Hormones and the Endocrine System
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Transcript Hormones and the Endocrine System
Chemical Coordination
Chapter 34
Hormones
A hormone is a chemical signal that is secreted into
the circulatory system and communicates regulatory
messages within the body.
Hormones may reach all parts of the body, but only
certain types of cells, target cells, are equipped to
respond.
Systems of Internal Communication
Animals have two systems of internal communication
and regulation:
The nervous system
The endocrine system
Systems of Internal Communication
The nervous system conveys high-speed electrical
signals along specialized cells called neurons.
The endocrine system, made up of endocrine
glands, secretes hormones that coordinate slower but
longer-acting responses to stimuli.
Hormones
Advantages of using chemical messengers:
Chemical molecules can spread to all tissues through the
blood.
Chemical signals can persist longer than electrical ones.
Many different kinds of chemicals can act as hormones;
different hormones can target different tissues.
Glands
Many hormones are secreted by ductless endocrine
glands.
Obtain raw materials from and secrete hormones directly
into the bloodstream.
Exocrine glands have ducts for discharging secretions
onto a free surface.
Sweat glands, salivary glands, enzyme-secreting glands
in the digestive tract.
Hormones
Hormones convey information via the bloodstream to
target cells throughout the body.
Pheromones carry messages outside the body – to
other individuals.
Hormones
Three major classes of molecules function as
hormones in vertebrates:
Proteins and peptides
Amines derived from amino acids
Steroids
Hormones
Signaling by any of these molecules involves three key
events:
Reception
Signal transduction
Response
Hormones
The hypothalamus regulates the neuroendocrine
system, maintaining homeostasis in the body.
The hypothalamus can use motor nerves to send shortlived electrical messages or hormones to send chemical
messages with a longer duration.
The Chain of Command
The hypothalamus produces seven different
“releasing” hormones that travel to the
pituitary gland.
Each releasing hormone stimulates the pituitary
to release a corresponding hormone which
travels to an endocrine gland and causes it to
start producing a particular endocrine hormone.
Membrane-Bound Receptors
Many hormones are too
large, or too polar, to
pass through plasma
membranes.
Bind to transmembrane
proteins that act as
receptor sites on target
cell membranes.
Hormone is first
messenger.
Causes activation of a
second messenger in
the cytoplasm.
cAMP
Nuclear Receptors
Steroid hormones are
lipid soluble molecules
that bind to hormone
receptors in the cytoplasm
of the target cell.
Site of activity is the
nucleus.
Steroids are manufactured
from cholesterol.
Estrogen, progesterone,
testosterone, cortisol.
Nuclear Receptors
Thyroid hormones and
insect-molting
hormone (ecdysone)
also act through
nuclear receptors.
Binds to
transmembrane
protein that uses ATP
to move it into the cell.
Control Pathways and Feedback
Loops
A common feature
of control pathways
is a feedback loop
connecting the
response to the
initial stimulus.
Negative feedback
regulates many
hormonal pathways
involved in
homeostasis.
Invertebrate Hormones
Ecdysone
regulates
molting in
insects.
Juvenile
hormone
favors the
retention of
juvenile
characteristics.
The Pituitary
The pituitary gland is
located below the
hypothalamus.
Nine major hormones
are produced here.
These hormones act
primarily to influence
other endocrine
glands.
The Pituitary
The posterior lobe of the pituitary regulates water
conservation, milk letdown, and uterine contraction in
women.
The anterior lobe regulates the other endocrine
glands.
The Anterior Pituitary
Thyroid stimulating hormone (TSH) – stimulates the
thyroid gland to produce thyroxine which stimulates
oxidative respiration.
Luteinizing hormone (LH) plays an important role in
the menstrual cycle. It also stimulates the production of
testosterone in males.
The Anterior Pituitary
Follicle-stimulating hormone (FSH) – plays
an important role in the menstrual cycle. In
males, it causes the testes to produce a
hormone that regulates sperm production.
Adrenocorticotropic hormone (ACTH) –
stimulates the adrenal gland to produce steroid
hormones. Some regulate glucose production,
others balance sodium & potassium in the
blood.
The Anterior Pituitary
Growth hormone (GH) – stimulates the growth of
muscle and bone.
Prolactin – stimulates milk production.
Melanocyte-stimulating hormone (MSH) – in reptiles
& amphibians, this hormone stimulates color change.
