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ENDOCRINE SYSTEM
Endocrine Gland: a group of cells which secret “messenger” molecules directly into the bloodstream
Hormone: the bioactive “messenger” molecule secreted by an endocrine gland into the blood.
Endocrine: relates to hormone’s action on target cells at a distance from source
Paracrine: relates to hormone’s action on nearby target cells e.g. within immediate area around source
Autocrine: relates to hormone having an effect on its own immediate source
Cryptocrine: a term devised to indicate that a hormone can have an effect within its own cell of production, i.e. hidden
The Endocrine System is made up of a number of glands called endocrine glands.
A gland is a structure which secretes a specific chemical substance or substances.
There are two types of gland in the body, exocrine glands and endocrine glands.
An exocrine gland is one which secretes its product into a duct, for example the sweat gland which secretes sweat into tubes called
sweat ducts that lead to the surface of the skin. An endocrine gland has the following characteristics:
 it secretes chemicals called hormones
 it has no duct (a ductless gland), instead, the hormone is secreted directly into the bloodstream;
 it has a rich supply of blood with a relatively large number of blood vessels
Some glands have both endocrine and exocrine functions, such as the pancreas, which secretes the hormones insulin and glucagon,
and also secretes pancreatic juice containing digestive enzymes into the pancreatic duct that leads to the gut.
A hormone is a chemical messenger. It has the following properties:
 it travels in the blood;
 it has its effect at a site different from the site where it is made, called the target, hence the term messenger;
 it fits precisely into receptor molecules in the target like a key in a lock - it is therefore specific for a particular target;
 it is a small soluble organic molecule;
 it is effective in low concentrations.
Release of hormones
The mechanisms controlling the release of hormones by glands are as follows:
 presence of a specific metabolite in the blood, for example excess glucose in the blood causes the release of insulin
from the pancreas which lowers the blood glucose level.
 presence of another hormone in the blood, for example many of the hormones released from the anterior pituitary
gland are 'stimulating' hormones which cause the release of other hormones from other glands in the body.
 stimulation by neurones from the autonomic nervous system, for example adrenaline and noradrenaline are released
from the cells of the adrenal medulla by the arrival of nerve impulses in situations of anxiety, stress and danger.
Chemical nature of hormones
Chemical group Hormones
Peptides and proteins Growth Hormone, Oxytocin, ADH(Vasopressin), Parathormone, Calcitonin,
Insulin and glucagon, Secretin

Amines Adrenaline and noradrenaline

Thyroxine and triiodothyronine
Releasing and inhibiting hormones and factors of the hypothalamus
FSH, LH, Prolactin, TSH, ACTH
Steroids Testosterone, Oestrogen, Corticosteroids
Fatty acids Prostaglandins

Effects on target cells

Hormones are very specific and only exert their effects on target cells which possess the particular protein receptors that recognize
the hormone. Non-target cells lack these receptors and therefore do not respond to the circulating hormone. Once attached to a
receptor, the hormone may exert its effect in a number of ways. Three of the most important are through effects on:
 the cell membrane

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 enzymes located in the cell membrane (second messenger mechanism);

 genes.
1. Cell membrane
Insulin exerts one of its effects by increasing the uptake of glucose into cells. It binds with a receptor she and alters the
permeability of the membrane to glucose. Adrenaline works on smooth muscle cells by opening or closing ion channels for
sodium or potassium ions or both, changing membrane potentials and either stimulating or inhibiting contraction as a result.

2. second messenger mechanism of hormone action

Adrenaline and many peptide hormones bind to receptor sites on
the cell membrane but cannot enter the cells themselves. Instead
they cause the release of a 'second messenger' which triggers a
series of enzyme-controlled reactions. These eventually bring
about the hormonal response. In many cases this 'second
messenger' is the nucleotide cyclic AMP (cyclic adenosine
monophosphate). when the hormone binds to the receptor site it
activates the receptor protein to become the enzyme adenyl
cyclase. This converts ATP to cyclic AMP. cyclic AMP
triggers a wide range of response depending on the particular
cell stimulated.
Example

3. Genes
Steroid hormones (the sex hormones and hormones secreted
by the adrenal cortex) pass through the cell surface
membrane and bind to a receptor protein in the cytoplasm.
The complex formed passes to the cell nucleus where the
hormones exert a direct effect upon the chromosomes by
switching on genes and stimulating transcription
(messenger RNA formation). The messenger RNA enters
the cytoplasm and is translated into new proteins, such as
enzymes, which carry out a particular function. For example,
the hormone thyroxine passes through the cell surface
membrane and binds directly to receptor proteins in the
chromosomes, switching on certain gene

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The hypothalamus
The hypothalamus is situated at the base of the fore brain immediately beneath the thalamus and above the pituitary
gland. It contains several distinct regions of nerve cells whose axons terminate on blood capillaries in the hypothalamus and
posterior pituitary. The pituitary gland can be divided into two parts, the anterior pituitary and the posterior pituitary. Despite its
small size it performs many vital functions.
 It regulates activities such as thirst, sleep and temperature control.
 It monitors the level of hormones and other chemicals in the blood passing through it.
 It controls the functioning of the anterior pituitary gland.
 It produces antidiuretic hormone and oxytocin which are stored in the posterior pituitary gland.

