Pleura

The pleura are a pair of mesothelial cell sheets which surround the lungs on both sides. They divide the thoracic cavity up into two discrete pleural cavities.

The pleural membranes can be divided into:

• visceral pleura, which lines the outer surface of the lung
• parietal pleura, which lines the inside of the thoracic cavity and lies outside of visceral pleura

The visceral and parietal pleura are smooth and secrete a small amount of fluid which occupies the space between them; this space is termed the pleural cavity. The fluid permits free movement of the lung as its expands and contracts during the phases of respiration. It is present at negative pressure within a pleural cavity and this encourages the visceral pleura to expand against the natural elasticity of the deeper lung. Hence, the majority of the pleural membrane surface is in close apposition.

The pleura may be affected by a range of conditions e.g. infection - pleuritis or pleurisy - and malignancy - mesothelioma. The potential space of the pleural cavities can be occupied by a variety of pathologies including air - pneumothorax - pus - empyema - and blood - haemothorax.