Which of the following best describes the action of the external intercostal muscles during inhalation?

The action of the external intercostal muscles during inhalation is best described by their role in expanding the thoracic cavity. When these muscles contract, they pull the rib cage upward and outward. This movement increases the volume of the thoracic cavity, which in turn lowers the pressure within the cavity, allowing air to flow into the lungs.

This process is crucial for effective breathing, as the expansion of the thoracic cavity enables a negative pressure gradient that draws air in. In addition, the contraction of the external intercostals works in conjunction with the diaphragm, which also contracts and moves downward during inhalation to further assist in this expansion.

This physiological mechanism is fundamental to understanding how breathing works, as inhalation relies on the active contraction of certain muscles to create the necessary conditions for air to enter the lungs efficiently.