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In humans the respiratory tract is the part of the anatomy that
has to do with the process of respiration or breathing.
This includes the nose and nasal passages, the pharynx, the trachea, bronchi and bronchioles which lead to the alveoli of the lungs.
The respiratory tract is a common site for infections. Upper respiratory tract infections are probably the most common infections in the
world.
Most of the respiratory tract exists merely as a piping system for air to travel in the lungs; alveoli are the only part of the lung that exchanges oxygen and carbon dioxide with the blood.
Moving down the respiratory tract starting at the trachea, the tubes get smaller and divide more and more. There are estimated
to be about 20 to 23 divisions, ending up at an alveolus.
Even though the cross-sectional area of each bronchus or bronchiole is smaller, because there are so many, the total surface
area is larger. This means there is less resistance at the terminal bronchioles. (Most resistance is around the 3-4 division from
the trachea due to turbulence.)
General histology
The respiratory tract is covered in an epithelium, the type of which varies
down the tract. There are glands and mucus
producing 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. 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 from the bronchioles
onwards. 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 brochi and
bronchioles which it completely encircles.
Instead of hard cartilage, the bronchi and brochioles have a lot of elastic tissue.
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