Each lung is invested by an exceedingly delicate serous membrane, the pleura, which is arranged in the form of a closed invaginated sac. A portion of the serous membrane covers the surface of the lung and dips into the fissures between its lobes; it is called the visceral pleura or (pulmonary pleura). The visceral pleura is derived from mesoderm.
The visceral pleura is attached directly to the lungs, whilst the parietal pleura is attached to the opposing thoracic cavity. The space between these two delicate membranes is known as the pleural cavity or intrapleural space. Contraction of the diaphragm causes a negative pressure within this space which forces the lungs to expand, resulting in passive exhalation and active inhalation. This process can be made forceful through the contraction of the external intercostal muscles, forcing the rib cage to expand and aiding to the negative pressure within the intrapleural space, which causes the lungs to fill with air.