General Histology
The respiratory tract is covered in an epithelium, the type of which varies down the tract. There are glands and mucus produced by goblet cells in parts, as well as smooth muscle, elastin or cartilage.
Most of the epithelium (from the nose to the bronchi) is covered in pseudostratified columnar ciliated epithelial cells, commonly called respiratory epithelium. The cilia beat in one direction, moving mucus towards the throat where it is swallowed. Moving down the bronchioles, the cells get more cuboidal in shape but are still ciliated.
Cartilage is present until the small bronchi. In the trachea they are C-shaped rings, whereas in the bronchi they are interspersed plates.
Glands are abundant in the upper respiratory tract, but there are fewer lower down and they are absent starting at the bronchioles. The same goes for goblet cells, although there are scattered ones in the first bronchioles.
Smooth muscle starts in the trachea, where it joins the C-shaped rings of cartilage. It continues down the bronchi and bronchioles, which it completely encircles.
Instead of hard cartilage, the bronchi and bronchioles are composed of elastic tissue.
Read more about this topic: Respiratory Tract
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