Explain the role of the ribs, the internal and external intercostal muscles and the diaphragm in producing volume and pressure changes in the thorax, causing the movement of air into and out of the lungs (breathing)

The process of breathing, or ventilation, involves complex interactions between the ribs, intercostal muscles, and diaphragm, which work together to create changes in volume and pressure within the thoracic cavity. This mechanism facilitates the movement of air into and out of the lungs. Here’s a detailed explanation of how each component contributes to this process:

Role of the Ribs

• Structure: The ribs form a protective cage around the thoracic cavity and are attached to the spine at the back and the sternum at the front.
• Function: During inhalation, the ribs elevate and expand outward. This upward movement increases the thoracic cavity’s volume, which helps lower the pressure inside the thorax relative to atmospheric pressure, allowing air to flow into the lungs. Conversely, during exhalation, the ribs return to their resting position, decreasing thoracic volume and increasing pressure, which pushes air out of the lungs.

Role of External Intercostal Muscles

• Structure: These muscles are located between each rib and run diagonally from the lower edge of one rib to the upper edge of the rib below.
• Function: The external intercostal muscles contract during inhalation. This contraction lifts the ribs and expands the chest wall, further increasing thoracic volume. As a result, intrathoracic pressure decreases, facilitating air intake into the lungs. This action is often described as “bucket handle” movement due to how it raises the rib cage.

Role of Internal Intercostal Muscles

• Structure: Situated deeper than the external intercostals, these muscles run perpendicular to them.
• Function: The internal intercostal muscles are primarily involved in forced exhalation. When they contract, they pull the ribs downward and inward, reducing thoracic volume and increasing intrathoracic pressure. This action helps expel air from the lungs more forcefully during activities like exercise or coughing.

Role of the Diaphragm

• Structure: The diaphragm is a dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. It is attached to the lower ribs and spine.
• Function: The diaphragm is the primary muscle responsible for breathing. During inhalation, it contracts and flattens, moving downwards towards the abdominal cavity. This contraction increases thoracic volume significantly and creates a vacuum effect that pulls air into the lungs (reducing intrathoracic pressure). During exhalation, when the diaphragm relaxes, it returns to its dome shape, decreasing thoracic volume and pushing air out of the lungs.

Summary of Breathing Mechanics

1. Inhalation:
   • The diaphragm contracts and flattens.
   • External intercostal muscles contract to elevate ribs.
   • Thoracic volume increases; intrathoracic pressure decreases.
   • Air flows into lungs due to higher atmospheric pressure.
2. Exhalation:
   • The diaphragm relaxes and rises back into a dome shape.
   • Internal intercostal muscles contract (during forced exhalation) to depress ribs.
   • Thoracic volume decreases; intrathoracic pressure increases.
   • Air is expelled from lungs due to higher intrathoracic pressure compared to atmospheric pressure.