2. Pleura

The pleura (or strictly speaking the pleurae, as there are two), is the serous membrane that invests the lungs as the visceral pleura, and lines the thoracic cavity as the parietal pleura. They extend from the dome of the pleura superiorly, to the diaphragm inferiorly. The pleural membranes of the left and right lungs do not connect with each other, being separated by the mediastinum.

Parietal pleura: this is continuous but for descriptive purposes is divided into separate parts. These are the cupola (cervical pleura), the costal pleura, the mediastinal pleura and the diaphragmatic pleura

Visceral pleura. The parietal pleura is reflected onto the lung, at which point it becomes the visceral pleura. It covers the entire surface of the lung, including the fissures which separate the lobes.

Pleural space. This is a potential space between the layers of the visceral and parietal pleura which contains around 10–20 ml of pleural fluid (it has been calculated as being 0.3 ml kg–1 body weight). This is produced by the serous membrane and is similar in composition to plasma apart from its much lower protein count. It allows movement between the chest wall and the lung.

Innervation: the visceral pleura receives autonomic innervation via the pulmonary plexus and like the visceral peritoneum is sensitive to stretch but not to pain or temperature. The parietal pleura is innervated by the intercostal nerves and by some branches of the phrenic nerve, which means that pain can be referred to the ipsilateral shoulder

Vascular supply: the visceral pleurae as well as the parenchyma of the lung are supplied by the internal thoracic arteries. The parietal pleurae are supplied by the intercostal arteries. There is rich lymphatic drainage.

pleura is vulnerable to damage from amongst other things trauma, infection and malignancy. There are a number of anaesthetic procedures that can also put it potentially at risk

Pleural effusion. The small volume of fluid in the pleural cavity is maintained by the balance of oncotic and hydrostatic pressure and by efficient lymphatic drainage. Increases in the normal volume of pleural fluid are always abnormal, and effusions are classified traditionally as transudative (low protein) exudative (high protein

Causes include congestive cardiac failure, hypoproteinaemia, cirrhosis of the liver (ascites can cross the diaphragm into the pleural cavity) and the nephrotic syndrome. These are essentially ultrafiltrates produced by alterations in the balance between the oncotic and hydrostatic pressures.

Exudative effusions: these are associated with inflammatory pathologies that include infection, primary and secondary malignancy (mesothelioma being the classic tumour of the pleural mesothelial cells) and autoimmune conditions such as rheumatoid arthritis. There may be increased capillary disruption, impaired lymphatic drainage and increased fluid production by the pleural membranes themselves. Exudative effusions can also occur after pulmonary embolism. (As a practical point, it requires the accumulation of 250 ml of fluid before an effusion is apparent on a chest X-ray