The lungs are covered by a thin serous layer (the visceral pleura). The pleura is then reflected on to the chest wall and pericardium (the parietal pleura). The lung hila 'connect' the visceral and parietal pleura. There is normally a small amount of fluid in the 'pleural space' between the parietal and visceral pleura, which lubricates movement between them. A pleural effusion occurs when the volume of this fluid is substantially greater than normal.
This is a companion discussion topic for the original article at https://patient.info/doctor/pleural-effusion-pro