PITUITARY GLAND
The pituitary gland is a small red-grey gland, about the
size of a pea, hanging from the base of the brain by a
short stalk. It is divided into two lobes of different
origin, the anterior pituitary and the posterior
pituitary.
The anterior pituitary is a region of glandular tissue
which communicates with the hypothalamus by means
of tiny blood vessels (Hypophyseal portal system).
The posterior pituitary is of nervous origin and is in
effect an outgrowth of the hypothalamus.
Communication with the hypothalamus is by nerves
rather than blood vessels.

The hypothalamus is the link between the nervous and endocrine systems. By monitoring the level of hormones in the blood, the
hypothalamus is able to exercise homeostatic control of them. For example, the control of thyroxine production by the thyroid
gland is achieved by this, means:
 The hypothalamus produces thyrotrophin releasing factor (TRF) which passes to the pituitary along blood vessels.
 TRF stimulates the anterior pituitary gland to produce thyroid stimulating hormone (TSH).
 TSH stimulates the thyroid gland to produce thyroxine.
As the level of thyroxine builds up in the blood it suppresses TRF production from the hypothalamus and TSH production by the
anterior pituitary gland. By this form of negative feedback the level of thyroxine in the blood is maintained at a constant level
ANTERIOR PITUITARY
This portion of the pituitary gland produces six hormones. Most have other endocrine glands as their target organs. These
hormones, called trophic hormones, stimulate the activity of their respective endocrine glands. The only non-trophic hormone
is growth hormone which, rather than influencing other endocrine glands, affects body tissues in general. The production of all
these hormones is determined by small peptide molecules produced by the hypothalamus and passed to the pituitary via small
connecting blood vessels.
The posterior pituitary
This portion of the pituitary gland stores two hormones: antidiuretic hormone (ADH) or vasopressin and oxytocin. Both have
remarkably similar chemical structures, differing in just one of their nine amino acids.
Functions of hormones secreted by the pituitary

Hormone Function
Anterior Pituitary TSH -Stimulate the growth of the gland

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Thyroid stimulating Hormone -Stimulates the thyroid gland to produce hormones like thyroxine
Adrenocortocotrophic hormone ACTH -Regulates the growth of the adrenal cortex
-Stimulates the adrenal cortex to produce its hormones like cortisol
and aldosterone
Follicle stimulating hormone FSH -Cause development of the Graafian follicles
-Initiates sperm formation in the testes
Luteinising hormone LH -Causes release of the ovum from the ovary
(interstitial cell stimulating (ICSH) -Stimulates secretion of testosterone from interstitial cells in the
hormone) testes
Prolactin LTH -Maintains progesterone production from the corpus luteum
(Luteotrophic hormone, -Induces milk production in pregnant females.
luteotrophin)
Growth hormone GH -Promotes growth of skeleton and muscles
-Controls protein synthesis and general body metabolism
Posterior pituitary ADH -Causes water reabsorption by the kidney tubules
Antidiuretic hormone(vasopressin) -Raise blood pressure by constricting arterioles
Oxytocin -Induces parturition(birth) by causing uterine contractions
-Induces lactation(secretion of milk from the nipples)

THYROID GLAND
Found in the neck close to the larynx, the thyroid gland, produces three hormones; triiodothyronin (T3), thyroxine (T4) and
calcitonin.
Triiodothyronin and thyroxine are very similar chemically and functionally. They regulate the growth and development of
cells. In this respect they are especially important in young mammals. In addition, these hormones increase the rate at which
glucose is heat and these hormones are therefore produced when an organism is exposed to severe cold. Emotional stress and
hunger may elicit a similar production of these hormones. The overall effect is to control the metabolic rate of cells and in so
doing the hormones work in close conjunction with insulin, adrenaline and cortisone.
Both triiodothyronin and thyroxine are derivatives of the amino acid tyrosine and both contain iodine. Thyroxine possesses
four iodine molecules while triiodothyronin has only three. In times of iodine shortage, the latter is produced in preference to
the former in order to make maximum use of the limited iodine. If the iodine supply is severely reduced, the thyroid is unable to
make adequate supplies of these hormones and underactivity of the thyroid results.

