The wall of the trachea is made up of four distinct tissue
layers.
Along the luminal surface, the trachea is lined by respiratory mucosa (mucous
membrane). Goblet cells found in the epithelium produce mucus, which warms, moistens,
and removes foreign particles from the air as it flows through the trachea.
Deep to the mucosa is the submucosa. Like the lamina propria, the submucosa is
primarily composed of loose (= areolar) connective tissue. Many blood vessels,
neurons, and glands are also present. The (seromucous) glands secrete a combination
of water and mucus to the luminal surface of the trachea through narrow ducts.
The mucus adds to that secreted by the goblet cells.
External to the submucosa is a layer containing sixteen to twenty hyaline cartilage
rings. In back, the cartilaginous rings are incomplete, giving them a characteristic
C-shape. When large food particles pass down the esophagus, the tracheal wall
is able to partially collapse due to the open rings. The opposing ends of each
open ring are attached by the trachealis muscle. This small band of smooth muscle
controls the lumen size of the trachea. When breathing out the muscle contracts
to help force the air out of the trachea, and when breathing in the muscle relaxes.
The outer-most layer of the trachea, the adventitia, is a band of loose connective
tissue that primarily functions to loosely hold the trachea in place in the chest
cavity.
