Which of the following factors contribute to the reduction of the pulmonary volume ?

Options

A. Relaxation of diaphragm
B. Relaxation of the intercostal muscles
C. Return of sternum to their normal positions
D. All of the above

Answer

To solve the question regarding the factors that contribute to the reduction of pulmonary volume, we can break down the process of exhalation and how various anatomical structures play a role in this process.

### Step-by-Step Solution:

1. **Understanding Pulmonary Volume**:
- Pulmonary volume refers to the amount of air that the lungs can hold. It changes during the process of breathing, specifically during inhalation and exhalation.

2. **Exhalation Process**:
- Exhalation is the process of releasing air from the lungs, which involves a reduction in pulmonary volume. This is primarily due to the relaxation of certain muscles.

3. **Role of the Diaphragm**:
- The diaphragm is a dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. During exhalation, the diaphragm relaxes and moves back to its original dome shape, which decreases the volume of the thoracic cavity and pushes air out of the lungs.

4. **Role of Intercostal Muscles**:
- The intercostal muscles are located between the ribs. During exhalation, these muscles also relax, allowing the rib cage to move downward and inward. This further contributes to the reduction of thoracic volume.

5. **Return of the Sternum**:
- The sternum (breastbone) also plays a role in the mechanics of breathing. When the intercostal muscles relax, the sternum returns to its normal position, which aids in reducing the volume of the thoracic cavity.

6. **Conclusion**:
- All of the mentioned factors (relaxation of the diaphragm, relaxation of intercostal muscles, and return of the sternum to its normal position) contribute to the reduction of pulmonary volume during exhalation. Therefore, the correct answer to the question is option 4: "All of the above."

### Final Answer:
The factors that contribute to the reduction of pulmonary volume are:
- Relaxation of the diaphragm
- Relaxation of intercostal muscles
- Return of the sternum to its normal position
Thus, the answer is **4. All of the above**.