 Activate or inactive enzymes

ENDOCRINE 


Stimulate or inhibit cell division
Promote or inhibit secretion of a product
Turn on or turn off transcription of certain genes

SYSTEM
ENDOCRINE SYSTEM
(such as those encoding proteins or regulatory
molecules)

DIRECT GENE ACTIVATION

- releases chemicals called hormones that regulate - there are really only two mechanisms by which
complex body processes. hormones trigger changes in cells
- Hormones released by endocrine glands travel - Steroid hormones (and, strangely, thyroid hormone)
through the blood and alter the activity of target can use the mechanism of direct gene activation
cells - Because they are lipid-soluble molecules, the
steroid hormones can diffuse through the plasma
- regulates complex processes such as growth and membranes of their target cells
development, metabolism, and reproduction. - Once inside, the steroid hormone enters the
- Along with the nervous system, it coordinates and nucleus, and binds to a specific hormone receptor,
directs the activity of the body’s cells. the hormone-receptor complex then binds to
specific sites on the cell’s DNA activating certain
- the major processes that hormones control are genes to transcribe messenger RNA (mRNA). The
reproduction; growth and development; mobilizing mRNA is translated in the cytoplasm resulting in the
body defenses against stressors; maintaining synthesis of new proteins
electrolyte, water, and nutrient balance of the
blood; and regulating cellular metabolism and SECOND-MESSENGER SYSTEM
energy balance - Steroid hormones can influence cell activity either
by direct gene activation or by the indirect pathway
THE CHEMISTRY OF HORMONES of activating a second messenger
- Protein and peptide hormones are not water-
HORMONES soluble and are unable to enter target cells directly
- are chemical substances secreted by endocrine cells - they bind to hormone receptors situated on
into the extracellular fluids that regulate the the target cell’s plasma membrane and use a
metabolic activity of other cells in the Body second-messenger system
- nearly all of them can be classified chemically as - the hormone (first messenger) binds to the
either amino acid–based molecules (including receptor protein on the membrane, and the
proteins, peptides, and amines) or steroids. activated receptor sets off a series of reactions (a
cascade) that activates an enzyme. The enzyme, in
HORMONE ACTION turn, catalyzes reactions that produce second
- Although hormones circulate to all the organs of -messenger molecules (in this case, cyclic AMP, also
the body via blood, a given hormone affects only known as cAMP, or cyclic adenosine
certain tissue cells or organs, referred to as its monophosphate) that oversee additional
target cells or target organs. intracellular changes that promote the typical
- For a target cell to respond to a hormone, specific response of the target cell to the hormone.
protein receptors to which that hormone can attach
must be present on the cell’s plasma membrane or
in its interior
- term hormone comes from a Greek word meaning
“to arouse.” body’s hormones “arouse,” or bring
about their effects on, the body’s cells primarily by
altering cellular activity—that is, by increasing or
decreasing the rate of a normal, or usual, metabolic
process rather than by stimulating performance of
a new one
- Hormones can:

