Lungs do not collapse between breaths and some air always remains in the lungs which can never be expelled because

Options

A. there is a negative pressure in the lungs
B. there is a negatvie intrapleural presure lulling at the lung walls
C. there is a positive intrapleural pressure
D. pressure in the lungs is higher than the atmospheric pressure

Answer

### Step-by-Step Solution:

1. **Understanding Lung Mechanics**: The lungs are designed to never fully collapse due to the presence of air even between breaths. This is crucial for maintaining efficient gas exchange.

2. **Intrapleural Pressure**: The pressure within the pleural cavity (the space between the lungs and the chest wall) is known as intrapleural pressure. This pressure is typically negative, which helps keep the lungs inflated.

3. **Role of Elastic Forces**: The lungs have elastic properties that tend to pull them inward, while the chest wall has elastic properties that tend to push outward. These opposing forces create a negative pressure in the pleural cavity.

4. **Air Remaining in the Lungs**: The air that remains in the lungs after exhalation is known as residual volume. This volume of air cannot be expelled and is essential for preventing lung collapse.

5. **Surfactant Function**: The alveoli (air sacs in the lungs) contain a substance called surfactant, which reduces surface tension and helps keep the alveoli open, further preventing collapse.

6. **Conclusion**: Therefore, the reason the lungs do not collapse and some air always remains in them is primarily due to the negative intrapleural pressure created by the opposing elastic forces of the lungs and chest wall.

### Final Answer:
The lungs do not collapse between breaths, and some air always remains in the lungs which can never be expelled because of the negative intrapleural pressure created by the opposing elastic forces of the lungs and chest wall.

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