Lung cancer has the highest mortality rate among all cancers and is one of the most threatening malignant tumors to the health and lives of people, which is very harmful to patients, so once the symptoms of lung cancer appear, they need to be treated actively. Among them, coughing up blood is a common symptom of lung cancer patients, so what are the causes of coughing up blood in lung cancer? Causes of coughing up blood in lung cancer: Blood comes out from the lungs through the airway due to injury of lung collaterals, or pure blood is bright red, or both sputum and blood, or blood in sputum. It is also called blood coughing and blood hemoptysis. External evil attacking the lung, phlegm stasis blocking the lung, liver fire offending the lung, lung and kidney yin deficiency, and qi deficiency not being taken in, all cause damage to the lung ligaments, rebellion of the lung qi, and blood overflowing into the airway. Generally, external coughing of blood has a short duration and an acute onset, with fever and malignant chills at the beginning; internal coughing of blood has a slow onset and a long duration, with the manifestation of deficiency or prevalence of yin and yang qi and blood in the internal organs. Patients with lung cancer, due to the tumor, block the passage of gas and stimulate the bronchial walls, causing the trachea to react by trying to expel the foreign body, thus producing a cough, sometimes coughing up blood in the sputum, which is what we call coughing up blood. When there is a tumor in the lung, it will cause local inflammation of the lung tissue, which will increase the permeability of capillaries and cause blood to flow into the respiratory tract.