Pleural cavity

The pleural cavity is a space surrounding the lungs; The lungs are surrounded by two membranes, the pleurae. The outer pleura covers and is attached to the chest wall and is known as the parietal pleura. The inner one covers and is attached to the lung and other visceral tissues i.e. vessels, bronchi and nerves and is known as the visceral pleura. In between the two is an actual thin space known as the pleural cavity or pleural space normally containing a small amount of pleural fluid.

The parietal pleura is highly sensitive to pain; the visceral pleura is not.

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Functions

Pleural fluid serves several functions. It lubricates the pleural surfaces and allows the pleural layers to slide against each other easily during ventilation. Pleural fluid also provides the surface tension that keeps the lung surface in close apposition with the chest wall. This allows optimal inflation of alveoli during respiration. It also directly transmits pressures from the chest wall to the visceral pleural surface (and hence, the lung). Therefore, movements of the chest wall during breathing are coupled closely to movements of the lungs.

Blood supply

The visceral pleura has a dual blood supply from the bronchial and pulmonary arteries.

Fluid

It is filled with pleural fluid, a serous fluid produced by the pleura. A normal 70 kg human has approximately 12-15 mL of pleural fluid.

In normal pleurae, most fluid is produced by the parietal circulation (intercostal arteries) via bulk flow and reabsorbed by the lymphatic system. Thus, pleural fluid is continuously produced and reabsorbed. The rate of reabsorption may increase up to 40x before significant amounts of fluid accumulate within the pleural space.

In humans, there is no anatomical connection between the left and right pleural cavities, so in cases of pneumothorax (see below), the other hemithorax will still be able to function normally.

Diseases

Diseases involving the pleura include:

• Pneumothorax: a collection of air within the pleural cavity, arising either from the outside or from the lung. Pneumothoraces may be traumatic, iatrogenic, or spontaneous. A tension pneumothorax is a particular type of pneumothorax where the air may enter (though a defect of the chest wall, lung, or airways) on inspiration, but cannot exit on expiration. Each breath increases the amount of trapped air in the chest cavity, leading to further lung compression. This is a medical emergency.
• Pleural effusion: a fluid accumulation within the pleural space. Abnormal collections of pleural fluid may be due to excessive fluid volume (i.e. excess intravenous fluids, renal failure), decreased fluid protein (e.g. cirrhosis, proteinuria), heart failure, bleeding (hemothorax), infections (parapneumonic effusions, empyema), inflammation, malignancies, or perforation of thoracic organs (i.e. chylothorax, esophageal rupture).
• Pleural tumors: abnormal growths on the pleurae. These may be benign (i.e. pleural plaques) or malignant in nature. Mesothelioma is a type of malignant cancer associated with asbestos exposure.

See also

• Trachea
• Capillaries
• Larynx
• Pharynx
• Epiglottis
• Rings of cartilage
• Bronchus
• Bronchioles
• Thoracic cavity

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