Pleural cavity

In anatomy, the pleural cavity is the potential space between the lungs and the chest wall. It has virtually nothing in it in the normal non-diseased state, except a small amount of pleural fluid. The cavity is lined by specialized epithelium called pleura. The pleura that borders the chest wall is called the parietal pleura and is highly sensitive to pain. The pleura that borders the lung and other visceral tissues is called the visceral pleura, is not sensitive to pain and has a dual blood supply, from the bronchial and pulmonary arteries. The right pleural cavity is distinct from the left one. They do not have any communicating channels.

The pleura and pleural fluid function to preserve a vacuum state that enables breathing. This function of the pleural cavity can be disrupted in several ways:

pleural effusion - fluid can accumulate in the pleural space, compressing the lungs. This can occur as a consequence of malignancy, infection, heart failure, vascular obstruction, low serum oncotic pressure, lymphatic obstruction, or inability of the lung to expand. Congestive heart failure, bacterial pneumonia and malignancy constitute the vast majority of causes in the developed countries, although tuberculosis is a common cause in the developing world.
pneumothorax - when a pleural cavity communicates with the atmosphere, the lung on that side collapses. If instead of an open communication, a one-way valve is formed by tissue, air instead accumulates within the pleural cavity on that side, causing the heart to be pushed to the other side. This is called tension pneumothorax.
hemothorax - when blood fills a pleural cavity, the lung on that side is compressed.

In states of excess accumulation, pleural fluid can be sampled and evaluated to determine what disease state may be causing it. The evaluation consists of

Gram stain and culture - identifies bacterial infections
cell count and differential - differentiates exudative from transudative effusions
cytology - identifies cancer cells, may also identify some infective organisms
chemical composition including protein, lactate dehydrogenase, amylase, pH and glucose - differentiates exudative from transudative effusions
other tests as suggested by the clinical situation - lipids, fungal culture, viral culture, specific immunoglobulins