The rhythmic sounds of the heart provide an auditory signal of the continuous cardiac cycle. Listening to these sounds, a practice known as auscultation, is a quick and non-invasive way for medical professionals to assess heart function. The familiar “lub-dub” sound heard with a stethoscope is caused by the opening and closing of the heart’s four valves, which ensure blood flows in only one direction. These two distinct sounds are medically labeled S1 and S2, and understanding their timing reveals the heart’s two major mechanical phases.
Understanding the Heart’s Rhythmic Cycle
The heart operates through a precise sequence of contraction and relaxation known as the cardiac cycle. This cycle is divided into two primary phases that dictate the movement of blood. The first phase, known as systole, is the period of ventricular contraction when the ventricles squeeze forcefully to eject blood into the circulation.
The second phase is diastole, which represents the period of ventricular relaxation and filling. Following the ejection of blood, the ventricular muscles relax, allowing the chambers to expand. This creates a lower pressure environment, causing blood to flow passively from the atria into the ventricles, preparing the heart for the next contraction.
The First Sound: S1 and the “Lub”
The first heart sound, S1, is the “lub” component of the heartbeat and marks the beginning of ventricular contraction. This sound is generated by the simultaneous closure of the two atrioventricular (AV) valves: the mitral valve and the tricuspid valve. These valves sit between the atria and the ventricles, regulating blood flow.
As the ventricles begin to contract, pressure rapidly increases, forcing the mitral and tricuspid valve leaflets to snap shut. This prevents blood from flowing backward into the atria. The noise results from the vibrations created in the blood and cardiac structures as the flow is abruptly stopped.
The closure of the mitral valve (M1) typically occurs a fraction of a second before the tricuspid valve closure (T1). This is because the left ventricle contracts slightly earlier and is a high-pressure system. However, the time difference is so minuscule that the two components are heard as a single, unified S1 sound, which is usually lower in pitch and longer in duration than S2.
The Second Sound: S2 and the “Dub”
The second heart sound, S2, is the “dub” component of the heartbeat and marks the termination of ventricular contraction. This sound is caused by the closure of the two semilunar valves: the aortic valve and the pulmonic valve. These valves control the exit of blood from the ventricles into the great arteries.
After the ventricles eject blood, they begin to relax, causing internal pressure to fall rapidly. When ventricular pressure drops below the pressure in the aorta and pulmonary artery, blood attempts to flow backward. This backflow forces the aortic and pulmonic valves to suddenly close, preventing blood from re-entering the heart chambers.
The closure of the aortic valve (A2) and the pulmonic valve (P2) forms the S2 sound, which is typically higher in pitch and shorter in duration than S1. The aortic valve normally closes slightly before the pulmonic valve due to the higher pressure in the systemic circulation. This slight separation is known as a physiological split in S2 and becomes more noticeable with inspiration.
How S1 and S2 Relate to Systole and Diastole
The alternating sequence of S1 and S2 establishes the distinct periods of the cardiac cycle. S1 signifies the start of systole, the period of ventricular ejection. It is the acoustic marker that ventricular contraction is underway, immediately following the closure of the inlet valves.
S2 marks the end of systole and, simultaneously, the beginning of diastole, the period of ventricular relaxation and filling. The time interval between S1 and S2 is the systolic period, during which the ventricles actively pump blood out of the heart.
Following S2, the heart enters the diastolic period, which lasts until the next S1 is heard. This time between the “dub” and the subsequent “lub” is when the ventricles are refilling with blood. At normal resting heart rates, the diastolic interval is longer than the systolic interval, allowing time for the ventricles to fully relax and prepare for the next contraction.