Download The Endocrine System

Chapter 8
THE ENDOCRINE SYSTEM
Endocrine system
 2nd great controlling system (behind nervous
system)
 Not fast
 Uses chemical messages (hormones) to
control major processes
 Reproduction, growth and development
 Body defenses, controlling electrolytes, water,
nutrients in blood
 Regulates metabolism and energy balance
Chemistry of hormones
 Hormones-chemical substances secreted by
cells into extracellular fluid to regulate
metabolic activity of other cells
 Classified as amino-acid based
 Proteins, peptides, amines
 Or prostaglandins (made from lipids)
 Or steroids (made from cholesterol)
 Sex hormones from gonads
 Hormones from adrenal cortex
Mechanisms of hormone action
 Blood borne hormones circulate throughout
the body
 Only affect cells that contain specific protein
receptors (target cells)
 Hormones are used to alter cellular activity
Mechanisms of hormone action
 After hormone binding to the protein
receptor, one or more of the following occur:
 Changes in plasma membrane permeability or
electrical state
 Synthesis of proteins or regulatory molecules
(enzymes)
 Activation or inactivation of enzymes
 Stimulation of mitosis
Mechanisms cont.
 Steroid hormones (lipid-soluble)
 Diffuse through the plasma membrane of target
cell
 Inside the cell, steroid enters nucleus and binds a
specific receptor
 Hormone-receptor complex binds specific sites on
DNA to trigger transcription of mRNA
 mRNA then is translated into new proteins
Mechanisms cont
 Nonsteroidal hormones (proteins and
peptides) must bind receptors on the outside
of the target cell
 Hormone binds membrane receptor
 Causes a series of reactions that activate enzymes
 Enzymes catalyze reactions to produce a second
messenger molecule cAMP
 cAMP oversees the intracellular changes in
response to the hormone
Control of hormone release
 Negative feedback mechanisms control
release of most hormones
 Hormone secretion is triggered by an internal or
external stimulus
 Rising hormone levels inhibit further hormone
release
Stimuli that activate
endocrine organs
 Hormonal
 Most common
 Endocrine organs are activated by other
hormones
 Humoral
 Changing blood levels of certain ions and
nutrients
 Neural
 Activation by nerve fibers
Major endocrine organs
 Pituitary, thyroid, parathyroid, adrenal,
pineal, thymus, pancreas, gonads,
hypothalamus (also part of nervous system)
 Endocrine organs are duct-less, secreting
hormones produced into blood or lymph
Pituitary gland
 Hangs below hypothalamus
 Has two lobes
 Anterior pituitary (glandular tissue)
 Posterior pituitary (nervous tissue)
Hormones of anterior
pituitary
 Growth hormone and prolactin – exert effects on
nonendocrine targets
 Thyrotropic hormone, adrenocorticotropic
hormone, two gonadotropic hormones- are all
tropic and stimulate target organs (endocrine
glands) to secrete their own hormones
 All anterior pituitary hormones are
 Proteins, act through second messengers, regulated
by hormonal stimuli (negative feedback)
Growth hormone
 Effects directed to growth of skeletal muscles





and long bones of the body
Helps determine final body size
Causes amino acids to be built into proteins
Stimulate target cells to grow in size and
divide
Causes fats to be broken down for energy
Maintains blood sugar homeostasis
Homeostatic imbalance
 Hyposecretion of GH
 Pituitary dwarfism; body proportions are normal
but maximum height is 4 feet
 Hypersecretion of GH
 In childhood leads to gigantism (8-9 feet tall) with
normal body proportions
 After long bone growth has ended leads to
acromegaly (thickening of soft tissues,
malformation of facial features); can be treated
with GH
Prolactin
 PRL is a protein hormone
 Targets the breast to stimulate milk
production after childbirth
 No known function in males
Adrenocorticotropic hormone
 ACTH regulates endocrine activity of the
adrenal cortex
Thyroid-stimulating hormone
 TSH (or thyrotropic hormone – TH)
 Influences growth and activity of the thyroid
gland
Gonadotropic hormones
 Regulate hormonal activity of the gonads:
ovaries and testes
Follicle stimulating hormone
 FSH
 In females-stimulates follicle development in
ovaries producing estrogen as they mature;
prepares eggs for ovulation
 In males-stimulates sperm development in the
testes
Luteinizing hormone
 LH
 In females
 triggers ovulation of an egg from an ovary
 Causes ruptured follicle to become the corpus
luteum which then produces progesterone and
