HLTAP301A ANATOMY & PHYSIOLOGY
PART 1 – OVERVIEW OF THE ENDOCRINE SYSTEM
Along with the nervous system, the endocrine system co-ordinates and directs
the activity of the body’s cells. The nervous system is the ‘rapid response’
part of the body, using electrical impulses to generate messages, whereas the
endocrine system uses chemical messages (called hormones) which are
released into the blood and transported to the appropriate parts of the body.
Major processes controlled by hormones are:
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Reproduction
Growth and development
Mobilising body defenses
Maintaining water, electrolyte and nutrient balances of the blood
Regulation of cellular metabolism and energy balance
The organs of the endocrine system collectively weigh only 1kg and are
scattered around the body. Hormones are the product of the endocrine
system almost all of them are either amino acid-based molecules (proteins,
peptides, etc.) or steroids (mostly produced by either the ovaries or the testes,
or the adrenal cortex). Prostaglandins are a ‘local’ hormone made from lipids
within the cellular plasma membrane, and thus almost form a very small ‘subcategory’.
Hormones circulate throughout the body in the blood but they only affect a
certain cell type or organ – this is called the target cell or target organ.
In order for the target cell to respond to a particular hormone, specific protein
receptors must be present, so that that hormone can attach and thus
influence the workings of a cell. There are really only two ways that
hormones trigger changes in cells.
Steroids act on cells in the following manner:
 They diffuse through the plasma membrane of the target cell;
 once inside, the steroid hormone enters the nucleus;
 and binds to a specific protein receptor to form a complex.
 this complex binds to certain sites on the cell’s DNA (Deoxyribonucleic
Acid)
 which activates certain genes to tell the messenger RNA (Ribonucleic
Acid) to
 synthesize a new protein within the cytoplasm of the cell.
Non-Steroidal Hormones cannot enter the target cells and so
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they bind to receptors on the target cell’s plasma membrane.
this sets off a series of reactions that activates an enzyme which
starts a reaction that produces a second messenger molecule that
effects
the target cell’s functions so that something occurs – i.e. glycogen
breakdown.
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What prompts the endocrine system to release their hormones or not?
Usually negative feedback mechanisms trigger hormonal secretions as a
result of some internal or external stimulus. Rising hormonal levels in the
blood then inhibit further hormone releases even as the hormones are
promoting a response within the target cells or organs.
There are three major categories of stimuli that activates the endocrine
system – hormonal, humoral or neural:
Hormonal is the most common stimulus – for example the hypothalamus
secretes hormones that stimulate the anterior pituitary gland to secrete
hormones that then stimulate other endocrine glands to secrete hormones
that cause an effect on the body.
Humoral refers to body fluids (blood, bile, saliva, etc.) – for example, if blood
calcium levels are low, the parathyroid glands release parathyroid hormone
(PTH), thus causing blood calcium levels to rise which then ends the stimulus
to release PTH.
Neural stimuli is relatively rare (compared to the two others) and occurs when
nerve fibers stimulate a hormonal release. The Sympathetic Nervous System
stimulates the adrenal medulla to release EPH and nor-EPH during periods of
stress.
Organs of the Endocrine System
The major endocrine organs are:
 Pituitary gland
 Thyroid gland
 Parathyroid glands
 Adrenal glands
 Pancreatic Islets
 Thymus
 Pineal gland
 Gonads
 Hypothalamus (part of both the endocrine & nervous systems)
There are two types of glands – endocrine and exocrine. Endocrine glands
are ductless glands that empty their hormonal products directly into the blood.
Exocrine glands have ducts that carry their secretions to a specific site, such
as the pancreas and the gonads.
The pituitary gland hangs by a stalk inferior to the hypothalamus and has
two functional lobes – the anterior portion is comprised of glandular tissue and
the posterior lobe is nervous tissue.
The anterior lobe of the pituitary gland produces 6 hormones (refer table in
text book). Two (growth hormone and prolactin) exert their influence on nonendocrine targets.
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The other four are topic hormones, which means they stimulate their target
organs (which are also endocrine glands) to secret their hormones which then
exert an effect on other body organs or tissues.
