The lower airway begins below the larynx and is comprised of the trachea, bronchial passages, and the alveoli. The Trachea is a tube protected by 16 rings of cartilage, which provide structure and protect the trachea from collapsing. The top ring is the cricoid ring, which extends fully 360 degrees around. The other rings, the cartilage extends only about three-fourths of the way around and is connected posteriorly by smooth muscle. The trachea branches at the carina and forms two mainstem bronchi, which further subdivide to form smaller and smaller air passages called bronchioles. All the air passages are supported by cartilage and are lined with smooth muscle, which allows the bronchioles to change their internal diameter in response to specific stimulation. The bronchioles end at the alveoli.