The University of Southern California Keck School of Medicine describes the bronchioles as hollow structures that allow air to travel in and out of the lungs. The four types of passageways in the lungs are the trachea, bronchus, bronchi and bronchioles.
In addition to allowing air to travel through the lungs, the American Lung Association states that bronchioles and the other airways are one of the body's defenses against disease and irritants. Mucus and minute, hair-like projections called cilia cover the interior of the airways. Cilia move in a wave-like motion that pushes the mucus out of the lungs. Mucus traps infectious agents, pollen and other debris, and a person discharges these contaminants when coughing.
According to the University of Southern California Keck School of Medicine, air enters the body through the mouth or nose, where it travels through the larynx and into the trachea. The trachea splits into two bronchus, with one entering each lung. Each bronchus further splits into bronchi, which become progressively smaller in diameter as they continue branching. The bronchi eventually branch into bronchioles, the smallest of the lung's passageways. Each bronchiole terminates in a cluster of aveoli, air sacs, which are the structures where oxygen enters the bloodstream and carbon dioxide leaves it.