Final answer:
During inspiration, the inspiratory muscles contract, causing the rib cage to expand, which increases the volume and decreases the pressure within the pleural cavities. Consequently, air is drawn into the lungs. The diaphragm's contraction and flattening enhance this volume change and pressure decrease.
Step-by-step explanation:
During inspiration, the inspiratory muscles of the rib cage contract, causing the rib cage to expand and move outward. This expansion is due to the contraction of the external intercostal muscles which raise the rib cage, thereby increasing its volume. As the volume inside the pleural cavities increases, the pressure within them decreases, leading to a pressure gradient compared to the outside atmosphere. As a result, air will flow into the lungs to equalize the pressure.
The diaphragm also plays a crucial role in this process. As it contracts, it flattens and allows for a greater increase in the thoracic cavity volume. This action contributes further to the reduction in pressure within the pleural cavities and aids in drawing air into the lungs for gas exchange.