The lungs are enclosed in the pleural cavity and sur-
rounded by a pleural membrane. The pleural membrane
forms pleural fluid, which helps the lungs adhere to the
thoracic wall, allowing the movement of the thoracic wall
to draw air in and pushing it out of the lungs.
The lungs extend from the diaphragm to slightly above
the clavicles and lie against the ribs. The broad bottom por-
tion of each lung is its
base
; the narrow top portion is the
apex.
The left lung has an indentation in which the heart lies
(cardiac notch). Because of the space occupied by the heart,
the left lung is about 10% smaller than the right lung.
Deep grooves called
fissu res
divide each lung into lobes.
The left lung has only two lobes, the superior and inferior
lobes. The right lung has three lobes: the superior, middle,
and inferior lobes. Each lobe receives its own secondary
bronchus.
Within the lungs are tiny air
sacs called
alveoli
, which attach to
the alveolar ducts. Gas exchange
occurs between the lungs and the
blood at the alveoli. Around the alveoli, the pulmonary ar-
teriole and venule form lush networks of blood capillaries.
The alveoli have thin walls for gas exchange with the red
blood cells and plasma that carry oxygen and carbon diox-
ide (
Figure 13.2
). Gas exchange between the air spaces
in the lungs and the blood takes place by diffusion across
the alveolar and capillary walls. This exchange provides
the oxygen needed by our cells, eliminates carbon dioxide
produced by the body cells, and plays a crucial role in regu-
lating the pH of the blood (a topic we will examine more
closely in Chapter 15).
The
lungs
contain
roughly
300 million alveoli. They provide a
huge surface area for the exchange
of oxygen and carbon dioxide—
about 30 to 40 times greater than
the surface area of your skin or
half the size of a tennis court.
Within the alveoli, specialized
cells secrete
surfactant
, a chemi-
alveolus
(al-VE-o-lus;
singular is
a lv e o lu s )
An air sac in the lungs.
surfactant
(sur-FAK-
tant) A complex mix-
ture of phospholipids
and lipoproteins that
decreases surface ten-
sion in the alveoli and
makes the lungs more
compliant (flexible).
B ronchioles and alveoli • Figure 13.2
a.
Alveolar sacs are two or more alveoli that share a common
opening into an alveolar duct.
b.
The exchange of respiratory
gases occurs by diffusion across the alveolar and capillary walls,
which together form the respiratory membrane.
a.
Bronchioles and
alveolar
sacs
Terminal
bronchiole
BRANCHING OF
BRONCHIAL TREE:
Trachea
I
Primary bronchi
Respiratory
bronchiole
Alveoli
Alveolar
ducts
Alveolar
sac
Secondary bronchi
Tertiary bronchi
Bronchioles
Alveoli
I
Terminal bronchioles
1
Respiratorybronchioles
Microscopic
^
airways
Alveolar ducts
I
Alveolar sacs
I
Alveoli
374 CHAPTER 13
The Respiratory System
previous page 409 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online next page 411 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online Home Toggle text on/off