The respiratory tree is so named because it has the appearance of an inverted tree, with its trunk and branches. The "trunk" of the tree is the trachea. The trachea extends from the inferior margin of the larynx, down through the neck, and into the center of the thorax.
In the center of the thorax, the trachea divides into right and left primary bronchi. The right is somewhat more vertical than the left. At the terminals of the branching tubes are groups of spherical alveoli. This gives the appearance of a bunch of grapes. A variety of situations may occlude these tubular air passageways.
External respiration takes place in the alveoli. . As we indicated earlier, external respiration is a surface phenomenon in which the gases pass through the wall of the alveolus.
Numerous blood capillaries are adjacent to the walls of the alveoli, which contain a special chemical known as surfactant. . Because the surfaces of the alveoli are wet, one of the major fluid losses of the body is with the exhaled air.
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