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In anatomy, a potential space is a structure that normally is either empty or contains a small amount of substance, but can expand to much larger sizes as a result of different processes. The lungs and the heart are both surrounded by potential spaces, called the pleural space and pericardial space respectively. The brain has spaces that can be filled with fluid, including the epidural and subdural spaces. Some people also consider the vagina to be a potential space.
Normally small in size, a poential space can expand as a result of different conditions. This concept is best illustrated with an example. The lungs are covered by a thin membrane called the visceral pleura. This membrane contacts the parietal pleura, which is a membrane that lines the inner part of the rib cage. The space, called the pleural cavity, located between the visceral and parietal pleura, is normally only filled with a few milliliters of fluid; however, in pathologic conditions hundreds or thousands of milliliters of fluid can collect in this space.
Similar to how the lung is surrounded by the pleural cavity, the heart is surrounded by the pericardial cavity that is bounded by the epicardial surface lining the outside of the heart and the stiff pericardial sac that surrounds the heart. Normally this space contains a small amount of fluid that serves as a lubricant. With different diseases, fluid can accumulate in this space. If enough fluid collects, the ability of the heart to pump blood is compromised. This condition is called cardiac tamponade, and can be life-threatening without a surgical procedure to relieve the buildup of pressure in the pericardial space.
A number of different regions of the brain have potential spaces. The brain is lined by three different membranes, the outermost being a rigid dural layer, the middle a thin arachnoid layer, and an inner pial layer that adheres to the surface of the brain. Blood can accumulate in the space between the dural and arachnoid layers, in the subdural space. This occurs when the veins normally present in this space burst, a condition most often present as a result of trauma. The epidural space, the area between the skull and dura, can also accumulate blood if the artery located in this region ruptures.
Some people consider the vagina to be another type of potential space in the body. Unlike the other potential spaces discussed so far, this space is not defined by membranes. It is a cavity lined by a mucus membrane that opens to the external surface of the body. Normally the mucus membranes of the vagina contact each other, but this potential space can expand during sexual intercourse or childbirth.