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OCC103
Human Structure
& Function 1
Endocrine System
Week 13 Lecture 1
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Endocrine System: Overview
• acts with the nervous system to coordinate and
integrate the activity of body cells
⟶ Influences metabolic
activities by means of
hormones transported in
the blood
⟶ Responses occur
more slowly but tend to
last longer than those of
the nervous system
Learning outcome 1
Define the term
‘endocrine gland’ and
state the locations of the
major endocrine glands
endocrine
glands
Pineal gland
Hypothalamus
Pituitary gland
Thyroid gland
Parathyroid glands
The hypothalamus
has both neural and
endocrine functions
Thymus
Adrenal glands
Pancreas
Ovary (female)
Testis (male)
Other tissues that produce
hormones include adipose
cells, cells in the walls of the
small intestine, stomach,
kidneys, and heart
•
Functions under hormonal
control
The major processes coordinated
by hormones include:
⟶ growth and development
⟶ reproduction
⟶ maintenance of components of blood: erythrocytes,
water, electrolytes, nutrients
⟶ energy utilization, including the determination of
metabolic rate
⟶ mobilization of body defences including stress responses
All of these processes involve the coordination of many
physiological activities over long periods of time.
Chemical messengers
• Hormones: long-distance
chemical signals that travel in
the blood or lymph to target
cells
• Other hormone-like chemicals
act locally on nearby cells
and will not be considered
part of the endocrine system
e.g. cytokines produced by
the immune system
Target cells must
have specific
receptors to which
the hormone binds
Cells are activated
by hormones
binding to receptors
Target Cell Activation
• Target cell activation depends on three factors
1. Blood levels of the hormone
2. Relative number of receptors on or in the target cell
3. Affinity of binding between receptor and hormone
• Hormones influence the number of their receptors
⟶ Up-regulation—target cells form more receptors in
response to the hormone
⟶ Down-regulation—target cells lose receptors in response
to the hormone e.g. type 2 diabetes
Learning outcome 2
Distinguish between
water-soluble hormones
which bind to cell
membranes and lipidsoluble hormones which
pass through cell
membranes to interact
with DNA
hormone
s
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used with permission under license. These images and/or photos may not be copied or downloaded without permission from 123RF Limited.
Chemistry of hormones
Two main classes:
1. Amino acid-based hormones
⟶ Amines, thyroxine, peptides,
and proteins
2. Steroid hormones
⟶ Synthesized from cholesterol
in the testes/ovaries and adrenal
gland
Amino-acid image attributed to http://catalog.flatworldknowledge.com/
Steroid image attributed to McGraw-Hill Companies Ltd
Amino acid-based hormones
Water-soluble hormones (all amino acid–based
hormones except thyroid hormone)
•
Cannot enter the target cells
•
Act on plasma membrane receptors
•
Coupled by G proteins to intracellular second
messengers that mediate the target cell’s response
•
Activate or deactivate enzyme systems
http://www.youtube.com/watch?v=tMMrTRnFdI4&feature=related
Example: Glucagon
Receptor activates G protein
G protein activates
adenylate cyclase
cAMP acts as a 2nd messenger
Steroid hormones and thyroid
hormone
Lipid-soluble hormones
1. Diffuse into their target cells
and bind with intracellular
receptors
2. Receptor-hormone complex
enters the nucleus
3. Directly activate genes to
make proteins
Example: Cortisol
http://www.youtube.com/watch?v=oOj04WsU9ko&NR=1
Steroid
hormone
Extracellular fluid
Cytoplasm
Receptor
protein
Plasma
membrane
1 The steroid hormone
diffuses through the plasma
membrane and binds to an
intracellular receptor.
Receptorhormone
complex
Nucleus
Figure 16.3, step 1
Steroid
hormone
Extracellular fluid
Cytoplasm
Receptor
protein
Plasma
membrane
1 The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
Receptorhormone
complex
2 The receptor-
Nucleus
hormone complex enters
the nucleus.
Figure 16.3, step 2
Steroid
hormone
Extracellular fluid
Plasma
membrane
1 The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
Receptorhormone
complex
Cytoplasm
Receptor
protein
2 The receptor-
Nucleus
Hormone
response
elements
DNA
hormone complex enters
the nucleus.
