Air enters the lungs through the oral cavity or through the nasal passages, which join together in the pharynx (see Figure 48.10A). Below the pharynx, the esophagus conducts food to the stomach, and the trachea conducts air to the lungs. At the beginning of this airway is the larynx, or voice box, which houses the vocal cords. The larynx is the lump that you can see or feel on the front of your neck. The trachea is about 2 centimeters in diameter. C-shaped bands of cartilage prevent the thin walls of the trachea from collapsing as air pressure changes during the breathing cycle. If you run your fingers down the front of your neck just below your larynx, you can feel a few of these bands of cartilage.

The trachea branches into two bronchi, one leading to each lung. The bronchi branch repeatedly to generate a treelike structure of progressively smaller airways extending to all regions of the lungs. After four branchings, the cartilage supports disappear, marking the transition to bronchioles. After about 16 branchings, the bronchioles are less than a millimeter in diameter, and tiny, thin-walled air sacs called alveoli begin to appear. Alveoli are the sites of gas exchange. After the first alveoli there are about six more branchings of the airways that end in clusters of alveoli (see Figure 48.10B). Because the airways conduct air only to and from the alveoli and do not themselves participate in gas exchange, their volume is dead space.

Human lungs have about 300 million alveoli. Although each alveolus is very small, their combined surface area for diffusion of respiratory gases is about 70 square meters—about one-fourth the size of a basketball court. The walls of each alveolus are made of very thin cells. Wrapped around the alveoli are networks of capillaries that also have very thin walls. Thus where capillary meets alveolus, the diffusion path length between air and blood is less than 2 micrometers (see Figure 48.10C).

Diseases of the bronchioles and alveoli are the third leading cause of death in the United States as of 2014. Among these diseases, the most lethal is emphysema, a condition in which inflammation damages and eventually destroys the walls of the alveoli. As a result, the lungs have fewer but larger alveoli, the lungs lose elasticity, and the RV increases. The principal cause of emphysema is smoking.