The respiratory tract includes the nose, pharynx (throat), larynx (voice box), trachea (windpipe), bronchus, and lung in this order. Lining the tract is a special ciliated epithelium. The nasal cavities are fashioned into tortuous passageways, and the trachea and primary bronchi are composed of cartilaginous rings strung together by connective tissue and smooth muscle. We also find smooth muscle throughout the small bronchi and bronchioles.

The terminal bronchioles enter alveolar ducts, and these in turn enter alveolar sacs, each of which resembles a bunch of hollow grapes. Each "grape" of the sac, continuing with the analogy, is called an alveolus, and because the alveolar wall is only one cell thick, gases easily diffuse across it between the air within and the blood in the surrounding capillaries. The terminal bronchioles, the alveolar sac, and the associated vessels together make up what is called a pulmonary unit; millions of these units compose the substance of the lungs.

Breathing
The chest, or thorax, is a marvelous piece of engineering. When the external intercostal muscles (between the ribs) and the diaphragm contract, the bony cage increases in all dimensions, almost beyond expectation. As a result, the pressure within drops, and the lungs are sucked out, causing an inrush of air. This is called inspiration. To effect expiration the body merely relaxes these muscles; that is, the diaphragm returns from a stretched out horizontal position to its normal rounded dome shape, and the intercostal muscles drop the ribs.

Forced breathing calls other muscles into play: the rhomboid muscles and levator scapulae muscles in the case of forced inspiration, and the innermost intercostals and abdominal muscles in the case of forced expiration. The volume of air moving to and for during quiet, normal breathing averages about 0.5 liter. This is called tidal air. The greatest volume of air that one is able to expel after the greatest possible inhalation averages about 4.5 liters. This is called vital capacity.