Final answer:
During inspiration, the pressure inside the lungs is lower than the pressure outside, allowing air to flow in. During expiration, the pressure inside the lungs is higher than the pressure outside, causing air to flow out.
Step-by-step explanation:
During inspiration, the pressure inside the lungs is lower than the pressure outside. This is because the diaphragm and intercostal muscles contract, increasing the volume of the thoracic cavity and decreasing the pressure inside the lungs. As a result, air flows into the lungs. On the other hand, during expiration, the pressure inside the lungs is higher than the pressure outside. This is due to the relaxation of the diaphragm and intercostal muscles, which decreases the volume of the thoracic cavity and increases the pressure inside the lungs. As a result, air flows out of the lungs.