lobes of the right lung and 8 among the two lobes
of the left lung. A bronchial cluster—which
includes bronchi, bronchioles, alveoli, blood ves-
sels, LYMPH VESSELS, and nerves—branches into
each segment.
The cells of the respiratory tract are primarily
epithelial cells, the same kind of cells that make
up the SKIN. The epithelial cells lining the trachea
and bronchi contain cilia, tiny hairlike projections
that sweep debris, such as dust and pollen, and
excess mucus from the respiratory tract outward
to the throat for coughs to expel them from the
body. A tissue-thin membrane, the PLEURA, covers
the outer surface of the lungs. The pleura secretes
serous fluid to lubricate the lungs in their perpet-
ual movement, protecting them from friction and
irritation. Lung tissue is highly porous, with the
substance of the lungs being about 15 percent
solid tissue and 85 percent air and blood.
The heart pumps the body’s full volume of
blood—about five liters—through the lungs once
each minute to pick up oxygen and release carbon
dioxide. The blood, which flows through the lungs
in a closed circuit from the heart via the PUL-
MONARY ARTERIESand back to the heart via the PUL-
MONARY VEINS, pulses through a dense, meshlike
capillary network surrounding the alveoli. Pul-
monary and cardiovascular mechanisms maintain
an intricate balance between the flow of blood
and the flow of air, with the blood flow constantly
adjusting to flood into CAPILLARY BEDSsurrounding
alveoli that have strong air flow and recede from
those with diminished air flow. This balance pro-
vides the greatest efficiency for getting oxygen
into the bloodstream.
The bronchial tree: trachea, bronchi, bronchi-
oles, and alveoli Like a hollow trunk, the trachea
supports the treelike bronchial structure that
brings air into the lungs. The trachea extends from
the back of the throat about four and a half inches
down to the midchest, where it branches into the
right BRONCHUSand left bronchus. The spine in the
back and the sternum in the front protect the tra-
chea’s path. The trachea’s most vulnerable expo-
sure is at the front of the neck, where it passes in
front of the ESOPHAGUSbefore dropping behind the
sternal notch. C-shaped bands of CARTILAGEhelp
protect the trachea as well as give it the rigidity
necessary to maintain an open passageway against
ever-changing air pressures. Long fibers of smooth
MUSCLEcomplete the posterior wall of the trachea.
The trachea terminates with the branching of
the right main bronchus, going to the right lung,
and left main bronchus, going to the left lung.
Like the trachea, the bronchi contain smooth
muscle with rings of cartilage for support and
STRENGTH. The smooth muscle fibers of the trachea
and the bronchi contract and expand in response
to the air pressure changes of inhalation and
exhalation. Each bronchus quickly divides to
branch to the lung’s lobes (lobular bronchi), and
further subdivides into segmental bronchi,
branching bronchi, and ultimately the very tiny
and cartilage-free bronchioles. The alveoli cluster
at the ends of the bronchioles.
The alveoli are the work stations of the lungs,
and each lung contains about 150 million of them.
Each microscopic alveolus looks like a small sac;
an alveolar cluster contains dozens of alveoli that
bubble from the end of a bronchiole. The alveolar
membrane, the thickness of a single cell, forms the
interface between the pulmonary system and the
cardiovascular system, allowing the oxygen and
carbon dioxide molecules to cross from the air
within the lungs and the blood within the capillar-
ies. Their bunched arrangement vastly extends the
surface area available for oxygen exchange within
the confined space of the chest. The total surface
area of the alveoli, if spread out flat, is about the
size of a tennis court.
Breathing: mechanics, physics, and molecular
exchange The balance between oxygen and car-
bon dioxide in the blood regulates pulmonary res-
piration (BREATHING). As carbon dioxide from
cellular METABOLISMaccumulates in the blood, it
reaches a threshold that triggers a sequence of bio-
chemical signals to the brainstem. The brainstem
then initiates a RESPIRATORY CYCLE, sending NERVE
signals that trigger the diaphragm (the flat, broad
muscle that forms the base of the thoracic cavity)
and the intercostal muscles (the muscles between
the ribs) to contract. In response the diaphragm
flattens, pulling the floor of the thoracic cavity
downward. The intercostal muscles simultane-
ously contract to pull the ribs outward and
upward. The synchronized movements enlarge
the thoracic cavity, drawing air into the lungs.
When the diaphragm and the intercostal musclesThe Pulmonary System 177