Hormones 1. Introduction to the endocrine system of hormones
e.g. homeostasis and the function of the hormones adrenaline and thyroxine
including explaining negative feedback systems
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school biology revision notes: GCSE biology, IGCSE biology, O level
biology, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of biology
(other hormones are dealt with on
other pages - see links further down the page)
This page will help you answer questions
such as ... What do endocrine glands do? What do hormone molecules do? What is the role of thyroxine in our
metabolism? How does adrenaline prepare us in an
'emergency' situation? What is a negative feedback system? How does
a negative feedback system work - its function?
Sub-index for this page
(a)
What are
hormones?
(b)
Examples of
hormones and which gland or organ produces them
(c)
A comparison of
the nervous system and the endocrine hormone system
(d) The
function of the hormone adrenaline
(e)
The function of
the hormone thyroxine
See also
Homeostasis - introduction to how it functions (negative
feedback systems explained)
Hormone systems - menstrual cycle
Homeostasis - control of blood sugar level
- insulin and diabetes
Homeostasis - osmoregulation, ADH, water control, urea and ion
concentrations, kidney function, dialysis
and for plants see
Hormone control of plant growth and uses of plant hormones
gcse biology revision notes
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(a) What are
hormones?
Hormones are produced in the endocrine gland system and are transported
by the blood to their target cells, tissues or organs
Many
process within the
body are coordinated and controlled by chemical substances called hormones.
You can think of hormones as chemical
messengers sent around the bloodstream.
Hormones are often quite large
organic molecules.
Hormones and the nervous system
send information around the body.
Hormones, being directly
released into the blood, are quite rapidly carried to all parts of the body
BUT only affect the function of particular cells, tissue or organs -
the 'targets''.
Hormones, in acting as
'chemical messages', trigger particular biochemical reactions in
various types of tissue and organs.
Hormones control functions in
cells, tissue and organs that need constant adjustment
and their effect is relatively long-lasting
compared to eg the nervous responses of a reflex arc.
The activated cells are called
'target cells' and have a chemical receptor that responds to the hormone -
the hormones work on effectors.
You should appreciate that the nervous system and hormones enable us to respond to external changes
and also help us to control
conditions inside our bodies.
BUT, unlike the nervous system,
the hormone response times are slower e.g. minutes or hours, an
exception is adrenaline , which is the fastest acting hormone
response.
Know that hormones are used in some forms of
contraception
and in fertility treatments.
TOP OF PAGE and
sub-index
(b) Examples of
hormones and which gland or organ produces them
Hormones are produced in, and
secreted by, various glands called endocrine glands - hence the
overall description - the endocrine system - sources and examples of which
are described below.
Endocrine glands secrete hormones
directly into the bloodstream.
Pituitary gland
The pituitary is a small
gland at the base of the brain
The pituitary gland produces many
hormones that regulate conditions in the body and growth hormone is
important for the development of the body.
Some hormones have a direct
effect on the body, but others have an indirect effect by causing
other glands to release hormones.
Which is why the pituitary gland is sometimes
referred to as the master gland because these hormones act
on other glands causing them to release other hormones to
bring about changes somewhere in the body.
The pituitary gland produces the
hormones FSH and LH which are important control chemicals in the
female
menstrual cycle. These act on the ovaries and testes to release
reproductive hormones which control the release of eggs from the
ovaries and the birth of a baby.
FSH = Follicle
stimulating hormone and LH =
Luteinizing hormone
The pituitary gland secretes the
hormone TSH which acts on the thyroid gland - which in turn
secretes thyroxine hormone (see next section).
TSH = Thyroid
stimulating hormone
It also produces the hormone
ACTH which acts on the adrenal gland to secrete the hormone
adrenal hormone.
ACTH =
Adrenocorticotropic hormone
The pituitary produces the growth
hormone STH, which acts on the whole body - if very deficient
in STH for a long time you may be of short stature and dwarfism,
and, if too much of STH for a long time you have excessive growth in
stature, organ enlargement and suffer from functional disorders such
as diabetes and heart disease.
STH = somatotropin or
somatotropic hormone
ADH is a hormone that is produced
in a part of the brain called the hypothalamus. It is then stored
and released from the pituitary gland. ADH acts on the kidneys to
control the amount of water excreted in the urine.
