Download hormones

Anatomy & Physiology
Of Endocrine System
Jony Mallik
[email protected]
THE ENDOCRINE SYSTEM
• Two body systems are responsible for sending and
receiving sensory information and coordinating body
responses. These are the nervous system and the
endocrine system. Together, they are sometimes
referred to as the neuro-endocrine system.
• The endocrine system regulates body activities by
releasing hormones (chemical messengers) into the
bloodstream, where they are carried throughout the
entire body.
• Hormonal responses may be almost instantaneous,
or may occur days later.
• There is a wide variety of hormonal effects.
HORMONE FUNCTIONS
• Regulate the chemical composition and
volume of the internal environment
(extracellular fluid).
• Help regulate metabolism and energy balance.
• Help regulate contraction of smooth and
cardiac muscle fibers and secretion by glands.
• Help
maintain
homeostasis,
despite
disruptions, such as infection, trauma,
emotional stress, dehydration, starvation,
hemorrhage, and temperature extremes.
HORMONE FUNCTIONS
• Regulate certain activities of the immune
system.
• Play a role in the smooth, sequential
integration of growth and development.
• Contribute to the basic processes of
reproduction, including gamete production,
fertilization, nourishment of the embryo and
fetus, delivery, and nourishment of the
newborn.
EXOCRINE & ENDOCRINE GLANDS
• The body contains two kinds of glands:
– Exocrine glands secrete their products into body
ducts, which carry the products into body cavities,
the lumen of an organ, or the outer surface of the
body.
• Sudoriferous glands, sebaceous glands, mucous glands,
and digestive glands.
– Endocrine glands secrete their products
(hormones) into the extracellular space around
the secretory cells. The secretions diffuse into
capillaries and are carried throughout the body by
the circulatory system.
ENDOCRINE SYSTEM
• The endocrine system is composed of the
body’s endocrine glands. These include:
– The pituitary, thyroid, parathyroid, adrenal, and
pineal glands
• There are also many organs that have cells
which secrete hormones, but are not
exclusively endocrine organs. These include:
– The hypothalamus, thymus, pancreas, ovaries,
testes, kidneys, stomach, liver, small intestine,
skin, heart, and placenta.
ENDOCRINE SYSTEM
HORMONES
• Hormones can have very powerful effects, even
when present in very low concentrations.
• There are approximately 50 different hormones
produced in the human body. Most of these only
affect a few types of cells.
• The specific cells which are affected by a hormone
are called target cells.
• Hormones influence their target cells by binding to
proteins or glycoproteins in the cell membrane called
receptors. Only the target cells for a certain
hormone have receptors that will recognize and bind
to that hormone.
HORMONES
• Hormones that pass into the blood and act on distant
target cells are called circulating hormones or
endocrines.
– Circulating hormones may linger in the blood for minutes
to hours, exerting their effects for a prolonged period of
time.
– Eventually, circulating hormones are inactivated by the
liver and excreted by the kidneys.
• Hormones that act locally without first entering the
bloodstream are called local hormones.
– Local hormones that act on neighboring cells are called
paracrines.
– Local hormones that act on the same cell that secreted
them are called autocrines.
– Local hormones are usually inactivated very quickly.
HORMONES
• Chemically, there are four principal classes of
hormones:
– Steroids are lipids derived from cholesterol.
• Produced in the adrenal cortex, kidneys, testes, and
ovaries.
• Include aldosterone, cortisol, androgens, calcitriol,
testosterone, extrogens, and progesterone.
– Biogenic amines are structurally simple
molecules, often synthesized by modifying
tyrosine (an amino acid).
• Produced in the thyroid gland, adrenal medulla, mast
cells, blood platelets, and pineal gland.
• Include thyroid hormones (T3 and T4), epinephrine,
norepinephrine, histamine, serotonin, and melatonin.
HORMONES
– Peptides and proteins consist of chains of 3-200 amino
acids. Some also have carbohydrate groups attached,
making them glycoprotiens.
• Produced in the hypothalamus, anterior pituitary gland, pancreas,
parathyroid glands, thyroid gland, stomach, small intestine, and
kidneys.
• Include hypothalamic releasing and inhibiting hormones, oxytocin,
antidiuretic hormone, anterior pituitary gland hormones, insulin,
glucagon, somatostatin, pancreatic polypeptide, parathyroid
hormone, calcitonin, digestion-regulating hormones, and
erythropoietin.
– Eicosanoids are derived from arachidonic acid, a 20carbon fatty acid. They act as local hormones, and
sometimes also as circulating hormones.
• Produced by all cells except red blood cells.
• Include prostaglandins and leukotrienes.
MECHANISMS OF HORMONE ACTION
• The response to a hormone is dependent
upon both the hormone and the target cell.
The same hormone can have very different
effects on different target cells.