For example - follicle stimulating hormone (FSH) stimulated follicle
development in the ovaries, which, as they mature, produces estrogen and
eggs are readied for ovulation. Hyposecretion of FSH results in sterility,
whereas hypersecretion can result in multiple births.
Growth hormone is a general metabolic hormone whose major effects are
directed to the growth of skeletal muscles and long bones. Hyposecretion can
result in pituitary dwarfism, where the individual has normal body proportions
but reaches a maximum height of 1.2m (4 feet) tall.
Hypersecretion can result in gigantism – the opposite effect where the
individual still has normal body proportions but they can reach a height of
2.4m – 2.7 m (8-9 feet) tall. Hypersecretion after long bone growth is
completed results in acromegaly – a condition where the facial bones
(particularly the nose and lower jaw), hands and feet are enlarged.
The pituitary gland was considered the ‘master’ gland because it controls so
many of the other endocrine glands. However, the true master of the
endocrine system is the hypothalamus, which is the major integrating link
between the nervous and endocrine systems. The hypothalamus releases at
least nine different hormones, several of which control the release
mechanisms of the pituitary gland. It also produces oxytocin and ADH which
are stored in the posterior pituitary gland. Oxytocin stimulates the uterus
during labour and causes the letdown reflex when breast feeding. Antidiuretic Hormone (ADH) is discussed when we explore the urinary system.
Thyroid and Parathyroid Glands – the thyroid gland is located at the base of
the throat inferior to the Adam’s apple and secretes two hormones that
contain iodine, the major one being thyroxine. Thyroid hormones control the
rate at which glucose is ‘burned’ or oxidized and converted to chemical
energy and body heat. Enlargement of the thyroid gland is the result of a diet
deficient in iodine and ends up as a condition called a goiter.
The second important product is thryocalcitonin which helps to maintain the
Ca2 levels within the blood along with PTH, which is secreted by the
parathryoid gland located on the posterior surface of the thyroid gland. PTH
is a hypercalcemic hormone (increases blood levels of calcium), whereas
calcitonin is a hypocalcemic (decreased blood calcium levels).
The adrenal glands are similar to the pituitary gland, in that they are actually
two organs in one. It has glandular tissue (the adrenal cortex) and neural
tissue (the adrenal medulla). The medulla is enclosed by the cortex which
has three layers of cells. The adrenal cortex produces three types of steroid
hormones which are collectively called corticosteroids. The adrenal medulla
produces epinephrine and norepinephrine, which was discussed in detail in
the nervous system.
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The Pancreas is a mixed gland. The pancreatic islets (once called the islets
of langerhans) are little parts of hormone producing tissue scattered among
the enzyme-producing tissue of the pancreas. The exocrine part of the
pancreas secretes pancreatic juice into the small intestine via the pancreatic
duct. The endocrine part (the pancreatic islets) produce insulin and glucagon
which both help to regulate the amount of glucose (sugar) in the blood.
If blood glucose levels are high, the beta cells of the pancreatic islets secrete
insulin which stimulates the liver to take glucose out of the blood and store it
as glycogen. As the blood sugar levels decline to a specific point, the
stimulus for insulin release diminishes (insulin’s effect is hypoglycemic).
When blood sugar levels drop (ie. if you skip a meal) the alpha cells of the
pancreatic islets release glucagon into the blood – it’s target is the liver which
is stimulated to break down glycogen stores into glucose which is then
released into the blood (glucagon’s effect is hyperglycemic). Refer to diagram
in text book.
The Pineal Gland mostly secretes melatonin which is known as the ‘sleep
trigger’. Melatonin is also believed to be linked to delaying sexual maturity
until physical maturity is reached.
The Thymus Gland secretes thymosin, which is very important to the
immune response.
The Gonads - female gonads are the ovaries which produce estrogens and
progesterone – two hormones that work together in a cyclic manner that we
call the menstrual cycle.
Male gonads are the testes which produce androgens (male sex hormones) of
which testosterone is the major one. Testosterone is necessary for the
continuous production of sperm. Hyposecretion of gonadal hormones results
in sterility in both males and females.
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