The pleural cavity contains a little amount of liquid called as "pleural fluid".
Step-by-step explanation:
The two thin serum producing membrane which gathers back itself resulted in membrane cavity of two layer pleural sac, which surrounds the lungs is termed as "pleura".
This cavity contain small amount of liquid named as pleural fluid, its function is to provide lubrication during respiration when the lungs expand and contract. Most fluid is generated via bulk flow through the parietal circulation and is reabsorbed by the lymphatic system.
Thus pleural fluid is continuously produced and reabsorbed. A few millilitres of pleural fluid is always present inside the intrapleural space in a typical 70 kg human.