b. The interface, or meeting place, of the pulmonary pleura and the parietal
pleura is called the pleural cavity, but in health, it is only a potential, rather than an
actual cavity. Its walls are covered with a film of serous fluid that moistens them and
holds them together by hydraulic action in the presence of a barometric pressure that, in
health, is slightly lower than atmospheric pressure. This tight adherence of the
pulmonary and parietal pleurae facilitates movement of the lungs in the chest. The two
pleural cavities are entirely separate from one another; thus one lung can be collapsed
without affecting the other cavity or lung.
c. The costophrenic (phrenicostal) angle, or sinus, is the triangular space
between the wall of the thorax and the lower margin of the diaphragm. This angle is
readily seen in PA (posteroanterior) chest radiographs.
3-28. MECHANICS OF RESPIRATION
a. Respiration is the act, or function, of breathing. The process of breathing may
be subdivided into inspiration, or breathing in, and expiration, or breathing out. The
coordinated movements of the lungs, diaphragm, abdomen, and associated muscles
b. In inspiration, the intercostal muscles between ribs contract to raise the
anterior ends of the ribs. At the same time, a contraction flattens the dome-shaped
diaphragm muscle. By these combined actions, the thoracic cavity is enlarged in all
directions. As a result, each pleural cavity is also enlarged and, since it is completely
airtight, a partial vacuum is created. Due to the lowering of the pressure in the pleural
cavity and the elasticity of the lung, the air pressure within the lung is lowered. As the
interior of the lung communicates freely with the outside air, this air rushes into the lung
to equalize the pressure.
c. At the end of each inspiration, the intercostal muscles and the diaphragm
relax and the thoracic wall returns to its normal resting position, giving an expiration.
This decrease in size increases the pressure in the lungs. As a result, air flows out of
the lungs until the pressure within them is again equal to that of atmospheric pressure.
The lungs are prevented from collapsing by the negative pressure in the surrounding
d. The muscles of respiration normally act automatically. The respiratory cycle
consists of an active inspiration, followed by a passive expiration. The normal rate of
quiet respiration in an adult is between 16 to 20 cycles per minute.
e. On full inspiration, the right dome of the diaphragm lies approximately at the
level of the eleventh thoracic vertebra and the left dome at that of the twelfth thoracic
vertebra. On full expiration, the right dome of the diaphragm lies at about the same
level as the eighth thoracic vertebra and the left dome at that of the ninth thoracic
vertebra. These respiratory movements of the diaphragm are important in radiography.