512
UNIT THREE
A
bout 7% of the population of the United
States has some form of diabetes mel-
litus. In Latin,
diabetes
means “increased
urine output,” and
mellitus
means “honey,” refer-
ring to urine’s sugar content.
In type 1 diabetes mellitus (juvenile or insulin-
dependent diabetes mellitus), the pancreas can-
not produce insulin. Symptoms usually begin
before age twenty. About 15% of people with
diabetes mellitus have this form. It is an auto-
immune disorder in which the immune system
attacks pancreatic beta cells, ultimately destroy-
ing them and halting insulin secretion. Lack of
insulin decreases movement of glucose into skel-
etal muscle and adipose cells, inhibiting glycogen
formation. As a result, blood glucose concentra-
tion rises (hyperglycemia), and when it reaches
a certain level, the kidneys begin to excrete the
excess. Glucose appears in the urine (glycosuria).
Water follows the glucose by osmosis, causing
dehydration and intense thirst.
Untreated type 1 diabetes decreases protein
synthesis, shrinking tissues as glucose-starved
cells use protein for energy. Weight falls, and
wounds cannot heal. Fatty acids accumulate in
the blood as a result of decreased fat synthesis
and storage. Ketone bodies, a by-product of fat
metabolism, also build up in the blood. They are
excreted in the urine as sodium salts, and a large
volume of water follows by osmosis, intensifying
dehydration and lowering sodium ion concentra-
tion in the blood. Accumulation of ketones and
loss of sodium ions lead to metabolic acidosis,
a condition that lowers the pH of body fluids.
Acidosis and dehydration adversely a±
ect brain
neurons. Without treatment (insulin replace-
ment), the person becomes disoriented and may
enter a diabetic coma and die.
Daily life for a person with type 1 diabetes
mellitus means constant awareness of the illness—
insulin delivery; frequent finger punctures to
monitor blood glucose level; a restrictive diet;
and concern over complications, which include
loss of vision, leg ulcers, and kidney damage. The
many symptoms re²
ect disturbances in carbohy-
drate, protein, and fat metabolism.
In another form of diabetes, type 2 or noninsu-
lin-dependent diabetes mellitus, begins gradually,
usually in people over forty. Cells lose insulin recep-
tors and are less able to respond to insulin. Heredity
and a lifestyle of overeating and underexercising
are risk factors for developing type 2 diabetes.
Treatment includes avoiding foods that stimulate
insulin production, exercising, taking medication,
and maintaining desirable body weight.
About 54 million people in the United States
have prediabetes, which is an elevated blood glu-
cose level after eating, that is not high enough to
be considered diabetes. However, organ damage
may already be occurring. Adopting a healthy
diet and exercise habits can slow progression to
type 2 diabetes.
The oral glucose tolerance test is used to
diagnose both major types of diabetes mellitus
and indicate prediabetes. The patient ingests a
known quantity of glucose, and blood glucose
concentration is measured at intervals to assess
glucose use. If the person has diabetes, blood
glucose concentration rises greatly and remains
elevated for several hours. In a healthy person,
glucose rise is less dramatic, and the level returns
to normal in about an hour and a half.
13.4
CLINICAL APPLICATION
Diabetes Mellitus
cytes). The thymus gland plays an important role in immu-
nity and is discussed in chapter 16 (p. 623).
The reproductive organs that secrete important hor-
mones include the
testes,
which produce testosterone; the
ovaries,
which produce estrogens and progesterone; and the
placenta,
which produces estrogens, progesterone, and a
gonadotropin. Chapters 22 and 23 discuss these glands and
their secretions (pp. 846, 857, and 884).
The digestive glands that secrete hormones are generally
associated with the linings of the stomach and small intes-
tine. The small intestine alone produces dozens of hormones,
many of which have not been well studied. Chapter 17 (pp. 668
and 672) describes these structures and their secretions.
The pineal gland secretes a hormone,
melatonin,
that
is synthesized from serotonin. Varying patterns of light and
dark outside the body control the gland’s activities. In the
presence of light, nerve impulses from the eyes travel to
the hypothalamus, then to the reticular formation, and then
downward into the spinal cord. From here, the impulses
travel along sympathetic nerve ±
bers back into the brain,
and finally they reach the pineal gland, where they
decrease melatonin secretion. In the absence of light, nerve
impulses from the eyes decrease and secretion of melatonin
increases.
Melatonin secretion is part of the regulation of
circadian
rhythms,
which are patterns of repeated activity associated
with cycles of day and night, such as sleep/wake rhythms.
Melatonin binds to two types of receptors on brain neurons,
one that is abundant and one that is scarce. The major recep-
tors are on cells of the suprachiasmatic nucleus, a region
that regulates the circadian clock. Binding to the second, less
abundant receptors, however, induces sleepiness.
The
thymus
(thi
mus), which lies in the mediastinum
posterior to the sternum and between the lungs, is large in
young children but shrinks with age. This gland secretes a
group of hormones, called
thymosins,
that affect production
and differentiation of certain white blood cells (T lympho-
The fact that melatonin secretion is a response to the relative lengths
of day and night explains why traveling across several time zones
produces the temporary insomnia of jet lag. Melatonin supplements
are advertised as preventing jet lag, based on anecdotal reports and
small studies. However, the ³
rst large study, conducted on 257 doc-
tors traveling from Norway to New York, testing three nightly doses
of melatonin supplement versus placebo, showed no effect at all
from melatonin in preventing or alleviating jet lag.
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