Transudates from altered Starling forces; exudates from increased permeability
Expansion
Normally only 10 to 20 ml of pleural fluid is present, formed from parietal pleural capillaries and absorbed by parietal pleural lymphatics, which can increase drainage up to twentyfold.
Transudate - a Starling force problem, protein-poor
- Heart failure (the commonest cause overall), cirrhosis, nephrotic syndrome, hypoalbuminaemia
Exudate - a permeability or lymphatic problem, protein-rich
- Infection, malignancy, pulmonary embolism, connective tissue disease, pancreatitis
Light’s criteria distinguish them, and are deliberately weighted to avoid missing an exudate.
Physiological effects are those of a restrictive defect: reduced lung volumes, compression atelectasis with shunt, and a flattened or inverted hemidiaphragm in large effusions. Breathlessness correlates poorly with size, and relates more to diaphragmatic mechanics than to gas exchange, which is why drainage often relieves symptoms more than it improves oxygenation.