Pleural fluid is caused by an imbalance between pleural formation and absorption of pleural fluid, resulting in excessive clinical signs of fluid accumulation in the pleura. Under normal circumstances, the wall pleura continuously produces pleural fluid into the pleural cavity, while the dirty wall pleura continuously absorbs and drains pleural fluid, thus achieving a balance between production and drainage recovery. There is always a certain amount of fluid in the pleural cavity, but it is not easily detected by general positioning chest X-ray or fluoroscopy. In healthy people, there is a small amount of fluid in the pleural cavity. If a lateral posteroanterior radiograph is used, about 12.5% of people have a small amount of fluid in the pleural cavity, 22.8% of women in labor can find a small amount of fluid, and 4.3% of children have a small amount of pleural fluid. Pleural fluid-related diseases Ovarian hyperstimulation syndrome Chronic constrictive pericarditis Pulmonary parasitosis Liver tumors Mediastinal cysts and tumors Thoracic aortic aneurysm Hepatic pleural effusion Mediastinal non-Hodgkin’s malignant lymphoma Acute right-sided heart failure Pleural effusion and pleurisy in the elderly Pneumonia-like pleural effusion Tuberculous pleurisy Pediatric cirrhosis Pediatric tuberculous pleurisy Pediatric acute hematogenous pulmonary tuberculosis Pediatric primary pulmonary tuberculosis Dystrophic cirrhosis Cardiogenic cirrhosis Pediatric congestive heart failure Maternal and child blood group incompatibility hemolytic disease Cirrhosis Chronic suppurative pleurisy