Which of the following structure is not supported by in complete cartilaginous rings.

Options

A. Trachea
B. Secondary bronchi
C. Terminal bronchioles
D. Primary bronchi

Answer

To determine which structure is not supported by incomplete cartilaginous rings, we need to analyze the components of the respiratory system and their structural support.

### Step-by-Step Solution:

1. **Understand the Respiratory System**: The respiratory system is responsible for the exchange of gases, primarily oxygen and carbon dioxide. It includes various structures that facilitate this process.

2. **Identify Key Structures**: The main structures involved in the conduction of air include:
- **Nostrils**: Entry point for air.
- **Nasal Chamber**: Warms and moistens the air.
- **Pharynx**: Passageway for air.
- **Larynx**: Voice box and part of the airway.
- **Trachea (Windpipe)**: Main airway supported by incomplete cartilaginous rings.
- **Primary Bronchi**: Branches from the trachea that lead to each lung, also supported by cartilaginous rings.
- **Secondary Bronchi**: Branches from the primary bronchi, similarly supported.
- **Tertiary Bronchi**: Further divisions of the bronchi, still supported by cartilage.
- **Bronchioles**: Smaller branches that lead to the alveoli.

3. **Focus on Cartilaginous Support**: The trachea, primary bronchi, secondary bronchi, and tertiary bronchi all have incomplete cartilaginous rings to prevent collapse when air is not present. This structural support is crucial for maintaining the shape of these airways.

4. **Identify the Exception**: As we move down the respiratory tract, the bronchioles become smaller and finer. Specifically, the **terminal bronchioles** are the last branches before the alveoli and do not require cartilaginous rings for support. They are small enough that they do not collapse, as they are surrounded by smooth muscle and elastic fibers.

5. **Conclusion**: Based on the analysis, the structure that is not supported by incomplete cartilaginous rings is the **terminal bronchioles**.

### Final Answer:
The structure that is not supported by incomplete cartilaginous rings is the **terminal bronchioles**.

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