ADH = Anti-diuretic
hormone
See
Homeostasis - osmoregulation, ADH, water control
and kidney function

Thyroid gland
The thyroid gland is attached to
the trachea. The thyroid gland produces
thyroxine which takes part in regulating functions such as the
rate of chemical reactions in metabolism, heart rate and temperature control
- its production is triggered by the hormone TSH produced in the
pituitary gland.
See also
notes
on thyroxine and
Homeostasis - thermoregulation, control of temperature
Adrenal gland
The adrenal gland are the top of
the kidneys. The adrenal gland produces
adrenaline which is used by the body to prepare for 'fight or
flight' e.g. helps your body for action if you suffer trauma or find yourself in danger
- its production is triggered by the ACTH hormone from the pituitary
gland. (see
notes
on adrenaline)
The pancreas
The pancreas is situated below
the stomach.
The pancreas produces insulin
which regulates the glucose concentration ('level') in blood.
See
Homeostasis - control of blood sugar level
- insulin and diabetes
The ovaries (female only)
The ovaries produce the sex hormone
oestrogen which is part of the chemistry of the
menstrual cycle.
Oestrogen gives girls their
female features such as breasts, soft skin, feminine voice and
prepares the womb for a baby.
The testes (male only)
The testes produce
testosterone, a hormone that controls puberty and sperm
production in males.
Testosterone is a sex hormone
that gives boys their male features such as deeper voices and more
body hair than females.
TOP OF PAGE and
sub-index
(c) A comparison of
the nervous system and the endocrine hormone system
Hormones effectively act as
'chemical messages' to trigger particular biochemical reactions and their
effects are slower than nervous system.
The effect of hormones is more general around the body, but
tend to affect particular cells in particular organs, and relatively long-lasting
effect compared to eg the
fast but short-term nervous impulses and responses of a reflex arc.
Generally speaking, if your
body's response to a situation is relatively long lasting (e.g.
minutes or hours) its probably a function of the hormone system.
Some hormones like
adrenaline, can act quite quickly - see notes further
down.
Nerves: Compared to the hormone
system of response and control in the body, the
nervous system, using nerve signals which are
electrical in nature (not chemical).
The nervous system of neurones acts very fast
e.g. a short burst of a nerve impulse for a short time, acting from one precise area to
another in the body.
Generally speaking, if your
body's response is fast, its probably a nervous reaction.
There are situations when
information needs to be passed to your effectors quickly!
Examples of when nerve
signal information has to be passed to effectors quickly include nerve
signals from your retina, pain receptors, taste buds warning of
danger etc. must be processed in microseconds, NOT minutes! too
late!
Hormones act too slowly to be of
use in most dangerous 'split second' decision making situations.
Hormone levels and negative
feedback
Your body can control the level
of hormones in the blood using a negative feedback system.
If the body detects that a level
of a substance X is above or below the normal level it triggers a
response to bring the level of substance X back to normal again.
A good example is the way
thyroxine regulates metabolism (see
thyroxine notes
and graph below)
See also examples of
homeostasis
Homeostasis - introduction to how it functions (negative
feedback systems explained)
Homeostasis - control of blood sugar level
- insulin and diabetes
gcse biology revision notes
Homeostasis - osmoregulation, ADH, water control, urea and ion
concentrations and kidney function
Homeostasis - thermoregulation, control of temperature
gcse biology revision notes
TOP OF PAGE and
sub-index
(d) The
function of the hormone adrenaline
When you suddenly feel in danger or get a shock
(physical or mental) your adrenal gland quite rapidly releases
the hormone adrenaline into your bloodstream and distributed all
around your body.
The adrenal glands are found just above the
kidneys.
Adrenaline causes, what is often described as, the
'fight or fight' response - in other words your body is quite rapidly
(by hormonal standards) being prepared to deal with a threat of some kind.
This happens when your brain detects fear or
stress (dangerous situation, confrontation etc.) and immediately
sends nerve impulses to the adrenal glands
which then secrete the hormone adrenaline into the bloodstream to prepare your
body for action!
The initial stimulation might be
visual, physical or mental.
