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Anatomy & Physiology
Lesson 8
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, digestionregulating 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.