The pleural cavity is the name given to the gap between the inner and outer sheets of the pleura. The pleural cavity is filled with fluid to keep the two pleural sheets from rubbing against each other. When there is increased fluid accumulation in the pleural cavity, breathing becomes obstructed.
What is the pleural cavity?
In medical terminology, the pleural cavity is called cavitas pleuralis or cavum pleurae. Because the pleural cavity is rather small, it is also called the pleural space. It is located between the mural leaflet and the pulmonary leaflet of the pleura. Within the pleural cavity, physiologically, there are about five to a maximum of ten milliliters of fluid.
Anatomy and structure
The pleura is also known as the lung pleura or the pleura. It is a thin skin that lines the inside of the chest cavity and covers the lungs. The area that covers the lungs is accordingly called the pleura. The pleura, in turn, can be further subdivided into four areas. The pleural domes lie against the dome of the lungs. The pleura covers the inner sides of the ribs. The pars mediastinalis of the pleura lies in the area of the connective tissue of the mediastinal cavity, and the pars diaphragmatica is located on the upper side of the diaphragm. The pleura consists of two sheets, the visceral pleura and the parietal pleura. The visceral leaf is the inner leaf of the pleura. The parietal leaf faces outward. In the region of the pulmonary hilus, the internal leaflet merges with the external leaflet. The pulmonary hilus is where blood vessels, nerves, lymphatics, and bronchi enter the lungs. Between the parietal and visceral sheets of the pleura lies the pleural cavity. It is a very narrow gap rather than a cavity. The gap is filled by a few milliliters of fluid. The fluid is serous, meaning that it has a similar composition to blood serum.
Function and tasks
The fluid within the pleural cavity reduces friction between the two sheets of pleura. The two sheets are slidable on each other but cannot separate. This can be compared to two sheets of glass with a few milliliters of water between them. The water film on the glass allows the glass sheets to slide back and forth on each other. However, the adhesive forces prevent the two panes from separating. Because the outer sheet of the pleura is adherent to the chest cavity, the inner sheet is connected to the lungs, and the two sheets in turn adhere to each other through the fluid film, the pleural gap prevents the lungs from collapsing. As a sliding displacement layer, the pleura with the pleural cavity is also a prerequisite for the mobility of the lungs. At the same time, it helps to create suction during inhalation so that the respiratory air can flow in. When the chest expands during inspiration, the outer leaflet of the pleura follows. The two leaves are connected by the pleural space, so the inner pleural leaf must follow the movement. As this leaf is connected to the lung, the lung also expands. A negative pressure is created and the breathing air flows in. The pressure difference between the pleural cavity and the outside air is -800 pascals during inspiration. During exhalation, the pressure difference decreases to -500 pascals. If exhalation is very forced, the pressure inside the pleura can even reach positive values for a short time.
Diseases
If fluid accumulation in the pleural cavity exceeds physiologic amounts, respiratory distress occurs. Such excessive accumulation of fluid in the pleural cavity is also called pleural effusion. Pleural effusions are divided into low-protein transudates and high-protein exudates. The fluid may be bloody, purulent, or cloudy. Pleural effusions occur in the setting of infectious diseases such as tuberculosis or pneumonia, may be due to cardiac or renal insufficiency, or may be a consequence of cancer. Pleural effusions can also develop after trauma or in the course of autoimmune diseases. Smaller effusions of up to half a liter of fluid are often not even noticed. The cardinal symptom of larger fluid collections is shortness of breath. The lungs can no longer expand properly due to the fluid in the pleural space, consequently not enough breathing air flows into the vessels of the lungs. In the case of smaller effusions, shortness of breath only becomes apparent during physical exertion.Larger effusions are also noticeable at rest. In addition to shortness of breath, there may be irritation of the cough or respiratory-dependent chest pain. If pus collects in the pleural cavity instead of fluid, this is called pleural empyema. The most frequent cause of pleural empyema is pleurisy, i.e. inflammation of the pleura. However, hematogenous spread of pathogens is also conceivable, as is infection after trauma or after perforation of the esophagus. Usually, the disease is caused by streptococci, staphylococci, Escherichia coli, or Pseudomonas aeruginosa. Despite the accumulation of pus, the symptomatology of pleural empyema may be mild. Uncharacteristic symptoms such as fever, cough, and night sweats are typical. If air gets into the pleural space, it often has life-threatening consequences. In a pneumothorax, air enters the pleural space. As a result, the two pleural sheets lose their adhesive force and the lung collapses completely or in parts. Depending on the extent of the collapse, symptoms range from coughing to life-threatening shortness of breath. The skin turns blue, and there may be pain or a feeling of pressure in the chest area.
