Which action among the following will increase the volume of the thoracic cage?

• A. Contraction of the internal intercostal muscles
• B. Elevation of the arms over one's head
• C. Contraction of the abdominal muscles
• D. Alveolar pressure increasing above atmospheric pressure

Answer: (B)

The action of elevating the arms over one’s head contributes to increasing the volume of the thoracic cage because it involves the upward movement of the rib cage, which expands the space within the thoracic cavity. When the arms are raised, this can cause the shoulder girdle to move, leading to an upward and outward motion of the ribs. This movement helps facilitate inhalation by reducing pressure in the thoracic cavity, allowing fresh air to flow into the lungs.

In contrast, contraction of the internal intercostal muscles primarily assists in expiration, helping to pull the rib cage down and inward, thereby decreasing the thoracic cavity volume. The contraction of abdominal muscles also tends to expel air from the lungs, reducing thoracic volume as they push the diaphragm upward. An increase in alveolar pressure above atmospheric pressure indicates that air is being pushed out of the lungs during expiration, further decreasing the volume of the thoracic cavity. Therefore, the action of lifting the arms is the only choice that leads to an increase in the thoracic cage volume, facilitating improved ventilation.