The pleurae, which contain the lungs, lie on either side of the mediastinum within the chest cavity. Each pleura is made up of two layers: parietal layer and visceral layer. The parietal layer lines the thoracic wall and covers the thoracic surface of diaphragm. It also lines the lateral aspect of mediastinum and extends into the root of the neck where it lines the undersurface of supra-pleural membrane. The visceral layer completely covers the outer surface of the lungs and extends into the depths of interlobar fissures of lungs.
These two layers of each pleura become continuous with one another through a cuff that surrounds all the structures, which enter or leave the lungs at the hilum. To allow for the movements of pulmonary vessels and large bronchi during different phases of respiration, the pleural cuff hangs down as loose fold called the pulmonary ligament.
The parietal and visceral layers of each pleura are separated from one another by pleura cavity, which is a slit like space containing the pleural fluid. Pleural fluid is actually tissue fluid that covers the surfaces of both layers of pleura as thin film. It permits the two layers to slide on one another with minimum friction.
Divisions of parietal pleura:
Parietal layer of each pleura is customarily divided into a number of divisions based on the region where it is present.
- Cervical pleura: It extends up into the neck and lines the undersurface of the supra pleural membrane. At its very top, it reaches to a level, which is about 1.5 inches higher than the medial third of the clavicle.
- Costal pleura: It lines the inner surface of ribs and costal cartilages. The intercostals spaces, sides of vertebral bodies and back of sternum are also lined by this division of parietal pleura.
- Diaphragmatic pleura: It covers the thoracic of the diaphragm. It is related to a recess known as costodiaphragmatic recess, which is created by the apposition of costal and diaphragmatic pleurae with each other during quiet respiration. In deep inspiration, the margins of the base of the lungs descend the costal and diaphragmatic pleurae separate and the lungs expand into this recess.
- Mediastinal pleura: It covers and forms the lateral boundary of the mediastinum. At the hilum of each lung, it is reflected as a cuff around the blood vessels and bronchi. At this point it becomes continuous with the visceral pleura. At the hilum the lung is attached to blood vessels and bronchi and is free everywhere else.
It is a slit like space between the costal and diaphragmatic parietal pleura in each lung. During inspiration, the lower margins of the lungs descend into the recess. During expiration, the lower margins of the lungs ascend so that the costal and diaphragmatic pleurae come together again.
It is situated along the anterior margins of costal and mediastinal parietal pleura in both lungs. During inspiration and expiration, the anterior borders of the lungs slide in and out of this recess.
Nerve supply of the pleurae:
Parietal pleura: The parietal pleura is sensitive to common sensations like pain, temperature, touch, and pressure. It is innervated in the following order.
- Costal pleura is segmentally supplied by the intercostals nerves.
- The mediastinal pleura is supplied by phrenic nerves.
- The diaphragmatic pleura is supplied over the domes by phrenic nerve and towards the periphery by the lower six intercostals nerves.
- Visceral pleura: It receives an autonomic nerve supply from the pulmonary plexus. It is sensitive to stretch but it is insensitive to common sensations like pain and touch.