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Biology 55
Endocrine System
ENDOCRINE SYSTEM – regulates body functions
Endocrine Glands – ductless glands that secrete hormones
hormones – control functions of other organs or tissues in the body
usually diffuse into capillaries and circulate in blood
target cells have receptors for hormones – when the hormone binds to the
receptor, it changes the function of the cell in some way
Chemical Classes of Hormones
peptide hormones (modified amino acids or proteins)
act as first messengers that bind to receptors on the cell membrane
activates a second messenger inside the cell (eg. cAMP)
second messenger turns on chains of enzymatic reactions that alter cell
functions
steroid hormones – (eg. sex hormones)
enter cell and bind to receptor inside cell
hormone-receptor complex binds to DNA
turns genes on, resulting in synthesis of new proteins (enzymes) that
alter cell functions
Regulation of Hormone Secretion
nervous control – signals from neurons control the secretion of hormones
hormonal control – one hormone controls the secretion of another hormone
negative feedback – secretion of most hormones is controlled by feedback of
its product or a product of its action
eg. high blood glucose causes release of insulin, which lowers blood glucose
low blood glucose then inhibits release of insulin
HYPOTHALAMUS AND PITUITARY GLAND – function together to regulate most
aspects of growth, development, metabolism, and homeostasis in the body
HYPOTHALAMUS – link between nervous and endocrine systems
controls autonomic nervous system – emotions affect endocrine functions
receives sensory information – water balance, blood pressure, concentration of
various hormones in the blood, tactile sensations
produces hormones
hormones stored by posterior pituitary – released in response to a nerve
signal from hypothalamus
antidiuretic hormone (ADH), oxytocin
regulatory hormones – regulate secretion of anterior pituitary hormones
releasing hormones – stimulate anterior pituitary
inhibiting hormones – inhibit anterior pituitary
portal system – special blood network delivers regulatory hormones
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PITUITARY GLAND – connected to the hypothalamus at base of brain
anterior pituitary – "master gland" that controls secretion of other hormones
posterior pituitary – stores oxytocin and ADH and secretes them in response
to nerve signals from the hypothalamus
Hormones of Posterior Pituitary – produced by hypothalamus
oxytocin
stimulates uterine contractions during childbirth
stimulates milk let-down during nursing
positive feedback – hypothalamus signals secretion in response to
sensory stimuli (more stimulation = more hormone)
stretch of the cervix causes secretion – stops after birth
nursing of infant causes secretion
antidiuretic hormone (ADH) – decreases volume of urine (conserves water)
hypothalamus senses dehydration (blood concentrated)
signals posterior pituitary to secrete ADH
kidneys reabsorb more water, so less urine is formed
negative feedback – as blood dilutes, less ADH is secreted
Hormones of Anterior Pituitary
secretion of most anterior hormones is controlled by releasing and inhibiting
hormones from the hypothalamus
thyroid-stimulating hormone (TSH)
stimulates thyroid to secrete thyroid hormones
controlled by negative feedback of thyroid hormones
adrenocorticotropic hormone (ACTH)
stimulates adrenal cortex to secrete its hormones
controlled by negative feedback of adrenal cortex hormones
gonadotropinic hormones (FSH & LH)
female – stimulate ovaries to produce eggs and sex hormones
male – stimulate testes to produce sperm and sex hormones
controlled by negative feedback of sex hormones
prolactin (PRL)
stimulates mammary glands to produce milk
many hormones are involved in mammary development and secretion
secretion begins before childbirth and continues while child is nursing
growth hormone (GH)
stimulates cells in most body tissues to synthesize proteins, grow and
divide, and use fat stores for energy
controlled by negative feedback of GH
giantism – too much GH during childhood
pituitary dwarfism – too little GH during childhood
acromegally – too much GH during adulthood
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THYROID GLAND – attached to trachea, just below larynx
thyroid hormones (T4 and T3)
T4 (thyroxine) has 4 iodines (tetraiodothyronine)
T3 has 3 iodines (triiodothyronine)
iodine – required in diet for production of thyroid hormones
