Each lung is enclosed within a serous membrane named the pleura. It is a Greek word which means side. The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum.
The pleurae consist of two layers.
- The visceral pleura is the layer that is superficial to the lungs, and extends into and lines the lung fissures.
- In contrast, the parietal pleura is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm.
The visceral and parietal pleura are continuous at the hilum of the lung via the pulmonary ligament.
The pulmonary ligament hangs inferiorly from the hilum of each lung as a double fold of pleura and creates an empty space that allows for the expansion of vessels in the lung hilum as the diaphragm descends with inspiration.
The pleural cavity is the space between the visceral and parietal layers. The pleurae perform two major functions:
- They produce pleural fluid and create cavities that separate the major organs. Pleural fluid is secreted by mesothelial cells from both pleural layers and acts to lubricate their surfaces. This lubrication reduces friction between the two layers to prevent trauma during breathing, and creates surface tension that helps maintain the position of the lungs against the thoracic wall. This adhesive characteristic of the pleural fluid causes the lungs to enlarge when the thoracic wall expands during ventilation, allowing the lungs to fill with air.
- They create a division between major organs that prevents interference due to the movement of the organs, while preventing the spread of infection.
The parietal pleura is sensitive to pressure, pain, and temperature. It produces a well localised pain, and is innervated by the phrenic and intercostal nerves.
The blood supply is derived from the intercostal arteries.
The visceral pleura is not sensitive to pain, temperature or touch. Its sensory fibres only detect stretch. It also receives autonomic innervation from the pulmonary plexus which has both sympathetic and parasympathetic nerve fibres.
Arterial supply is via the bronchial arteries.
Borders and Regions of the Pleura
The borders of the pleura is an imaginary line that connects the following spots:
- Right 6th (left 4th) rib at the anterior midline
- 12th rib at the scapular line
- 8th rib at the midclavicular line
- 10th rib at the midaxillary line
- The costal pleura lines the cavity along the ribs
- The mediastinal pleura lines the cavity along the mediastinum
- The diaphragmatic pleura lines the cavity along the diaphragm
- The cervical pleura or cupula lines the dome that forms at the apex of the lung.
Pleural Recesses and Reflections
A pleural recess is a potential space created when the pleural touch one another in times of quiet respiration. These recesses are most likely to occur as a costomediastinal space or a costodiaphragmatic space.
The pleural cavity is not completely filled by the lungs anteriorly and posteriorly. This creates a pleural reflection whereby the pleura folds onto itself. Posteriorly there is a vertebral reflection, inferiorly a costal reflection and anteriorly a sternal reflection.