 Change plasma membrane permeability or

membrane potential (electrical state) by opening or
closing ion channels
STIMULI FOR CONTROL OF HORMONE RELEASE MAJOR ENDOCRINE ORGANS
- What prompts the endocrine glands to release or - major endocrine organs of the body include
not release their hormones? the pituitary, pineal, thyroid, parathyroid, thymus
and adrenal glands, pancreas, and gonads (ovaries
NEGATIVE FEEDBACK MECHANISMS and testes)
- the chief means of regulating blood levels of nearly - Hypothalamus which is part of the nervous system,
all hormones is also recognized as a major endocrine organ
- The stimuli that activate endocrine glands fall into because it produces several hormones
three major categories - Some hormone-producing glands (the anterior
- These three mechanisms represent the most pituitary, thyroid, parathyroids, and adrenals) have
common systems that control hormone release, but purely endocrine functions, but others (pancreas
they by no means explain all of them. Some and gonads) have both endocrine and exocrine
endocrine organs respond to many different stimuli functions and are thus mixed glands
- Both types of glands are formed from epithelial
1. HORMONAL tissue, but the endocrine glands are ductless glands
- most common stimulus in which endocrine organs that produce hormones that they release into the
are prodded into action by other hormones blood or lymph
- For example, hormones of the hypothalamus - exocrine glands release their products at the body’s
stimulate the anterior pituitary gland to secrete its surface or into body cavities through ducts (they
hormones, and many anterior pituitary hormones have an exit)
stimulate other endocrine organs to release their
hormones into the blood
- As the hormones produced by the final target PITUITARY GLAND AND HYPOTHALAMUS
glands increase in the blood, they “feed back” to
inhibit the release of anterior pituitary hormones PITUITARY GLAND
and thus their own release. - approximately the size of a pea
- Hormone release promoted by this mechanism - hangs by a stalk from the inferior surface of the
tends to be rhythmic, with hormone blood levels hypothalamus of the brain, where it is snugly
rising and falling again and again. surrounded by the sella turcica (“Turk’s saddle”) of
the sphenoid bone
- It has two functional lobe:
2. HUMORAL  Anterior pituitary (glandular tissue)
- Changing blood levels of certain ions and nutrients  Posterior pituitary (nervous tissue)
may also stimulate hormone release
- term humoral refers to the ancient use of the word
humor to indicate the various body fluids (blood, PITUITARY-HYPOTHALAMUS RELATIONSHIPS
bile, and others)
- For example, a decreasing blood calcium ion level in ANTERIOR PITUITARY GLAND
the capillaries serving the parathyroid glands - controls the activity of so many other endocrine
prompts the release of parathyroid hormone (PTH). glands that it has often been called the “master
- Because PTH acts by several routes to reverse that endocrine gland”
decline, the blood Ca2+ level soon rises, ending the - Its removal or destruction has a dramatic effect on
stimulus for PTH release the body
- Other hormones released in response to humoral - the adrenal and thyroid glands and the gonads
stimuli include calcitonin, released by the thyroid atrophy, and results of hyposecretion by those
gland, and insulin, produced by the pancreas glands quickly become obvious
- the release of each of its hormones is controlled by
3. NEURAL releasing hormones and inhibiting hormones
- In isolated cases, nerve fibers stimulate hormone produced by the hypothalamus. Hypothalamus
release, and the endocrine cells are said to respond liberates these regulatory hormones into the blood
to neural stimuli of the portal circulation, which connects the blood
- example is sympathetic nervous system stimulation supply of the hypothalamus with that of the
of the adrenal medulla to release the anterior pituitary
catecholamines norepinephrine and epinephrine
during periods of stress
HYPOTHALAMUS
- also makes two additional hormones, oxytocin and ANTERIOR PITUITARY HORMONES
antidiuretic hormone, which are transported along - produces several hormones that affect many
the axons of the hypothalamic neurosecretory cells body organs
to the posterior pituitary for storage. They are later - there are six anterior pituitary hormones
released into the blood in response to nerve  Growth hormone and Prolactin - exert their
impulses from the hypothalamus major effects on nonendocrine targets

