What is the residual volume?

Explanation

**Step-by-Step Solution:**

1. **Definition of Residual Volume**: Residual volume is defined as the amount of air that remains in the lungs after a person has exhaled as much air as possible. It is the volume of air that cannot be voluntarily expelled from the lungs.

2. **Measurement Context**: This volume is measured at the end of maximal exhalation. This means that after you breathe out all the air you can, there is still some air left in your lungs.

3. **Importance of Residual Volume**:
- **Prevention of Lung Collapse**: The residual volume is crucial because it prevents the lungs from collapsing completely. If the lungs were to empty completely, they could stick to the chest wall and collapse.
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