The first heart sound, lubb, is produced by
- A. blood rushing into the atria
- B. the closure of the semilunar valves
- C. contraction of the atrial walls
- D. the closure of the atrioventricular valves at the start of ventricular systole
Explanation
When the ventricles begin to contract the pressure inside them rises above atrial pressure, the tricuspid and bicuspid valves snap shut and the vibration gives the long low lubb. The shorter, sharper dupp comes later, when the semilunar valves close at the end of ventricular systole. A murmur heard between these sounds usually means a valve is leaking or narrowed.