Final answer:
During inhalation, the contraction of the diaphragm and external intercostal muscles increases the volume of the thoracic cavity, resulting in a decrease in pressure. This allows air to flow into the lungs.
Step-by-step explanation:
During inhalation, when the diaphragm and external intercostal muscles contract, the volume of the thoracic cavity increases. This expansion of the thoracic cavity leads to a decrease in pressure according to Boyle's Law. The decrease in pressure inside the thoracic cavity compared to the atmosphere causes air to rush into the respiratory passages and into the lungs, increasing their volume.