Note the interaction between the
nervous system (electrical impulses in nerve fibres -
neurones) and the endocrine system (secretion of hormone
molecules into the bloodstream).
There are nerve connections between the
brain and adrenal gland - a part of the adrenal gland called the
adrenal medulla responds to the nerve signal from the brain
(CNS) by releasing the hormone adrenaline.
The secreted adrenaline is carried round
in the blood and acts on various parts of the body.
The effects of adrenaline on the body
are described below,
The surge in adrenaline levels triggers an
increase in heart rate and breathing rate to increase the supply of
oxygen and glucose to the cells of your brain and muscles.
The increase in respiration releases more
thermal energy and your body temperature rises - but, if it
becomes too high, the thermoregulatory centre in the brain
detects this and the adrenaline secretion is blocked.
Note that the body's volume of blood is
fairly constant, so heart rate must increases to pump
more blood around the body at a greater rate to carry extra
glucose and oxygen to the muscle cells.
The adrenaline molecules do this by binding
to specific receptors in the heart causing the heart muscles to
contract more frequently and more forcefully - this increases
your heart rate and blood pressure, hence more glucose and oxygen to
your cells through your bloodstream e.g. it gives the cells of the
muscle tissue extra energy to contract and prepare to fight or flee!
Adrenaline also binds to receptors in the
liver causing the cells to increase in the rate of breakdown of glycogen
(chemical potential energy store) to increase the level of
glucose in the bloodstream for respiration - particularly muscle
cells (in limbs or heart).
To increase the rate of respiration you
also need more glucose, so the hormone adrenaline performs two
functions to increase energy output.
Note that the metabolism of glucose is
controlled by three hormones, here its adrenaline acting on
the liver, but there is also the action of
insulin and glucagon in
maintaining the balanced level of glucose in the blood.
Footnote - above is not quite the full
"fight or flight" story - another hormone comes into play too!
When the brain responds to the initial
stimulus and triggers the release of adrenaline, this hormone
from the adrenal gland, cannot alone do everything required in a
'fight or flight' situation.
Simultaneously, the brain also signals the
pituitary gland to release a hormone (name ?) that
acts on a different part of the adrenal gland to release a 2nd
hormone called cortisol, and it this steroid hormone
that sustains our response to danger - most cells in the
body have cortisol receptors.
This is another example of several
hormones jointly controlling a situation.
Also note that it is
the hypothalamus links the
nervous and endocrine systems by way of the
pituitary gland
- nervous responses working with hormone responses to keep us
alive!
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sub-index
(e) The function of
the hormone
thyroxine
Thyroxine is a hormone made from iodine and amino
acids, it is produced in, and released (secreted) by, the thyroid gland
in the neck.
Thyroxine has an important role in regulating
the basal metabolic rate, the basic rate (speed) at which the
chemical reactions of your body occur while your body is at rest.
Thyroxine increases the rate of metabolism
of all the body's cells.
e.g. increases the rate of respiration,
powering the cell's chemistry and releasing thermal energy
Thyroxine is also important for many other
biochemical processes including facilitating protein synthesis -
essential for growth and development.
Problems with an underactive thyroid gland
- symptoms of thyroxine deficiency
Tiredness, sluggishness, increase in
weight, slower heart rate,
If a child has too little thyroxine it leads
to slower growth and mental development.
This potentially harmful situation begins
in the uterus, continues in the embryo, through infancy and into
childhood, if there is insufficient thyroxine
A negative feedback system maintains the
thyroxine concentration in the blood at the correct level.
So, how does the negative feedback system
regulate the level of thyroxine in the blood?
Both the pituitary gland and hypothalamus
(a small region at the base of the brain) control the thyroid and it
is the hypothalamus, using TRH (thyrotropin releasing
hormone), that alerts the pituitary gland to produce TSH
(thyroid stimulating hormone).
Please note, from now on I'll just
use the abbreviations TRH and TSH.
Thyroxine is produced in the thyroid gland, in response to the
actions of two principal hormones:
The hormone TRH (from the
hypothalamus), stimulates the production of TSH which is made and secreted
from the pituitary gland into the bloodstream.
In turn, the production of TSH stimulates the
thyroid gland to make more thyroxine.