 Follicle-stimulating hormone, Luteinizing

POSTERIOR PITUITARY AND HYPOTHALAMIC hormone, Thyrotropic hormone, and
HORMONES Adrenocorticotropic hormone - tropic
hormones
POSTERIOR PITUITARY
- is not an endocrine gland in the strict sense because TROPIC HORMONES
it does not make the peptide hormones it releases - stimulate their target organs, which are also
- it acts as a storage area for hormones made by endocrine glands, to secrete their hormones, which
hypothalamic neurons in turn exert their effects on other body organs and
tissues
OXYTOCIN
- is released in significant amounts only during - All anterior pituitary hormones are:
childbirth and nursing  proteins (or peptides)
- it stimulates powerful contractions of the uterine  act through second-messenger systems
muscle during sexual relations, during labor, and  are regulated by hormonal stimuli and, in most
during breastfeeding cases, negative feedback
- It also causes milk ejection (the let-down reflex) in a
nursing woman GROWTH HORMONE (GH)
- Both natural and synthetic oxytocic drugs are used - a general metabolic hormone
to induce labor or to hasten labor that is - its major effects are directed to the growth of
progressing at a slow pace skeletal muscles and long bones of the body, and
- oxytocics are used to stop postpartum bleeding (by thus it plays an important role in determining final
causing constriction of the ruptured blood vessels body size
at the placental site) and to stimulate the let-down - a protein-sparing and anabolic hormone that causes
Reflex the building of amino acids into proteins and
stimulates most target cells to grow in size and
ANTIDIURETIC HORMONE (ADH) divide
- the second hormone released by the posterior - causes fats to be broken down and used for energy
pituitary while it spares glucose, helping to maintain blood
- Diuresis- urine production sugar homeostasis
- antidiuretic is a chemical that inhibits or prevents
urine production PROLACTIN (PRL)
- causes the kidneys to reabsorb more water from - is a protein hormone structurally similar to growth
the forming urine; as a result, urine volume hormone
decreases, and blood volume increases - only known target in humans is the breast
- Water- powerful inhibitor of ADH release - After childbirth, it stimulates and maintains
- in larger amounts, ADH also increases blood milk production by the mother’s breasts
pressure by causing constriction of the arterioles
(small arteries). It is sometimes referred to as
Vasopressin GONADOTROPIC HORMONES
- Drinking alcoholic beverages inhibits ADH secretion - regulates the hormonal activity of the gonads
and results in output of large amounts of urine (ovaries and testes)
- Certain drugs, classed together as diuretics,
antagonize the effects of ADH, causing water to be FOLLICLE-STIMULATING HORMONE (FSH)
flushed from the body. These drugs are used to - stimulates follicle development in the ovaries
manage the edema typical of congestive heart - as the follicles mature, they produce estrogen, and
failure eggs are readied for ovulation
- stimulates sperm development by the testes
 together, but thyroxine has four bound iodine
LUTEINIZING HORMONE (LH) atoms, whereas triiodothyronine has three
- triggers ovulation of an egg from the ovary and  Thyroxine (T4)
causes the ruptured follicle to produce  Triiodothyronine (T3)
progesterone and some estrogen
- stimulates testosterone production by the THYROXINE
interstitial cells of the testes. - major hormone secreted by the thyroid follicles

THYROTROPIC HORMONE (TH) TRIIODOTHYRONINE

- also called Thyroid-stimulating hormone (TSH) - mostly formed at the target tissues by conversion
- influences the growth and activity of the thyroid of thyroxine to triiodothyronine
gland

ADRENOCORTICOTROPIC HORMONE (ACTH) CALCITONIN

- regulates the endocrine activity of the cortex - decreases the blood calcium ion level by causing
portion of the adrenal gland calcium to be deposited in the bones
- acts antagonistically to parathyroid hormone
- made by the parafollicular cells found in the
connective tissue between the follicles
PINEAL GLAND - released directly to the blood in response to an
- a small, coneshaped gland that hangs from the roof increasing level of blood calcium ions
of the third ventricle of the brain - Few effects of hypo- or hypersecretion of calcitonin
- many chemical substances have been identified in are known, and calcitonin production is meager or
the pineal gland, only the hormone Melatonin ceases entirely in adults
appears to be secreted in substantial amounts - hypocalcemic hormone
- Melatonin is believed to be a “sleep trigger” that
plays an important role in establishing the body’s
sleep-wake cycle
- coordinates the hormones of fertility and to inhibit PARATHYROID GLANDS
the reproductive system (especially the ovaries of - are tiny masses of glandular tissue most often
females) until the body matures. found on the posterior surface of the thyroid gland
- there are two parathyroid glands on each thyroid
lobe, that is, a total of four parathyroids
- secretes:
THYROID GLAND
- located at the base of the throat, just inferior to the PARATHYROID HORMONE (PTH)
Adam’s apple - the most important regulator of calcium ion (Ca2+)
- a fairly large gland consisting of two lobes joined homeostasis of the blood
by a central mass, or isthmus - when the blood calcium ion concentration drops
- makes two hormone below a certain level, the parathyroids release PTH,
 Thyroid hormone which stimulates bone destruction cells (osteoclasts)
 Calcitonin - produced by the parafollicular to break down bone matrix and release calcium
cells ions into the blood
- composed of hollow structures called follicles which - a hypercalcemic hormone
store a sticky colloidal material - also stimulates the kidneys and intestine to absorb
more calcium ions
THYROID HORMONE
- often referred to as the body’s major metabolic
hormone THYMUS
- controls the rate at which glucose is “burned,” or - located in the upper thorax, posterior to the
oxidized, and converted to body heat and chemical sternum
energy (ATP) - it decreases in size throughout adulthood
- important for normal tissue growth and - by old age, it is composed mostly of fibrous
development, especially in the reproductive and connective tissue and fat
nervous systems
- two active iodine-containing hormones where each
is constructed from two tyrosine amino acids linked
THYMOSIN - are released from the adrenal cortex in response to
- hormone produced by Thymus a rising blood level of ACTH
- appear to be essential for normal development
of a special group of white blood cells (T
lymphocytes) and the immune response HORMONES OF THE ADRENAL MEDULLA

