What causes normal heart sounds, and how do changes in blood flow relate to the closure of the atrioventricular and semilunar valves?

Sourav PanNovember 9, 2024

Answered

Normal heart sounds, commonly referred to as “lub” (S1) and “dub” (S2), are produced by the closure of the heart valves during the cardiac cycle. Understanding the mechanisms behind these sounds and their relationship to blood flow changes provides insight into cardiac function.

Causes of Normal Heart Sounds

1. First Heart Sound (S1):
   • The “lub” sound corresponds to the closure of the atrioventricular (AV) valves—the mitral and tricuspid valves—at the beginning of ventricular contraction (systole). This closure prevents backflow of blood into the atria as the ventricles contract to pump blood into the aorta and pulmonary artery.
   • The sound is primarily caused by the rapid deceleration of blood flow and the vibration of the valve leaflets as they close.
2. Second Heart Sound (S2):
   • The “dub” sound is produced by the closure of the semilunar valves—the aortic and pulmonary valves—at the end of ventricular contraction as pressure in the ventricles falls below that in the aorta and pulmonary artery.
   • Similar to S1, this sound arises from the vibrations of the valve cusps and surrounding structures as they close, preventing backflow into the ventricles during diastole.

Relationship Between Blood Flow Changes and Valve Closure

1. Atrioventricular Valves Closure:
   • As ventricular pressure rises during systole, it exceeds atrial pressure, leading to the closure of the AV valves. This closure occurs just after atrial systole, which allows for maximal filling of the ventricles before they contract.
   • The rapid increase in ventricular pressure results in a swift closure of these valves, contributing to S1.
2. Semilunar Valves Closure:
   • After blood is ejected from the ventricles into the aorta and pulmonary artery, ventricular pressure decreases. Once it falls below arterial pressure, the semilunar valves close to prevent backflow into the ventricles.
   • This closure occurs at the end of ventricular systole and marks the beginning of diastole, producing S2. The dynamics of blood flow—specifically, a decrease in flow velocity as ventricular contraction ends—are crucial for this process.
3. Hemodynamic Changes:
   • During both phases of valve closure, changes in blood flow dynamics play a significant role. For instance, when blood is ejected from the ventricles, it creates high velocities that contribute to valve closure through increased pressure gradients across valve cusps.
   • The interplay between rising pressures during systole and falling pressures during diastole ensures that each valve closes properly without allowing regurgitation.