Within the islets are two types of cells: al-
pha cells, which secrete
glucagon
, and beta
cells, which secrete
insulin
. Both glucagon
and insulin are protein hormones that act
mainly on the liver and skeletal muscle;
they have opposite effects on the levels of
glucose in the blood (
Figure 9.13
). Main-
tenance of normal blood glucose levels is
important for the proper functioning of the
nervous system.
The secretions of glucagon and insulin
are
coordinated to prevent blood glucose
from rising too much after a meal and falling
too much between meals. Abnormal levels
of insulin, glucagon,
and/or
glucose
in
the blood can indi-
cate
hypoglycemia
,
hyperglycemia
,
or
the presence of dis-
eases such as diabe-
tes mellitus, the most
common disorder.
There are two types of
diabetes mel-
litus,
which affects about 7.8% of the U.S.
population: •
Type I .
Type I diabetes involves a lack of
insulin
because
the
patient’s
immune
system has destroyed the beta cells of the
pancreas. Type I diabetes usually develops
early in life and has been called juvenile
diabetes.
T y p e II.
Type II diabetes involves decreased
sensitivity of target cells to insulin. Type II
diabetes is often associated with obesity,
usually develops later in life, and has been
called adult-onset diabetes. Patients with
type II diabetes may have nearly normal
levels of insulin in their blood. This type of
diabetes is a growing epidemic in America,
mainly due to the rise in obesity.
Both types of diabetes are character-
ized by elevated blood glucose levels and
impaired glucose
tolerance
(see
W h a t
a
H e a lth P ro vid er S e e s),
presence of glucose in
the urine, excessive thirst, excessive hunger
and eating, and frequent urination. In both
type I and type II diabetes, the body acts
as if it is starving. Elevated glucagon and
cortisol levels cause the breakdown of liver
hypoglycemia
A condition in which
blood glucose levels
are below normal.
hyperglycemia
A condition in which
blood glucose levels
are above normal
R egu lato r o f b lo o d g lu co se by
g lu cag o n and insulin • Figure 9.13
Glucagon and insulin have opposite effects on skeletal
muscle to regulate blood glucose levels.
Low blood glucose
levels (hypoglycemia)
stimulate pancreatic
alpha cells to secrete
glucagon
High blood sugar
(hyperglycemia)
stimulates beta cells
to secrete insulin.
Glucagon
Insulin
a
a
I
Glucagon acts on
liver cells to break
down glycogen into
glucose and to make
glucose from other
sources (lactic acid,
amino acids).
i
Liver cells release
glucose into the
blood, causing
a rise in blood
glucose levels.
1
If blood glucose
rises above
normal levels
(hyperglycemia)
then alpha cells
will stop secreting
glucagon.
a
a
5
\
Insulin acts on all cells,
especially liver and
skeletal muscle, to take
up glucose from the
blood. It stimulates liver
and muscle cells to make
glycogen from glucose
and to take up amino
acids to make proteins.
1
The removal of glucose
by cells and the synthesis
of glycogen cause blood
glucose levels to fall.
Ï
If blood glucose levels
/
fall below normal, then
insulin secretion is
inhibited.
glycogen to release glucose, triglycerides to release fatty acids, and
muscle proteins to release amino acids. Because most cells of the body
do not take up excess glucose from the blood, the blood becomes hy-
pertonic (Chapter 3). This condition, along with increased excretion
by the kidneys, leads to dehydration, circulatory problems, and tissue
damage. Diabetes mellitus often leads to death due to cardiovascu-
lar damage and kidney failure. Individuals with type I diabetes can
be treated with blood glucose monitoring, changes in diet, exercise,
and administration of insulin. Individuals with type II diabetes can be
treated with blood glucose monitoring, changes in diet, exercise, and
medications that stimulate insulin secretion.
Endocrine Glands Regulate Key Body Functions
267
PROCESS DIAGRAM
previous page 302 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online next page 304 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online Home Toggle text on/off