Lung cancer tumours can grow in a way that block your airways, exert pressure on your lungs, or cause inflammation (redness and swelling) in your respiratory system. They can also cause fluid to build up around the lungs, making it difficult for the affected lung to fully expand when inhaling. All these can affect the proper functioning of the respiratory system, resulting in symptoms such as shortness of breath, or result in bleeding in the airway, which can cause you to cough up blood.