Final answer:
Intrapleural pressure must remain at approximately −4 mm Hg, a negative pressure relative to intra-alveolar pressure, to allow the lungs to inflate properly and facilitate normal breathing.
Step-by-step explanation:
In order for the lungs to function normally, the intrapleural pressure must remain negative relative to the intra-alveolar pressure. This negative pressure is typically around −4 mm Hg, and it is essential for the lungs to inflate properly during inspiration. The intrapleural pressure is influenced by the elasticity of the lung tissue, which pulls the lungs inward, and the surface tension in the alveolar fluid that also creates an inward pull. Counteracting these forces, the elasticity of the chest wall and the surface tension in the pleural cavity work to pull the lungs outward. The balance between these forces ensures that the intrapleural pressure remains negative, which is critical for normal lung function and the overall process of ventilation.