Final answer:
During the phase of ventilation called inspiration, contraction of respiratory muscles cause an increase in the volume of the thoracic cavity.
Step-by-step explanation:
Inspiration is an active process in which the diaphragm and external intercostal muscles contract. The diaphragm moves downward, and the ribcage expands as the external intercostal muscles lift the ribs.
This expansion of the thoracic cavity decreases the intra-alveolar pressure, creating a pressure gradient that allows air to flow into the lungs. The opposite phase, "expiration" or "expiration at rest," involves the relaxation of these muscles, leading to a decrease in thoracic volume, an increase in intra-alveolar pressure, and the expulsion of air from the lungs.