Page 880 - Fundamentals of anatomy physiology

Checkpoint
10. List functions of the trachea.
11. Why are the cartilages that reinforce the trachea
C-shaped?
12. If food accidentally enters the bronchi, in which
bronchus is it more likely to lodge? Why?
See the blue Answers tab at the back of the book.
23-5 Enclosed by pleural cavities,
the lungs are paired organs containing
alveoli, which permit gaseous exchange
Learning Outcome Describe the superficial anatomy of the lungs, the
structure of a pulmonary lobule, and the functional anatomy of alveoli. contain progressively less cartilage. In the secondary and
The left and right lungs are surrounded by the left and right
pleural cavities, respectively (Figure 23–7). Each lung is a blunt
cone. Its tip, or apex, points superiorly. The apex on each side
extends superior to the first rib. The broad concave inferior
portion, or base, of each lung rests on the superior surface of
the diaphragm.
Lobes and Surfaces of the Lungs
The lungs have distinct lobes that are separated by deep fis-
sures (Figure 23–7). The right lung has three lobes—superior,
middle, and inferior—separated by the horizontal and oblique
fissures. The left lung has only two lobes—superior and in-
ferior—separated by the oblique fissure. The right lung is
broader than the left, because most of the heart and great
vessels project into the left thoracic cavity. However, the left
lung is longer than the right lung, because the diaphragm
rises on the right side to accommodate the mass of the
liver. The lateral and medial lung surfaces are shown in
Figure 23–7b,c.
The heart is located to the left of the midline, so its cor-
responding impression is larger in the left lung than in the
right. In anterior view, the medial edge of the right lung forms
a vertical line, but the medial margin of the left lung is indented
at the cardiac notch (Figure 23–7). Figure 23–8 shows the rela-
tionship between the heart and the lungs.
The Bronchi
The primary bronchi and their branches form the bronchial
tree. Because the left and right primary bronchi are outside
the lungs, they are called extrapulmonary bronchi. As the primary
bronchi enter the lungs, they divide to form smaller passage-
ways (Figure 23–6a). The branches within the lungs are collec-
tively called the intrapulmonary bronchi.
Each primary bronchus divides to form secondary bronchi,
also known as lobar bronchi. In each lung, one secondary
Chapter 23 The Respiratory System 867
bronchus goes to each lobe, so the right lung has three second-
ary bronchi, and the left lung has two.
Figure 23–9 depicts the branching pattern of the left pri-
mary bronchus as it enters the lung. (The number of branches
has been reduced for clarity.) In each lung, the secondary
bronchi branch to form tertiary bronchi, or segmental bron-
chi. The branching pattern differs between the two lungs,
but each tertiary bronchus ultimately supplies air to a single
bronchopulmonary segment, a specific region of one lung
(Figure 23–9a). The right lung has 10 bronchopulmonary seg-
ments. During development, the left lung also has 10 seg-
ments, but adjacent tertiary bronchi fuse, generally reducing
the number to 8 or 9.
The walls of the primary, secondary, and tertiary bronchi
tertiary bronchi, the cartilages form plates arranged around
the lumen. These cartilages serve the same structural purpose
as the rings of cartilage in the trachea and primary bronchi.
As the amount of cartilage decreases, the amount of smooth
muscle increases. With less cartilaginous support, the amount
of tension in those smooth muscles has a greater effect on
bronchial diameter and the resistance to airflow. During a
respiratory infection, the bronchi and bronchioles can be-
come inflamed and constricted, increasing resistance. In this
condition, called bronchitis, the individual has difficulty
breathing.
The Bronchioles
Each tertiary bronchus branches several times within a bron-
chopulmonary segment, forming many bronchioles. These
bronchioles then branch into the finest conducting branches,
called terminal bronchioles. Nearly 6500 terminal bronchi-
oles arise from each tertiary bronchus. The lumen of each ter-
minal bronchiole has a diameter of 0.3–0.5 mm.
The walls of bronchioles lack cartilage but are dominated
by smooth muscle tissue (Figure 23–9c). In functional terms,
23bronchioles are to the respiratory system what arterioles are
to the cardiovascular system. Changes in the diameter of the
bronchioles control the resistance to airflow and the distribu-
tion of air in the lungs.
The autonomic nervous system controls the diameter
of the bronchioles. It does so by regulating the activity in
the smooth muscle layer. Sympathetic activation leads to
bronchodilation, the enlargement of the diameter of the airway.
Parasympathetic stimulation leads to bronchoconstriction, a
reduction in the diameter of the airway. Bronchoconstriction
also takes place during allergic reactions such as anaphylaxis,
in response to histamine released by activated mast cells and
basophils. p. 849
Bronchodilation and bronchoconstriction are ways of ad-
justing the resistance to airflow. These actions direct airflow to-
ward or away from specific portions of the respiratory exchange