Pleura

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Lungs in situ seen from the anterior view.

The pleura is a double-layered serous membrane that covers each lung and lines the thoracic cage. The outer layer (parietal pleura) attaches to the chest wall. The inner layer (visceral pleura) covers the lungs, neurovascular structures of the mediastinum and the bronchi. The space between the parietal and visceral pleurae is called the pleural cavity which contains a small amount of serous fluid (pleural fluid). It is important to note that there is no connection between the right and left pleural cavities. 

The parietal pleura is divided into three subdivisions:

• Mediastinal part - covers the mediastinum and its structures;
• Costal part - covers the inner surface of the thoracic cage including the ribs;
• Diaphragmatic part - covers the diaphragm.

In some areas the pleural cavity is not completely filled by lung parenchyma. This results in an area of the pleural cavity where two layers of parietal pleura are directly opposed against each other, separated by pleural fluid. The potential spaces that are formed are called pleural recesses. These places are usually the place for fluid accumulation and can be of significant clinical importance in cases of pleural effusion. There are two pleural recesses: 

• The costomediastinal recess is located between the mediastinal and costal pleura,  just posterior to the sternum.
• The costodiaphragmatic recess is situated between the diaphragmatic and costal pleura.

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.