The second heart sound (dupp) closely follows the closure of the semilunar valves. This occurs during the phase of the cardiac cycle called ventricular diastole, when the ventricles are relaxed and filling with blood.
To understand why the second heart sound follows the closure of the semilunar valves, let's go through the events of the cardiac cycle:
1. Atrial systole: The atria contract, forcing blood into the ventricles through the open atrioventricular valves (mitral and tricuspid valves).
2. Ventricular systole: The ventricles contract, causing the atrioventricular valves to close. This closure produces the first heart sound (lubb) and prevents the backflow of blood into the atria.
3. Isovolumetric contraction: The ventricles continue to contract, building up pressure until it exceeds the pressure in the aorta and pulmonary artery. Once this pressure threshold is reached, the semilunar valves (aortic and pulmonic valves) open, allowing blood to be ejected into the arteries.
4. Ventricular diastole: After the blood has been ejected, the ventricles relax. The pressure in the aorta and pulmonary artery drops, causing the semilunar valves to close. This closure of the semilunar valves produces the second heart sound (dupp).
Therefore, the second heart sound closely follows the closure of the semilunar valves (D). It is important to note that the closure of the atrioventricular valves (C) occurs earlier in the cardiac cycle and is associated with the first heart sound (lubb).