S1 marks the beginning of which cardiac event?

S1 marks the beginning of which cardiac event?

• A. Ventricular systole
• B. Ventricular diastole
• C. Atrial systole
• D. Atrial diastole

Answer: (A)

S1 occurs when the atrioventricular valves close as the ventricles begin to contract. This marks the start of ventricular systole. As the ventricles generate pressure, the mitral and tricuspid valves snap shut to prevent backflow into the atria, producing the first heart sound. The second heart sound, by contrast, comes later with the closure of the semilunar valves at the end of systole, signaling the start of diastole. The other options refer to atrial or ventricular relaxation and filling phases, which do not correspond to the onset of ventricular contraction.

S1 occurs when the atrioventricular valves close as the ventricles begin to contract. This marks the start of ventricular systole. As the ventricles generate pressure, the mitral and tricuspid valves snap shut to prevent backflow into the atria, producing the first heart sound. The second heart sound, by contrast, comes later with the closure of the semilunar valves at the end of systole, signaling the start of diastole. The other options refer to atrial or ventricular relaxation and filling phases, which do not correspond to the onset of ventricular contraction.