Hi, Thanks for writing in. Cough is a reflex action by which the body tries to throw out anything that is stuck in the air passage or any focus of irritation in the lungs. When a person has a lung cancer, there might be irritation of the small airways and larger airways. By coughing the person is trying to expel the contents in the lungs, bronchi, trachea or throat. These are the different areas which might get irritated and coughing helps to clear the passages and make the person feel better. In lung cancer there is destruction of the smaller passages and lung tissue. This leads to irritation and stimulation of nerves that carry pain sensation to be brain. Due to cancer the blood vessels also get affected and open up and the patient coughs out blood. Please do not worry.