Which muscle is primarily responsible for changing the pressure in the thoracic cavity during breathing?

The diaphragm is the primary muscle responsible for changing the pressure in the thoracic cavity during breathing. When the diaphragm contracts, it moves downwards, increasing the volume of the thoracic cavity. This increase in volume leads to a decrease in pressure in the thoracic cavity, allowing air to flow into the lungs. As the diaphragm relaxes, the volume of the thoracic cavity decreases, increasing the pressure and causing air to be expelled from the lungs.

The intercostal muscles support this process by aiding in the expansion and contraction of the rib cage but are not the primary muscle for changing thoracic cavity pressure. Meanwhile, the sternocleidomastoid helps with accessory breathing, primarily during strenuous activity and is not a major player in regular respiration. The abdominal muscles assist with forced expiration and support movement during exhalation but are not primarily responsible for pressure changes during the inhalation process. Thus, the diaphragm is uniquely suited as the main driver of respiratory pressure changes.