Which of the following happens during expiration?

Options

A. Increase in the pulmonary volume decreases the intrapulmonary pressure
B. Increase in the pulmonary volume increases the intrapulmonary pressure
C. Decrease in the thoracic volume decreases the intrapulmonary pressure
D. Decrease in the thoracic volume slightly increases the intrapulmonary pressure

Answer

### Step-by-Step Solution:

1. **Understanding Expiration**:
- Expiration refers to the process of breathing out air from the lungs. It is the opposite of inhalation.

2. **Diaphragm Action**:
- During expiration, the diaphragm, which is a dome-shaped muscle at the base of the thoracic cavity, relaxes.

3. **Effect on Thoracic Cavity**:
- When the diaphragm relaxes, the thoracic cavity becomes smaller. This is because the diaphragm moves upwards into a more dome-like shape.

4. **Pressure Changes**:
- As the thoracic cavity decreases in size, the intrapulmonary pressure (the pressure within the lungs) increases. This increase in pressure helps push air out of the lungs.

5. **Evaluating Options**:
- Now, let’s analyze the options given in the question:
- **Option A**: Increase in pulmonary volume - This is incorrect because the volume decreases during expiration.
- **Option B**: Increase in pulmonary volume - This is also incorrect for the same reason.
- **Option C**: Decrease in the thoracic cavity - This is correct, but it states that intrapulmonary pressure decreases, which is incorrect.
- **Option D**: The thoracic volume decreases while the intrapulmonary pressure increases - This is the correct statement.

6. **Conclusion**:
- Therefore, the correct answer is **Option D**: The thoracic volume decreases while the intrapulmonary pressure increases.