12
UNIT ONE
bleeding (see chapter 14, pp. 539–541). Preventing blood loss
following an injury is critical to sustaining life. Similarly, a
positive feedback mechanism increases the strength of uter-
ine contractions during childbirth.
Positive feedback mechanisms usually produce unstable
conditions, which might not seem compatible with homeo-
stasis. However, the few examples of positive feedback
associated with health have very speciF
c functions and are
short-lived.
Homeostatic mechanisms maintain a relatively constant
internal environment, yet physiological values may vary
slightly in a person from time to time or from one person
to the next. Therefore, both normal values for an individual
and the idea of a
normal range
for the general population
are clinically important. Numerous examples of homeostasis
are presented throughout this book, and normal ranges for a
number of physiological variables are listed in
Appendix B,
Laboratory Tests of Clinical Importance, pages 940–943
.
PRACTICE
13
Which requirements of organisms does the external environment
provide?
14
What is the relationship between oxygen use and heat
production?
15
Why is homeostasis so important to survival?
16
Describe three homeostatic mechanisms.
1.6
ORGANIZATION OF THE
HUMAN BODY
The human organism is a complex structure composed of
many parts. The major features of the human body include
cavities, various types of membranes, and organ systems.
Body Cavities
The human organism can be divided into an
axial
(ak
se-al)
portion,
which includes the head, neck, and trunk, and an
appendicular
(ap
en-dik
u-lar)
portion,
which includes
the upper and lower limbs. Within the axial portion are
the
cranial cavity,
which houses the brain; the
vertebral
canal
(spinal cavity), which contains the spinal cord and
is surrounded by sections of the backbone (vertebrae);
the
thoracic
(tho-ras
ik)
cavity;
and the
abdominopelvic
(ab-dom
ı˘-no-pel
vik)
cavity.
The organs within these last
two cavities are called
viscera
(vis
er-ah).
Figure 1.9
shows
these major body cavities.
The thoracic cavity is separated from the lower abdomi-
nopelvic cavity by a broad, thin muscle called the
dia-
phragm
(di
ah-fram). When it is at rest, this muscle curves
upward into the thorax like a dome. When it contracts dur-
ing inhalation, it presses down upon the abdominal viscera.
The wall of the thoracic cavity is composed of skin, skeletal
muscles, and bones. Within the thoracic cavity are the lungs
and a region between the lungs, called the
mediastinum
(me
de-as-ti
num). The mediastinum separates the thorax
into two compartments that contain the right and left lungs.
The remaining thoracic viscera—heart, esophagus, trachea,
and thymus—are within the mediastinum.
The abdominopelvic cavity, which includes an upper
abdominal portion and a lower pelvic portion, extends from
the diaphragm to the fl
oor of the pelvis. Its wall primarily
consists of skin, skeletal muscles, and bones. The viscera
within the
abdominal cavity
include the stomach, liver,
spleen, gallbladder, and the small and large intestines.
The
pelvic cavity
is the portion of the abdominopelvic
cavity enclosed by the pelvic bones. It contains the terminal
end of the large intestine, the urinary bladder, and the inter-
nal reproductive organs.
Smaller cavities within the head include the following
g. 1.10)
:
1.
Oral cavity,
containing the teeth and tongue.
2.
Nasal cavity,
located within the nose and divided into
right and left portions by a nasal septum. Several air-
F
lled sinuses are connected to the nasal cavity.
These include the sphenoidal and frontal sinuses
(see F
g. 7.25).
3.
Orbital cavities,
containing the eyes and associated
skeletal muscles and nerves.
4.
Middle ear cavities,
containing the middle ear bones.
Thoracic and Abdominopelvic
Membranes
Thin
serous membranes
line the walls of the thoracic and
abdominal cavities and fold back to cover the organs within
these cavities. These membranes secrete a slippery serous
uid that separates the layer lining the wall of the cavity
(parietal layer) from the layer covering the organ (visceral
layer). ±or example, the right and left thoracic compart-
ments, which contain the lungs, are lined with a serous
membrane called the
parietal pleura.
This membrane folds
back to cover the lungs, forming the
visceral pleura.
A thin
F
lm of serous fl
uid separates the parietal and visceral
pleural
(ploo
ral)
membranes.
Although there is normally no space
between these two membranes, the potential space between
them is called the
pleural cavity.
The heart, located in the broadest portion of the medi-
astinum, is surrounded by
pericardial
(per
ı˘-kar
de-al)
membranes.
A thin
visceral pericardium
(epicardium) cov-
ers the heart’s surface and is separated from the
parietal
pericardium
by a small volume of serous fl
uid. The potential
space between these membranes is called the
pericardial cav-
ity.
The parietal pericardium is covered by a much thicker
third layer, the
f
brous pericardium.
Figure 1.11
shows the
membranes associated with the heart and lungs.
previous page 42 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 44 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off