Final answer:
During exhalation, the rib cage becomes smaller and the thoracic cavity's volume decreases, which increases air pressure inside the lungs, causing air to flow out.
Step-by-step explanation:
When the rib cage becomes smaller during respiration, air will flow out of the lungs. This process is known as exhalation, and it involves the diaphragm and intercostal muscles relaxing, causing the thoracic cavity to decrease in volume. As the thoracic cavity becomes smaller, the pressure inside the lungs increases relative to the pressure outside the body. According to Boyle's Law, this change in volume and pressure causes air to rush out from the lungs to the atmosphere. Thus, breathing is a mechanical process in which the diaphragm and intercostal muscles play a crucial role in changing the volume and pressure within the pleural cavities, allowing the inflow and outflow of air during the respiratory cycle.