518
U N I T 6
Respiratory Function
convert angiotensin I to angiotensin II, and serve as a
reservoir for blood storage. Heparin-producing cells
are particularly abundant in the capillaries of the lung,
where small clots may be trapped.
Pleura
The lungs are encased in a thin double-layered closed
sac, called the
pleura
(see Fig. 21-1). The outer pari-
etal layer of the pleural sac lines the pulmonary cavities
and adheres to the thoracic wall, the mediastinum, and
the diaphragm. The inner visceral pleura closely covers
the lung and is adherent to all its surfaces. It is con-
tinuous with the parietal pleura at the hilus of the lung,
where the major bronchus and pulmonary vessels enter
and leave the lung. A thin film of serous fluid separates
the two layers, allowing them to glide over each other,
yet hold together so there is no separation between the
lungs and the chest wall. The pleural cavity, or space
between the two pleural layers, is also a potential space
in which serous fluid or inflammatory exudate can accu-
mulate. The term
pleural effusion
is used to describe an
abnormal collection of fluid in the pleural cavity.
Respiratory Lobules
The gas exchange function of the lung takes place in
the
respiratory lobules
of the lungs. Each lobule, which
is the smallest functional unit of the lung, is supplied
by a terminal bronchiole, alveoli, and pulmonary blood
vessels (Fig. 21-7). Blood enters the lobules through a
pulmonary artery and exits through a pulmonary vein.
Lymphatic structures surround the lobule and aid in the
removal of plasma proteins and other particles from the
interstitial spaces.
The alveoli, which consist of alveolar ducts and sacs,
are the terminal air spaces of the respiratory tract and
the actual sites of gas exchange between the air and the
blood. The
alveolar ducts
are elongated airways that
have almost no walls at their peripheral boundary. The
alveolar sacs
are cup-shaped, thin-walled structures
that are separated from each other by thin alveolar
septa. A single network of capillaries occupies most
of the septa, so blood is exposed to air on both sides.
There are approximately 300 million alveoli in the
adult lung, with a total surface area of approximately
50 to 100 m
2
. Unlike the bronchioles, which are tubes
with their own separate walls, the alveoli are intercon-
necting spaces that have no separate walls. As a result
of this arrangement, there is a continual mixing of air
in the alveolar structures. Small holes in the walls of
adjacent alveoli, the minute
pores of Kohn
, contribute
to the mixing of air.
The alveolar epithelium is composed of two types of
cells: type I and type II alveolar cells (Fig. 21-8).
Type I
alveolar cells
are extremely thin squamous cells with a
thin cytoplasm and flattened nucleus that occupy about
95% of the surface area of the alveoli. Type I alveolar
cells are not capable of regeneration.
Type II alveolar
cells,
which are found interspersed between the type I
alveolar cells, are secretory cells that produce the sur-
face-active agent called
surfactant.
In addition to secret-
ing surfactant, type II alveolar cells are the progenitor
cells for type I cells. Following lung injury, they prolifer-
ate and restore both type I and type II alveolar cells. The
alveoli also contain brush cells and macrophages. The
brush cells, which are few in number, are thought to act
as receptors that monitor the air quality of the lungs.
The
surfactant molecules
produced by the type II
alveolar cells reduce the surface tension at the air-epithe-
lium interface, and they modulate the immune functions
of the lung. Recent research has identified four types of
surfactant, each with a different molecular structure:
surfactant proteins A, B, C, and D. Surfactants B and C
serve to reduce the surface tension at the air-epithelium
interface and increase lung compliance and ease of lung
inflation. Surfactant B is particularly important to the
generation of the surface-reducing film that makes lung
expansion possible (to be discussed). Surfactants A and
D do not reduce surface tension, but contribute to innate
immune defenses that protect against pathogens that
have entered the lung. They bind pathogens, damage
microbial membranes, regulate microbial phagocytosis,
and activate or deactivate the inflammatory response
(see Chapter 16).
The
alveolar macrophages
, which are present in
both the connective tissue of the septum and in the air
spaces of the alveolus, are responsible for the removal
of offending substances from the alveoli (see Fig. 21-8).
In the air spaces, they scavenge the surface to remove
inhaled particulate matter, such as dust and pollen.
Some macrophages move up the bronchial tree in the
mucus and are disposed of by swallowing or coughing
when they reach the pharynx. Others enter the septal
connective tissue where, filled with phagocytosed mate-
rials, they remain for life. Thus, at autopsy, the lungs
Lymphatics
Bronchiole
Smooth
muscle
Pulmonary
artery
Alveoli
Pulmonary
capillaries
Pulmonary
vein
Air
Pores
of Kohn
FIGURE 21-7.
Lobule of the lung, showing the bronchial
smooth muscle fibers, pulmonary blood vessels, and
lymphatics.