CONTROL OF THYROXINE PRODUCTION

This is achieved by a negative feedback process. The shedding of
TSH into the bloodstream is triggered by a hormone secreted by
the hypothalamus. This is called thyrotrophic hormone. This
stimulates the Thyrotrope cells of the anterior pituitary gland to
release TSH. TSH travels in blood and stimulate the thyroid gland
to release thyroxine hormone. Excess thyroxine hormone produced
inhibits both the hypothalamus and the pituitary gland.

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Underactivity of the thyroid gland Underactivity of the thyroid0783 686735
gland
(hypothyroidism) (hypothyroidism)

 Causes a swelling in the neck known at a goitre  Causes a swelling in the neck known at a goitre
 A lack of TSH production by the anterior pituitary,  A lack of TSH production by the anterior pituitary,
iodine deficiency in the diet or failure of enzyme systems iodine deficiency in the diet or failure of enzyme
involved in thyroxine production may result in systems involved in thyroxine production may result in
hypothyroidism. hypothyroidism.
 In infants will lead to poor growth and mental  In infants will lead to poor growth and mental
retardation, a condition known as cretinism retardation, a condition known as cretinism
 Thyroxine deficiency in later life gives rise to a condition  Thyroxine deficiency in later life gives rise to a
known as myxoedema and the symptoms are a reduction condition known as myxoedema and the symptoms are
in metabolic rate accompanied by decreased oxygen a reduction in metabolic rate accompanied by decreased
consumption, ventilation, heart rate and body oxygen consumption, ventilation, heart rate and body
temperature. temperature.
 Mental activity and movement become slower and  Mental activity and movement become slower and
weight increases due to the formation and storage of a weight increases due to the formation and storage of a
semi-fluid material under the skin. This causes the face semi-fluid material under the skin. This causes the face
and eyelids to become puffy, the tongue swells, the skin and eyelids to become puffy, the tongue swells, the skin
becomes rough and hair is lost from the scalp and becomes rough and hair is lost from the scalp and
eyebrows. eyebrows.
All of these symptoms can be eliminated and the condition All of these symptoms can be eliminated and the condition
treated by taking thyroxine tablets. treated by taking thyroxine tablets.

Calcitonin
Calcitonin is concerned with calcium metabolism. Calcium, in addition to being a major constituent of bones and teeth, is
essential for blood clotting and the normal functioning of muscles and nerves. In conjunction with parathormone from the
parathyroid gland, calcitonin controls the level of calcium ions (Ca2+) in the blood. calcitonin is produced in response to high
levels of Ca2+ in the blood and it causes a reduction in the Ca2+

ADRENAL GLANDS
Situated above each kidney in humans is a collection of cells weighing about 5 g. These are the adrenal glands. They have two
separate and independent parts:
 The adrenal cortex consisting of the outer region of the glands.
 The adrenal medulla consisting of the inner region of the glands.

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The adrenal cortex

Making up around 80% of the adrenal gland, the cortex produces a number of hormones which have relatively slow, long-lasting
effects on body metabolism, kidney function, salt balance and blood pressure. All the hormones produced are steroids formed
from cholesterol. Being lipid-soluble they are able to pass across cell membranes. Hormones from the adrenal cortex are
collectively called corticoids and fall into two groups.
 Glucocorticoids which are concerned with glucose metabolism.
 Mineralocorticoids which are concerned with mineral metabolism.

Glucocorticoid hormones
This group of hormones includes ►Raising the blood sugar level,
Cortisol which is produced in response partly by inhibiting insulin and
to stress. In stressful situations like partly by the formation of
shock, pain, emotional distress, extreme glucose from fats and proteins.
cold or infection, the hypothalamus
induces the anterior pituitary gland to ►Increasing the rate of
produce adrenocorticotrophic glycogen formation in the liver.
hormone (ACTH). This in turn causes ►Increasing the uptake of
the adrenal cortex to increase its amino acids by the liver. These
production of glucocorticoids, including may either be deaminated to
Cortisol. Where stress is prolonged, the form more glucose or used in
size of the adrenal glands increases. The enzyme synthesis
glucocorticoid hormones combat stress
in a number of ways:

Mineralocorticoid hormones
This group of hormones includes aldosterone which regulates water retention by controlling the distribution of sodium and other
minerals in the tissues. Aldosterone cannot increase the total sodium in the body, but it can conserve that already present. This
it achieves by increasing the reabsorption of sodium (Na+) and chloride (Cl- ) ions by the kidney, at the expense of potassium ions
which are lost in urine. Control of aldosterone production is complex. In response to a low level of sodium ions in the blood, or a
reduction in the total volume of blood, special cells in the kidney produce renin which in turn activates a plasma protein called
angiotensin. It is angiotensin which stimulates production of aldosterone from the adrenal cortex. This causes the kidney to
conserve both water and sodium ions. Angiotensin also affects centres in the brain, creating a sensation of thirst, in response to
which the organism seeks and drinks water, thus helping to restore the blood volume to normal.

