Each lung is surrounded by a double membrane called a pleura. The outer membrane is the parietal pleura, which is attached to the thoracic (chest) wall and contains nerve receptors that detect pain. The inner membrane is the visceral pleura, which adheres to the lung and covers the lung fissures, hilar bronchi and blood vessels (Figure 12.21) (Tortora and Derrickson [272]).

The pleurae help to maintain the negative pressure required to prevent the lung from collapsing. Before inspiration, the pressure within the pleural space is equal to −5 cmH₂O. During inspiration, there is a decrease in intrapleural pressure due to the diaphragm being drawn down and the outward expansion of the chest. The pressure falls to −7.5 cmH₂O, which causes air to be drawn in for gaseous exchange. During expiration, the lung returns to its pre‐inspiratory state by elastic recoil, with collapse of the chest wall and diaphragm and the exhalation of gas. The intrapleural pressure subsequently rises to −4 cmH₂O (Noorani and Abu‐Omar [201]). If either of the pleura is damaged, partial or total lung collapse will occur due to loss of the normally negative pressure (Woodrow [290]).

The space that exists between the parietal and visceral pleurae is commonly known as the pleural space, pleural cavity or potential space. While approximately 1 to 2 L of serous fluid moves across the thin and porous pleural membranes each day, only 3–5 mL of fluid actually fills the space. The serous fluid allows the pleurae to move in unison with the chest wall on inspiration and enables the membranes to be held closely together by surface tension forces (George and Papagiannopoulos [88]). The amount of fluid present in the pleural space is maintained by the oncotic and hydrostatic pressures across the pleurae and lymphatic drainage network (Chadwick et al. [42]). Disruption in this balance results in an accumulation of fluid within the pleural space.