The air passage branches repeatedly into smaller tubules which end in ________

Explanation

**Step-by-Step Solution:**

1. **Understanding the Respiratory System**: The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) in the body. It consists of various structures that facilitate this process.

2. **Air Passage**: When air enters the body, it travels through a series of passages. It starts from the nostrils, moves to the nasal cavity, then to the pharynx, and continues down the trachea (windpipe).

3. **Division of Air Passage**: The trachea branches into two primary bronchi, which lead into each lung. These bronchi further divide into smaller branches known as secondary bronchi, and then tertiary bronchi.

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