The conducting part of the respiratory system is

Options

A. The part starting with trachea up to the terminal bronchioles
B. The part starting with external nostrils up to the terminal bronchioles
C. The part starting with the initial bronchioles up to the terminal bronchioles
D. The part starting with the external nostrils up to the respiratory bronchioles

Answer

To solve the question regarding the conducting part of the respiratory system, we will analyze each option provided and determine which one correctly identifies the conducting pathways.

### Step-by-Step Solution:

1. **Understanding the Conducting Part of the Respiratory System**:
- The conducting part of the respiratory system is responsible for the passage of air to the lungs and does not participate in gas exchange. It includes structures that filter, warm, and humidify the air.

2. **Analyzing Option A**:
- **Description**: The part starting with the trachea up to the terminal bronchioles.
- **Evaluation**: This option includes the trachea and bronchi, which are part of the conducting zone. However, it ends at the terminal bronchioles, which are considered part of the respiratory zone where gas exchange begins. Thus, this option is incorrect.

3. **Analyzing Option B**:
- **Description**: The part starting with the external nostrils up to a terminal bronchus.
- **Evaluation**: This option includes the external nostrils, nasal cavity, pharynx, larynx, trachea, and bronchi, which are all part of the conducting zone. It correctly identifies the pathway for air to travel to the lungs without including the respiratory bronchioles, making this option correct.

4. **Analyzing Option C**:
- **Description**: The part starting with the initial bronchioles up to the terminal bronchiole.
- **Evaluation**: This option is incorrect because it starts with the bronchioles, which are not part of the conducting zone. The bronchioles are involved in gas exchange, thus making this option wrong.

5. **Analyzing Option D**:
- **Description**: The pathway starting with external nostrils up to the respiratory bronchiole.
- **Evaluation**: This option is incorrect as it includes the respiratory bronchioles, which are part of the respiratory zone where gas exchange occurs. Therefore, this option does not represent the conducting part.

6. **Conclusion**:
- The correct answer is **Option B**: the part starting with the external nostrils up to a terminal bronchus.

### Final Answer:
The conducting part of the respiratory system is **Option B**: the part starting with the external nostrils up to a terminal bronchus.