ADRENAL MEDULLA
ADRENAL GLANDS - like the posterior pituitary, is a knot of nervous
- two adrenal glands curve over the top of the tissue
kidneys like triangular hats - When the medulla is stimulated by sympathetic
- it is structurally and functionally two endocrine nervous system neurons, its cells release two
organs in one similar hormones into the bloodstream
- it has parts made of glandular (cortex) and neural  Epinephrine - also called adrenaline
tissue (medulla). The central medulla region is  Norepinephrine - noradrenaline
enclosed by the adrenal cortex, which contains - Collectively, these hormones are called
three separate layers of cells catecholamines (increase heart rate, blood
pressure, and the blood glucose level and dilate
the small passageways of the lungs)
HORMONES OF THE ADRENAL CORTEX - When you are (or feel) threatened physically
or emotionally, your sympathetic nervous system
ADRENAL CORTEX brings about the “fight-or-flight” response to help
- produces three major groups of steroid hormones, you cope with the stressful situation. One of the
which are collectively called corticosteroids organs it stimulates is the adrenal medulla, which
- mineralocorticoids, glucocorticoids, and sex literally pumps its hormones into the bloodstream
hormones to enhance and prolong the effects of the
neurotransmitters of the sympathetic nervous
MINERALOCORTICOIDS system
- mainly aldosterone
- produced by the outermost adrenal cortex cell layer
- are important in regulating the mineral (or salt) PANCREATIC ISLETS
content of the blood, particularly the
concentrations of sodium and potassium ion PANCREAS
- targets the kidney tubules that selectively reabsorb - located close to the stomach in the abdominal
the minerals or allow them to be flushed out of the cavity, is a mixed gland
body in urine
- help regulate both water and electrolyte balance in - also called the islets of Langerhans
body fluids - are little masses of endocrine (hormone-producing)
tissue scattered among the exocrine (enzyme
GLUCOCORTICOIDS -producing) tissue of the pancreas.
- is what the middle cortical layer mainly produces
- includes cortisone and cortisol EXOCRINE, OR ACINAR
- promote normal cell metabolism and help the body - part of the pancreas acts as part of the digestive
to resist long-term stressors, primarily by increasing system
the blood glucose level
- when blood levels of glucocorticoids are high, fats - two important hormones produced by the islet cells
and even proteins are broken down by body cells are insulin and glucagon
and converted to glucose, which is released to the - Islet cells act as fuel sensors, secreting insulin and
blood glucagon appropriately during fed and fasting states
- are said to be hyperglycemic hormones
- controls the more unpleasant effects of
inflammation by decreasing edema, and they INSULIN
reduce pain by inhibiting the pain-causing - A high level of glucose in the blood stimulates the
prostaglandins release of insulin from the beta cells of the islets
- Because of their anti-inflammatory properties, they - acts on just about all body cells, increasing their
are often prescribed as drugs to suppress ability to import glucose across their plasma
inflammation for patients with rheumatoid arthritis. membranes. Once inside the cells, glucose is
 oxidized for energy or converted to glycogen or fat uterine lining (the menstrual cycle, or
for storage menstruation)
- Insulin also speeds up these “use it” or “store it”
activities. Because insulin sweeps the glucose out PROGESTERONE
of the blood, its effect is said to be hypoglycemic. - acts with estrogen to bring about the menstrual
- As the blood glucose level falls, the stimulus for cycle
insulin release ends—another classic case of - during pregnancy, it quiets the muscles of the
