Ch. 16 Notes: Endocrine System
General Principle of Endocrine System Operation
Specific Endocrine Organs and Hormones
Ch. 16 Notes: Endocrine System
General Principles
Endocrine system is a control system: compare/contrast to nervous system
Chemical Messengers: Hormones compared to other chemical messengers
Hormones: Classification by chemical structure. Most are either:
Amino-acid based: Hydrophilic, water-soluble, don’t cross the cell membrane
Cholesterol-based: Hydrophobic, poorly soluble in water, do cross cell membrane into cytoplasm
Hormone mechanisms of action
Amino acid based (except thyroid hormone): Work via plasma membrane receptors and second
messengers. The membrane receptors are usually coupled to G proteins which activate when the
hormone binds to the rector. (“G-protein coupled receptors”). Know general principles of G protein
coupling & 2nd messengers and a couple of examples.
Steroid hormones and thyroid hormone: work via cytoplasmic receptors – know general principles of
how these can regulate gene expression , protein synthesis
Other: Insulin receptor is a tyrosine kinase. Cyclic GMP is a second messenger in some cases.
Target Cell Specificity
Refers to ability of a given hormone to have different effects in different tissues, even though the
hormones “go everywhere” since they are carried by blood to all tissues.
Possible due to fact that different cells have different receptors and/or different intracellular pathways
“connected to” those receptors
Half-life, onset, duration
Regulated by rate of production and rate of removal from the system
Removed by metabolic breakdown in various tissues, esp. liver, and by excretion, esp by kidneys
Interactions at target cells: permissive, synergistic, antagonistic
Control of Hormone Release
Almost always involves negative feedback to keep some physiological quantity in the “good” range
Humoral, i.e. controlled by concentration of a substance in the blood. Examples: glucose, calcium, etc
Neural. Example: action potentials on sympathetic preganglionic fibers to adrenal medulla cause epi &
norepi release from adrenal medullary cells
Hormonal: Many hormones are regulated by other hormones, or regulate another hormone, or both
Specific Endocrine Organs
Hypothalamus & Pituitary (functionally & anatomically linked)
Pituitary gland – “master gland”
Other name = hypophysis. Stalk = infundibulum. Location = hypophyseal fossa of sphenoid.
Two parts: posterior pituitary = neurohypophysis; anterior pituitary = adenohypophysis
Post. Pit.:
Secretes oxytocin & ADH (also known as vasopressin) – know what they regulate and
what would cause more or less secretion of both. Both are peptides.
Secretion is from axons of hypothalamic neurons (i.e. neurons whose cell bodies are in
hypothalamus)
Ant. Pit.
Secretes GH, TSH, ACTH, PRL, FSH, LH. Know what each does and what directly or
indirectly would cause more or less secretion of each. LH, FSH covered more in
reproduction chapter.
Hormones of ant. pit. are made in the ant. pit., unlike post. pit. hormones that are made in
hypothalamus.
Ant pit hormones are controlled by hormones made in other hypothalamic nuclei, which
secrete into capillary network at top of infundibulum. Portal veins carry the “control”
hormones down the infundibulum to capillaries of the ant pit. (An example of a portal
circulation – what does that mean?)
Growth hormone (GH): General metabolic stimulation & more protein synthesis.
Releases fat stores into blood for use as fuel for growth. Reulates by 2 hypothalamic
hormones which come via the hypophyseal portal circulation: GHRH & GHIH. (Know
what the abbrevs in this chapter stand for.) Gigantism, dwarfism if too much or too little
during childhood.
TSH (thyrotropin): Stimulates thyroid gland, making it make more thyroid hormone
(TH). Regulated by TRH which comes from hypothalamus via portal circulation. Both
TRH release from hypothalamus and TSH release from ant pit are under negative
feedback control by TH: more TH suppresses TRH and TSH release.
ACTH (=corticotropin): Stimulates adrenal cortex to make & release more cortisol &
cortisone, which are themselves hormones (“glucocorticoids”). Stressors of various types
can cause more ACTH release. CRH, from hypothalamus, stimulates ACTH synthesis &
release from ant pit. Glucocorticoids (cortisol, cortisone) exert negative feedback control
over release of CRH and ACTH.
Prolactin, PRL: Stimulates mammary glands to produce milk. PRL production & release
is inhibited by PIH, from the hypothalamus. PRL is the slow, long-term stimulator of
breast milk synthesis in lactating women. (Oxytocin is the short-term stimulus for milk
“letdown” during nursing. Its production & release are stimulated via neural pathways
driven by suckling.)
FSH & LH = gonadotropins. LH stimulates ovaries & testes to make more estrogen,
progesterone, testosterone. FSH stimulates sperm maturation in men. FSH & LH work
together in women to cause one egg per month (usually) to mature and become ready for
fertilization. Controlled by GnRH from hypothalamus: more GnRH causes more
production & release of LH & FSH. There is negative feedback control of GnRH, LH,
and FSH by the gonadal hormones (testosterone, estrogen testosterone).
Thyroid Gland
Thyroid hormones: thyroxine (T4) & tri-iodothyronine (T3) have 4 & 3 iodine atoms
TH stimulates metabolism throughout the body and increases body temperature
Parathyroid Glands
Make PTH
Regulates plasma calcium concentration
Low [Ca] causes more PTH release, which causes
Vitamin D activation, which causes more Ca uptake from gut
Less Ca loss in urine
Stimulation of osteoclasts, which dissolve bone matrix and put Ca into the circulation
Adrenal Glands
Adrenal Cortex
Mineralocorticoids: mainly aldosterone:
Aldo causes [Na] up by reducing the loss of Na in urine, [K] down
Aldo release is controlled by angiotensin (more ang.II causes more aldo release), which is
controlled by renin (more renin causes more ang.II to be created), which is controlled by
sympathetic nerves and blood pressure (low BP and high symp nerve activity cause more
renin release).
Whole system is called renin-angiotensin-aldosterone system. R-A-A system is a key
slow and long-term regulator of blood pressure
Glucocorticoids
Cortisol, cortisone, corticosterone – mostly the first. All are steroid-backbone hormones.
Regulated by ACTH, which is regulated by CRH – see above. More ACTH causes more
cortisol release. Cortisol feeds back to inhibit release of CRH and ACTH, so the system
controls itself by negative feedback.
Stress stimulates more glucocorticoid release. Glucocorticoids helps keep blood glucose
levels and blood pressure up. High levels of glucocorticoids inhibit immune system
function and inhibit inflammatory responses. This can be good or bad. Prednisone is a
potent synthetic analog of cortisol. May be prescribed to help during a flare-up of
rheumatoid arthritis and other autoimmune diseases, or even for a bad case of poison ivy,
but side effects of prednisone make its long-term use dangerous.
Adrenal Medulla
Epi & norepi – mediate fight-or-flight responses
Pineal Gland
Makes & secretes melatonin, which promotes sleep
Bright light inhibits melatonin release
Other Endocrine Organs
These organs have endocrine and non-endocrine functions. This list is not complete.
Pancreas
Insulin
Glucagon
Diabetes mellitus (as opposed to diabetes insipidus, a disease caused by absence of ADH)
Pancreas is also an exocrine organ – more in GI chapter
Gonads & Placenta
Make gonadal hormones: estrogen, progesterone, testosterone
Also make eggs & sperm
Heart: ANP
Kidneys: erythropoietin (EPO) & renin
Copyright © 2015, William C. Rose