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A pleura is a serous membrane that folds back on itself to form a two-layered membranous pleural sac. The outer layer is called the parietal pleura and attaches to the chest wall. The inner layer is called the visceral pleura and covers the lungs, blood vessels, nerves, and bronchi. There is no anatomical connection between the right and left pleural cavities. [rx] With the addition of pleural fluid, the lung pleura allows for easy movement of the lungs and inflation during breathing.
The mediastinum is a central compartment in the thoracic cavity between the pleural sacs of the lungs. It is divided into two major parts, the superior and inferior portions. The inferior portion is then further divided into the anterior, middle, and posterior portion. Each region of the mediastinum contains specific groups of structures. [rx]
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Superior mediastinum – Organs: thymus, trachea, esophagus; Arteries: aortic arch, brachiocephalic trunk, left common carotid artery, left subclavian artery; Veins and lymphatics: superior vena cava, brachiocephalic vein, thoracic duct; Nerves vagus nerve, left recurrent laryngeal nerve, cardiac nerve, phrenic nerve
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Anterior mediastinum – Organs: thymus; Arteries: small arterial branches; Veins and lymphatics: small branches; Nerves: none
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Middle mediastinum – Organs: heart, pericardium; Arteries: ascending aorta, pulmonary trunk, pericardiacophrenic arteries; Veins and lymphatics: superior vena cava, azygos vein, pulmonary vein, pericardiacophrenic vein; Nerve: phrenic
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Posterior mediastinum – Organs: esophagus; Arteries: thoracic aorta; Veins and lymphatics: Azygos vein, hemiazygos vein, thoracic duct; Nerve: vagus nerve
Structure of Pleura
The pleural cavity is a space between the visceral and parietal pleura. The space contains a tiny amount of serous fluid which has two key functions.
The serous fluid continuously lubricates the pleural surface and makes it easy for them to slide over each other during lung inflation and deflation. The serous fluid also generates surface tension, which pulls the visceral and parietal pleura adjacent to each other. This function will allow the thoracic cavity to expand during inspiration.
NB; when air enters the pleural space, the surface tension will disappear, and the resulting condition is known as a pneumothorax.
Pleural Recesses
Located posterior and anterior, there are spaces where the pleural cavity is not totally filled by the lung parenchyma. This space is known as the recess – an area where the adjacent surfaces of the parietal pleura come into contact. The two recesses in the pleural cavity include the following:
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Costomediastinal recess which is found between the mediastinal and costal pleura. Space is located just posterior the sternum.
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Costodiaphragmatic recess is found between the diaphragmatic and costal pleura.
The reason these recesses are important is that they provide a space for fluid to accumulate. Pleural effusions usually collect in the costodiaphragmatic recess.
Blood Supply of Pleura
The bronchial arteries predominantly supply the visceral pleura. Blood returns to the heart via the pulmonary system. The parietal pleura, however, receives supply by the systemic arteries that overlie it (internal thoracic, intercostal, and phrenic arteries). Venous return for the parietal pleura occurs similarly.
Lymphatic drainage from the pleural cavity occurs through mostly parietal pleura involvement. The parietal pleura contains several openings (stomata) that allow fluid drainage to the subpleural lymphatic system. These drain to the diaphragmatic, internal mammary, retrosternal, paraesophageal, and celiac lymph node stations.[rx]
The visceral pleura does have a significant role in draining fluid from the interstitium of the lung parenchyma. The lymphatic channels run through the basement membrane and mostly drain to the infralobar and hilar lymph nodes.[rx]
Nerves of Pleura
Historically, the parietal pleura was considered to be the only innervated part of the pleura. The intercostal nerves that run parallel to the ribs give off branches that innervate the costal pleura. Alternatively, the phrenic nerve is the primary innervation for the diaphragmatic and mediastinal pleurae.
There is a long convention that visceral pleura has no innervation and no role in pain. Interestingly, recent research has shown that the visceral pleura may be innervated by what researchers have referred to as visceral pleura receptors (VPRs) that may be associated with the elastic fibers of the lung. They may be involved in the reflexive dyspnea that is present in some pulmonary illnesses.[rx]
Muscles
Muscles involved in the pleural cavity are typically associated with muscles of respiration. These include the diaphragm and intercostal muscles. Additional accessory muscles include the sternocleidomastoid and scalene muscles. The pectoralis major and minor can also help in forceful respiration.
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