Pleural effusion is the buildup of excess fluid in the space between the membrane around the lungs and the membrane covering the ribcage.
Pleural effusion prevents the lungs from fully expanding during breathing.
The space between the membrane around the lungs and the membrane covering the ribcage is the pleural cavity. Normally, there is a small amount of fluid in this cavity that acts as a lubricant and allows the lungs to expand smoothly during breathing. When there is too much fluid in the cavity, the lungs cannot expand as well. Certain conditions can cause excess fluid to build up in the pleural cavity. The excess fluid is either thin, almost like water, or thick, like pus.