Pleural effusion occurs when any cause accelerates the production of fluid in the chest cavity and/or reduces its absorption, so pleural effusion is not a disease, but a manifestation of some diseases in the chest cavity. Etiology: 1. Hydrostatic pressure augmentation in pleural capillaries: such as congestive heart failure, constrictive pericarditis, rising blood volume, superior vena cava compression, etc. leads to the production of leakage fluid . 2.Pleural capillary wall permeability pressurization: such as pleural inflammation, connective tissue disease, pleural tumor, pulmonary infarction, subphrenic inflammation, etc. lead to the production of exudate. 3, decreased colloid osmotic pressure in pleural capillaries: such as hypoproteinemia, cirrhosis, nephrotic syndrome, acute glomerulonephritis, mucinous edema, etc. leading to the production of leakage fluid. 4.Mural lymphatic drainage disorders: such as cancerous lymphatic duct obstruction, abnormal drainage of developmental lymphatic ducts, etc. lead to the generation of exudate. 5.Intra-thoracic hemorrhage due to injury, etc.: such as ruptured aortic aneurysm, ruptured esophagus, etc. leading to hemothorax, pus thorax, celiac thorax. Clinical manifestations: manifestations of the original disease + manifestations of pleural effusion Pleural effusion: small amount: no obvious abnormality, there may be a sense of pleural friction and pleural friction sounds. Large amount: feeling of chest tightness, shortness of breath, palpitation, chest pain; tracheal displacement, fullness of the affected side of the chest, reduced respiratory kinetics, diminished fibrillation, cloudy to solid percussion, and diminished to absent breath sounds.