Bronchioles are tiny airways that carry oxygen to alveoli, or air sacs, in the lungs and help stabilize breathing in the respiratory system, according to About.com. Bronchioles are lined with smooth muscular walls that constrict or expand to regulate airflow and prevent irritants from passing further along the respiratory tract.
When humans breathe, air travels from the mouth and nasal cavity to the larynx, or voice box, which feeds air into a central windpipe known as the trachea, according to the National Heart, Lung, and Blood Institute. The trachea branches off into right and left bronchial tubes, and each passageway is connected to multiple branches of bronchioles.
Bronchioles are divided into a three-tier hierarchy. Lobular bronchioles are directly attached to the bronchial tubes and branch off into tinier terminal bronchioles, notes About.com. Respiratory bronchioles extend from the terminal branch to the alveoli ducts, which move air to alveoli clusters to begin the process of distributing oxygen throughout the body and removing carbon dioxide.
When bronchioles are sensitive to harmful irritants, such as allergens, cold air and fumes, they may become dangerously constricted and block the lungs from receiving sufficient airflow, according to About.com. Asthma and wheezing are common conditions that are caused by constricted bronchioles.