Chapter 16
The Endocrine System
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16
thyroid hormone, and glucocorticoids have proved vulnerable
to the effects of such pollutants. Interference with glucocorti-
coids, which turn on many genes that may suppress cancer, may
help to explain high cancer rates in certain areas of the country.
Endocrine Function Throughout Life
Barring exposure to environmental pollutants, and hypersecre-
tory and hyposecretory disorders, most endocrine organs oper-
ate smoothly until old age. Aging may alter the rates of hormone
secretion, breakdown, and excretion, or the sensitivity of target
cell receptors. Endocrine function in the elderly is difficult to re-
search, however, because it is frequently affected by the chronic
illnesses common in that age group.
Structural changes in the anterior pituitary occur with age.
Te amount of connective tissue increases, vascularization de-
creases, and the number of hormone-secreting cells declines.
Tese changes may or may not affect hormone production. In
women, for example, blood levels and the release rhythm of
AC±H remain constant, but levels of gonadotropins increase
with age. GH levels decline in both sexes, which partially ex-
plains muscle atrophy in old age.
Te adrenal glands also show structural changes with age, but
normal controls of cortisol persist as long as a person is healthy
and not stressed. Chronic stress, on the other hand, drives up
blood levels of cortisol and appears to contribute to hippocam-
pal (and memory) deterioration. Plasma levels of aldosterone
fall by half in old age, but this change may reflect a decline in
renin release by the kidneys, which become less responsive to
renin-evoking stimuli. No age-related differences have been
found in the release of catecholamines by the adrenal medulla.
Te gonads, particularly the ovaries, undergo significant changes
with age. In late middle age, the ovaries become smaller and
unresponsive to gonadotropins. As female hormone production
declines dramatically, the ability to bear children ends, and prob-
lems associated with estrogen deficiency appear, such as atheroscle-
rosis and osteoporosis. ±estosterone production by the testes also
wanes with age, but this effect usually is not seen until very old age.
Glucose tolerance (the ability to dispose of a glucose load
effectively) begins to deteriorate as early as the fourth decade
of life. Blood glucose levels rise higher and return to resting
levels more slowly in the elderly than in young adults. Te fact
that the islet cells continue to secrete near-normal amounts of
insulin leads researchers to conclude that decreasing glucose
tolerance with age may reflect declining receptor sensitivity to
insulin (pre–type 2 diabetes).
Tyroid hormone synthesis and release diminish somewhat
with age. ±ypically, the follicles are loaded with colloid in the el-
derly, and the gland becomes fibrosed. Basal metabolic rate declines
with age. Mild hypothyroidism is only one cause of this decline.
Te increase in body fat relative to muscle is equally important,
because muscle tissue is more active metabolically than fat.
Te parathyroid glands change little with age, and P±H levels
remain fairly normal throughout life. Estrogens protect women
against the demineralizing effects of P±H, but estrogen produc-
tion wanes aFer menopause, leaving older women vulnerable
to the bone-demineralizing effects of P±H and to osteoporosis.
initiate the renin-angiotensin-aldosterone mechanism of aldo-
sterone release described earlier.
Skeleton
Osteoblasts in bone secrete
osteocalcin
, a hormone that
prods pancreatic beta cells to divide and secrete more insulin. It
also restricts fat storage by adipocytes, and triggers the release of
adiponectin. Tis improves glucose handling and reduces body fat.
Interestingly, insulin promotes the conversion of inactive os-
teocalcin to active osteocalcin in bone, forming a two-way con-
versation between bone and the pancreas. Osteocalcin levels are
low in type 2 diabetes, and increasing its level may offer a new
treatment approach.
Skin
Te skin produces
cholecalciferol
, an inactive form of
vitamin D
3
, when modified cholesterol molecules in epidermal
cells are exposed to ultraviolet radiation. Tis compound then
enters the blood via the dermal capillaries, is modified in the
liver, and becomes fully activated in the kidneys. Te active
form of vitamin D
3
,
calcitriol
, is an essential regulator of the
carrier system that intestinal cells use to absorb Ca
2
1
from food.
Without this vitamin, bones become soF and weak.
Thymus
Located deep to the sternum in the thorax is the lobu-
lated
thymus
(see ²igure 16.1). Large and conspicuous in in-
fants and children, the thymus shrinks throughout adulthood.
By old age, it is composed largely of adipose and fibrous con-
nective tissues.
Tymic epithelial cells secrete several different families of
peptide hormones, including
thymulin, thymopoietins
, and
thymosins
(thi
9
mo-sinz). Tese hormones are thought to be
involved in the normal development of
T lymphocytes
and the
immune response, but their roles are not well understood. Al-
though still called hormones, they mainly act locally as para-
crines. We describe the thymus in Chapter 20 in our discussion
of lymphoid organs and tissues.
Check Your Understanding
17.
Which hormone does the heart produce and what is its
function?
18.
What is the function of the hormone produced by the skin?
For answers, see Appendix H.
Developmental Aspects
of the Endocrine System
Describe the effects of aging on endocrine system function.
Hormone-producing glands arise from all three embryonic
germ layers. Endocrine glands derived from mesoderm pro-
duce steroid hormones. All others produce amines, peptides, or
protein hormones.
Effects of Environmental Pollutants
Exposure to environmental pollutants—many pesticides, in-
dustrial chemicals, arsenic, dioxin, and other soil and water pol-
lutants—can disrupt endocrine function. So far, sex hormones,
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