The "lubb" sound (first heart sound) is caused by

Options

A. filling of the ventricles
B. ventricular contraction
C. closing of the AV valves
D. closing of semilunar valves

Answer

To answer the question about what causes the "lubb" sound (the first heart sound), we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Heart Sounds**: The heart produces two main sounds during its cycle, commonly referred to as "lubb" (first heart sound) and "dupp" (second heart sound).

2. **Identifying the Timing of the "Lubb" Sound**: The "lubb" sound occurs at the beginning of ventricular contraction, also known as ventricular systole. This is when the ventricles are preparing to pump blood out of the heart.

3. **Recognizing the Role of Valves**: The heart has several valves that regulate blood flow. The two main types of valves involved in the first heart sound are the atrioventricular (AV) valves and the semilunar valves.

4. **Focusing on the Atrioventricular Valves**: The AV valves consist of the tricuspid valve (on the right side of the heart) and the bicuspid or mitral valve (on the left side of the heart).

5. **Understanding the Mechanism**: When the ventricles contract, the pressure in the ventricles rises, causing the AV valves to close. This closure prevents the backflow of blood into the atria and is responsible for the "lubb" sound.

6. **Conclusion**: Therefore, the "lubb" sound is specifically caused by the closing of the atrioventricular (AV) valves during ventricular contraction.

### Final Answer:
The "lubb" sound (first heart sound) is caused by the closing of the AV valves.