Coughing up blood generally refers to hemoptysis, and hemoptysis of lung cancer can be fresh blood, dark red blood or rust-colored sputum, there is no such thing as coughing up bruises. Hemoptysis occurs when lung cancer cells invade blood vessels and cause destruction of blood vessels. If it invades capillaries or alveoli, it will be manifested as blood in sputum or a small amount of fresh blood. If it invades larger blood vessels, such as bronchial artery or pulmonary artery, it will be manifested as fresh blood in the whole mouth or even hemoptysis. If the bleeding volume is less than 200ml, it may not cause hemoptysis, but it will be deposited in the lungs, forming rust-colored sputum, or dark red bloody sputum. If the amount of hemoptysis is more than 100ml, or the amount of hemoptysis is more than 500ml in 24h, it is called hemoptysis, and if the treatment is not timely, complications such as asphyxiation, hemorrhagic shock, aspiration pneumonia and so on may occur. If hemoptysis symptoms appear in lung cancer, one should go to hospital for treatment in time to avoid serious consequences.