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Hormones and Homeostasis
Some cells are in contact with the external environment, majority of cells are in
contact with the internal environment
...
Homeostasis is the maintenance of a constant internal environment,
through the regulation of the activities of organs and organ systems
...

The internal state of an animal's body is best described as being a dynamic
equilibrium rather than constant
...
So conditions of the internal environment
are kept within an optimum range
...


Negative feedback:
This is a change in the internal environment leads to a response, which counteracts
the change and restores the system to its original state; increasing the stability of
systems
Negative feedback keeps the temperature 36-38°C
...
g
...

- pH as well as oxygen and carbon dioxide

2 | P a g e


Positive feedback:
A change produces a response that amplifies the original change
...
E
...
during birth: early
contractions begin to force the baby's head against the cervix, which makes the
cervix dilate (open)
Stretch receptors respond to this expansion by sending signals to the hypothalamus
which releases oxytocin that stimulates more and stronger uterine contractions,
which stimulates the release of more oxytocin
...
E
...
urination, defaecation,
sneezing and vomiting
...
g
...
Rich supply of blood
with a lot of blood vessels; make up endocrine system
...


Hormones:
• regulatory
• secreted
• small

by an endocrine gland

and soluble

• organic
• travel
• have

chemical messengers

molecules

in the blood

an effect at a target site (different from the production site)

• target

specific

• effective

in low concentration

A hormone is the secretion of an endocrine gland that is released into the blood and
is thus carried to a target organ on which it exerts its effects
...
Amines: derived from single amino acid molecules (tyrosine -> adrenaline +
thyroxine)
2
...
Steroids:

derivatives

of

cholesterol

(sex

hormones:

oestrogen,

progesterone, testosterone)
4
...
Glycoproteins (follicle stimulating hormone and lutenising hormone)
Steroids and fatty acids are lipid-soluble transported bound to carrier proteins
...

Mode of action of hormones:
Hormones are released:
1
...
e
...
high levels of glucose
stimulate release of insulin from the pancreas
...
As a result of nervous stimulation e
...
adrenaline and noradrenaline are
released by the arrivals of nerve impulses
3
...
g
...
Act on cell membranes and modify their permeability by increasing carrier
proteins
...
g
...
Adrenaline
works on smooth muscle cells by opening - closing ion channels)
2
...
g
...
Activate genes
...
This stimulates the transcription a specific genes to mRNA
but may also suppress the expression of other genes
...
Second messenger mechanism
...

They induce membrane bound enzymes to release a 'second messenger'
which triggers a series of enzyme-controlled reactions
...

When adrenaline binds to the receptor sites, it activates a G protein (in
membrane) which activate the enzyme adenylyl cyclase (converts ATP into
cAMP)
...


Process: cascade amplification/effect: increase in number of

molecules
...


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The hypothalamus and pituitary gland
Homeostatic control is under nervous and hormonal control
...
It contains nerve
cells with axons that terminate on blood capillaries
...
Information is given to the pituitary
gland by the release of neurohormones from neurosecretory cells
...
It is divided into two:

Anterior pituitary
ü Endocrine cells that synthesis and secrete tropic hormones
...

Nerve terminals release nerohormones into the blood, which include:
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− Releasing factors: induce anterior pituitary to secrete hormones
− Inhibiting factors: prevent anterior pituitary from secreting hormones
Tropic hormone: stimulates other endocrine glands to release their hormones
Hypothalamus

Anterior pituitary hormone and Site of action

neurohormone

response

Growth hormone releasing Stimulates
factor

growth

hormone Wide

(GH) release

GH release-inhibiting factor Inhibits

variety

of

variety

of

tissues

growth

hormone Wide

(somatostatin)

release

tissues

Prolactin releasing factor

Stimulates prolactin secretion

Ovaries

and

mammary glands
Prolactin inhibiting factor

Inhibits prolactin secretion

Ovaries

and

mammary glands
Luteinising

hormone Stimulates release of LH and Ovaries and testies

releasing

factor follicle stimulating hormone

(gonadotrophin)
Thyrotrophin

releasing Stimulates release of thyroid Throid gland

factor

stimulating hormone

Adrenocorticotrophin

Stimulates

releasing factor

adrenocrticotrophic hormone

release

of Adrenal cortex

Hypothalamus
Adrenocorticotrophin releasing factor
Anterior Pituitary

Cascade Effect: enables the
release of a small amount of
hormone to become
amplified at each stage

Adrenocorticotrophic hormone
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Adrenal cortex
Cortisol
Liver
Glycerol à glucose

Posterior Pituitary
It releases two hormones: antidiuretic hormone (ADH) and oxytocin
...
It causes constriction of peripheral blood vessels- to increase
blood pressure
...
These
are produced by neurosecretory cell bodies in the hypothalamus
...

Nerosecretory cells are specialised and form structures consisting of swollen synapse
at the end of the neuron attached to a capillary
...
Lack of glucose in the brain results to fainting
...
Fall in glucose level
(hypoglycaemia) inhibits insulin secretion and stimulates secretion of glucagon and
other hormones (e
...
adrenaline)
Pancreas (has cells that surround numerous branches of the pancreatic duct
...


Exocrine functions (acinar cells secrete enzymes of the pancreatic juice into
the duct)

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Endocrine functions (consists of islets of Langerhans – rich supply of blood
...
Its main effect is on the liver, muscle and fat cells
...
An increase in the rate of conversion of glucose -> glycogen (glycogenesis) in
liver and muscle
2
...
An increase in the use of glucose
4
...
A decrease in glucose production (gluconeogenesis)
Production of insulin is regulated by a negative feedback mechanism
...
It stimulates:
1
...
Breakdown of proteins into amino acids and fats into glycerol and fatty acids
...
Conversion

of

amino

acids,

glycerol

and

lactic

acid

into

glucose

(gluconeogenesis)
Glucagon operates by a second messenger mechanism
...

-

Adrenaline released from the adrenal medulla of the adrenal glands stimulate
the conversion of glycogen into glucose (fight or flight situations)

-

Cortisol secreted from the adrenal cortex of the adrenal glands stimulates the
release of amino acids, glycerol and fatty acids and increase the rate of
synthesis or enzymes in the liver that convert amino acids and glycerol into
glucose
...

Diagnosis: excretion of sugary urine as blood glucose concentration becomes too
high
...

TYPE I: (juvenile/insulin –dependent) around 40 years old
Pancreas does not produce insulin due to an autoimmune disorder that destroys
islets of Langerhans
...
One must follow
a planned diet, exercise, and glucose monitoring and insulin injections – daily
...


TYPE II: (maturity onset/insulin – independent) 40 years
-

The pancreas produces a low amount of insulin

-

Body cells do not respond to insulin

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Low fat diet and regular exercise/ oral drugs (cells more sensitive to the effects of
insulin / stimulate pancreas to secrete more insulin)
They rely on fats, high circulating levels of lipids, therefore cholesterol

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