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We need to identify the process when the diaphragm relaxes, moving up into the thoracic cavity.

Expiration, or the exhalation phase of breathing, happens when the diaphragm relaxes and returns to its resting position in the thoracic cavity. The thoracic cavity's volume shrinks, which squeezes the lungs and reduces their volume. Because the pressure in the lungs is now higher than the pressure in the environment, air is forced out by expiration.

We need to identify the process when volume of lungs expands.

According to Boyle's law, as the volume of the lungs increases, the pressure inside the lungs drops, eventually falling below the atmospheric pressure. The lungs and the atmosphere are separated by a pressure gradient caused by the difference in pressures. Molecules flow from a higher pressure location to a lower pressure area in a pressure gradient. Inspiration occurs when air rushes into the lungs during the inhalation phase of breathing.

We need to identify the process the pressure within the lungs is higher than that of the atmosphere.

Expiration, or the exhalation phase of breathing, happens when the diaphragm relaxes and returns to its resting position in the thoracic cavity. The thoracic cavity's volume shrinks, which squeezes the lungs and reduces their volume. Because the pressure in the lungs is now higher than the pressure in the environment, air is forced out by expiration.