Read the following statement about human respiration (i) trachea divides at the level of 6th thoracic vertebra
(ii) terminal bronchioles alveoli and their ducts form the respiratory part of this system
(iii) contraction of diaphragm increases volume of thoracic chamber dorsoventrally
(iv) the internal intercostals help in inspiration

Options

A. all except (iv) are true
B. only (iii) and (iv) are false
C. only (i) is true
D. none is true

Answer

To solve the question regarding the statements about human respiration, we will analyze each statement one by one to determine their correctness.

### Step-by-Step Solution:

1. **Statement (i)**: "Trachea divides at the level of 6th thoracic vertebra."
- **Analysis**: The trachea actually bifurcates into the primary bronchi at the level of the **5th thoracic vertebra**, not the 6th. Therefore, this statement is **incorrect**.

2. **Statement (ii)**: "Terminal bronchioles, alveoli, and their ducts form the respiratory part of this system."
- **Analysis**: The terminal bronchioles are part of the **conducting zone** of the respiratory system. The respiratory part includes the **respiratory bronchioles, alveolar ducts, and alveoli**. Thus, this statement is also **incorrect**.

3. **Statement (iii)**: "Contraction of diaphragm increases volume of thoracic chamber dorsoventrally."
- **Analysis**: When the diaphragm contracts, it flattens and increases the volume of the thoracic cavity **anteroposteriorly** (not dorsoventrally). Therefore, this statement is **incorrect**.

4. **Statement (iv)**: "The internal intercostals help in inspiration."
- **Analysis**: The internal intercostal muscles are primarily involved in **expiration**, as they help decrease the volume of the thoracic cavity. Therefore, this statement is also **incorrect**.

### Conclusion:
All four statements are incorrect regarding human respiration.

The correct answer to the question is that **none of the statements are true**.

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