The Endocrine System

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The Endocrine System Hormones are secreted into
the blood by endocrine glands See table 1- some
endocrine organs are specialized for hormone
secretion Some (e.g., skin, stomach, liver,etc.)
have additional functions
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(specialized)
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What is a hormone? Small molecule that affects
metabolism of target organ Amines- from
tyrosine and tryptophan adrenal medulla,
thyroid, pineal glands Polypeptides and
proteins more than 100 amino acids example
growth hormone
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Glycoproteins large chain of amino acids with
carbohydrates attached (FSH and
LH) Steroids derived from cholesterol progester
one, cortisol, testosterone (only by adrenal
gland and gonads) Some are synthesized as a
precursor and later activated Some are polar,
some lipid-soluble- these can pass through cell
membranes if small enough
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Endocrine vs neural control A lot of
overlap some polypeptides are hormones
AND neurotransmitters Neurotransmitters do not
travel in the blood, but across a synaptic
cleft Neural control generally considered
faster- acting
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Common requirements target cells must have
specific receptors for the hormone/neurotransmitt
er binding to receptor must trigger
specific changes in the target cell mechanism
for quickly switching off the activity (removal
or inactivation of the hormone/neurotransmitter)
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Effects of hormones One tissue can respond to
many hormones Synergistic effect- hormones work
together to produce a result Example
epinephrine and norepinephrine have an additive
effect to increase heart rate FSH and
testosterone have complementary effect on sperm
production
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Permissive effect- one hormone enhances
the effect of another Glucocorticoids enhance
effectiveness of catecholamines (epinephrine
and norepinephrine) Antagonist effect- one
hormone opposes the effect of another insulin
promotes fat formation glucagon promotes fat
breakdown
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Modulation of hormone effect Hormones do not
usually accumulate in blood half-life usually
several hours usually deactivated in
liver Concentration is important Physiological
range- normal activity Pharmacological (high)
range may affect other cells may affect other
hormone levels
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Priming hormone binds?more receptors
synthesized ? more hormone can bind
cell Downregulation- prolonged exposure to
high hormone levels can reduce
receptor expression. Cells are therefore
desensitized Some hormones are therefore
secreted in spurts
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Mechanisms of hormone action Lipophilic
(steroids and thyroxine) pass through membrane b
ind receptors inside target cells in cytoplasm
or nucleus
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Water-soluble hormones cant pass
through membrane bond to receptors on cell
(membrane) surface Second messenger
activation required adenylate cyclase-cAMP phosp
holipase C- calcium
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Some important endocrine glands Pituitary
gland anterior lobe secretes its own
hormones regulated by hypothalamus feedback
control posterior lobe- neural tissue stores
and releases products of hypothalamus
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Adrenal glands- paired organs that sit on
top of the kidneys Outer cortex and inner
medulla have different functions Medulla-
catecholamines (epinephrine, norepinephrine) sti
mulated by sympathetic nerves Cortex- controlled
by ACTH from anterior pituitary
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Cortex secretes steroid hormones (corticosteroids
) Mineralocorticoids- regulate Na and K
balance aldosterone Glucocorticoids- regulate
glucose metabolism cortisol Androgens,
supplement sex steroids secreted by gonads
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Adrenal medulla- innervated by sympathetic nervou
s system Fight or flight rise in blood
glucose rise in blood fatty acids Sustained
stress- general adaptation syndrome 1. Alarm 2.
Resistance 3. Exhaustion
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Thyroid and parathyroid glands Thyroglobulin
iodine thyroxine (T4) and triiodothyronine
(T3) Released from precursor (thyroxine)
through action of TSH Protein synthesis Maturatio
n of nervous system Increase rate of cell
respiration
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Calcitonin released by parafollicular
cells works with parathyroid hormone inhibits
dissolution of bone stimulates excretion of
calcium in urine (lowers blood calcium levels)
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Thyroid diseases Iodine deficiency goiter cant
make enough T3 and T4 no inhibition of
TSH stimulates abnormal thyroid
growth Hypothyroid low metabolic rate
inability to adapt to cold myxedema (swelling)
in adults
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Lots of possible causes for hypothyroidism Lack
of thyrotropin-releasing hormone from
hypothalamus insufficient TSH from
pituitary iodine deficiency (goiter) Hyperthyroi
d Graves disease tumors metabolic rate is too
high irritability intolerance of heat high
blood pressure See Table 11.8 for comparison
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Children with thyroxine deficiencies will lack
normal gowth and nervous system development
(cretinism) (lack of growth hormone does not
affect intelligence) Immediate treatment with
thyroxine will restore intelligence Children are
now routinely tested at birth for thyroid
function
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Review other hormones for where produced and
what are target organs/tissues how regulated
(feedback, pituitary, etc.) disorders
associated with overproduction or
underproduction