Section IV. DIAPHRAGMATIC ("ABDOMINAL") BREATHING
7-14. PHYSICAL CHARACTERISTICS OF THE ABDOMINOPELVIC CAVITY
a. The abdominopelvic cavity is a closed system filled with a fluid (water)
b. The abdominopelvic cavity is inclosed by essentially muscular barriers.
(1) The inferior end is closed off by the pelvic diaphragm.
(2) The cylindrical walls of the abdomen are composed of three muscular
sheets. Their orientation is similar to plywood. These muscles are kept taut by their
intrinsic tone, but they are capable of additional contraction.
(3) Forming the top of the abdominopelvic cavity is the thoracic diaphragm.
We discuss the thoracic diaphragm in the next paragraph.
7-15. THORACIC DIAPHRAGM
The thoracic diaphragm is attached to the inferior margin of the rib cage and to
the bodies of the lumbar vertebrae behind. As a muscular membrane, it domes upward
into the thoracic cavity. Upon contraction, the fibers of the thoracic diaphragm shorten
and pull downward. This downward motion produces a piston-like pressure on the
contents of the abdominopelvic cavity.
7-16. DIAPHRAGMATIC INHALATION
a. As the thoracic diaphragm contracts and lowers, the vertical diameter of the
thoracic cavity is increased. This increases the volume of the thoracic cavity. Thus,
according to Boyle's law, the pressure of the air in the lungs decreases. The relatively
higher atmospheric pressure outside pushes the air into the lungs, and the alveoli are
b. At the same time, the thoracic diaphragm produces a pistol-like pressure
upon the noncompressible fluid continuum in the abdominopelvic cavity. By Pascal's
law, the resulting pressure is distributed equally to the elastic walls of the cavity. As
these walls are stretched by the added pressure, they "store" potential energy.
7-17. DIAPHRAGMATIC EXHALATION
a. When the thoracic diaphragm relaxes, it no longer pushes down upon the
contents of the abdominopelvic cavity. The potential energy stored in the stretched
muscular walls becomes kinetic energy, and the walls rebound. This energy is sufficient
for exhalation during quiet breathing.