What is the function of pleural fluid?

Options

A. (1) reduces friction on the lung surface
B. (2) protection from external shocks
C. (3) provide moisture
D. (4)all of the above

Answer

### Step-by-Step Solution:

1. **Understanding the Pleural Fluid**:
- The pleural fluid is found in the pleural cavity, which is the space between the pleural membranes surrounding the lungs.

2. **Function of Pleural Fluid**:
- **Reduces Friction**: The primary function of pleural fluid is to act as a lubricant. It allows the pleura (the membranes surrounding the lungs) to slide smoothly against each other during respiratory movements such as inhalation and exhalation. This reduces friction between the lung surfaces and the ribs.

3. **Protection from External Shock**:
- The pleural fluid also provides a cushioning effect, protecting the lungs from external shocks and impacts. This helps maintain the integrity of lung tissues during physical activities.

4. **Providing Moisture**:
- Additionally, pleural fluid helps to keep the pleura moist, which is essential for the proper functioning of the lungs and for facilitating smooth respiratory movements.

5. **Conclusion**:
- Given the functions of pleural fluid (reducing friction, providing moisture, and protection from external shocks), the correct answer to the question is that all of the above functions are performed by pleural fluid.

### Final Answer:
The function of pleural fluid includes reducing friction on the lung surface, providing moisture, and protection from external shock. Therefore, the correct option is **all of the above**.

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