Section III. PHYSIOLOGY OF RESPIRATION
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 the connected spaces of air have different
pressures, the air moves from the space with greater pressure to the one with lesser
b. 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.
c. 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
1-11. 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 and from front to back. 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.
1-12. DIAPHRAGMATIC (ABDOMINAL) BREATHING
a. Inhalation. As the diaphragm contracts, the dome flattens, and the
diaphragm descends. This increases the depth (vertical diameter) of the thoracic cavity
thus increasing the cavity's volume. At the same time, air pressure in the thoracic cavity
decreases. Now, air pressure outside the body is greater, a condition which forces air
into the lungs.
b. Exhalation. As the diaphragm relaxes, the elastic abdominal wall forces the
diaphragm back up by pushing the watery tissues of the abdomen against the underside
of the relaxed diaphragm. The dome extends upward. Air is forced out of the lungs.