The body creates a difference between atmospheric and intrapulmonary pressures to facilitate airflow into and out of the lungs through the process of ventilation, which involves the mechanics of breathing. This process is driven by the contraction and relaxation of specific respiratory muscles, leading to changes in the volume and pressure within the thoracic cavity.

### Inhalation (Inspiration)

1. **Muscles Involved**:

   - **Diaphragm**: The primary muscle involved. During inhalation, the diaphragm contracts and flattens, moving downward and increasing the vertical dimension of the thoracic cavity.

   - **External Intercostal Muscles**: These muscles contract, lifting the ribs upward and outward, which increases the lateral and anteroposterior dimensions of the thoracic cavity.

   

2. **Mechanism**:

   - The contraction of the diaphragm and external intercostal muscles expands the thoracic cavity.

   - This expansion decreases the intrapulmonary (alveolar) pressure relative to atmospheric pressure.

   - Air flows into the lungs from the higher-pressure atmosphere to the lower-pressure alveoli.

### Exhalation (Expiration)

1. **Muscles Involved**:

   - **Relaxation of the Diaphragm and External Intercostal Muscles**: During normal, passive exhalation, these muscles relax, causing the diaphragm to move upward and the ribs to move downward and inward.

   - **Internal Intercostal Muscles and Abdominal Muscles**: These muscles can be recruited during forceful exhalation, such as during exercise or coughing, to actively decrease the thoracic cavity volume further.

2. **Mechanism**:

   - The relaxation of the diaphragm and external intercostal muscles decreases the thoracic cavity volume.

   - This increases the intrapulmonary pressure relative to atmospheric pressure.

   - Air flows out of the lungs from the higher-pressure alveoli to the lower-pressure atmosphere.

### Summary of Pressure Changes

- **Inhalation**: The expansion of the thoracic cavity reduces intrapulmonary pressure below atmospheric pressure, causing air to flow into the lungs.

- **Exhalation**: The reduction in thoracic cavity volume increases intrapulmonary pressure above atmospheric pressure, causing air to flow out of the lungs.

### Additional Muscles (during forced breathing)

- **Sternocleidomastoid and Scalenes**: Assist in elevating the rib cage during deep or forced inhalation.

- **Abdominal Muscles (Rectus Abdominis, Transversus Abdominis, Internal and External Obliques)**: Contract to push the diaphragm up more forcefully during forced exhalation.

In summary, the coordination of these muscles and the resulting changes in thoracic cavity volume and pressure facilitate the flow of air into and out of the lungs, ensuring efficient gas exchange.