The Trachea

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The trachea (or windpipe) is a wide, hollow tube that connects the larynx (or voice box) to the bronchi of the lungs. It is an integral part of the body's airway and has the vital function of providing air flow to and from the lungs for respiration.

The trachea begins at the inferior end of the larynx in the base of the neck. It is located along the body's midline, anterior to the esophagus and just deep to the skin, so that it is possible to feel the larynx through the skin of the neck. From its origin at the larynx, the trachea extends inferiorly into the thorax posterior to the sternum. In the thorax, the trachea ends where it splits into the left and right bronchi, which continue onward toward the lungs.

Viewed in cross section, the trachea is about one inch (2.6 cm) in diameter. It has a thin, membranous wall with C-shaped rings of cartilage embedded into it. Between sixteen and twenty cartilage rings are stacked along the length of the trachea, with narrow membranous regions spaced between the cartilage rings. The open ends of the cartilage rings face the posterior of the trachea near the esophagus.

Four layers of tissues make up the walls of the trachea:

• The mucosa is the innermost layer and consists of ciliated pseudostratified columnar epithelium with many goblet cells. Goblet cells produce sticky mucus to coat the inner lining of the trachea and catch any debris present in inhaled air before it reaches the lungs. On the surface of the columnar cells, long, hair-like cilia beat together to push mucous away from the lungs like a microscopic conveyor belt. Mucus from the trachea, along with any trapped contaminants, makes its way to the larynx, where it is either expelled during coughing or swallowed and digested in the stomach.
• Deep to the mucosa is the submucosa layer, which is made of areolar connective tissue containing blood vessels and nervous tissue. Many collagen, elastin and reticular protein fibers give soft support and elasticity to the wall of the trachea, while blood vessels and nerves support the other layers of the tracheal wall. Longitudinal smooth muscle fibers are present in the posterior trachea between the ends of the cartilage rings. This smooth muscle tissue allows the trachea to adjust its diameter as needed.
• Surrounding the submucosa is a layer of hyaline cartilage that forms the supportive rings of the trachea. Hyaline provides a strong, yet flexible structure that maintains an open airway and is resistant to external stresses.
• The outermost layer of the trachea is the adventitia, a layer of areolar connective tissue that loosely anchors the trachea to the surrounding soft tissues.

While the trachea plays a vital role as a passive air passageway, it also performs several other important functions as well. The trachealis muscle in the posterior wall allows the trachea to contract and reduce its diameter, which makes coughs more forceful and productive. During the process of swallowing food, the esophagus expands into the space normally occupied by the trachea. The incomplete cartilage rings of the trachea allow it to narrow and permit the esophagus to expand into its space. Finally, the loose connection of the adventitia allows the trachea to move within the neck and thorax, aiding the lungs in their expansion and contraction during breathing.