Lungs are made up of air-filled sacs, the alveoli . They do not collapse even after forceful expiration because of

Options

A. residual volume
B. inspiratory reserve volume
C. tidal volume
D. expiratory reserve volume

Answer

### Step-by-Step Solution:

1. **Understanding Alveoli**: The lungs are composed of tiny air-filled sacs called alveoli. These structures are crucial for the process of gas exchange, allowing oxygen to enter the blood and carbon dioxide to be expelled.

2. **Forceful Expiration**: During forceful expiration (exhaling), one might think that all the air in the lungs would be expelled, potentially leading to the collapse of the alveoli.

3. **Residual Volume**: However, even after a forceful expiration, the lungs do not completely empty. There is always a certain volume of air that remains in the lungs, known as the **residual volume**. This volume is crucial because it prevents the alveoli from collapsing.

4. **Function of Residual Volume**: The residual volume not only keeps the alveoli open but also ensures that there is a continuous supply of oxygen available for the cells, even after exhalation.

5. **Conclusion**: Therefore, the reason the alveoli do not collapse after forceful expiration is due to the presence of residual volume in the lungs.

### Final Answer:
The alveoli do not collapse even after forceful expiration because of the **residual volume** of air that remains in the lungs.

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