The Pleurae refer to the serous membranes that line the lungs and thoracic cavity, allowing respiration to occur in an efficient and effortless manner. This article will outline the structure and function of the pleurae, as well as considering the clinical correlations.
There are two pleurae in the body, one associated with each lung. Each pleura consists of a serous membrane, which is supported by connective tissue and is known as the mesothelium. This serous membrane is composed of a layer of simple squamous cells. The two pleurae can be divided into two further parts, namely; the Visceral pleura, which covers the lungs, and the Parietal pleura, which covers the internal surface of the thoracic cavity.
The two parts of the pleurae are continuous with each other at the hilum of each lung, forming a potential space between the viscera and parietal pleura, known as the Pleural Cavity. This small space houses a small volume of serous fluid, which has two main functions; to lubricate the surfaces of the pleurae, allowing them to slide over each other, and to produce a surface tension that pulls the parietal and visceral pleura together, ensuring that the lung expands as the thorax does. This surface tension is lost if air enters the pleural cavity, a condition known as a pneumothorax.
Anteriorly and posteroinferiorly, the pleural cavity is not completely filled by the lungs, creating two recesses in each cavity; the Costodiaphragmatic, located between the costal pleurae and the diaphragmatic pleura, and the Costomediastinal, located between the costal pleurae and the mediastinal pleurae, behind the sternum. These recesses provide a space for fluid to collect, such as in a pleural effusion, and are therefore of clinical importance.
The parietal and visceral pleura both receive a different neurovascular supply; the parietal pleura is sensitive to pressure, pain, and temperature, produces a well localised pain, and is innervated by the phrenic and intercostal nerves, with its blood supply being derived from the intercostal arteries. The visceral pleura, meanwhile, is only sensitive to stretch, and receives autonomic innervation from the pulmonary plexus as well as arterial supply via the bronchial arteries.
Pneumothorax is a condition that occurs when air or gas is present within the pleural space. It is commonly referred to as a collapsed lung, and can occur if air enters the pleural cavity, leading to a loss of the surface tension between the parietal and visceral pleura.
In conclusion, the pleurae perform an essential role in respiration, and the structure and function of these serous membranes is highly complex. In addition, the clinical correlations of the pleurae can provide important insight into the diagnosis and management of a number of conditions.
The pleurae are two thin membranes which line the thoracic cavity and cover the lungs. They are continuous with each other at the hilum of each lung, and the potential space between the viscera and parietal pleura is known as the pleural cavity. We shall consider the structures of the pleurae in more detail.
The parietal pleura covers the inner surface of the thoracic cavity. It is thicker than the visceral pleura, and can be subdivided according to the part of the body that it is in contact with:
The visceral pleura covers the outer surface of the lungs, and is continuous with the parietal pleura at the hilum. It also extends into the interlobar fissures.
The pleural cavity, which is a potential space between the parietal and visceral pleura, contains a small volume of serous fluid. This fluid has two main functions: to lubricate the surfaces of the pleurae so they can slide over each other, and to create a surface tension which pulls the parietal and visceral pleura together. This ensures that when the thorax expands, the lung expands too and fills with air.
Anteriorly and posteroinferiorly, the pleural cavity is not completely filled by the lungs. This gives rise to recesses, where the opposing surfaces of the parietal pleura touch. These are the costodiaphragmatic recess (between the costal pleurae and the diaphragmatic pleura) and the costomediastinal recess (between the costal pleurae and the mediastinal pleurae, behind the sternum).
The two parts of the pleurae receive a different neurovascular supply:
A pneumothorax (commonly referred to as a collapsed lung) occurs when air or gas is present within the pleural space. This removes the surface tension of the serous fluid and reduces lung extension. Symptoms include chest pain and shortness of breath, and upon percussion, the affected side may be hyper-resonant. It may be classified as spontaneous (no underlying respiratory disease) or traumatic (caused by blunt or penetrating chest trauma, such as a rib fracture). Treatment depends on identifying the underlying cause.
Small primary pneumothoraces generally require minimal intervention, whereas secondary and traumatic pneumothoraces may require decompression to remove the extra air/gas so the lung can reinflate. This is achieved by inserting a chest drain.
