In anatomy, a potential space is an inappropriate term used to describe the small space between two adjacent structures that are normally in contact one another. Examples are the pleural, the peritoneal and pericardial spaces. In other words, they are like an almost empty plastic bag that has not been opened (two walls collapsed against each other; small but not zero interior volume) or a balloon that has not been inflated. The pleural space, between the visceral and parietal pleura of the lung, is a potential space.[1] Though it only contains a small amount of fluid normally, it can sometimes accumulate fluid or air that widens the space.[2] The pericardial space is another potential space that may fill with fluid (effusion) in certain disease states (e.g. pericarditis; a large pericardial effusion may result in cardiac tamponade).