Which of the following statement is incorrect w.r.t the mechanism of breathing ?

Options

A. Movement of air into and out of the lungs is carried out by creating a pressure gradient between the
lungs and the atmosphere
B. Inspiration is initiated by contraction of diaphragm which increases the volume of thoracic chamber
C. The contraction of external intercostal muscles lifts up the ribs and the sternum causing decrease in
the volume of thoracic chamber
D. On an average, a healthy human breathes 12 - 16 times/minute.

Answer

To determine which statement is incorrect with respect to the mechanism of breathing, we can analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Breathing Mechanism**:
- Breathing involves the movement of air into and out of the lungs, which is facilitated by creating a pressure gradient between the lungs and the atmosphere.

2. **Evaluating Each Statement**:
- **Statement 1**: "The movement of air into and out of the lung is carried out by creating a pressure gradient between lungs and atmosphere."
- This statement is **correct**. Breathing relies on pressure differences to allow air to flow in and out of the lungs.

- **Statement 2**: "Inspiration is initiated by contraction of diaphragm which increases the volume of thoracic chamber."
- This statement is **correct**. The diaphragm contracts and flattens, which increases the volume of the thoracic cavity, allowing air to enter the lungs.

- **Statement 3**: "The contraction of external intercostal muscles lifts up the ribs and sternum causing a decrease in the volume of the thoracic chamber."
- This statement is **incorrect**. When the external intercostal muscles contract, they lift the ribs and sternum, which actually **increases** the volume of the thoracic chamber, not decreases it.

- **Statement 4**: "On an average, a healthy person breathes 12 to 16 times a minute."
- This statement is **correct**. The average respiratory rate for a healthy adult is indeed within this range.

3. **Identifying the Incorrect Statement**:
- Based on the evaluations, the **third statement** is incorrect because it incorrectly states that the contraction of the external intercostal muscles decreases the volume of the thoracic chamber.

### Final Answer:
The incorrect statement with respect to the mechanism of breathing is: **"The contraction of external intercostal muscles lifts up the ribs and sternum causing a decrease in the volume of the thoracic chamber."**

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