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The cardiac cycle is a precisely coordinated sequence of electrical activation, pressure changes, valve motion, and blood flow that allows the heart to pump efficiently with every beat. At a normal heart rate of 75 beats/min, one complete cardiac cycle lasts approximately 0.8 seconds: - Atrial systole: 0.1 s - Ventricular systole: 0.3 s - Complete cardiac diastole (joint relaxation): 0.4 s It can be divided into two major phases: 🟥Diastole: Ventricular relaxation and filling. ⬜ Systole: Ventricular contraction and blood ejection. The sequence begins with electrical activation at the sinoatrial (SA) node, causing both atria to contract. After a brief delay at the atrioventricular (AV) node, both ventricles contract. Mechanical contraction always follows electrical depolarization by a short interval, which is why the QRS complex precedes ventricular systole on the ECG. The left ventricle passes through four key mechanical phases: 🔻Isovolumic relaxation: Aortic valve closes, mitral valve remains closed. Ventricular pressure falls while volume remains unchanged. 🔻 Ventricular filling: Mitral valve opens, allowing blood to flow into the relaxing ventricle. 🔻 Isovolumic contraction: Mitral valve closes, producing the first heart sound (S1). Pressure rises rapidly, but no blood is ejected because both valves are closed. 🔻 Rapid ventricular ejection: Once ventricular pressure exceeds aortic pressure, the aortic valve opens and blood is ejected. A normal ventricle ejects more than 60% of its end-diastolic volume. Closure of the aortic valve produces the second heart sound (S2). Pressure gradients, not the valves themselves, determine valve opening and closure. Valves simply open and close in response to changing pressures, ensuring one-way blood flow. Typical left-sided pressures: - Left ventricle: 120/15 mmHg - Aorta: 120/80 mmHg Understanding the cardiac cycle is fundamental to clinical cardiology. It explains: - Normal and abnormal heart sounds - The timing of systolic and diastolic murmurs - Pressure-volume loops and the Wiggers diagram - Echocardiographic assessment of chamber function and valve disease - Hemodynamic changes seen in heart failure, valvular disease, arrhythmias, and congenital heart defects Every heartbeat is a synchronized interaction between electrical conduction, myocardial contraction, valve function, and pressure gradients. Mastering this physiology is essential for interpreting ECGs, echocardiography, cardiac catheterization, and bedside cardiac examination.