11. The pleural surface at the lower left demonstrates areas of yellow-tan purulent exudate. Pneumonia may be complicated by a pleuritis. Initially, there may just be an effusion into the pleural space. There may also be a fibrinous pleuritis. However, bacterial infections of lung can spread to the pleura to produce a purulent pleuritis. A collection of pus in the pleural space is known as empyema.