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MDCAT Biology QUESTION #11259
Question 1
In the cardiac cycle the 'lub' sound is produced by:
  • Closure of the semilunar valves
  • Closure of the atrioventricular (AV) valves✔️
  • Opening of the AV valves
  • Contraction of the atria
Correct Answer Explanation
The first heart sound ('lub') is produced by the closure of the atrioventricular (AV) valves — the tricuspid and mitral (bicuspid) valves — at the beginning of ventricular systole. The 'dub' sound is produced by closure of the semilunar valves.