A general term for the system of tubes that runs from the back of the mouth and nose into the lungs. The largest is the trachea or windpipe. In the chest, the trachea divides into two smaller tubes called bronchi. Each bronchus then supplies one lung. After entering the lung, the bronchi divide further into narrower tubes called bronchioles and these supply the air sacs of the lung. The airways contain a thin lining on the inside moistened with a little mucus. Barring choking or drowning, from the moment you are born your airways are kept clear and, unless you smoke, fairly clean. Asthma sufferers find that periodically their airways narrow and they can't breathe normally. The smooth muscle is contracting and so narrowing the airway, and at the same time the inner lining becomes inflamed and reddened. It may also swell. Extra mucus is produced, sometimes thicker than usual, and it can partially clog or obstruct the airway.