Figure 1-4. Infralaryngeal structures.
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