estrogen
 In males
 (interstitial cell-simulating hormone) stimulates
testosterone production by interstitial cells of
testes
Homeostatic imbalance
 Hyposecretion of FSH or LH leads to sterility
in both males and females
 Hypersecretion does not seem to cause
problems but multiple births are common
Pituitary/hypothalamus
relationship
 Pituitary release of hormones is controlled by
releasing or inhibiting hormones from the
hypothalamus
 Hypothalamus sends those hormones into the
portal circulation (connects hypothalamus to
pituitary)
 Hypothalamus also produces oxytocin and
antidiuretic hormone (which travel down the
neurosecretory cells) to posterior pituitary for
storage
 Later released into blood after nerve impulses
from hypothalamus
Hormones of posterior
pituitary
 Hormones are made by hypothalamic
neurons
 Oxytocin – released only during childbirth
and nursing women; stimulates contractions
of the uterus during labor, sex, and breastfeeding; causes let-down of milk
 Antidiuretic hormone (ADH) prevents urine
production and causes kidneys to reabsorb
more water; blood volume increases as does
blood pressure
Homeostatic imbalance
 ADH is inhibited by alcohol consumption
resulting in output of large amounts of water
(urine); dry mouth and intense thirst are often
part of the “hangover”
 Diuretics antagonize ADH and result in more
urine output; these are used to manage
edema and congestive heart failure
 Hyposecretion of ADH leads to excessive
urine output or diabetes insipidus resulting in
the consumption of large amounts of water
Thyroid gland
 Located at the base of the throat, inferior to
adam’s apple
 Has 2 lobes joined by the isthmus
 Produces 2 hormones
 Thyroid hormone
 calcitonin
Thyroid gland
 Thyroid hormone is 2 active iodine-
containing hormones: thyroxine and
triiodothyronine (T4 and T3 respectively)
 T4 has 4 iodine atoms, T3 has 3 iodine atoms
 Controls the rate at which glucose is burned and
converted to heat / energy
 Needed for normal tissue growth and
development, especially reproductive and nervous
Homeostatic imbalance
 Lack of iodine causes goiter, enlarged
thyroid, which fails to inhibit FSH release
 Cretinism is hyposecretion in childhood
which does not stimulate TH; results in
dwarfism, mental retardation
 In adults, hypothyroidism results in
myxedema, physical and mental
sluggishness but no retardation, poor muscle
tone, low body temperature, obesity, dry skin
Homeostatic imbalance
 Hyperthyroidism results from a tumor in the
thyroid gland producing high metabolic rate,
intolerance of heat, rapid heartbeat, weight
loss, nervous behavior
 Grave’s disease is hyperthyroidism with
enlarged thyroid and bulging eyes
Thyroid gland
 Calcitonin (thyrocalcitonin) decreases blood
calcium levels by depositing calcium into
bones
 Secretion of calcitonin declines in adults with
age resulting in progressive decalcification of
bones
Parathyroid glands
 Tiny masses of glandular tissue on the
posterior surface of the thyroid gland
 Secrete parathyroid hormone (PTH) which
helps reglulate Ca in the blood
 Ca levels decline, PTH stimulates destruction of
osteoclasts which release Ca to the blood
 PTH also stimulates kidneys and intestine to
absorb more Ca
Adrenal gland
 Bean-shaped glands that curve over the top
of both kidneys
 Has glandular cortex and neural tissue
(medulla)
 Medulla is enclosed by the cortex
Hormones of adrenal cortex
 3 groups of corticosteroids: mineralcorticoids,
glucocorticoids, and sex hormones
 Mineralcorticoids
 Aldosterone – from outer adrenal cortex
 Regulate mineral (salt) content of blood
 Help regulate water and electrolyte balance
Hormones of adrenal cortex
 Glucocorticoids (cortisone, cortisol)
 Promote normal cell metabolism and resist
stressors by increasing blood glucose levels
 Can also be used to reduce inflammation paincausing molecules called prostaglandins
 Glucocorticoids are released in response to high
blood levels of ACTH
Hormones of adrenal cortex
 Sex hormones – produced in adrenal cortex
for both sexes in small amounts
 Androgens are the majority (male hormones)
but small amounts of estrogen are also
produced
Homeostatic imbalance
 Addison’s disease – hyposecretion of adrenal
cortex hormones
 Bronze color of skin
 Problems with water/electrolyte balance
 Weak muscles, hypoglycemia
 Cushing’s disease-tumor in middle cortex
 Edema, hypertension, hyperglycemia, weak bones
 Hypersecretion – masculinization (masculine