The Posterior Pituitary
Antidiuretic hormone
(ADH) regulates the
kidney’s retention of water.
Oxytocin initiates uterine
contraction during
childbirth and milk release
in mothers.
These hormones are
actually synthesized in the
hypothalamus and stored
in the posterior pituitary.
Biological Clocks
The pineal gland is
located in the brain of
most vertebrates.
Evolved from a light
sensitive “third eye”.
Primitive fish & some
reptiles still have a third
eye.
Biological Clocks
In other vertebrates it functions as an
endocrine gland secreting melatonin.
Melatonin controls color change in amphibians
& reptiles.
Release of melatonin is controlled by light/dark
cycles.
The primary functions of melatonin appear to
be related to biological rhythms associated with
reproduction.
Circadian rhythms – 24 hours long.
The Thyroid
The thyroid gland, located
in the neck, produces:
Thyroxine – increases
metabolic rate and promotes
growth.
Two iodine-containing
hormones, triiodothyronine
(T3) and thyroxine (T4).
Calcitonin – stimulates
calcium uptake by bones.
The Thyroid
The hypothalamus and
anterior pituitary
control the secretion of
thyroid hormones
through two negative
feedback loops.
The Thyroid
The thyroid
hormones play
crucial roles in
stimulating
metabolism and
influencing
development
and maturation.
The Parathyroids
The parathyroid glands are
four small glands attached to
the thyroid.
The hormone they produce
is parathyroid hormone
(PTH) which regulates the
level of calcium in the blood.
Essential that calcium is
kept within narrow limits
for muscle contraction,
including the heart.
Calcium Homeostasis
Two antagonistic
hormones,
parathyroid
hormone (PTH) and
calcitonin, play the
major role in calcium
(Ca2+) homeostasis in
mammals.
Calcium Homeostasis
Calcitonin, secreted by the thyroid gland,
stimulates Ca2+ deposition in the bones and
secretion by the kidneys, thus lowering blood
Ca2+ levels.
PTH, secreted by the parathyroid glands, has
the opposite effects on the bones and kidneys,
and raises Ca2+ levels.
Also has an indirect effect, stimulating the kidneys to
activate vitamin D, which promotes intestinal uptake
of Ca2+ from food.
The Adrenals
Mammals have an adrenal gland above each kidney.
Adrenal medulla is the inner core which produces
adrenaline (epinephrine) and norepinephrine.
Adrenal cortex is the outer shell that produces the
steroid hormones cortisol and aldosterone.
Adrenal Medulla
The adrenal medulla releases adrenalin (epinephrine)
and norepinephrine in times of stress.
Identical to the effects of the sympathetic nervous
system, but longer lasting.
Accelerated heartbeat, increased blood pressure, higher
levels of blood sugar and increased blood flow to heart and
lungs.
Adrenal Cortex
The adrenal cortex produces the steroid hormone
cortisol (hydrocortisone).
Reduces inflammation.
Synthetic derivatives such as prednisone are used as antiinflammatory agents.
Stimulates carbohydrate metabolism.
Adrenal Cortex
The adrenal cortex also produces aldosterone.
Aldosterone acts in the kidney to promote the uptake of
sodium & other salts from the urine.
These salts are important in nerve conduction.
Aldosterone and PTH are the only two hormones
essential for survival.
The Pancreas
The pancreas is located
behind the stomach and is
connected to the small
intestine by a small tube.
It secretes digestive
enzymes into the digestive
tract (exocrine function).
Endocrine function –
production of insulin and
glucagon.
Glucose Homeostasis
The islets of
Langerhans in the
pancreas secrete
insulin and glucagon.
Insulin removes
glucose from the
blood.
Glucagon returns
glucose to the blood.
Diabetes
Diabetes mellitus, perhaps the best-known endocrine
disorder, is caused by a deficiency of insulin or a
decreased response to insulin in target tissues.
Marked by elevated blood glucose levels.
Diabetes
Type I diabetes mellitus (insulin-dependent
diabetes) is an autoimmune disorder in which
the immune system destroys the beta cells of
the pancreas.
Type II diabetes mellitus (non-insulindependent diabetes) is characterized either by
a deficiency of insulin or, more commonly, by
reduced responsiveness of target cells due to
some change in insulin receptors.