Which of the following factors contributes to the increase of the pulmonary volume ?

Options

A. Relaxation of the external intercostal muscles
B. Relaxation of the internal intercostal muscles
C. Contraction of the internal intercostal muscles
D. Relaxation of the diaphragm

Answer

To determine which factor contributes to the increase of pulmonary volume, we need to analyze the role of various muscles involved in the breathing process. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Role of Intercostal Muscles
- The intercostal muscles are located between the ribs and are crucial for the mechanics of breathing. There are two types:
- **External Intercostal Muscles (EICM)**: These muscles are located on the outer side of the ribs.
- **Internal Intercostal Muscles (IICM)**: These muscles are located on the inner side of the ribs.

**Hint**: Remember that the external intercostal muscles are involved in inhalation, while the internal intercostal muscles are primarily involved in exhalation.

### Step 2: Analyze the Contraction of External Intercostal Muscles
- When the EICM contract, they pull the ribs and sternum upward and outward, which increases the thoracic volume.
- An increase in thoracic volume leads to an expansion of the lungs, which in turn increases pulmonary volume.

**Hint**: Think about how expanding the chest cavity allows more air to fill the lungs.

### Step 3: Analyze the Relaxation of External Intercostal Muscles
- If the EICM relax, the opposite occurs. The ribs and sternum move downward and inward, which decreases thoracic volume and thus decreases pulmonary volume.

**Hint**: Relaxation of muscles typically leads to a decrease in the volume they help create.

### Step 4: Analyze the Internal Intercostal Muscles
- When the IICM contract, they pull the ribs downward and inward, decreasing thoracic volume and pulmonary volume.
- If the IICM relax, the thoracic volume can increase, which can lead to an increase in pulmonary volume.

**Hint**: Consider how the internal intercostal muscles assist in forced expiration.

### Step 5: Analyze the Diaphragm
- The diaphragm is a dome-shaped muscle that, when contracted, moves downward into the abdominal cavity, increasing thoracic volume and consequently increasing pulmonary volume.
- If the diaphragm relaxes, it returns to its dome shape, decreasing thoracic volume and thus decreasing pulmonary volume.

**Hint**: The diaphragm plays a significant role in breathing; its contraction is essential for inhalation.

### Step 6: Evaluate the Options
1. **Relaxation of External Intercostal Muscles**: This would decrease pulmonary volume.
2. **Relaxation of Internal Intercostal Muscles**: This can lead to an increase in pulmonary volume.
3. **Contraction of Internal Intercostal Muscles**: This decreases pulmonary volume.
4. **Relaxation of the Diaphragm**: This decreases pulmonary volume.

### Conclusion
The factor that contributes to the increase of pulmonary volume is the **relaxation of internal intercostal muscles**, as it allows for an increase in thoracic volume, leading to an increase in pulmonary volume.

**Final Answer**: The correct option is **relaxation of internal intercostal muscles**.