What causes the intrapleural pressure to drop during inspiration?

During inspiration, the intrapleural pressure drops primarily due to the expansion of the thoracic cavity. When the diaphragm contracts and the intercostal muscles elevate the ribs, the volume of the thoracic cavity increases. This increase in volume leads to a decrease in intrapleural pressure because of the relationship defined by Boyle’s law, which states that pressure and volume are inversely related in a closed system. As the volume within the pleural cavity expands, it creates a more negative pressure relative to atmospheric pressure.

Increased elastic recoil strength of the lungs also plays a role, as lungs naturally resist expansion. However, it is the expansion of the thoracic cavity that primarily drives the decrease in intrapleural pressure. The correct answer highlights the interplay of increased volume and the dynamics of lung recoil, which together enable air to flow into the lungs during inspiration. This simultaneous increase in volume and maintenance of elastic tension allow for efficient air intake while establishing the necessary negative intrapleural pressure.