pleura

The pleura is the lung’s slippery double wrapping — two thin membranes, one hugging the lung surface and one lining the inside of the chest wall, with a whisper-thin film of fluid between them. Think of two sheets of cling film with a drop of water sandwiched in between, so they glide freely but cannot easily be pulled apart.

The inner layer (visceral pleura) coats the lung; the outer layer (parietal pleura) lines the chest wall, diaphragm and mediastinum. The thin film of pleural fluid lubricates their sliding during each breath, while surface tension makes the two layers cling together, so the lung is pulled open as the chest expands.

Trouble in this potential space is common. Fluid collecting there is a pleural effusion, air leaking in is a pneumothorax, and inflammation produces sharp, breath-catching pleuritic chest pain. Because the outer layer is pain-sensitive but the inner is not, pleural pain comes from irritation of the parietal surface.