TSH binds to receptors located on the cells
of the thyroid gland to stimulate production of thyroxine.
We now put these two hormonal actions,
'forward and reverse' into the negative feedback system.
I've also added a graph to go with the text
below.
If your body detects that the level of
thyroxine has risen above 'normal', the hypothalamus stops
releasing TRH.
This tells the pituitary gland to stop
producing TSH (blocks secretion) inhibiting the production of thyroxine in the
thyroid.
In reducing the amount of thyroxine secreted
from the pituitary gland, the thyroxine level falls down to normal
(1st half of graph below) and your metabolic rate is reduced to
'normal' i.e. becomes stabilised again.
Apparently, a higher than normal thyroxine
level also reduces the secretion of TSH from the pituitary gland
(i.e. without the intervention of the level of TRH from the
hypothalamus).
If your body detects the level of thyroxine has
fallen below 'normal', the hypothalamus is stimulated to release
TRH.
The release of TRH stimulates the pituitary
gland to release TSH.
The TSH stimulates the thyroid gland to
produce more thyroxine, whose level rises back to normal (2nd
half of graph) and your metabolic rate increases to 'normal' i.e.
becomes stabilised again.
Note:
If the body temperature falls, the body
produces more thyroxine to increase the rate of respiration and
release more thermal energy.
But, since the increase in respiration
releases more thermal energy and your body temperature rises, if
it becomes too high, the thermoregulatory centre in the brain
detects this and the adrenaline secretion is blocked.
The negative feedback system is illustrated in
the graph below.
General comment on the graph and negative feedback systems
Using a negative feedback system, your
body controls the levels of hormones, and other substances in
the blood.
When your body detects that the level of a
substance X is too high above the 'normal' level, or too low
below the 'normal' level, it triggers a response to bring the
level of substance X back up/down to its normal level.
Thyroid gland problems
e.g. if you have an underactive thyroid gland,
it can cause your body to gain unnecessary weight.
This is because too little thyroxine is
produced and your metabolic rate falls.
As a result, less of the glucose from your
food intake is used up in respiration, so the excess glucose is
converted to, and stored as, fat.
Fortunately, the remedy, in most cases, is to
take thyroxine tablets every day.
TOP OF PAGE and
sub-index
See also
Hormone systems - menstrual cycle
and for plants see
Hormone control of plant growth and uses of plant hormones
gcse biology revision notes
General HUMAN BIOLOGY revision notes
Introduction to the organisation of cells =>
tissues => organs => organ systems (e.g. in humans)
Examples of surfaces for the exchange of substances in
animal organisms gcse biology revision notes
See also
Enzymes - section on digestion and synthesis gcse
biology revision notes
The human circulatory system - heart, lungs, blood,
blood vessels, causes/treatment of cardiovascular disease
Homeostasis - introduction to how it functions (negative
feedback systems explained) gcse
biology revision notes
Homeostasis - control of blood sugar level
- insulin and diabetes
gcse biology revision notes
Homeostasis - osmoregulation, ADH, water control, urea and ion
concentrations and kidney function, dialysis
Homeostasis - thermoregulation, control of temperature
gcse biology revision notes
The brain - what the different parts do and the dangers
if damaged gcse biology revision notes
An introduction
to the nervous system including the reflex arc
gcse biology revision notes
Hormone systems - Introduction to the endocrine
system - adrenaline & thyroxine hormones
gcse biology revision
Hormone systems - menstrual cycle, contraception,
fertility treatments
gcse biology revision notes
Respiration - aerobic and anaerobic in plants and animals. gcse
biology revision notes
Keeping healthy - communicable diseases -
pathogen infections gcse
biology revision notes
Keeping healthy - non-communicable diseases
- risk factors for e.g. cancers gcse
biology revision notes
Keeping healthy - diet and exercise
gcse biology revision notes
Keeping healthy - defence against
pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies
See also
Culturing microorganisms like bacteria - testing
antibiotics/antiseptics gcse
biology revision
Food tests for reducing sugars, starch, proteins and
lipids gcse
biology revision notes
The eye - structure and function - correction of vision
defects gcse
biology revision notes
Optics - lens types (convex, concave, uses),
experiments, ray
diagrams, correction of eye defects (gcse physics)
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