Doctors use stethoscope to hear the sounds produced during each cardiac cycle. The second sound is heard when

Options

A. AV valves open up
B. Ventricular walls vibrate due to gushing in of blood from atria.
C. Semilunar valves close down after the blood flows into vessels from ventricles
D. AV node receives signal from SA node

Answer

### Step-by-Step Solution:

1. **Understanding Heart Sounds**: The human heart produces two main sounds during each cardiac cycle, commonly referred to as the first heart sound (S1) and the second heart sound (S2).

2. **Identifying S1**: The first heart sound (S1) occurs when the atrioventricular (AV) valves (tricuspid and mitral valves) close. This happens when the ventricles are filled with blood and begin to contract, preventing backflow into the atria.

3. **Identifying S2**: The second heart sound (S2) occurs when the semilunar valves (pulmonary and aortic valves) close. This happens after the ventricles have contracted and blood is ejected into the pulmonary artery and aorta.

4. **Mechanism of S2**: When the ventricles contract, blood is pushed into the arteries. Once the blood has been ejected, the semilunar valves close to prevent the backflow of blood into the ventricles, producing the second heart sound, often described as "dub."

5. **Analyzing the Options**:
- **Option 1**: "AV valves open up" - This is incorrect because the AV valves do not open during the second sound; they are closed.
- **Option 2**: "When the ventricular walls vibrate due to gushing in of blood from the atria" - This is also incorrect as it describes a different phase of the cardiac cycle.
- **Option 3**: "The semilunar valves close down after the blood flows into the vessels from the ventricles" - This is the correct answer as it accurately describes the event that produces the second heart sound.
- **Option 4**: "AV node receives the signal from SA node" - This is unrelated to the heart sounds and is therefore incorrect.

6. **Conclusion**: The correct answer is Option 3, which states that the second heart sound is heard when the semilunar valves close after the blood flows into the vessels from the ventricles.