ABSTRACT
The pleural mesothelium is a monolayer of cells that may vary in shape from flattened ovoid to columnar or cuboidal. The mesothelium adheres to a basement membrane that comprises a matrix of connective tissue.1 The basement membrane of the pleura is a complex structure that plays an important role in inflammation of the pleural space.2 The visceral pleura is nourished by a large network of capillaries that originate from the bronchial arteries. The parietal and diaphragmatic surfaces of the pleura are supplied by the blood vessels local to those areas. Together, the visceral, parietal and diaphragmatic surfaces of the pleura form a closed boundary around the pleural space. Under normal conditions the volume of fluid in the pleural space is very small, in the range of 0.2-0.5 mL. The pleural space may expand to accommodate much larger volumes under certain conditions, such as pleural effusion. Although the pleural space is drained by lymphatic vessels, they are not present in large numbers. Indeed, the lymphatic vessels draining the pleural space may grow in size and number according to need.3 Among the lymph nodes that drain pleural structures are the mediastinal, intercostal and sternal lymph nodes. Normally, pleural fluid is characterized by low protein concentration and low cellularity with an absence of inflammatory cells.4
