Bronchi are the transition structures that air travels through between the right and left main bronchus, and the very small bronchioles. The air first travels from the mouth or nose through the larynx and trachea, and then the main bronchus.
When air is first inhaled, the oxygen in it cannot be immediately absorbed and used by the body. Each subsequent part of the respiratory system leading up to the bronchioles is smaller than the last. The bronchioles turn into small air sacs called alveoli, which allows oxygen to be taken in by the body and carbon dioxide to be eliminated.