k. Diaphragm. The diaphragm is the most important muscle connected directly
with the respiratory system. It is a domeshaped muscle, which lies between the
abdominal and thoracic cavities and completely separates the two cavities. As you
inhale, the diaphragm tends to flatten out thus helping to draw more air into the lungs.
The visceral pleura is pulled down as the diaphragm flattens out. As you exhale, the
diaphragm assumes its original dome shape, pushed up by the viscera.
l. Ribs. The intercostal muscles (between the ribs) assist the breathing
mechanism. These structures enlarge the lung cavity laterally, anteriorly, and
posteriorly. Thus, the ribs and their muscles, along with the diaphragm, constitute the
main part of the mechanical process of breathing.
230. MECHANICS OF BREATHING
a. Control Center.
(1) Medulla. The portion of the brain that controls respiration is the lower
portion of the brain, the medulla oblongata.
(2) Phrenic nerve. In connection with and coming from the medulla
oblongata are the right and left phrenic nerves which run from the brain to the
diaphragm. If one of these nerves is severed, the corresponding portion of the
diaphragm ceases to function.
(1) Inhalation. Inhalation occurs when the diaphragm contracts and its
domed upper surface flattens out and, as a result, the size of the chest cavity increases.
This causes decreased pressure in the chest cavity and a partial vacuum in the lungs;
thus, air is drawn into the lungs. The contraction of the diaphragm is accompanied by
the contraction of certain muscles of the chest wall, particularly the intercostal muscles
between the ribs. The contraction of the intercostal muscles causes the chest cavity to
enlarge from side to side and from front to back, and thus assist in expanding the lungs.
These muscles, then, are accessory muscles of respiration.
(2) Expiration. Expiration of air from the lungs is a passive process caused
by the relaxation of the diaphragm, which moves upward, and by the relaxation of the
accessory muscles, allowing the ribs to compress the lungs. This action causes the
thoracic cavity to decrease in size so that the air in the lungs is driven out, due to the
greater pressure. Then the cycle repeats itself.
c. Respiration. Respiration is a complex process by which oxygen is supplied
to the tissues and CO2 removed. Respiration involves the following processes:
(1) Pulmonary ventilation, which is concerned with the distribution and
volume of air ventilating the alveoli.