which of the following structures of respiratory passage is devoid of incomplete cartilaginous rings?

Options

A. primary bronchi
B. secondary bronchi
C. Tertiary bronchi
D. Alveolar bronchi

Answer

To solve the question "Which of the following structures of the respiratory passage is devoid of incomplete cartilaginous rings?" we need to analyze the respiratory structures listed in the options: primary bronchi, secondary bronchi, tertiary bronchi, and alveolar bronchi.

### Step-by-Step Solution:

1. **Understanding the Respiratory Passage**:
The respiratory passage begins at the nostrils, continues through the nasal cavity, pharynx, trachea, bronchi (primary, secondary, tertiary), and ends at the alveoli.

2. **Identifying the Components**:
- **Conducting Portion**: This includes the nostrils, nasal cavity, pharynx, trachea, and bronchi (primary, secondary, tertiary). These structures are primarily responsible for conducting air but do not participate in gas exchange.
- **Respiratory Portion**: This includes the alveoli, where the actual exchange of gases occurs.

3. **Role of Cartilaginous Rings**:
The incomplete cartilaginous rings (C-shaped) are present in the conducting portion of the respiratory system to prevent the collapse of these structures during breathing. They are essential for the trachea and bronchi as they maintain the open passage for air.

4. **Analyzing Each Option**:
- **Primary Bronchi**: Contains incomplete cartilaginous rings to prevent collapse.
- **Secondary Bronchi**: Also contains incomplete cartilaginous rings.
- **Tertiary Bronchi**: Contains incomplete cartilaginous rings as well.
- **Alveolar Bronchi**: These structures are part of the respiratory portion where gas exchange occurs. Alveoli do not require cartilaginous support as they are surrounded by air and do not collapse.

5. **Conclusion**:
Since the alveolar bronchi are involved in gas exchange and do not have the need for incomplete cartilaginous rings, the correct answer is **alveolar bronchi**.

### Final Answer:
**Alveolar bronchi** are devoid of incomplete cartilaginous rings.

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