The production of pleural effusion is mainly related to the exudation and leakage of capillaries and lymphatic vessels in the pleural cavity, and the common causes include lung diseases, immune system diseases, other systemic diseases and trauma. 1. Lung diseases: mainly include pneumonia, tuberculosis, lung cancer, etc., which cause pleural irritation and inflammatory reaction as well as abnormal blood reflux, leading to pleural effusion. 2. Immune system diseases: immune system diseases, such as systemic lupus erythematosus, can cause metabolic inflammation in the body, which can also lead to fluid leakage from the local pleural cavity and cause effusion. 3. Other systemic diseases: such as liver and kidney function abnormalities, advanced tumors, etc., leading to hypoproteinemia, causing plasma colloid osmotic pressure to decrease, which will also lead to fluid exudation, causing pleural effusion. 4. Trauma: directly causing damage to pleural capillaries and lymphatic vessels, leading to direct leakage of fluid into the chest cavity, causing pleural effusion. When pleural effusion is accompanied by obvious chest tightness and restricted breathing, it is necessary to consult a doctor in time for observation and standardized treatment under the guidance of a clinician after confirming the specific condition.