What happens with the external intercostals during thoracic breathing?

During thoracic breathing, the external intercostal muscles play a crucial role in the mechanics of respiration. When these muscles contract, they lift the rib cage upward and outward. This action increases the volume of the thoracic cavity, thereby facilitating the influx of air into the lungs during the inspiratory phase of breathing.

This lifting of the rib cage is essential for effective inhalation as it helps to expand the lung volume, allowing air to flow in more easily. The coordinated contraction of the external intercostals, along with the diaphragm, contributes to the efficient movement of air into the lungs.

Other options do not accurately describe the role of the external intercostals during thoracic breathing. For instance, the complete relaxation of these muscles, their contraction during expiration, or pressing the diaphragm down do not align with their primary function of assisting with the inhalation process. Hence, the correct understanding emphasizes the lifting of the rib cage during inspiration facilitated by the contraction of the external intercostals.