Your temperature regulatory mechanism works fairly
well for moderate changes in temperature. However, in
extreme cases of
h y p o th e r m ia
(in which core body tem-
perature is lower than 35°C /94°F), the system may be ac-
tive but unable to compensate for the heat losses. Some
conditions that cause hypothermia include extreme cold
stress (for example, immersion in icy water or exposure to
very cold air), metabolic diseases (hypoglycemia, hypothy-
roidism, adrenal insufficiency), drugs (alcohol, antidepres-
sants, sedatives, tranquilizers), burns, and malnutrition.
Symptoms of hypothermia include cold sensation, shiver-
ing, vasoconstriction, slow heart rate, loss of spontaneous
movement, and coma. Death from hypothermia is usually
due to cardiac arrhythmias and cardiac arrest. Generally,
elderly people are at increased risk for hypothermia be-
cause their BMR is already reduced.
The hypothalamic thermostat can be reset to higher
temperatures by infections
(viruses, bacteria), bacterial
toxins, hyperthyroidism, tumors, and immune reactions to
vaccinations. The result is a fever. When phagocytes fight
infections, they secrete chemicals
called
p y ro g en s
that reset the hy-
pothalamic thermostat to a higher
temperature by causing it to secrete
prostaglandins
. Often, this higher
body temperature helps fight the
infection. However, at the higher
p rostaglandin
(pros'-ta-GLAN-din) A
membrane-associated
lipid that is released
in small quantities and
act as a local hormone.
set point (higher temperature), your body senses the sur-
rounding temperature as colder and adapts to the higher set
point (see Figure 14.17); this is why a feverish person may
feel chilled and shiver. The fever may “break” when the origi-
nal cause of the change in the set point is eliminated. This
“breaking” of the fever is usually accompanied by sweating.
While some fever may be beneficial, it is important to
prevent the core body temperature from rising to a life-
threatening level of 40-42°C or 104-106°F. Temperatures
above 42°C are generally fatal. To reduce fevers, you can
take pain relievers that reduce prostaglandin production,
such as aspirin, acetaminophen (Tylenol™), and ibuprofen
(Advil™). Aspirin should not be given to feverish children
and teenagers because it has been associated with Reye’s
syndrome, a disorder characterized by vomiting and brain
dysfunction that often progresses to coma and death.
1.
What
is the main source of heat production in
your body?
2.
Where
is the thermostat of your temperature
regulation system located?
3.
How
does your body respond to control your
temperature when you are working outside on a
hot summer day?
TH E PLANNER ^
Summary
1
L et's Journey Through the Digestive System
400 •
• The digestive system, as shown, consists of organs of the
GI tract (mouth, esophagus, stomach, small intestine, large
intestine) and accessory organs (salivary glands, teeth, liver,
pancreas, gallbladder).
• As food passes through the GI tract, it is digested into
smaller nutrient molecules, including simple sugars, tri-
glycerides, and amino acids. This occurs both physically
(through chewing and mixing) and chemically (via various
secretions containing enzymes, such as gastric juice, pan-
creatic juice, and bile). Most of the nutrients and water get
absorbed in the small intestine and large intestine. Water-
soluble nutrients pass through the blood to the liver, where
they are processed and then make their way to other cells in
the body where they are stored or metabolized. Fat-soluble
nutrients are directed into the lymphatic system and then
into the blood.
T he d ig e stiv e sy stem • Figure 14.1
Liver
Stomach
Small intestine
Large intestine
430 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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