Why is there an film of liquid in the pleural cavity?

Answer 1

The pleural space contains this fluid to reduce friction between the interior chest wall lining and the exterior lung surface lining. Think of two pieces of flat glass pane one on top of the other. With a tiny amount of liquid between them the glass pieces slide across each other easily; without the liquid the glass pieces are more difficult to move. The pleural liquid provides this ability for the two linings to move easily as the lung and chest wall expand and contract during inspiration and expiration. The pleural space is not truly a space as we might think of as an air containing room, but rather a compartment filled with a thin serous type fluid. The fluid is rather static. That is to say the body physiologically maintains the amount of fluid at a constant level. Given infections or other intrusions into the pseudo space, the fluid content may increase and become painful; ie pleurisy. That is the overall simplified version to answer your question. There are many other issues surrounding the pleural space but that would go much further into your question.