Final answer:
During expiration, the diaphragm and intercostals relax, causing the thoracic cavity to decrease in volume, which increases intrathoracic pressure and causes air to flow out of the lungs passively.
Step-by-step explanation:
The actions of the diaphragm, intercostals, the rib cage, and the changes in intrathoracic and intrapleural pressures all contribute to the movement of air during expiration. When we exhale, the diaphragm relaxes and moves upward into the chest cavity, which compresses the lungs and increases the pressure inside the thorax. This causes internal pressure to rise above external atmospheric pressure. The intercostals relax along with the diaphragm, aiding in reducing the volume within the thoracic cavity. As a result, air flows out of the lungs, similar to deflating a balloon because the intrathoracic pressure is higher than the outside atmospheric pressure. During a forced expiration, muscles like the internal intercostals and abdominal muscles work harder to push air out rapidly.