Pleura

The pleura is a thin serous membrane that envelops each lung (visceral pleura) and lines the walls of its pleural cavity (parietal pleura). It is formed by a thin connective tissue base covered by flat epithelium (mesothelium) located on the basement membrane. The cells of the mesothelium are flat in shape, have numerous microvilli on the apical surface, and poorly developed organelles. The connective tissue base is formed by alternating lattice-like layers of collagen and elastic fibers; it contains individual bundles of smooth myocytes and an insignificant number of connective tissue cells.

It covers the lung parenchyma, mediastinum, diaphragm and lines the inner surface of the chest. The parietal and visceral pleurae are covered by a single layer of flat mesothelial cells.

The visceral (pulmonary) pleura (pleura visceralis, s.pulmonalis) covers the lung on all sides, firmly grows together with its surface, and enters the gaps between the lobes. Along the anterior and posterior surfaces of the root of the lung, the visceral pleura passes into the parietal (mediastinal) pleura. Below the root of the lung, the anterior and posterior sheets of the visceral pleura form a vertically oriented fold - the pulmonary ligament (lig.pulmonale), descending all the way to the diaphragm. This ligament is located in the frontal plane between the medial surface of the lung and the sheet of the parietal pleura adjacent to the mediastinum.

The parietal (pleura parietalis) is a continuous sheet that forms a receptacle for the lung in each half of the chest cavity, merging with the inner surface of the chest cavity and the surface of the mediastinum. The parietal is divided into costal, mediastinal and diaphragmatic parts.

In the parietal, the mesothelial cells are located directly on the connective tissue layer. In the visceral, the mesothelial cell layer is located on a thin connective tissue layer, which is associated with a deeper connective tissue layer (the main connective tissue layer). Between the main layer of the visceral pleura and the border subpleural layer of the lung, there is a vascular layer. The vascular layer contains lymphatic vessels, veins, arteries, and capillaries, with the diameter of the capillaries being significantly larger than the diameter of the capillaries in other tissues of the body, which helps maintain low capillary pressure in the visceral pleura. There are differences in the ratio of blood and lymphatic vessels in the visceral and parietal pleura. In the parietal there are 2-3 times more lymphatic vessels than blood vessels, in the visceral - the ratio is reversed - there are more blood vessels than lymphatic vessels. The most active is the intercostal (costal) pleura, it has lymphatic "hatches" of a round or oblong shape, with the help of which the lymphatic vessels of the parietal (costal) pleura are connected with the pleural cavity.

The costal pleura (pleura costalis) covers the inner surface of the ribs and the intercostal spaces from the inside. In front, at the sternum, and behind, at the spine, the costal pleura passes into the mediastinal pleura.

Mediastinal (pleura mediastinalis) limits the organs of the mediastinum from the lateral side, separating them from the pleural cavity of the corresponding lung (right or left). The mediastinal pleura goes from the inner surface of the sternum in front to the lateral surface of the spinal column at the back. The mediastinal is fused with the pericardium, in the area of the root of the lung it passes into the visceral pleura.

Above, at the level of the head of the 1st rib, the costal and mediastinal pleurae merge into each other, forming the pleural dome (cupula pleurae). The subclavian artery and vein are adjacent to the pleural dome in front and medially. Below, the costal and mediastinal pleurae pass into the diaphragmatic pleura. The diaphragmatic (pleura diaphragmatica) covers the diaphragm from above, except for its central areas, to which the pericardium is adjacent.

The pleural cavity (cavitas pleuralis) is located between the parietal and visceral in the form of a narrow slit, it contains an insignificant amount of serous fluid that moistens the pleural sheets, helping to reduce friction of the visceral and parietal pleura against each other during respiratory movements of the lungs. In the areas of transition of the costal pleura into the mediastinal and diaphragmatic pleura, the pleural cavity has depressions - pleural pockets (sinuses). They are reserve spaces of the pleural cavity, which are filled with the lungs during breathing. Pleural sinuses (recessus pleurales) can be places of accumulation of serous or other fluid in diseases or injuries of the lung, pleura. The costophrenic sinus (recessus costodiaphragmaticus) is located at the transition of the costal pleura into the diaphragmatic. Its greatest depth (9 cm) corresponds to the level of the midaxillary line. The phrenic-mediastinal sinus (recessus phrenicomediastindlis) is a shallow sagittally oriented fissure of the pleural cavity at the transition of the lower part of the diaphragmatic pleura into the mediastinal. The costomediastinal sinus (recessus costomediastinalis) is a small fissure located at the transition of the anterior part of the costal pleura into the mediastinal.

The blood supply of the parietal pleura is carried out by the vessels of the systemic circulation. The costal pleura is supplied by branches of the intercostal arteries, the mediastinal pleura by the pericardiodiaphragmatic artery, the diaphragmatic pleura by the superior diaphragmatic and muscular-diaphragmatic arteries.

The visceral pleura is supplied with blood from the bronchial artery system and the pulmonary artery.

Normally, the parietal and visceral layers are separated by a very thin layer of fluid. It has been established that, according to Starling's law of transcapillary exchange, fluid normally moves from the capillaries of the parietal pleura into the pleural cavity and is then absorbed by the visceral pleura (Ligt, 1983).

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Topography of the pleura

The dome is located on the right and left 1.5-2 cm above the clavicle. The anterior and posterior borders of the parietal correspond to the borders of the right and left lungs. The lower border of the parietal pleura is located one rib (2-3 cm) below the corresponding border of the lung. Passing downwards and laterally, the lower border of the costal pleura crosses the 7th rib along the midclavicular line, the 8th rib - along the anterior axillary, the 9th rib - along the middle axillary, the 10th - along the posterior axillary, the 11th - along the scapular line, and at the level of the 12th rib it sharply passes into the posterior border. The anterior borders of the right and left costal pleura run from the 2nd to the 4th rib almost parallel to each other, and diverge at the top and bottom, forming interpleural fields. The upper interpleural field is directed with its apex downwards, is located behind the manubrium of the sternum. The thymus is located in this field. The lower interpleural field is triangular in shape and is located behind the lower half of the body of the sternum and the adjacent cartilages of the IV and V ribs. In the lower interpleural field, the anterior surface of the heart, covered with pericardium, is adjacent to the anterior chest wall.

The pleura in a newborn is thin, loosely connected to the intrathoracic fascia, and mobile during respiratory movements of the lungs. The upper interpleural space is wide (occupied by a large thymus). With aging, adhesions (adhesions) appear in the pleural cavity between the parietal and visceral pleural layers. The lower border in older people is somewhat lower than at the age of 30-40 years.