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Hormones as signals
The endocrine system
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Communication system between cells in the body using hormones – released as a result of a stimuli
Hormones are signalling molecules which are transported in the blood stream
Has a longer lasting effect that the nervous system however it is a slower response
The endocrine system is anatomically discontinuous meaning that there does not have to a link between the
secretory organ and the receptive organ/ gland/ cell etc
The endocrine system controls
o Homeostasis
o Growth
o Metabolism
o Sexual development and functions
o Calcium levels
o Circadian rhythm
o Fight/flight responses
There are primary and secondary endocrine glands
o Primary glands have a main function of secreting hormones
o Secondary glands/ organs do not have hormone secretion as the main function eg the kidney or liver

Negative feedback

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One endocrine gland may
produce a trophic hormone
which will activate the
production of another hormone
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Often, they are produced as large
molecular weight precursors which are proteolytically cleaved in order to
form the active hormone
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These hormones are water soluble but cannot pass the cell surface membrane
or cross membranes generally
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They
are small and often water soluble and can cross membranes
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Tyrosine is a precursor of thyroid hormone
and tryptophan is a precursor of melatonin

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Steroids – They are derived from cholesterol and are small and fat soluble
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They are
able to pass through membranes

They are necessary for communication in multicellular organisms as they help to
coordinate cell functions and allow for differentiated cell function

Transport of hormones
Hydrophilic hormones
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Dissolved in plasma
Hydrophobic hormones
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Bound to carrier proteins
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Only free hormone can be metabolized
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can be stored in adipose tissue

Some Definitions
Synergistic – The effects of two hormones favour each other and the net effects exceed the sum of the individual effects
Antagonistic – The effects of two hormones oppose each other
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Many of these hormones regulate the secretion of other hormones- Master
gland
The pituitary is divided into two structurally and functionally distinct sections:
The posterior pituitary –neurohypophysis
o Posterior lobe of the pituitary contains neural tissue consisting of the ending of neurones originating in the hypothalamus
o These neural endings secrete ADH and oxytocin
o Antidiuretic hormone (ADH or vasopressin)
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Secreted by paraventricular nucleus
o Water balance and osmolarity
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Oxytocin
Secreted by supraoptic nucleus
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Milk ejection
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Following synthesis in these neurones, these hormones are packed into secretory vesicles which are transported to the neural
ending of the posterior pituitary, where they are secreted when these neurones receive the signal, usually from other neurones
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The anterior pituitary-adenohypophysis
Posterior lobe of the pituitary contains neural tissue consisting of the ending of neurons originating in the hypothalamus
o These neural endings secrete ADH and oxytocin
o Antidiuretic hormone (ADH or vasopressin)
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Secreted by paraventricular nucleus
o Water balance and osmolarity
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Oxytocin
o Secreted by supraoptic nucleus
o Milk ejection
The hypothalamus and anterior pituitary are connected by the Hypothalamic-Pituitary Portal System
A specialised arrangement of blood vessels in which two capillaries beds are located one after the others
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Tropic name comes from the Greek word tropos, which means to change or to turn

Control of hormone secretion
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The rate of hormone secretion varies from hour to hour and day to day
Hormone secretion occurs in response to changes in the composition of blood or surrounding tissues-Homeostasis
The control is based on negative feedback
Negative feedback: the response reverse the effect of a stimulus/ An action which reduce the effects of a stimulus

How does the endocrine system control puberty?
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Humans are not born with the ability to reproduce but acquire that ability during puberty, a period of sexual
maturation
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Dramatic effects on body weight and appetite, studies showed that leptin also has a role in regulating human
development in puberty, and immune function
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The effects of leptin on hypothalamic, pituitary and peripheral hormone levels, as well as the presence of the leptin
receptor in a variety of tissues, suggest both an endocrine and a paracrine mode of action
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Age: Females 8-13 y; males: 9-14 y
Stimulus:
o Multi factorial
o Body weight of 47kg – this is a better indicator than age
o Leptin involvement – hormone of adipose tissue
Response:
o Growth spurt (also increased GH secretion)
o Secondary sexual characteristics
o Fertility

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Reproductive hormone cycle in females
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b
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d
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Which promotes androgen
secretion (androgen such testosterone
inhibits its own release
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Leydig cells Clusters of cells in the
testis Leydig cells which are responsible
to secrete testosterone
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And
stimulates spermatogenesis or sperm
production
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in response of FSH they
increase
they secrete inhibin which suppresses
FSH and gonadotropin releasing
hormone production

b
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d
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f
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Gonadotrophin releasing factor GnRH is
synthesised in hypothalamus, and
transported to the AP gland in portal vein,
it act on gonadotroph cells to stimulate
the release of gonadotrophin LH and
FSH
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Both LH and FSH are involved in
oestrogen synthesis
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Oestrogen and progesterone feedback to
the anterior pituitary and hypothalamus to
regulate their release
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Before ovulation the feedback became
positive triggering the surge in LH and
FSH release that causes ovulation

Endocrine control of growth
a
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Growth hormone was described
as a pituitary hormone that acts directly on tissues instead of
stimulating peripheral endocrine tissues like other pituitary
hormones
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Fat cells
(adipocytes), for example, have growth hormone receptors,
and growth hormone stimulates them to break down
triglyceride and suppresses their ability to take up and
accumulate circulating lipids
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Factors increasing GHRH release
c
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Decreases in fatty acids increases GHRH release
e
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Sleep
g
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Stress
(There are more)

Endocrine reproductive disorders
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Polycystic ovarian syndrome (PCOS)
Multiple immature follicles in ovaries
Excess of LH & deficiency of FSH from AP
Symptoms:
o Amenorrhoea or infrequent periods
o Infertility
o Hirsutism (male pattern hair growth)
o Acne
o Weight gain

Polycystic ovary syndrome
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P
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syndrome is a common but very complicated disease characterised by several endocrine abnormalities and
multiple cysts in the ovaries
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The endocrine abnormalities causes the following symptoms
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Further treatment is aimed to relieve the symptoms:
Infertility is treated by raising pituitary FSH secretion, often by using the anti-oestrogen clomiphene
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What are the different actions of growth hormone?
Promote growth
o Increases cell size (hypertrophy)
o Increases cell number (hyperplasia)
Metabolic actions supporting growth
o Inhibit glucose uptake into adipose tissue and skeletal muscle
o Stimulate lipolysis in adipose tissue
o Stimulate gluconeogenesis in liver
o Increase uptake of amino acids into cells

Hypothalamus control of endocrine cells of the adrenal medulla
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Sympathetic innervation - So when the sympathetic division of the autonomic nervous system is activated, the
adrenal medulla release the hormones into the blood stream
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Acts as an endocrine organ - Release ADH , reduces water loss from kidney oxytocin stimulates contractions of
smooth muscles in uterus and release of milk in females
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These
hormones are synthesised by the cells in the hypothalamus-neurohormones-they are neurones cells but secrete
hormones as well

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