pharynx (throat) and the
larynx (voice box), as
it makes its way to the
trachea (Figure 134). The trachea's primary job is to carry exhaled air back
out of the body and to direct inhaled air to the lungs. The trachea of a human being is a tube with a
diameter of 2 cm and a length of 10 to 12 cm. It is located in front of the esophagus and extends from
the larynx into the chest cavity before splitting into two primary bronchi at the midthorax. It is
constructed of incomplete hyaline cartilage rings and smooth muscle (Figure 135). Goblet cells that
produce mucus and ciliated epithelia line the trachea. The cilia direct foreign objects entrapped in
mucus in the direction of the pharynx. To keep the trachea open, the cartilage gives it strength and
support. When the smooth muscle contracts, the trachea's diameter decreases, which increases the
force with which exhaled air rushes from the lungs. When we cough, the forced exhalation helps clear
the mucus. Smooth muscle can contract or relax, depending on stimuli from the external environment
or the body’s nervous system.
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(credit: modification of work by NCI)
198
Figure 135. The trachea and bronchi are made of incomplete rings of cartilage
106
.
Lungs: Bronchi and Alveoli The left and right lungs are reached by the bifurcation (dividing) of the trachea's end. The
lungs do not match up exactly. The larger right lung has three lobes, while the smaller left lung only
has two lobes (Figure 136). The muscular
diaphragm , which facilitates breathing, is inferior to
(below) the lungs and marks the end of the thoracic cavity.
Figure 136. In the lungs, the trachea divides into the right and left bronchi. The right lung is bigger and has
three lobes. The left lung has only two lobes and is smaller to accommodate the heart.
106
(credit: modification of work by Gray's Anatomy)
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In the lungs, air is diverted into smaller and smaller passages, or
