515
CHAPTER THIRTEEN
Endocrine System
Parathyroid function differs between the sexes with
age. Secretion peaks in males at about age F
fty, whereas in
women, the level of parathyroid hormone decreases until
about age forty, after which it rises and contributes to osteo-
porosis risk. ±at accumulates between the cells of the para-
thyroid glands.
The adrenal glands illustrate the common theme of
aging-related physical changes, yet continued function.
±ibrous connective tissue, lipofuscin pigment, and increased
numbers of abnormal cells characterize the aging adrenal
glands. However, thanks to the F ne-tuning of negative feed-
back systems, blood levels of glucocorticoids and mineralo-
corticoids usually remain within the normal range, although
the ability to maintain homeostasis of osmotic pressure,
blood pressure, acid/base balance and sodium and potas-
sium ion distributions may falter with age.
The most obvious changes in endocrine function that
occur with age involve blood glucose regulation. The pancreas
may be able to maintain secretion of insulin and glucagon, but
lifestyle changes, such as increase in fat intake and less exer-
cise, may increase the blood insulin level. The development of
insulin resistance—the decreased ability of muscle, liver, and
fat cells to take in glucose even in the presence of insulin—
refl ects impaired ability of these target cells to respond to the
hormone, rather than compromised pancreatic function. Blood
glucose buildup may signal the pancreas to secrete more insu-
lin, setting the stage for type 2 diabetes mellitus.
ACTH, which increases the adrenal cortex’s secretion of
cortisol. Cortisol supplies cells with amino acids and extra
energy sources and diverts glucose from skeletal muscles to
brain tissue (F g. 13.37). Stress can also stimulate release of
glucagon from the pancreas, growth hormone (GH) from the
anterior pituitary, antidiuretic hormone (ADH) from the pos-
terior pituitary gland, and renin from the kidney.
Glucagon and growth hormone help mobilize energy
sources, such as glucose, glycerol, and fatty acids, and
stimulate cells to take up amino acids, facilitating repair of
injured tissues. ADH stimulates the kidneys to retain water.
This action decreases urine output and helps to maintain
blood volume—particularly important if a person is bleed-
ing or sweating heavily. Renin, by increasing angiotensin II
levels, helps stimulate the kidneys to retain sodium (through
aldosterone), and through the vasoconstrictor action of
angiotensin II contributes to maintaining blood pressure.
Table 13.13
summarizes the body’s reactions to stress.
PRACTICE
47
What is stress?
48
Distinguish between physical stress and psychological stress.
49
Describe the general stress syndrome.
13.12
LIFE-SPAN CHANGES
With age, the glands of the endocrine system generally
decrease in size and increase in the proportion of each gland
that is F brous in nature. At the cellular level, lipofuscin pig-
ment accumulates as glands age. ±unctionally, hormone
levels may change with advancing years. Treatments for
endocrine disorders associated with aging supplement deF -
cient hormones, remove part of an overactive gland or use
drugs to block the action of an overabundant hormone.
Aging affects different hormones in characteristic ways.
±or growth hormone, the surge in secretion that typically
occurs at night lessens somewhat with age. Lower levels of
GH are associated with declining strength in the skeleton and
muscles with advancing age. However, supplementing older
people with GH in an attempt to duplicate the effects of exer-
cise can dangerously raise blood pressure and blood glucose
levels and enlarge the spleen, liver, and kidneys.
Levels of antidiuretic hormone increase with age, but
this is due to slowed breakdown in the liver and kidneys,
rather than increased synthesis. As a result, the kidneys are
stimulated to reabsorb more water.
The thyroid gland shrinks with age, as individual fol-
licles shrink and increasing amounts of F brous connective
tissue separate them. Thyroid nodules, which may be benign
or cancerous, become more common with age, and are often
F rst detected upon autopsy. Although blood levels of T
3
and
T
4
may diminish with age, in general, the thyroid gland’s
control over the metabolism of various cell types is main-
tained throughout life. Calcitonin levels decline with age,
which raises the risk of osteoporosis.
TABLE
13.13
|
Major Events in the General
Stress Syndrome
1. In response to stress, nerve impulses are transmitted to the
hypothalamus.
2. Sympathetic impulses arising from the hypothalamus increase blood
glucose concentration, blood glycerol concentration, blood fatty acid
concentration, heart rate, blood pressure, and breathing rate. They
dilate air passages, shunt blood into skeletal muscles, and increase
secretion of epinephrine from the adrenal medulla.
3. Epinephrine intensiF
es and prolongs sympathetic actions.
4. The hypothalamus secretes CRH, which stimulates secretion of ACTH
by the anterior pituitary gland.
5. ACTH stimulates release of cortisol by the adrenal cortex.
6. Cortisol increases the concentration of blood amino acids, releases
fatty acids, and forms glucose from noncarbohydrate sources.
7. Secretion of glucagon from the pancreas and growth hormone from
the anterior pituitary increase.
8. Glucagon and growth hormone aid mobilization of energy sources
and stimulate uptake of amino acids by cells.
9. Secretion of ADH from the posterior pituitary increases.
10. ADH promotes the retention of water by the kidneys, which increases
blood volume.
11. Renin increases blood levels of angiotensin II, which acts as a
vasoconstrictor and also stimulates aldosterone secretion by the
adrenal cortex.
12. Aldosterone stimulates sodium retention by the kidneys.
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