The bronchii are the tubes which carry ~air~ from the |trachea| to the inner recesses of the |lungs|, where it can transfer oxygen to the ~blood~ in small ~air~ sacs called |alveoli|. Two main bronchii, the right and left |bronchus|, branch off of the low end of the |trachea| in what is called the tracheal bifurcation. One |bronchus| extends into each of the right and left |lung|. The bronchii continue to divide into smaller passageways, called |bronchioles|, forming a tree-like network of branches which extends throughout the spongy |lung| tissue. The exterior of the bronchii are composed of cartilaginous, elastic |fiber| and feature annular reinforcements of |smooth muscle| tissue. The bronchii are able to distend during inspiration, to allow the |lungs| to expand, and contract during expiration as ~air~ is exhaled.
