The larynx, commonly called the voice box (Fig. 14-3), is located between the pharynx and the trachea. It has a framework of cartilage, part of which is the thyroid cartilage that protrudes in the front of the neck. The projection formed by the thyroid cartilage is commonly called the Adam’s apple because it is considerably larger in the male than in the female. Folds of mucous membrane used in producing speech are located on both sides at the superior portion of the larynx. These are the vocal folds, or vocal cords (Fig. 14-4), which vibrate as air flows over them from the lungs. Variations in the length and tension of the vocal cords and the distance between them regulate the pitch of sound. The amount of air forced over them regulates volume. A difference in the size of the larynx and the vocal cords is what accounts for the difference between adult male and female voices. In general, a man’s larynx is larger than a woman’s. His vocal cords are thicker and longer, so they vibrate more slowly, resulting in a lower range of pitch. Muscles of the pharynx, tongue, lips, and face also are used to form clear pronunciations. The mouth, nasal cavities, paranasal sinuses, and the pharynx all serve as resonating chambers for speech, just as does the cabinet for an audio speaker. The space between the vocal cords is called the glottis. This is somewhat open during normal breathing but widely open during forced breathing (see Fig. 14-4). The little leaf-shaped cartilage that covers the larynx during swallowing is called the epiglottis.
The glottis and epiglottis help keep food and liquids out of the remainder of the respiratory tract. As the larynx moves upward and forward during swallowing, the epiglottis moves downward, covering the opening into the larynx. You can feel the larynx move upward toward the epiglottis during this process by placing the flat ends of your fingers on your larynx as you swallow. Muscles in the larynx assist in keeping foreign materials out of the respiratory tract by closing the glottis during swallowing. Muscles also close the glottis when one holds his or her breath and strains, as to defecate or lift a heavy weight.
Figure 14-3 The larynx. (A) Anterior view. (B) Posterior view.
Figure 14-4 The vocal cords, superior view. (A) The glottis in closed position. (B) The glottis in open position.
The trachea, commonly called the windpipe, is a tube that extends from the inferior edge of the larynx to the upper part of the chest superior to the heart.
The trachea’s purpose is to conduct air between the larynx and the lungs. A framework of separate cartilages reinforces the trachea and keeps it open. These cartilages, each shaped somewhat like a tiny horseshoe or the letter C, are found along the trachea’s entire length. The open sections in the cartilages are lined up at their posterior so that the esophagus can expand into this region during swallowing.
At its inferior end, the trachea divides into two primary, or main-stem, bronchi, which enter the lungs (see Fig. 14-2). The right bronchus is considerably larger in diameter than the left and extends downward in a more vertical direction. Therefore, if a foreign body is inhaled, it is likely to enter the right lung. Each bronchus enters the lung at a notch or depression called the hilum. Blood vessels and nerves also connect with the lung in this region.
Figure 14-2 The respiratory system. (A) Overview. (B) Enlarged section of lung tissue showing the relationship between the alveoli (air sacs) of the lungs and the blood capillaries. (C) A transverse section through the lungs.
The Lining of the Air Passageways The trachea, bronchi, and other conducting passageways of the respiratory tract are lined with a special type of epithelium (Fig. 14-5). Basically, it is simple columnar epithelium, but the cells are arranged in such a way that they appear stratified. The tissue is thus described as pseudostratified, meaning “falsely stratified.” These epithelial cells have cilia to filter out impurities and to create fluid movement within the conducting tubes. The cilia beat to drive impurities toward the throat, where they can be swallowed or eliminated by coughing, sneezing, or blowing the nose.
Figure 14-5 Microscopic view of ciliated epithelium. Ciliated epithelium lines the respiratory passageways, as shown here in the trachea.