The lungs are full of tiny branching airways, known as bronchi. Oxygen travels through these airways, ends up in tiny sacs called alveoli, and from there is absorbed into the bloodstream. The inside walls of the bronchi are coated with sticky mucus, which protects against damage from particles moving down into the lungs. In bronchiectasis, one or more of the bronchi are abnormally widened. This means more mucus than usual gathers there, which makes the bronchi more vulnerable to infection. If an infection does develop, the bronchi may be damaged again, so even more mucus gathers in them, and the risk of infection increases further. Over time, this cycle can cause gradually worsening damage to the lungs.