lobe is supplied by a secondary or lobar bronchus. Each segment is supplied by a
tertiary or segmental bronchus, a branch of the lobar bronchus.
d. Pleural Cavities. See paragraph 3-14 to review a description of pleural cavities.
That paragraph indicates that each serous cavity has inner and outer membranes. In
the case of the lungs, the inner membrane is known as the visceral pleura which very
closely covers the surface of the lungs. The outer membrane is known as the parietal
pleura, forming the outer wall of the cavity. The pleural cavities are the potential spaces
between the inner and outer membranes. The pleural cavities allow the lungs to move
freely with a minimum of friction during the expansion and contraction of breathing.
Section II. BREATHING AND BREATHING MECHANISMS IN HUMANS
a. Boyle's law tells us that as the volume (V) of a gas-filled container increases,
the pressure (P) inside decreases; as the volume (V) of a closed container decreases,
the pressure (P) inside increases. When two connected spaces of air have different
pressures, the air moves from the space with greater pressure to the one with lesser
pressure. In regard to breathing, we can consider the air pressure around the human
body to be constant. The pressure inside the lungs may be greater or less than the
pressure outside the body. Thus, a greater internal pressure causes air to flow out; a
greater external pressure causes air to flow in.
b. We can compare the human trunk to a hollow cylinder. This cylinder is
divided into upper and lower cavities by the diaphragm. The upper is the thoracic cavity
and is essentially gas-filled. The lower is the abdominopelvic cavity and is essentially
7-7. COSTAL (THORACIC) BREATHING
a. Inhalation. Muscles attached to the thoracic cage raise the rib cage. 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. The "bucket handle" is lifted
by the overall movement upward and outward of the rib cage. These movements
increase the thoracic diameters from right to left (transverse) and from front to back
(A-P). Thus, the intrathoracic volume increases. Recalling Boyle's law, the increase in
volume leads to a decrease in pressure. The 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. Thus, intrathoracic volume decreases. The intrathoracic
pressure increases and forces air outside the body.