There are many reasons for coughing up blood, commonly found in coughing too hard, tuberculosis, bronchial dilatation and so on. 1. Coughing too hard: when the patient coughs too hard, it will lead to the tearing of the capillaries on the bronchial tubes, which will cause bleeding and the phenomenon of coughing up blood. 2. Tuberculosis: patients with tuberculosis have hollow foci in the lungs, and blood vessels are exposed inside the hollows, which can cause rupture of pulmonary blood vessels inside the hollows after coughing, resulting in coughing up blood. 3. Bronchial dilatation: due to the bronchial structure changes, caused by bronchial dilatation, and the pulmonary artery and bronchial artery end-to-end anastomosis, which leads to peribronchial capillary dilatation, encounter inflammation stimulation will occur when coughing cough bleeding phenomenon. There are many other reasons for coughing up blood, such as lung cancer, etc. Patients need to go to the hospital in time, after a clear diagnosis under the guidance of the doctor for standardized treatment.