What heart valves are closed during the S1 heart sound?

The S1 heart sound, often associated with the "lub" of the heart sounds, occurs when the ventricles contract, which leads to the closure of the atrioventricular (AV) valves. These valves include the mitral and tricuspid valves. The closure of these valves marks the beginning of ventricular systole, ensuring that blood flows from the ventricles into the aorta and pulmonary artery without backflow into the atria.

When the ventricles contract, the pressure rises inside them, causing the mitral valve (which separates the left atrium from the left ventricle) and the tricuspid valve (which separates the right atrium from the right ventricle) to close. This closure prevents the backflow of blood into the atria and is what generates the distinct sound of S1.

In contrast, the aortic and pulmonary valves, which are located at the exits of the ventricles, remain open during S1 to allow for blood ejection during the contraction. Therefore, understanding the role of the mitral and tricuspid valves in this process clarifies why this choice is correct.