Residual air mostly occurs in

Options

A. alveoli
B. bronchus
C. nostrils
D. trachea

Answer

**Step-by-Step Solution:**

1. **Understanding Residual Air:**
- Residual air, also known as residual volume, refers to the amount of air that remains in the lungs after a person has exhaled as much air as possible.

2. **Identifying Where Residual Air is Found:**
- This residual volume is primarily located in the alveoli, which are the tiny air sacs in the lungs where gas exchange occurs.

3. **Significance of Residual Air:**
- The presence of residual air is crucial because it prevents the lung tissues from collapsing. If the lungs were to empty completely, the lung tissues could stick together, making it difficult to breathe.

4. **Conclusion:**
- Given that the question asks where residual air mostly occurs, the correct answer is the alveoli, as this is where the residual volume is retained after exhalation.

**Final Answer:**
- Residual air mostly occurs in the **alveoli**.

---