The pleural cavity is a real rather than potential space, containing a thin layer of fluid and lined with a double‐layered membrane covering the thoracic cavity (parietal pleura) and outer lung surface (visceral pleura) whose precise purpose and structure are incompletely understood. The gaps in our knowledge are best illustrated by the unexplained anatomical variations among different mammals. In humans, the left and right pleural cavities are separated by the mediastinum, but in species as diverse as the mouse and bison there is a single pleural cavity, allowing free communication of fluid and air between right and left. The elephant has evolved to have no cavity at all – instead having loose connective tissue between the two pleural membranes. In time, it may be that describing how and why these differences have evolved will help us to understand the role the pleural cavity has in humans. This chapter focuses on the key features of human pleural anatomy and physiology.

Embryology

The human body contains three mesothelium‐lined cavities – two large (pleural, peritoneal) and one small (pericardial) – derived from a continuous mesodermal structure called the intra‐embryonic coelom as it is partitioned at 4–7 weeks’ gestation. Arising from a medially placed foregut structure that will ultimately form the mediastinum, primordial lung buds grow out into the laterally placed pericardio‐peritoneal canals, taking a layer of lining mesothelium that will become the visceral pleura in the process. As the lungs rapidly enlarge, they enclose the heart and widen the pericardio‐peritoneal canals to form the pleural cavities. These are separated from the pericardial space by the pleuro‐pericardial membranes, whilst the septum transversum (an early partial diaphragm) joins the pleuro‐peritoneal membranes to partition each pleural cavity from the peritoneal space. The mesothelium lining the pericardio‐peritoneal canals as they become the pleural cavities goes on to form the parietal pleura.

Macroscopic anatomy

The pleura is a double‐layered serous membrane overlying the inner surface of the thoracic cage (diaphragm, mediastinum and rib cage) and outer surface of the lung, with an estimated total area of 2000 cm² in the average adult male. Between lies the pleural cavity, a sealed space maintained 10–20 micrometres across and filled with a thin layer of fluid to maintain apposition and provide lubrication during respiratory movement. The left and right pleural cavities are completely separated by the mediastinum.

The visceral pleura is tightly adherent to the entire lung surface, not only where it is in contact with chest wall, mediastinum and diaphragm, but also into the interlobar fissures. The parietal pleura is subdivided into four sections according to the associated intrathoracic structures: costal (overlying ribs, intercostal muscles, costal cartilage and sternum); cervical (extending above the first rib over the medial end of the clavicle); mediastinal; and diaphragmatic. Inferiorly, the parietal pleura mirrors the lower border of the thoracic cage but may extend beyond the costal margin, notably at the right lower sternal edge and posterior costovertebral junctions.

The visceral and parietal pleura meet at the lung hilae, through which the major airways and pulmonary vessels pass. Posteriorly, where a double layer of parietal pleura has been pulled into the thoracic cavity during lung development, lie the pulmonary ligaments extending from hilum to diaphragm bilaterally. These are thought to prevent torsion of the lower lobes, and are important intra‐operatively as they may contain vessels, lymphatics or tumour.