Bronchus (plural bronchi, adjective bronchial) is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes, which in turn become bronchioles. No gas exchange takes place in this part of the lungs. The hyaline cartilage forms an incomplete ring in the bronchi, giving them a "D"-shaped appearance in the larger bronchi (as compared with the "O"-shaped complete cartilaginous rings of the trachea), and as small plates and islands in the smaller bronchi. Smooth muscle is present continuously around the bronchi.