At the fifth thoracic vertebra in the mediastinum, the trachea divides into the right and left primary bronchi. The bronchi branch out into the volume of the lungs and get smaller and smaller until they transform into tiny air sacs called alveoli. Structurally similar to the trachea of cartilage and mucous membrane, the right bronchus is slightly larger than the left one, so often foreign objects breathed into the lungs more often end up in the right bronchus. Deeper into the lungs, each bronchus branches out further into five smaller secondary bronchi that provide air to the different lobes of the lungs. As the branching out continues, the amount of hyaline cartilage in the walls decreases until it is absent in the smallest bronchioles. The amount of smooth muscle, on the other hand, increases as the hyaline cartilage decreases. Also, the mucous membrane transitions from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium. In each lung, there are as many as 30,000 tiny bronchioles. This whole section of bronchi and bronchioles is called the Bronchial tree.