Chapter 22
The Respiratory System
811
22
Te tissue composition of the walls of the main bronchi
mimics that of the trachea. However, as the conducting tubes
become smaller, the following structural changes occur:
Support structures change.
Irregular
plates
of cartilage re-
place the cartilage rings, and by the time the bronchioles are
reached, the tube walls no longer contain supportive carti-
lage. However, the tube walls throughout the bronchial tree
contain elastic fibers.
Epithelium type changes.
Te mucosal epithelium thins
as it changes from pseudostratified columnar to columnar
and then to cuboidal in the terminal bronchioles. Mucus-
producing cells and cilia are sparse in the bronchioles. For
this reason, most airborne debris found at or below the level
of the bronchioles must be removed by macrophages in the
alveoli.
Amount of smooth muscle increases.
Te relative amount of
smooth muscle in the tube walls increases as the passageways
become smaller. A complete layer of circular smooth mus-
cle in the bronchioles and the lack of supporting cartilage
(which would hinder constriction) allows the bronchioles to
provide substantial resistance to air passage under certain
conditions (as we will describe later).
Respiratory Zone Structures
Defined by the presence of thin-walled air sacs called
alveoli
(al-
ve
9
o-li;
alveol
5
small cavity), the respiratory zone begins as the
terminal bronchioles feed into
respiratory bronchioles
within
the lung (Figure 22.8). Protruding from these smallest bron-
chioles are scattered alveoli. Te respiratory bronchioles lead
into winding
alveolar ducts
, whose walls consist of diffusely
arranged rings of smooth muscle cells, connective tissue fibers,
and outpocketing alveoli. Te alveolar ducts lead into terminal
clusters of alveoli called
alveolar sacs
or
alveolar saccules
.
Many people mistakenly equate alveoli, the site of gas ex-
change, with alveolar sacs, but they are not the same thing. Te
alveolar sac is analogous to a bunch of grapes, and the alveoli
are the individual grapes. Te 300 million or so gas-filled alveoli
in the lungs account for most of our lung volume and provide a
tremendous surface area for gas exchange.
The Respiratory Membrane
Te walls of the alveoli are com-
posed primarily of a single layer of squamous epithelial cells,
called
type I alveolar cells
, surrounded by a flimsy basement
membrane. Te thinness of their walls is hard to imagine, but a
sheet of tissue paper is 15 times thicker!
Te external surfaces of the alveoli are densely covered with
a “cobweb” of pulmonary capillaries
(Figure 22.9)
. ±ogether,
the capillary and alveolar walls and their fused basement mem-
branes form the
respiratory membrane
, a 0.5-μm-thick
blood
air barrier
that has blood flowing past on one side and gas on
the other (Figure 22.9c). Gas exchanges occur readily by simple
diffusion across the respiratory membrane—O
2
passes from the
alveolus into the blood, and CO
2
leaves the blood to enter the
gas-filled alveolus.
Scattered amid the squamous type I alveolar cells that form
the major part of the alveolar walls are cuboidal type II al-
veolar cells (Figure 22.9c).
Type II alveolar cells
secrete a
fluid containing a detergent-like substance called
surfactant
that coats the gas-exposed alveolar surfaces. (We describe sur-
factant’s role in reducing the surface tension of the alveolar
fluid later in this chapter.) ±ype II alveolar cells also secrete a
number of antimicrobial proteins that are important elements
of innate immunity.
Trachea
Superior lobe
of right lung
Middle lobe
of right lung
Inferior lobe
of right lung
Superior lobe
of left lung
Left main
(primary)
bronchus
Lobar (secondary)
bronchus
Segmental (tertiary)
bronchus
Inferior lobe
of left lung
Figure 22.7
Conducting zone passages.
The air pathway inferior to the larynx consists of
the trachea and the main, lobar, and segmental bronchi, which branch into the smaller bronchi
and bronchioles until reaching the terminal bronchioles of the lungs.
previous page 845 Human Anatomy and Physiology (9th ed ) 2012 read online next page 847 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off