tend to collapse inward. However, they adhere to one
another through the negative pressure of the pleural
space and suction created by the adhesive properties of
pleural fluid, so they act in unison, much like a spring.
Muscles Contract and Relax
to Move Air into and out
of the Lungs During Ventilation
Breathing in is called inhalation or inspiration. When the
respiratory muscles (such as the diaphragm and external
intercostals; Figure 13.3) contract during inhalation, they
expand the thoracic cavity (like pulling on a spring) and,
subsequently, the lungs. The increased volume of the tho-
racic cavity and lungs results in decreased pressure in the
alveoli. The alveolar pressure is now less than atmospheric
pressure, so air flows into the lungs. The more muscles
involved in the process, the more air that can be moved
into the lungs.
Breathing out is called exhalation or expiration
and begins when the respiratory muscles relax. During
normal exhalation, the elastic properties of the lungs
bring the thoracic cavity back to its original volume
(like letting go of a spring). The decreased volume of
the thoracic cavity increases the pressure in this space.
The pressure in the alveolus is now greater than the
atmospheric pressure, so air flows out of the lungs (see
Unlike quiet inhalation, there are no muscular con-
tractions involved in quiet exhalation; it is a passive
process. Exhalation becomes active only during forceful
breathing, such as in playing a flute or during exercise.
Forced exhalation involves contraction of muscles of the
thorax and abdomen (for example, internal intercostals,
external and internal obliques, transversus and rectus
abdominis), causing a further decrease in the size of the
thorax. This additional decrease in thoracic size increas-
es the pressure, and more air leaves the lungs.
MUSCLES OF INHALATION
the sternum during deep inhalations.
MUSCLES OF EXHALATION
two uppermost ribs during
pull up and
out on the ribs, thereby contributing
to expanding the thoracic cavity
during normal breathing.
is the major muscle of
inhalation. As the diaphragm
contracts, it descends and flattens,
thereby increasing the volume of the
External oblique muscles
move the lower ribs
downward and inward
during forced exhalation.
the abdominal viscera and force
the diaphragm upward during
• Internal oblique
• Transversus abdominis
• Rectus abdominis
Breathing involves Changes in Pressures and Volumes