What occurs to the ribs and chest when the internal intercostal muscles contract during expiration?

• A. They elevate to increase thoracic volume
• B. They expand the thorax
• C. They are pulled down to decrease thoracic volume
• D. They remain unchanged

Answer: (C)

When the internal intercostal muscles contract during expiration, they pull the ribs downwards, which decreases the thoracic volume. This action is crucial for the process of expiration, as reducing the volume of the thoracic cavity leads to an increase in pressure within the lungs, facilitating the expulsion of air from the respiratory system.

The contraction of these muscles works in opposition to the external intercostal muscles, which are responsible for elevating the ribs during inhalation to expand the thoracic cavity. Therefore, during expiration, the internal intercostal muscles play a vital role in actively compressing the thorax, supporting the exit of air from the lungs. This process is essential for effective breathing and overall respiratory function.