Biology
Cardiac Cycle Phases and Heart Sound Generation
Quick fact
The 'lub' (S1) is caused by the closure of the mitral and tricuspid valves at the start of ventricular contraction, and the 'dub' (S2) is caused by the closure of the aortic and pulmonary valves at the end of ventricular contraction. Each heartbeat lasts about 0.8 seconds when at rest.
Why this is interesting
You've likely heard your heart's 'lub-dub' sound, but did you know those sounds are not the heart contracting? The sounds are actually the snapping shut of valves, marking the precise moments when blood flow changes direction. What do these sounds tell us about the health of your heart?
Read the full explanation
Understanding Cardiac Cycle Phases and Heart Sound Generation
Imagine the heart as a two-story house: the top floors are the atria (receiving rooms), and the bottom floors are the ventricles (pumping rooms). The cardiac cycle is the choreographed sequence of filling and pumping. First, both atria contract (atrial systole) to give a final push of blood into the already relaxed ventricles. Then, the ventricles contract (ventricular systole). As they do, pressure rises, slamming shut the doors (valves) between atria and ventricles—this produces the first heart sound, 'lub.' For a brief moment, both sets of doors are closed (isovolumetric contraction), and pressure builds until it forces open the exit doors (semilunar valves) leading to the lungs (pulmonary artery) and the body (aorta). Blood is ejected, and then the ventricles relax (ventricular diastole). Now, the pressure in the exit arteries is higher, so the exit doors slam shut—this is the 'dub' sound (S2). For another moment, all doors are again closed (isovolumetric relaxation), and then the inlet doors open, and blood flows passively into the relaxed ventricles, filling them for the next beat. This entire cycle—filling, contracting, relaxing—takes about 0.8 seconds at a resting heart rate of 75 beats per minute.
A deeper explanation
The cardiac cycle is driven by pressure gradients, achieved by coordinated contraction and relaxation of cardiac muscle cells. The heart sounds S1 and S2 are not caused by the muscle contraction itself, but by the abrupt closure of valves. The 'lub' (S1) occurs at the beginning of ventricular systole when the atrioventricular (AV) valves (mitral and tricuspid) close due to rising ventricular pressure exceeding atrial pressure. The 'dub' (S2) occurs at the beginning of ventricular diastole when the semilunar valves (aortic and pulmonary) close because arterial pressure exceeds the falling ventricular pressure. These sounds are brief, high-frequency vibrations of the blood and valve tissues. During the isovolumetric phases, all valves are closed, preventing backflow. The timing and intensity of these sounds provide crucial clinical information: a heart murmur is an abnormal sound caused by turbulent blood flow, often due to defective valves that fail to close completely (regurgitation) or open fully (stenosis). Understanding the precise phases—atrial systole, ventricular systole (with isovolumetric contraction and ejection), early diastole (with isovolumetric relaxation), and late diastole (passive filling)—is fundamental to interpreting pressures and flows in the heart and relates directly to cardiac output and blood pressure.