Final answer:
When thoracic volume decreases during expiration, alveolar pressure increases, which creates a pressure gradient that expels air from the lungs. This process is passive, relying on the elasticity of the lung tissue and the decrease in thoracic volume to raise the intrapulmonary pressure.
Step-by-step explanation:
When thoracic volume decreases during expiration, the diaphragm and intercostal muscles relax, causing the lung volume to decrease as well. According to Boyle's Law, which states that the pressure of a given mass of gas is inversely proportional to its volume (assuming constant temperature), this decrease in lung volume leads to an increase in alveolar pressure. Consequently, alveolar pressure becomes greater than atmospheric pressure, creating a pressure gradient that facilitates the expulsion of air from the lungs.
It's worth noting that the process of expiration is mainly passive, depending on the elasticity of the lung tissue. No energy is required for this process; air is simply pushed out of the lungs by the rise in intrapulmonary pressure as the thoracic cavity volume decreases.