What mechanism allows the lungs to expand during inhalation?

The expansion of the lungs during inhalation is primarily facilitated by the contraction of the diaphragm and the resulting negative pleural pressure. When the diaphragm contracts, it flattens and moves downward, creating a larger thoracic cavity volume. This increase in volume leads to a decrease in intrathoracic pressure (the pressure in the pleural cavity), allowing air to flow into the lungs.

This process is crucial for adequate ventilation, as the difference in pressure between the atmosphere and the pressures within the thoracic cavity encourages air to rush into the lungs. The relationship between diaphragm movement and pressure changes is fundamental to respiratory mechanics, making this option the correct answer.