The bronchii, |bronchioles|, and alveolar ducts of the |lungs| are composed of both smooth muscular tissues and elastic, cartilaginous |fibers|. The elastic fibers comprise the bulk of the passageways, facilitating expansion and |contraction| during respiration. The smooth muscular tissues are arranged in reinforcing rings around the bronchii and |bronchioles|. They can constrict involuntarily, as a response to cold ~air~ or allergens which irritate the |lungs|. One of the symptoms of |asthma|, an often debilitating |lung| disorder, is the hypercontraction of these |muscle fibers|, making respiration difficult.