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The adrenal medulla

The central portion of the adrenal gland is called the adrenal medulla. It produces two hormones, adrenaline (epinephrine)
and noradrenaline (norepinephrine). Both are important in preparing the body for action. The cells producing them are
modified neurones, and noradrenaline is produced by the neurones of the sympathetic nervous system. These hormones
therefore link the nervous and endocrine systems. They are sometimes called the 'flight of fight hormones' as they prepare an
organism to either flee from or face an enemy or stressful situation. The effects of both hormones are to prepare the body for
exertion and to heighten its responses to stimuli.
Effects of adrenaline hormone
 Dilates bronchioles for more intake of gases(oxygen) for oxidation of glucose to provide energy
 Relaxes smooth muscle of the gut
 Induces the conversion of glycogen in the liver to glucose
 Increase heartbeat, blood pressure, rate to increase volume of blood pumped for faster release of metabolites.
 Reduces blood supply to the digestive system and diverted to muscles, lungs and liver
 Reduces peristalsis and digestion
 Increases sensory perception, mental awareness
 Dilates the pupils of the eyes
 Contract the hair erector muscles

At a cellular level, adrenaline acts as the

first messenger and combines with receptors
on the membrane of liver and muscle cells.
The hormone-receptor complex on the outer
face of the membrane activates the enzyme
adenylate cyclase which is on the inner face of
the membrane. The adenylate cyclase causes
the conversion of ATP to cyclic AMP. The
cyclic AMP acts as a second messenger
(intracellular mediator) in that it moves within
the cell to activate enzymes such as those
involved in glycogen breakdown. The process
is a complex series of enzyme reactions in a
chain reaction known as a cascade effect. The
cascade effect amplifies the response.

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In one respect adrenaline and noradrenaline differ. Whereas adrenaline dilates blood vessels, noradrenaline constricts them.
This difference explains the constriction of blood vessels around the gut while those supplying muscles, lungs and liver are
dilated. It appears that receptors on some blood vessels are sensitive to noradrenaline and so constrict while others are sensitive
to adrenaline and so dilate.
The pancreas
The structure of the pancreas and its role as
an exocrine gland are dealt with in
digestion. At intervals within the exocrine
cells are the Islets of Langerhans, which
are part of the endocrine system. Cells
known as alpha-cells produce the hormone
glucagon whereas beta-cells secrete the
hormone insulin. The two operate
antagonistically, with glucagon stimulating
the breakdown of glycogen to glucose
while insulin initiates the conversion of
glucose to glycogen.
Insulin
Insulin is a small protein composed of 51 amino acids, it is released in response to a rise in blood glucose level above 90mg per
100cm1 blood. It is carried in the blood plasma and has an important effect on every organ of the body, although its main effect is
on the liver and muscle. Receptor sites on cell surface membranes bind insulin, and this reaction leads to changes both in cell
permeability and the activity of enzyme systems within the cell, the overall effect being to reduce blood sugar
 increase in the rale of conversion of glucose to glycogen, called glycogenosis - this takes place mainly in the liver and
muscle.
 increase in the rate of uptake of glucose by cells, especially skeletal muscle;
 increase in the use of glucose rather than other substances, such as fat. as a source of energy for cell respiration:
 increase in the conversion of glucose into fatty acids and fats, and fat deposition:
 increase in the rate of uptake of amino acids into cells and the rate of protein synthesis;
 decrease in gluconeogenesis (production of glucose).
Glucagon
Glucagon is a protein composed of 29 amino acids and is released, along with several other hormones, in response to a fall in blood
glucose level below normal. This is usually the result of an increase in metabolic demand, for example as a result of exercise. Its
role is to increase blood glucose level. Its main target is the liver. Glucagon stimulates the conversion of glycogen to glucose
(glycogenolysis). It also stimulates the breakdown of proteins and fats to glucose and conversion of lactic acid to glucose, processes
known as gluconeogenesis

Effects of insulin
deficiency excess
High blood glucose(hyperglycaemia) Low blood glucose (hypoglycaemia)
Breakdown of muscle tissue Hunger
Loss of weight Sweating
tiredness Double vision

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OTHER GLANDS
Gland Hormone Function
Stomach Gastrin Secretion of gastric juices
Duodenum Secretin Secretion of pancreatic juice
Inhibits gastric secretion
Cholecystokinin(pancreozymin) Emptying of gall bladder and release of pancreatic juice into duodenum.
Kidney Renin Conversion of angiotensinogen into angiotensin
Erythropoeitin Increases rate of red blood formation

Ovary Oestrogen Female secondary sex characteristics, oestrous cycle.

Progesterone Gestation, inhibition of ovulation
Placenta Chorionic gonadotrophin Maintenance of corpus luteum
Human placental lactogen Stimulate mammary growth
Testis Testosterone Male secondary sexual characteristic

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