shaped sheet of skeletal muscle called the
wall of the thorax is formed by the spinal column, the ribs,
the breastbone (sternum), and several groups of muscles that
run between the ribs that are collectively called the
The thoracic wall also contains large amounts of
connective tissue with elastic properties.
Each lung is surrounded by a completely closed sac, the
consisting of a thin sheet of cells called
two pleural sacs are completely separate from each other. The
relationship between a lung and its pleural sac can be visualized
by imagining what happens when you push a ﬁ st into a ﬂ
ﬁ lled balloon. The arm shown in
major bronchus leading to the lung, the ﬁ st is the lung, and the
balloon is the pleural sac. The ﬁ st becomes coated by one sur-
face of the balloon. In addition, the balloon is pushed back upon
itself so that its opposite surfaces lie close together but are sepa-
rated by a thin layer of ﬂ uid. Unlike the hand and balloon, the
pleural surface coating the lung known as the
Functions of the Conducting Zone
of the Airways
1. Provides a low-resistance pathway for air ﬂ ow. Resistance
is physiologically regulated by changes in contraction of
airway smooth muscle and by physical forces acting upon the
2. Defends against microbes, toxic chemicals, and other
foreign matter. Cilia, mucus, and macrophages perform this
3. Warms and moistens the air.
4. Phonates (vocal cords).
Relationships between blood vessels and airways.
(a) The lung appears transparent so that the
relationships are visible. The airways beyond the
bronchioles are too small to be seen. (b) An enlargement
of a small section of Figure 13–3a shows the
continuation of the airways and the clusters of alveoli at
their ends. Virtually the entire lung, not just the surface,
consists of such clusters. Red represents oxygenated
blood; blue represents deoxygenated blood.