The normal value of a small amount of pleural effusion is usually maintained at 5 to 10 ml. Pleural effusion mainly serves as a lubricant and facilitates thoracic movement, as well as respiratory movement of the lungs. If pleural effusion is significantly excessive, corresponding respiratory symptoms may manifest. When the amount of pleural effusion is less than 300 ml, the symptoms are less obvious, however, if the effusion is mainly inflammatory fibrinous exudation, a small amount of effusion can cause chest pain on the affected side and irritating dry cough. When the amount of fluid increases, the dirty layer and the wall pleura are separated, which makes the chest pain disappear or reduce. When the amount of pleural effusion is more than 500 ml, the patient may have chest tightness and shortness of breath. In addition, due to the compression of mediastinal organs by a large amount of pleural effusion, palpitations, dyspnea, cyanosis and other symptoms involving the cardiac system may occur. When the above symptoms appear, it is recommended to consult a professional doctor in time to avoid delaying the condition.