Bronchioles end up in small balloon-like structures called _______

Explanation

**Step-by-Step Solution:**

1. **Identify the Structure**: The question asks about the small balloon-like structures at the end of bronchioles.

2. **Recall the Function**: These structures are involved in the process of gas exchange in the respiratory system.

3. **Name the Structure**: The small balloon-like structures at the end of bronchioles are called **alveoli**.

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