H.3.5.6 Plant Growth Regulators
and Animal Hormones (E.S)
Objectives – What you will need to
know from this section
 Describe the feedback mechanism of any one animal
hormonal system.
 Homeostasis is the maintenance of a constant internal
environment and is achieved by ‘feedback mechanisms’.
Homeostasis is necessary if an organism is to be
independent of its surroundings, and if its metabolism is to
function efficiently.
Most homeostatic mechanisms work by negative feedback,
i.e. if there is a change away from the normal optimum value,
action is automatically taken to reverse this change.
The thermostat in an oven
is an example of how this
negative feedback system
works.
 In negative feedback, the response reduces the strength
of the original stimulus—very useful for controlling the
release of hormones.
There must be a sensor to detect the change, and an
effector to reverse the change.
Control of water, thyroxine & sugar levels are examples of
how homeostasis works in humans.
Control of Water Levels
 The kidney regulates the amount of water in the body by
varying the amount of urine produced.
 This is known as osmoregulation, and it is an example of
homeostasis.
 ADH [Anti-diuretic hormone]
controls whether the distal
tubule and collecting ducts
reabsorb water or not.
If you drink a great deal of water
 the hypothalamus in the brain detects the diluted
blood and turns off ADH production.
 Less water is reabsorbed, so more water is allowed
to escape to the bladder,
 and a larger volume of dilute urine is produced.
When the body is low on water
 ADH is secreted from the pituitary gland.
 More water is reabsorbed
 and only a small volume of urine is produced.
 When the body is low on water, ADH is secreted from the
pituitary gland. More water is reabsorbed and only a small
volume of urine is produced.
If Water Levels Fall in the Body…..
If Water Levels Rise in the Body …..
 When the body has excess water, ADH is NOT secreted from
the pituitary gland. Less water is reabsorbed and so a larger
volume of urine is produced.
LEARNING CHECK
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What is homeostasis?
What is osmoregulation?
What is ADH?
Where is it made and where does it act?
What happens in the kidney if we take in a
lot of fluid?
• What happens in the kidney if we are short
of water in the body?
Control of Thyroxine Levels
 The regulation of thyroid hormone secretion depends on a
negative feedback loop between the pituitary and the thyroid
gland:
 High concentration of thyroxine --- inhibits pituitary ---> less
TSH --- > less thyroxine --> level drops
TSH = Thyroid Stimulating Hormone
 Low concentration of thyroxine ---> stimulates pituitary --->
more TSH ---> more thyroxine --> level rises
THYROXINE control – Feedback
Control of Sugar Levels
If Sugar Levels Rise in the Body …..
If Sugar Levels Fall in the Body …..
LEARNING CHECK
• What is meant by feedback?
• What is negative feedback?
• Can you find out what positive feedback is
and give an example from everyday life?
• What’s the difference between TSH and
Thyroxine?
• What is the difference between insulin and
glucagon?