CHAPTER 1
Anatomy and Physiology of the Pleura

John P. Corcoran and Najib M. Rahman

Oxford Centre for Respiratory Medicine, Churchill Hospital, Oxford, UK

OVERVIEW

• The pleural space is a real rather than potential space, containing a small amount (<20 mL) of pleural fluid.
• Mesothelial cells line the visceral and parietal pleura, with size and shape varying according to position. They are metabolically active and can perform a variety of functions.
• The parietal pleura is innervated whereas the visceral pleura has no nerve supply (and hence does not produce pain in pathological conditions).
• The pleural space is normally under negative pressure.
• Pleural fluid is secreted from the systemic vessels of the parietal pleura, and is drained through lymphatic channels in the parietal pleura. The normal drainage capacity is very large compared to the secretion capacity.

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.

Microscopic anatomy

The pleura is composed of a monolayer of mesothelial cells overlying layers of connective tissue; its precise structure varies between visceral and parietal pleura and according to anatomical position. Up to five layers can be identified histologically, consisting of the mesothelial cellular surface and four subcellular layers (basal lamina and thin connective tissue; thin superficial elastic tissue; loose connective tissue containing nerves, vessels and lymphatics; and a deep fibroelastic layer, often fused to the underlying tissue). These subcellular layers tend to be better defined when overlying looser substructures such as the mediastinum, than rigid tissue such as ribs or intercostal muscle. The parietal pleura is approximately 30 micrometres thick and overlies the deeper endothoracic fascia. The visceral pleura is between 30 and 80 micrometres thick with denser connective tissue layers that both contribute to elastic recoil of the lungs during expiration, and protect the lungs during inspiration by limiting their volume and expansion.

Mesothelial cells

The mesothelial cells lining the visceral and parietal pleura are the predominant cell type within the pleural cavity, forming an active multipotent layer capable of sensing and responding to external stimuli. Mesothelial cells dislodged from the pleural surface to float freely within the fluid‐filled space can transform into macrophages with immunological roles; whilst various studies have also proven them capable of producing growth factors, extracellular matrix proteins and a range of cytokines. They are metabolically active and have both secretory and absorptive roles, with electron microscopy demonstrating abundant pinocytic vesicles, polyribosomes and mitochondria amongst their intracellular structures. Injury or disruption of the monolayer is repaired through mitosis and migration of adjoining cells or incorporation of free‐floating mesothelial cells from pleural fluid.

Their size, shape and surface structure vary subtly according to location within the pleural space, although no major cytological differences have been found between mesothelial cells of pleural, pericardial or peritoneal origin. Each cell has a carpet of microvilli at the pleural surface whose precise role is still unknown; however, the density of microvilli is greatest in the inferior parts of the thorax, and greater in visceral than parietal pleura at corresponding levels. Parietal mesothelial cells in the apices are flatter with fewer microvilli; whilst basally the cells are cuboidal, more numerous per unit area and have a higher density of microvilli. These adaptations may relate to variable lung and chest wall movement at different thoracic levels.

Innervation, blood supply and lymphatics

Innervation

The visceral pleura is innervated by the vagal and sympathetic trunks which do not have pain fibres. Only the parietal pleura contains pain fibres, with pleuritic pain consequently implying involvement of this surface. It is innervated by intercostal nerves and refers pain to the corresponding area of the chest wall; with the exception of the diaphragm which being supplied by the phrenic nerve refers pain to the ipsilateral shoulder.

Blood supply

The parietal pleura is supplied by systemic capillaries according to anatomical location. (Figure 1.1) These originate from the intercostal and internal mammary arteries for the costal pleura; pericardiophrenic artery for the mediastinal pleura; subclavian arteries for the cervical pleura; and superior phrenic and musculophrenic arteries for the diaphragmatic pleura. Venous drainage follows arterial supply into either the superior or inferior vena cava depending on location.

Figure 1.1 CT angiogram demonstrating the course and variability of the intercostal arteries posteriorly.

The arterial supply of the visceral pleura is somewhat controversial. It is generally agreed that the bronchial arteries supply the majority of the visceral pleura, although supply of the lung apex and its convex surface is debated and may involve the pulmonary circulation. Venous drainage occurs via the pulmonary veins.

Lymphatics

The visceral pleural lymphatics constitute a superficial network of interconnecting vessels over the surface of and through the lung along the interlobular septae. Lymph flows via the bronchovascular bundles towards the lung hilae, with a greater density of lymphatics in dependent areas of lung with higher intravascular pressures.

Lymphatic plexuses are found in the parietal pleura overlying intercostal spaces, mediastinum and the diaphragm, but are essentially absent over the ribs. The costal pleura drains anteriorly into internal mammary...