The pleura is the lining of the outside of the lung and the inside of the ribcage (chest cavity). The area between the two surfaces of the pleura is called the pleural space. Usually, there is a very small amount of fluid in the pleural space (fewer than 10ml) which helps to lubricate the pleura when the lungs are expanding and contracting whilst breathing.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/pleurisy-causes