Final answer:
After a very deep exhalation, the diaphragm and intercostal muscles relax, causing the volume of the thoracic cavity to decrease and the air pressure inside the lungs to increase. This results in air being forced out of the lungs.
Step-by-step explanation:
After a very deep exhalation, the diaphragm relaxes and moves upward, while the intercostal muscles also relax. This causes the volume of the thoracic cavity to decrease and the air pressure inside the lungs to increase. As a result, air is forced out of the lungs and expelled into the atmosphere. Exhalation is typically a passive process that occurs mainly due to the elasticity of the lungs.