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
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
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
Laboratory Tests of Clinical Importance, pages 940–943
Which requirements of organisms does the external environment
What is the relationship between oxygen use and heat
Why is homeostasis so important to survival?
Describe three homeostatic mechanisms.
ORGANIZATION OF THE
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.
The human organism can be divided into an
which includes the head, neck, and trunk, and an
the upper and lower limbs. Within the axial portion are
which houses the brain; the
(spinal cavity), which contains the spinal cord and
is surrounded by sections of the backbone (vertebrae);
The organs within these last
two cavities are called
these major body cavities.
The thoracic cavity is separated from the lower abdomi-
nopelvic cavity by a broad, thin muscle called the
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
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
include the stomach, liver,
spleen, gallbladder, and the small and large intestines.
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
containing the teeth and tongue.
located within the nose and divided into
right and left portions by a nasal septum. Several air-
lled sinuses are connected to the nasal cavity.
These include the sphenoidal and frontal sinuses
containing the eyes and associated
skeletal muscles and nerves.
Middle ear cavities,
containing the middle ear bones.
Thoracic and Abdominopelvic
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
This membrane folds
back to cover the lungs, forming the
lm of serous ﬂ
uid separates the parietal and visceral
Although there is normally no space
between these two membranes, the potential space between
them is called the
The heart, located in the broadest portion of the medi-
astinum, is surrounded by
ers the heart’s surface and is separated from the
by a small volume of serous ﬂ
uid. The potential
space between these membranes is called the
The parietal pericardium is covered by a much thicker
third layer, the
membranes associated with the heart and lungs.