COSTAL (THORACIC) BREATHING
Muscles attached to the thoracic cage (rib cage) cause the chest to expand
during inhalation and return to normal during exhalation.
a. Inhalation. Muscles attached to the thoracic cage contract and raise the ribs.
A typical rib might be compared to a bucket handle, attached at one end to the sternum
(breastbone) and at the other end to the vertebral column (spine). The "bucket handle"
is lifted by the overall upward and outward movement of the rib cage. These
movements increase the thoracic diameters from right to left (transverse) and from front
to back. Thus, the volume within the chest increases. Recalling Boyle's law, the
increase in volume leads to a decrease in pressure. The higher air pressure outside the
body then forces air into the lungs and inflates them.
b. Exhalation. The rib cage movements and pressure relationships are
reversed for exhalation. The muscles relax, the ribs return to their normal position, and
the size of the chest decreases. The pressure within the chest increases and forces air
outside the body.
DIAPHRAGMATIC (ABDOMINAL) BREATHING
The diaphragm is a thin, but strong, dome-shaped muscular membrane that
separates the abdominal and thoracic cavities. The abdominal wall is elastic in nature.
The abdominal cavity is filled with soft, watery tissues.
a. Inhalation. As the diaphragm contracts, the dome flattens and the
diaphragm descends. This increases the depth (vertical diameter) of the thoracic cavity
and thus increases its volume. This decreases air pressure within the thoracic cavity.
The greater air pressure outside the body then forces air into the lungs.
b. Exhalation. As the diaphragm relaxes, the elastic abdominal wall forces the
diaphragm up again by pushing the watery tissues of the abdomen against the
underside of the relaxed diaphragm. The dome moves upward, the volume of the
thoracic cavity is decreased, and air is forced from the lungs and into the atmosphere.
NERVOUS CONTROL OF BREATHING
Breathing can be controlled voluntarily to some extent, such as holding your
breath for a short period of time or breathing deeply during a medical examination.
However, breathing is usually controlled by the nervous system without the need to
consciously order the body to inhale or exhale. Respiratory reflexes are controlled by
the respiratory center found in the medullar portion of the brainstem. The respiratory
center coordinates the actions of costal breathing and diaphragmatic breathing to
ensure that they work together (chest and abdominal muscles contract at the same time
and relax at the same time). The amount of carbon dioxide (CO2) in the circulating
blood is one of the major influences on the actions of the respiratory center.