Bronchi
Once air enters the trachea, or windpipe, it passes into the bronchi, or bronchial passages. These are the primary airways in the lungs; they deliver air to the alveoli, where the actual exchange of gases (oxygen for CO2) takes place.
The trachea divides into the two "mainstem" bronchi (left and right) just behind the sternum. Because of the location of the heart, the right bronchial passage is larger and wider than the left. The one on the right divides into three secondary, or lobar bronchi; the left divides into two. The lobar bronchi then divide into several tertiary bronchi. These in turn branch into several bronchioles, which are known as primary and terminal. The terminal bronchioles branch into the alveolar ducts, each of which lead to five or six alveolar sacs.The entire arrangement resembles an inverted tree.
The tertiary bronchial systems are divided into redundant sections, which are known as bronchopulmonary segments. Because of this arrangement, it is possible to have a portion of a lung removed without the patient losing all respiratory function.
Sources
Maton, Anthea, et. al. Human Biology and Health. (Englewood Cliffs: Prentice Hall, 1993).
University of Maryland Medical Center, "Respiratory Disease: Anatomy of the Respiratory System." http://www.umm.edu/respiratory/anatomy.htm (accessed 9 October 2007).