Human Anatomy - Kotsan I. Ya. 2009

Respiratory System
Pleura

Each lung is enclosed in the Pleura. Like the Peritoneum, the pleura is a smooth, glossy serous membrane consisting of two layers: visceral and parietal. The visceral (pulmonary) pleura (pleura visceralis (pulmonalis)) is tightly fused to the lung tissue, covers the lung on all sides, and extends into the fissures between its lobes. The parietal pleura (pleura parietalis) is a continuous membrane fused with the inner surface of the thoracic wall and the mediastinal Organs, forming a closed sac that contains the lung covered by the visceral pleura. Depending on its Location, the parietal pleura is divided into three parts: costal, mediastinal, and diaphragmatic. The costal part (pars costalis) covers the inner surface of the Ribs and intercostal spaces. Anteriorly near the Sternum and posteriorly near THE Vertebral Column, it transitions into the mediastinal part (pars mediastinalis), which lies adjacent to the mediastinal organs. The mediastinal part of the pleura is fused with the Pericardium. In the region of the lung ROOT, the mediastinal pleura wraps around it and continues as the diaphragmatic part (pars diaphragmatica), which covers the Diaphragm except for its central regions where the pericardium is fused with the diaphragm.

Between the visceral and parietal pleura lies a slit-like space—the pleural cavity (cavitas pleuralis)—containing a small amount (1–2 ml) of serous fluid that lubricates the opposing pleural surfaces, thereby reducing friction between them. The pressure within the pleural cavity is lower than atmospheric pressure, which creates favorable conditions for the Lungs to fill with air during inhalation. The right and left pleural cavities do not communicate with each other. In a healthy person, the pleural cavity is macroscopically invisible.

Where the parietal pleura transitions from one part to another, it forms recesses (sinuses), which serve as reserve spaces for the right and left pleural cavities, as well as reservoirs where serous fluid can accumulate when its production or absorption processes are impaired. Below, at the transition of the costal part of the parietal pleura into the diaphragmatic part, lies the rather deep costodiaphragmatic recess (recessus costodiaphragmaticus). During inspiration, the lower margin of the lung descends into this recess, but never reaches its very bottom, even during maximal inhalation. In Pleurisy, pus entering the pleural cavity accumulates primarily in this recess. At the transition of the mediastinal part of the pleura into the diaphragmatic part lies the shallow phrenicomediastinal recess (recessus phrenicomediastinalis). Along the anterior border of the lung, where the costal part of the parietal pleura transitions into the mediastinal part, lies the costomediastinal recess (recessus costomediastinalis); this recess is more pronounced on the left side.

Superiorly, at the level of the superior thoracic aperture, the costal and mediastinal PARTS OF THE pleura transition into each other to form the cupola of the pleura (cupula pleurae), bounded laterally by the scalene Muscles. Anteriorly and medially, the Subclavian Artery and vein lie adjacent to the pleural cupola. Posterior to the cupola are the HEAD of the first rib and the longus colli Muscle, covered by the prevertebral layer of the Cervical fascia, to which the pleural cupola is anchored.

The pleura plays a major role in the transudation (secretion) and resorption (absorption) of serous fluid, the normal balance of which is significantly disrupted in Diseases of the thoracic organs.

Despite their microscopic homogeneity and similar histological Structure, the parietal and visceral pleuras perform different Functions, which is likely due to their distinct embryonic origins. The visceral pleura, in which Blood Vessels predominate over Lymphatic vessels, functions primarily in fluid secretion. The parietal pleura, which contains more lymphatic vessels than blood vessels, performs the absorptive function.