A pleural effusion is the accumulation of extra fluid in the pleural cavity.
A pleural effusion can develop if the normal physiological flow of pleural fluid is disturbed by pleural inflammation (including tumor infiltration) or obstruction of the pulmonary lymphatic or venous channels.

Causes
Inflammation of pleural surface:
- infiltration by tumor (exudate)
- infection, infarction, irradiation (exudate)
Lymphatic obstruction:
- peripheral obstruction by tumor (transudate or exudate)
- central (mediastinal) obstruction (exudate)
Raised pulmonary venous pressure:
- local venous obstruction by tumor (transudate or exudate)
- cardiac failure, pericardial tamponade (transudate)
Other edematous conditions:
- hypoproteinemia, renal or liver failure (transudate)

Pleural effusions cause dyspnea, a nonproductive cough and chest pain that may be dull and aching or pleuritic. Dyspnea varies from mild to severe and depends on both the size and speed of accumulation of the effusion.
Small effusions and those occurring in patients who are relatively immobile are unlikely to cause significant symptoms.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. 4th ed. Oxford University Press, 2004. p. 199.