Two layers, one thin film of fluid
The pleura is a smooth, double-layered membrane wrapped around each lung. The inner layer (visceral pleura) hugs the lung surface; the outer layer (parietal pleura) lines the inside of the chest wall, the diaphragm, and the mediastinum. Between them is the pleural space — normally a potential space, not an open cavity, holding only a few milliliters of lubricating fluid.
That thin film of fluid does two jobs. First, it lubricates, so the two layers slide over each other without friction as you breathe. Second, surface tension in the film makes the layers cling together like two wet glass slides — easy to slide, hard to pull apart. This clinging is what couples the lung to the chest wall.
Negative pressure holds the lung open
The lung always wants to collapse inward because of its elastic recoil; the chest wall wants to spring outward. These opposing pulls create a slight vacuum in the pleural space. This intrapleural pressure is negative relative to the atmosphere — roughly −5 cm H₂O at rest, becoming more negative during inspiration.
The pressure difference across the lung wall — the transpulmonary pressure — is what keeps the alveoli inflated. As long as the pleural space stays sealed and negative, the lung follows the chest wall outward on every breath. Break the seal, and the lung's recoil pulls it inward, away from the chest wall: that is collapse.
When the space speaks: pleuritic pain
The visceral pleura has no pain fibers, but the parietal pleura is richly innervated. When it becomes inflamed or rubs against an irritated surface, you feel sharp [[pleuritic-chest-pain|pleuritic chest pain]] — pain that stabs when you breathe in, cough, or move, and eases when you hold your breath. That breath-linked pattern is the classic fingerprint of pleural trouble.