intrapleural pressure
Intrapleural pressure is the pressure in the thin, sealed space between the lung and the chest wall — and it is normally slightly negative, meaning lower than the air pressure inside the lungs. That gentle suction is what holds the lung pressed open against the chest wall.
The negativity arises from a standoff: the lung constantly tries to recoil inward while the chest wall springs outward, and because the pleural space is sealed and almost empty, these opposing pulls create a vacuum-like sub-atmospheric pressure between them, roughly minus 5 centimetres of water at rest.
It becomes more negative during inspiration as the chest enlarges, deepening the suction that pulls the lung open wider, and returns toward less negative values on expiration. This rhythmic swing is what couples chest-wall movement to lung inflation.
If the seal is broken — a punctured chest wall or a leaking lung lets air rush into the pleural space (pneumothorax) — the negative pressure is lost, the suction holding the lung open vanishes, and the lung recoils and collapses. Restoring the negative pressure, often with a chest tube, lets it re-expand.
In a tension pneumothorax, air keeps leaking into the pleural space with each breath but cannot escape, so the pressure turns positive, crushing the lung and even pushing on the heart — a true emergency.
Lost negative pressure collapses the lung.
The negative intrapleural pressure is what links the chest wall and lung into one moving unit. When the diaphragm pulls the chest open, that suction drags the lung along with it — break the seal and the coupling, and breathing, fail.