Intrapleural cavity
The space between the visceral pleura, which is attached to the external surface of the lungs, and the parietal pleura, connected to the chest wall and ribs. Both types of pleura are made out of the same membrane, which folds in on itself, forming a space between two layers. The intrapleural cavity is crucial to the breathing processes. It enables the lungs to expand and recoil depending on the stage of breathing.
Figure 1. Illustration of the pleural cavity indicating its pressure.