negative feedback control. Many hormones have uterus so that an implanted embryo will not be
hyperglycemic effects (glucagon, glucocorticoids, aborted and helps prepare breast tissue for
and epinephrine, to name a few), but insulin is the lactation
only hormone that decreases the blood glucose - Ovaries are stimulated to release their estrogens
level and progesterone in a cyclic way by the anterior
- Insulin is absolutely necessary for the use of pituitary gonadotropic hormones
glucose by body cells. Without it, essentially no
glucose can get into the cells to be used
HORMONES OF THE TESTES
GLUCAGON - paired oval testes of the male are suspended in a
- acts as an antagonist of insulin sac, the scrotum, outside the pelvic cavity
- it helps to regulate the blood glucose level but - the testes also produce male sex hormones, or
in a way opposite that of insulin androgens, of which testosterone is the most
- Its release by the alpha cells of the islets is important
stimulated by a low blood level of glucose
- its action is basically hyperglycemic TESTOSTERONE
- its primary target organ is the liver, which it - at puberty, it promotes the growth and maturation
stimulates to break down stored glycogen to of the reproductive system organs to prepare the
glucose and to release the glucose into the blood young man for reproduction
- no important disorders resulting from hypo- or - causes the male’s secondary sex characteristics
hypersecretion of glucagon are known (growth of facial hair, development of heavy bones
and muscles, and lowering of the voice) to appear
and stimulates the male sex drive
GONADS - In adults, it is necessary for continuous production
- female and male gonads produce sex cells (an of sperm
exocrine function
- produce sex hormones that are identical to those
produced by adrenal cortex cells OTHER HORMONE-PRODUCING TISSUES AND ORGANS
- major differences from the adrenal sex hormone
production are the source and relative amounts of PLACENTA
hormones produced - is a remarkable organ formed temporarily in the
uterus of pregnant women
- respiratory, excretory, and nutrition-delivery
HORMONES OF THE OVARIES systems for the fetus
- female gonads, or ovaries, are paired, slightly larger - produces several protein and steroid hormones
than almond-sized organs located in the pelvic that help to maintain the pregnancy and pave the
cavity way for delivery of the baby
- ovaries produce two groups of steroid hormones,
estrogens and progesterone HUMAN CHORIONIC GONADOTROPIN (hCG)
- during very early pregnancy, this hormone is
ESTROGENS produced by the developing embryo and then by
- are responsible for the development of sex the fetal part of the placenta
characteristics in women (primarily growth and - stimulates the ovaries to continue producing
maturation of the reproductive organs) and the estrogen and progesterone so that the lining of the
appearance of secondary sex characteristics (for uterus is not sloughed off in menses
example, hair in the pubic and axillary regions) at - In the third month, the placenta assumes the job of
puberty producing estrogen and progesterone, and the
- acting with progesterone, estrogens promote ovaries become inactive for the rest of the
breast development and cyclic changes in the pregnancy
- the high estrogen and progesterone blood levels
maintain the lining of the uterus (thus, the
pregnancy) and prepare the breasts for producing
milk

HUMAN PLACENTAL LACTOGEN (hPL)

- works cooperatively with estrogen and
progesterone in preparing the breasts for lactation

RELAXIN
- placental hormone
- causes the mother’s pelvic ligaments and the pubic
symphysis to relax and become more flexible, which
eases birth passage

REFERENCE:
Essentials of Human Anatomy & Physiology 12th Edition.
Marieb, E.N & Keller, 2016. S.M. Boston : Pearson. (PDF)