It sometimes refers as “water in the lungs”. Includes fluid collection in the pleural cavity-

1. Hydrothorax:- Accumulation of serous fluid within the pleural cavities. It may be unilateral or bilateral. It is due to congestive heart failure, renal failure, cirrhosis or liver, pulmonary oedema, or tumours of lungs. It is clear, straw coloured, transudate in nature, protein below 1gm/dl or specific gravity under 1.012. In this condition, a person is having dyspnoea which is relieved by the withdrawal of fluid.
2. Haemothorax:- Accumulation of pure blood in the pleural cavity. A most common cause is trauma to the chest wall, or to thoracic viscera.
3. Chylothorax:- There is an accumulation of milky fluid of lymphatic into the pleural cavity. It occurs due to rupture of thoracic duct by trauma or by obstruction of thoracic duct by malignancy. Chylous effusion is milky due to the high content of emulsified fat in chyle.