Dynamics of breathing 5
The simplified illustration of the human body
in figure 1.4 shows that the torso consists of two
cavities, thoracic and abdominal. These cavities
share some properties, and they have important
distinctions as well. Both contain vital organs: The
thoracic cavity contains the heart and lungs, and the
abdominal cavity contains the stomach, liver, gall
bladder, spleen, pancreas, small and large intestines,
kidneys, and bladder.
Both cavities open at one end to the external
environment—the thoracic at the top, and the
abdominal at the bottom. The cavities open to each
other 3 by means of an important shared, dividing
structure, the diaphragm. Another important shared
property is that both cavities are bound posteri-
orly by the spine. The two cavities also share the
quality of mobility—they change shape. This shape-
changing ability is most relevant to breathing; with-
out this movement, the body cannot breathe at all.
Although both the abdominal and thoracic cavi-
ties change shape, an important structural differ-
ence exists in how they do so.
the water Balloon and the accordion
The abdominal cavity changes shape like a flexible,
fluid-filled structure such as a water balloon. When
you squeeze one end of a water balloon, the other
end bulges (figure 1.5).
That is because water is noncompressible. Your
hand’s action only moves the fixed volume of water
from one region of the flexible container to another.
The same principle applies when the movements
of breathing compress the abdominal cavity; a
squeeze in one region produces a bulge in another.
In the context of breathing, the abdominal cavity
changes shape but not volume. In the context of
life processes other than breathing, the abdominal
cavity does change volume. When you drink a
large volume of liquid or eat a big meal, the overall
volume of the abdominal cavity increases as a result
of expanded abdominal organs (stomach, intestines,
and bladder). Any volume increase in the abdominal
cavity produces a corresponding decrease in the
volume of the thoracic cavity. That is why it is more
difficult to breathe after a big meal, before a big
bowel movement, or when pregnant.
(^3) The three openings (hiati) in the diaphragm are for the arterial supply to the lower body (aortic hiatus), the venous return
from the lower body to the heart (inferior vena cava) and the esophagus (esophageal hiatus). Hiatus is the Latin past
participle of hiare—to stand open or yawn.
Figure 1.4 E5267/Kaminoff/fig1.4/417552/alw/pulled-r1Breathing is thoracoab-
dominal shape change between (a)
inhalation and (b) exhalation.
a b
E5267/Kaminoff/fig 1.5/417553/JG/R
Figure 1.5 The water balloon
changes shape but not volume.