• Possible hormone effects include synthesis of
new molecules, changing the permeability of
the plasma membrane, stimulating transport
of a substance into or out of the target cell,
altering the rate of a metabolic reaction, or
causing contraction of smooth or cardiac
muscles.
MECHANISMS OF HORMONE ACTION
• Lipid-soluble hormones (including steroids
and thyroid hormones) are able to pass
through the plasma membrane of the target
cell, so their receptors are located within the
cell.
• These hormones affect cell function by
altering gene expression.
MECHANISMS OF HORMONE ACTION
• Water-soluble hormones (including catecholamine,
peptide, and protein hormones) cannot pass through
a plasma membrane, so must attach themselves to
integral protein receptors that extend into the
interstitial fluid. Since these hormones can only
deliver their messages to the plasma membrane,
they are called first messengers. A second
messenger is required to relay the message into the
interior of the cell.
• These hormones affect cell function by activating
plasma membrane receptors, initiating a cascade of
intracellular events.
CONTROL OF HORMONE SECRETION
• Secretion of hormones is regulated to prevent
oversecretion or undersecretion.
• Hormone secretion is controlled by signals
from the nervous system, chemical changes in
the blood, or other hormones.
• Usually, negative feedback systems regulate
hormonal secretions.
THE PITUITARY GLAND
• The pituitary gland or hypophysis is attached to the
hypothalamus at the base of the brain.
• The hypothalamus is the major integrating link
between the nervous and endocrine systems.
• Although the pituitary gland is sometimes called the
“master” endocrine gland, it is actually subject to
control by the hypothalamus, which receives input
from other regions of the brain.
• Together, the hypothalamus and pituitary gland
regulate virtually all aspects of growth, development,
metabolism, and homeostasis.
• The pituitary gland can be divided into the posterior
pituitary and anterior pituitary.
THE POSTERIOR PITUITARY
• The posterior pituitary works as a unit with
the hypothalamus.
• Although the posterior pituitary does not
synthesize its own hormones, it does store
and release oxytocin (OT) and antidiuretic
hormone (ADH) produced in the
hypothalamus.
– OT controls uterine contractions during delivery
and milk ejection during breastfeeeding.
– ADH causes retention of body water, controlling
the body’s water-balancing mechanism
THE ANTERIOR PITUITARY
• The anterior pituitary secretes hormones that control
a wide range of bodily activities.
• The hypothalamus regulates the anterior pituitary by
producing releasing hormones that stimulate release
of anterior pituitary gland hormones and inhibiting
hormones that suppress release of anterior pituitary
gland hormones.
• The Anterior pituitary has five princple types of cells
which secrete seven major hormones.
ANTERIOR PITUITARY CELLS
• Somatotrophs produce human growth hormone (hCG) or
somatotropin, which stimulates general body growth and
regulates certain aspects of metabolism.
• Thyrotrophs produce thyroid stimulating hormone (TSH),
which controls secretions and other activities of the thyroid
gland.
• Gonadoptrophs produce follicle-stimulating hormone (FSH)
and luteinizing hormone (LH). Together FSH and LH stimulate
the secretion of estrogen and progesterone and the
maturation of oocytes in the ovaries and the secretion of
testosterone and sperm production in the testes.
• Lactotrophs procude prolactin (PRL), which initiates milk
production in the mammary glands.
• Corticotrophs produce adrenocorticotropic hormone (ACTH)
and melanocyte-stimulating hormone (MSH). ACTH
stimulates the adrenal cortex to secrete glucocorticoids. MSH
affects skin pigmentation.
THE THYROID GLAND
• The thyroid gland is shaped like a butterfly, located
in the front center of the neck, just inferior to the
larynx.
• The thyroid is responsible for regulating metabolism,
growth, and development (including bone, nerve,
and muscle tissues).
• The thyroid is the only endocrine gland that stores
large quantities of its secretory product (usually
about a 100 day supply).
• Disorders associated with malfunctioning of the
thyroid gland include cretinism, myxedema, Graves’
disease, and goiter.
THE THYROID GLAND
• The thyroid gland produces three hormones:
Thyroxine or tetraiodothyronine (T4),
triiodothyronine (T3) (which are called thyroid
hormones) and calcitonin.
• Calcitonin helps to regulate calcium homeostasis.
• T3 is the more potent of the thyroid hormones, but
T4 is more abundant.
• Both T3 and T4 are synthesized from iodine and the
amino acid tyrosine, under the influence of a
negative feedback loop controlled by the
hypothalamus.
• Most thyroid hormones becomes bound to blood
proteins, making them unavailable for the body to
use. Only about 1% of thyroid hormones remain in
the free, unbound form which can be utilized by the
body.
THE PARATHYROID GLANDS
• The four parathyroid glands are located on
the posterior surface of the thyroid gland.
• The parathyroid glands produce three forms of
parathyroid hormone (PTH) or
parathormone.
• PTH responds to Ca2+ levels in the blood to
regulate the homeostasis of calcium and
phosphate ions. When Ca2+ levels fall in the
blood, PTH prompts the release of calcium
from the bones.
