Intrapleural pressure

The lungs are a pair of air-filled organs that, like alveoli, tend to recoil due to the elastic fibers in their connective tissue and the surface tension of the film of fluid that lines the alveoli. On the other hand, the chest wall, which protects vital organs like the lungs, tends to expand, creating a negative pressure that will act as a suction for the lungs and will keep them inflated. That is how the intrapleural pressure prevents the lungs from recoiling freely and, consequently, collapsing. At the moment of birth, the intrapleural pressure is 0.

Figure 1. Illustration of the intrapleural cavity indicating its pressure.