patter of body hair, more pronounced in
females
Hormones of adrenal medulla
 Stimulated by sympathetic nervous system
 Releases catecholamines for “fight-or-flight”
response to stressors
 epinephrine (adrenaline) into the blood stream
 Norepinephrine (noradrenaline)
 Increase heart rate, blood pressure, blood glucose
levels, dilates passages in lungs
Pancreatic islets




Called islets of Langerhans
Endocrine organ located near the stomach
Over a million islets
Produce insulin and glucagon
 Insulin is released from beta cells, increases cells’
abilities to transport glucose across the plasma
membrane; removes glucose from blood
 Glucagon is released from alpha cells; targets the
liver; stimulates breakdown of glycogen to
glucose to increase blood glucose levels
Homeostatic imbalance
Diabetes mellitus caused by sluggish pancreatic
islets that don’t produce enough insulin
3 signs of diabetes
1. polyuria – excessive urination to flush
out sugars and ketones
2. polydipsia – excessive thirst due to
water loss
3. polyphagia – hunger due to inability to
use glucose; loss of fat and proteins
Pineal gland
 Found on roof of third ventricle in brain
 Releases melatonin, levels rise and fall with
cycles of day and night
 Peak levels occur at night, lowest levels at
noon
 Melatonin is a “sleep trigger”
 Melatonin regulates mating behavior in some
animals; in humans it coordinates hormones
of fertility and prevents sexual maturity
during childhood
Thymus
 Found in upper thorax, posterior to sternum
 Large in infants and children; decreases in
size with adulthood
 Produces thymosin
 During childhood incubates white blood cells
called T lymphocytes used in the immune
response
Hormones of ovaries
 Paired, almond-sized organs in the pelvic cavity
 Produce ova (eggs) and two hormones
 Estrogens (estrone and estradiol)
 Stimulate secondary sex characteristics (maturation of
reproductive organs and hair in pubic and axillary
regions)
 Prepares uterus to receive the zygote (begins menses)
 Help maintain pregnancy (but estrogens come from
placenta at this time)
 Prepare breasts for lactation
Hormones of ovaries
 Progesterone (acts with estrogens) to begin
menses
 Quiets uterine muscles in pregnancy to prevent
abortion
 Prepares breasts for lactation
 Can also be secreted by corpus luteum
Hormones of testes
 Paired testes suspended in the scrotum,
outside the pelvic cavity
 Produces sperm
 Produces male hormones called androgens
 Testosterone from interstitial cells
 Development of male secondary sex characteristics
 Maturation of reproductive organs; stimulates sex
drive
 Needed for continuous production of sperm in
adulthood
Other hormone-producing
tissues/organs
Hormone producing cells can be found in
 Small intestine
 Stomach
 Kidneys
 Heart
 Placenta
 Some tumors (lung and pancreatic cancers)
 See Table 9.2 on page 301 for hormones produced
by these structures
Placenta
 Temporary organ produced in the uterus during
pregnancy
 Has roles as respiratory, excretory, and nutrition
delivery systems for the fetus
 Also maintains pregnancy and prepares for birth
 Produces human chorionic gonadotropin (hCG)
 Stimulates corpus luteum to continue making the
estrogens and progesterone
 Prevents sloughing of uterine lining (no periods)
 Eventually placenta takes over producing hormones
 To prepare for birth
 To prepare for lactation
Placenta
 Human placental lactogen (hPL) works with
estrogens and progesterone to prepare
breasts for lactation
 Relaxin causes mother’s pelvic ligaments and
pubic symphysis to relax and become flexible
for passage of infant through birth canal
Developmental aspects
 Embryonic development of endocrine glands varies
by gland (some are neural, some are eptithelial)
 For the most part, endocrine glands work smoothly
until old age (barring any diseases)
 Menopause occurs in women during late middle age
 Ovaries begin to atrophy and are not efficient
 Child bearing years are over
 Other problems begin with decreased estrogens:
arteriosclerosis, osteoporosis, decreased skin elasticity,
“hot flashes”, fatigue, anxiety, and depression are common
Developmental aspects
 Men typically continue to produce
testosterone in adequate amounts
throughout life
 For both sexes
 Decline in overall endocrine function
 Many elderly become hypothyroid and have
decreased ability to resist stress and infection
 Decreased immune function is due to decreased
melatonin production