Before revision: It is caused by the closure of the semilunar valves (the aortic valve and pulmonary valve) at the end of ventricular systole and the beginning of ventricular diastole.
After revision: It is caused by the closure of the semilunar valves (the mitral valve and the tricuspid valve) at the end of ventricular systole and the beginning of ventricular diastole.