spreads through both atria, causing them to contract. During this phase, the atrioventricular (AV) valves, namely the tricuspid and mitral valves, remain open, allowing blood to move from the atria into the ventricles. Atrial contraction contributes the final 20–30% of ventricular filling, while the majority of ventricular filling occurs passively during ventricular relaxation. Following atrial systole, the electrical impulse reaches the AV node, where it is briefly delayed before passing through the bundle of His, bundle