Mark the correct pair of muscles involved in the normal breathing in humans.

Options

A. External and internal intercostal mucles
B. Diaphragm and abdominal muscles
C. Diaphragm and external intercostal muscles
D. Diaphragm and intercostal muscles

Explanation

To answer the question regarding the correct pair of muscles involved in normal breathing in humans, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Breathing Mechanism**:
- Breathing consists of two main processes: inhalation (inspiration) and exhalation (expiration).
- Inhalation is when air enters the lungs, and exhalation is when air leaves the lungs.

2. **Role of Muscles in Breathing**:
- The diaphragm is a dome-shaped muscle located below the lungs. It plays a crucial role in the breathing process.
- Intercostal muscles, which include both external and internal intercostal muscles, are located between the ribs and assist in expanding and contracting the thoracic cavity during breathing.

3. **Pressure Gradient**:
- Breathing is controlled by pressure gradients. During inhalation, the pressure inside the lungs becomes negative relative to the outside atmosphere, allowing air to flow in. During exhalation, the pressure becomes positive, pushing air out.

4. **Identifying the Correct Pair**:
- The diaphragm is essential for both inhalation and exhalation.
- The intercostal muscles (both external and internal) also play a significant role in the mechanics of breathing.
- Therefore, the correct pair of muscles involved in normal breathing is the diaphragm and the intercostal muscles.

5. **Conclusion**:
- The correct answer is: **Diaphragm and Intercostal Muscles (External and Internal)**.