pleura

The pleura is the membrane that wraps each lung and lines the chest cavity it sits in. It has two layers: the visceral pleura lies on the lung itself, and the parietal pleura lines the inside of the chest wall and the diaphragm. Between them is the pleural cavity, a potential space that holds only a thin film of fluid. That film does two things — it lets the two surfaces slide over each other as the lung moves, and it holds them together, so when the chest wall moves out the lung is pulled with it. That pull is the reason a lung follows the ribs at all. If air gets into the cavity the link is broken and the lung collapses; if fluid collects, the same space fills and the lung cannot expand. The pericardium is the same design around the heart.