<P> Glands are abundant in the upper respiratory tract, but there are fewer lower down and they are absent starting at the bronchioles . The same goes for goblet cells, although there are scattered ones in the first bronchioles . </P> <P> Cartilage is present until the small bronchi . In the trachea they are C - shaped rings of hyaline cartilage, whereas in the bronchi the cartilage takes the form of interspersed plates . Smooth muscle starts in the trachea, where it joins the C - shaped rings of cartilage . It continues down the bronchi and bronchioles, which it completely encircles . Instead of hard cartilage, the bronchi and bronchioles are composed of elastic tissue . </P> <P> The lungs are made up of thirteen different kinds of cells, eleven types of epithelial cell and two types of mesenchymal cell . The epithelial cells form the lining of the tracheal, and bronchial tubes, while the mesenchymal cells line the lungs . </P> <Ul> <Li> <P> Differences in cells along the respiratory tract . </P> </Li> <Li> <P> Transverse section of tracheal tissue . Note that image is incorrectly labeled "ciliated stratified epithelium" at upper right . </P> </Li> </Ul>

Where is the airway located in the body