Figure 3-37.Pulmonary exchange at alveoli.
a diffusion process through the alveolar and
capillary cell walls (fig. 3-37). The LUNGS are
cone-shaped organs that lie in the thoracic cav-
ity. Each lung contains thousands of alveoli with
their capillaries. The right lung is larger that the
left and is divided into superior, middle, and in-
ferior lobes. The left lung has two lobes, the
superior and the inferior.
The PLEURAE are airtight membranes that
cover the outer surface of the lungs and line the
chest wall. They secrete a serous fluid that
prevents friction during movements of respiration.
Pleurisy is a painful inflammation of the pleural
The MEDIASTINUM is the interpleural space
between the two lungs. It extends from the ster-
num to the thoracic vertebrae and from the fascia
of the neck to the diaphragm. It contains the
heart, the great blood vessels, the esophagus, a
portion of the trachea, and the primary bronchi.
The DIAPHRAGM is the primary muscle of
respiration. It is dome-shaped and separates the
thoracic and abdominal cavities. Contraction of
the muscle flattens the dome and expands the ver-
tical diameter of the chest cavity.
The INTERCOSTAL MUSCLES are situated
between the ribs. Their contraction pulls the ribs
upward and outward, resulting in an increase in
the transverse diameter of the chest (chest
INHALATION is the direct result of the ex-
pansion caused by the action of the diaphragm
and intercostal muscles. The increase in chest
volume creates a negative (below atmospheric)
pressure in the pleural cavity and lungs. Air rushes
into the lungs through the mouth and nose to
equalize the pressure. EXHALATION results
when the muscles of respiration relax. Pressure
is exerted inwardly as muscles and bones return
to their normal position, forcing air from the
THE PROCESS OF RESPIRATION
The rhythmical movements of breathing are
controlled by the respiratory center in the brain.
Nerves from the brain pass down through the neck
to the chest wall and diaphragm. The nerve to the
diaphragm is called the phrenic nerve; the nerve
to the larynx is the vagus nerve; and those to the
muscles between the ribs are the intercostal nerves.
The respiratory center is stimulated by
chemical changes in the blood, especially if it
becomes acidic. When too much carbon dioxide
accumulates in the blood stream, the respiratory
center signals the lungs to breathe faster to get
rid of the carbon dioxide.
The respiratory center can also be stimulated
or depressed by a signal from the brain. For ex-
ample, changes in ones emotional state can alter
respiration through laughter, crying, emotional
shock, or panic.
The muscles of respiration normally act
automatically, with normal respiration being 14
to 18 cycles per minute. The lungs, when filled
to capacity, hold about 6,500 ml of air, but only
500 ml of air is exchanged with each normal
respiration. This exchanged air is called TIDAL
AIR. The amount of air left in the lungs after
forceful exhalation is about 1,200 ml and is known
as RESIDUAL AIR. The existence of this reserve
is the basis for administering the abdominal thrust
maneuver, described in the chapter entitled First
Aid and Emergency Procedures. In this life-
saving procedure, the residual air is used to force
a foreign object out of the trachea.
ABNORMALITIES OF BREATHING
The following terms are used to describe
breathing and significant variations in exchanges
of respiratory gases:
EUPNEA is ordinary quiet respiration.