stimulates most body tissues to increase metabolic rate
controlled by TSH and negative feedback of T3
goiter – enlarged thyroid
calcitonin
lowers blood calcium (increases bone density)
inhibits osteoclasts – decreases resorption of bone
controlled by negative feedback of blood calcium
PARATHYROID GLANDS – 4 small glands attached to thyroid gland
parathyroid hormone (PTH) – major regulator of calcium level in blood
increases blood calcium (decreases bone density)
stimulates osteoclasts – increases resorption of bone
controlled by negative feedback of blood calcium
hypocalcemia – low blood calcium; may be caused by low PTH
causes nerve depolarization and muscle spasms (tetany)
ADRENAL GLANDS – found on top of each kidney
Adrenal Medulla – central portion of adrenal gland
epinephrine and norepinephrine
controlled by sympathetic nervous system
fight-or-flight responses – secreted during stress
Adrenal Cortex – outer portion of adrenal gland; produces 3 types of steroid hormones
mineralocorticoids – regulate mineral(Na+ and K+) and water homeostasis
aldosterone – main mineralocorticoid produced
stimulates kidneys to conserve Na+ and water and excretion of K+
(increases blood pressure and blood Na+, decreases blood K+ )
controlled by renin-angiotensin-aldosterone (RAA) pathway
1) low blood volume and pressure
2) kidneys secrete renin (enzyme) into blood
3) renin converts angiotensinogen into angiotensin I
4) lungs convert angiotensin I to angiotensin II
5) angiotensin II stimulates aldosterone secretion
6) blood volume and pressure increase
also controlled by negative feedback of blood Na+ and K+
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glucocorticoids – increases blood glucose and helps resist stress
cortisol (hydrocortisone) – main glucocorticoid produced
increases blood glucose available for brain (main energy source for brain)
stimulates break down of fats and proteins(muscle)
stimulates most cells use of these other nutrients for energy
stimulates liver to convert these other nutrients to glucose
reduces pain
anti-inflammatory
suppresses immune system
secreted in response to ACTH and prolonged stress
controlled by negative feedback of glucocorticoids
sex hormones
amount is low compared to that produced in gonads
PANCREATIC ISLETS – clusters of endocrine cells in pancreas
pancreas – between stomach and sm. intestine
99% exocrine glands producing digestive enzymes
1% pancreatic islets (islets of Langerhans) – produce hormones
glucagon – increases blood glucose (between meals)
stimulates break down of fat to supply energy for cells
stimulates liver to convert other nutrients to glucose
controlled by negative feedback of blood glucose
insulin – decreases blood glucose (after eating)
stimulates most body cells to take in glucose (carrier proteins) and use it
cells use glucose for ATP synthesis
glucose stored as glycogen in liver and muscles
glucose converted to fat by adipose tissue
controlled by negative feedback of glucose
diabetes mellitus – inability to produce or respond to insulin
cells can’t take in or use glucose
Type I (insulin-dependent) – no insulin produced
Type II (insulin-resistant) – cells don't respond to insulin
hyperglycemia – high blood glucose
frequent urination – glucose gets in urine, water follows(osmosis)
hunger and weight loss (can't use carbohydrates for energy)
insulin shock – low blood glucose due to insulin overdose or not eating
brain can't function without glucose
PINEAL GLAND – found in diencephalon of brain regulated by light and darkness
melatonin – sets circadian rhythm (sleep and awakening cycles)
controlled by light and darkness
THYMUS – found over base of heart and distal trachea
produces hormones that regulate development of immune cells
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GONADS – testes and ovaries
Testes – found in scrotum of male
androgens – male sex hormones (mainly testosterone)
stimulates development of male reproductive organs in fetus
and their growth at puberty
causes development of male secondary sexual characteristics
beard, body hair, deepening of voice
anabolic – increases growth of tissues, especially bones and
muscles
Ovaries – found in pelvic cavity of female
estrogens and progesterone
stimulates development of female reproductive organs in fetus
and their growth at maturity
causes development of female secondary sexual characteristics
breasts, body hair and fat, wider pelvis
cause menstrual cycles
anabolic – though less so than testosterone
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