• Too little Ca2+ results in tetany.
THE ADRENAL GLANDS
• The paired adrenal (suprarenal) glands lie superior
to the kidneys.
• The adrenal glands are composed of the outer
adrenal cortex and the inner adrenal medulla.
• The adrenal cortex produces a total of about 40
different hormones which are collectively known as
corticosteroids.
• The complete loss of adrenocorticol hormones leads
to death within a few days to a week, due to
dehydration and electrolyte imballances.
THE ADRENAL GLANDS
• The adrenal cortex produces three classes of hormones:
mineralocorticoids (affect mineral homeostasis, help control
water and mineral balance), glucocorticoids (affect glucose
homeostasis, help regulate normal metabolism and stress
resistance), and androgens (male sex hormones).
• Aldosterone accounts for about 95% of mineralocorticoid
activity. It regulates salt excretion by the kidneys and is
involved in the use of carbohydrates.
• Cortisol (hydrocortisone), corticosterone, and cortisone are
the glucocorticoids. They aid in the production and storage of
glucose, reduce fat inflammation following trauma, and are
essential to immune function. Cortisol accounts for about
95% of glucocorticoid acctivity.
THE ADRENAL GLANDS
• The adrenal medulla contains hormoneproducing cells called chromaffin cells.
• Chromaffin cells are under direct control of
the ANS (autonomic nervous system), so
hormone release can occur very quickly.
• Chromaffin cells produce epinephrine
(adrenaline) and norepinephrine
(noradrenalin or NE), which are involved in
the “fight or flight” response, help resist
stress, and help regulate blood pressure.
THE PANCREAS
• Since the pancreas is classified as both and
endocrine organ and an exocrine organ, it will also be
discussed again with the digestive system.
• The pancreas is a flat organ located posterior and
slightly inferior to the stomach.
• About 99% of pancreatic cells are arranged in
clusters called acini. These exocrine cells produce
digestive enzymes.
• Located between the acini are tiny groups of
endocrine tissue called pancreatic islets or islets of
Langerhans.
THE PANCREAS
• Each pancreatic islet contains four types of hormonesecreting cells: Alpha or A cells, beta or B cells, delta or D
cells, and F cells.
• Alpha cells secrete glucagon, which increases blood glucose
levels.
• Beta cells secrete insulin, which decreases blood glucose
levels.
• Delta cells secrete somatostatin (identical to growth
hormone inhibiting hormone secreted by the hypothalamus),
which inhibits insulin release and may slow the absorption of
nutrients from the GI tract.
• F cells secrete pancreatic polypeptide, which inhibits
secretion of somatostatin, contraction of the gallbladder, and
secretion of pancreatic digestive enzymes.
THE OVARIES AND TESTES
• The ovaries and testes are paired oval organs referred to as
gonads.
• The ovaries are the female gonads, located in the pelvic
cavity. They secrete estrogens and progesterone, which are
responsible for the development and maintenance of female
sexual characteristics, as well regulating the female
reproductive system [in conjunction with gonadotropic
hormones from the pituitary gland]). The ovaries also
produce relaxin, which soften connective tissues in
preparation for childbirth.
• The testes are the male gonads, located in the scrotum. They
secrete testosterone, which is responsible for male sexual
characteristics, and inhibin, which controls sperm production
by inhibiting follicle stimulating hormone.
THE PINEAL GLAND
• The pineal gland is attached to the roof of the third
ventricle of the brain. It is covered by a capsule
formed by the pia mater and consists of masses of
neuroglia and secretory cells called pinealocytes.
• The physiological role of the pineal gland remains
unclear.
• The pineal gland secretes melatonin, which is
thought to promote sleepiness and help regulate the
body’s biological clock. In animals that breed during
specific seasons, melatonin apparently alters their
capacity for reproduction, but it has not been shown
to have a similar effect on humans.
THE THYMUS
• The thymus gland was previously discussed in
the lymphatic system lesson.
• Hormones produced by the thymus gland are
thymosin, thymic humoral factor (THF),
thymic factor (TF), and thymopoietin. These
hormones promote the proliferation and
maturation of T cells (white blood cells which
destroy microbes and foreign substances).
Thymic hormones may also help to retard the
aging process.
OTHER ENDOCRINE TISSUES
• The GI tract synthesizes several digestive hormones,
including gastrin, gastric inhibitory peptide (GIP),
secretin, and cholecystokinin (CCK).
• The placenta produces the pregnancy-related
hormones human chorionic gonadotropin (hCG),
estrogens, progesterone, relaxin, and human
chorionic somatomammotropin.
• When deprived of oxygen, the kidneys release
erythropoietin, which stimulates red blood cell
production. The kidneys also produce calcitriol, the
active form of vitamin D.
• The atria of the heart produce atrial natriuretic
peptide (ANP), which helps regulate blood pressure.