Final answer:
The rib cage returns to its original position as a result of the elastic recoil of the lungs when the inspiratory muscles relax.
Step-by-step explanation:
When the inspiratory muscles relax, the rib cage returns to its original position as a result of elastic recoil of the lungs. The inspiratory muscles, including the intercostal muscles and the diaphragm, during the inspiration phase, contract to expand the chest cavity, reducing pressure inside the lungs and allowing air to enter. As they relax during exhalation, the chest wall and diaphragm return to their resting positions because of the lungs' elastic nature. This part of the breathing cycle is passive; no energy is required as no muscles actively contract to expel the air. The high surface tension in the alveoli helps push the air out, assisted by surfactant, which reduces surface tension but does not directly cause the lungs to recoil.