What is residual volume? How much is it in a normal adult man?

Explanation

**Step-by-Step Solution:**

1. **Definition of Residual Volume**:
- Residual volume is the amount of air that remains in the lungs after a person has exhaled as much air as possible. This is important because it prevents the lungs from collapsing and ensures that there is always some air available for gas exchange.

2. **Understanding Maximum Expiration**:
- Maximum expiration refers to the process of forcefully expelling air from the lungs. Even after this effort, there is still some air left in the lungs, which is not possible to exhale.

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