Food or liquid forces the esophagus open during swallowing; between swallows, this tube collapses. The trachea cannot collapse, for air is not pushed into this tube. It is sucked in, and the tube must stay open at all times. Its wall is therefore braced and stiffened with C-shaped rings of cartilage. The open ends of each ring are linked by smooth muscle and elastic connective tissue that forms the back wall of the trachea. The trachea extends from the larynx to just above the level of the heart, where it divides to form the primary bronchi; the left primary bronchus carries air to the left lung and the right primary bronchus carries it to the right. In the lungs, the primary bronchi branch into secondary bronchi, which carry air into each of the lung lobes. There, the secondary bronchi branch into tertiary bronchi; these in turn branch into the bronchioles. Further branching yield the terminal bronchioles, which deliver air to the actual tissue of the lungs.

Like the trachea, the bronchi are stiffened and kept open by rings of cartilage. As the branching of the bronchi proceeds, the rings are replaced by plates of cartilage. They disappear entirely in the bronchioles. As the amount of cartilage diminishes, the relative amount of smooth muscle in the walls of the respiratory tubes increases. Irritation of the lining of the trachea, bronchi, and bronchioles, as by an allergy or infection, can cause spasms of the smooth muscle that narrow or close the airway. Such spasms coupled with the increased secretion of mucus that accompanies an allergic response, account for the wheezing and respiratory difficulty of asthma.

The trachea, bronchi, and bronchioles are lined with ciliated epithelium coated with a layer of mucus. The mucus traps dust in inhaled air. The cilia then propel the dust-laden mucus upward to the pharynx, where it can be swallowed. Tobacco smoke contains substances that both cause the epithelial cells to lose their cilia and inhibit the action of remaining cilia. In smokers, mucus transport is thus impaired. Yet the mucus must still be removed from the airway. Smokers accomplish this task with the hacking cough for which they are so infamous.