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

Options

A. Pressure in the lungs is higher than the atmospheric pressure
B. There is a negative pressure in the lungs
C. There is a negative intrapleural pressure pulling at the lung walls
D. There is a positive intrapleural pressure

Answer

To answer the question "Lungs do not collapse between breaths and some air always remains in the lungs which can never be expelled because...", we need to analyze the reasons behind the mechanics of lung function.

### Step-by-Step Solution:

1. **Understanding Lung Mechanics**: The lungs are surrounded by a pleural cavity that contains pleural fluid. This cavity creates a pressure difference between the lungs and the outside atmosphere.

2. **Intrapleural Pressure**: The pressure within the pleural cavity (intrapleural pressure) is always negative relative to the atmospheric pressure. This negative pressure is crucial as it helps keep the lungs inflated.

3. **Lung Expansion**: When we inhale, the diaphragm and intercostal muscles contract, increasing the volume of the thoracic cavity. This expansion further decreases the intrapleural pressure, allowing air to flow into the lungs.

4. **Residual Volume**: After exhalation, not all air is expelled from the lungs. The air that remains is known as the residual volume. This is due to the negative intrapleural pressure that prevents the lungs from fully collapsing.

5. **Conclusion**: The reason why the lungs do not collapse and some air always remains in them is primarily due to the negative intrapleural pressure that pulls on the lung walls, keeping them expanded.

### Final Answer:
Lungs do not collapse between breaths and some air always remains in the lungs which can never be expelled because there is a negative intrapleural pressure pulling at the lung walls.

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