3 The receptor- hormone
complex binds a hormone
response element (a
specific DNA sequence).
Figure 16.3, step 3
Steroid
hormone
Extracellular fluid
Plasma
membrane
1 The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
Receptorhormone
complex
Cytoplasm
Receptor
protein
2 The receptor-
Nucleus
Hormone
response
elements
DNA
mRNA
hormone complex
enters the nucleus.
3 The receptor- hormone
complex binds a hormone
response element (a
specific DNA sequence).
4 Binding initiates
transcription of the
gene to mRNA.
Figure 16.3, step 4
Steroid
hormone
Plasma
membrane
Extracellular fluid
1 The steroid hormone
diffuses through the plasma
membrane and binds an
intracellular receptor.
Cytoplasm
Receptor
protein
Receptorhormone
complex
2 The receptor-
Nucleus
Hormone
response
elements
DNA
mRNA
hormone complex enters
the nucleus.
3 The receptor- hormone
complex binds a hormone
response element (a
specific DNA sequence).
4 Binding initiates
transcription of the
gene to mRNA.
5 The mRNA directs
New protein
protein synthesis.
Figure 16.3, step 5
Hormone transport in blood
• Hormones circulate in the blood either free or
bound
⟶ Steroids and thyroid hormone are attached to plasma
proteins
⟶ All others circulate without carriers
• The concentration of a circulating hormone
depends on the:
⟶ rate of release from endocrine glands
⟶ speed of inactivation and removal from the body by
metabolism in the liver and excretion by the kidneys
Control of hormone release
• Blood levels of hormones
⟶ Are controlled by negative feedback systems
⟶ Vary only within a
narrow desirable range
• Three factors
stimulate release
of hormones:
(a) Humoral Stimulus
1 Capillary blood contains
low concentration of Ca2+,
1. Blood levels of
ions/nutrients stimulate
hormone release
Example: Ca2+ ions in the
blood
Declining blood Ca2+
concentration stimulates
the parathyroid glands to
secrete parathyroid
hormone (PTH)
which stimulates…
Capillary (low
Ca2+ in blood)
Parathyroid
glands
Thyroid gland
(posterior view)
PTH
2
Parathyroid
glands
…secretion of
parathyroid hormone (PTH)
by parathyroid glands*
PTH causes
Ca2+
concentrations
to rise and the
stimulus is
removed
Figure 16.4a
(b) Neural Stimulus
1 Preganglionic sympathetic
fibers stimulate adrenal
2. Nerves stimulate
hormone release
Sympathetic nervous
system fibers stimulate
the adrenal medulla to
secrete epinephrine and
norepinephrine
medulla cells…
CNS (spinal cord)
Preganglionic
sympathetic
fibers
Medulla of
adrenal
gland
Capillary
2 …to secrete catecholamines
(epinephrine and norepinephrine)
Figure 16.4b
(c) Hormonal Stimulus
1 The hypothalamus secretes
hormones that…
3. Hormones
stimulate other
endocrine
organs to
release their
hormones
Hypothalamus
2 …stimulate
the anterior
pituitary gland
to secrete
Pituitary
hormones
gland
that…
Thyroid
Adrenal
Gonad
gland
cortex
(Testis)
3 …stimulate other endocrine
http://www.youtube.com/watch?v=hLeBNyB1qKU
glands to secrete hormones
Figure 16.4c
Nervous System Modulation
• The nervous system modifies the
stimulation of endocrine glands
and their negative feedback
mechanisms
For example: under severe stress,
the hypothalamus and the
sympathetic nervous system are
activated
⟶ As a result, body glucose levels rise
Learning outcome 3
Describe the structure of
the pituitary gland and its
vascular and neural
relationships to the
hypothalamus
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used with permission under license. These images and/or photos may not be copied or downloaded without permission from 123RF Limited.
The Pituitary Gland and
Hypothalamus
• The pituitary gland
(hypophysis) is located at the
base of the brain, immediately
below the hypothalamus. It is
connected to the hypothalamus
by a pituitary stalk
Pituitary-Hypothalamic
relationships
•
The pituitary gland comprises 2 lobes: posterior (neurohypophysis)
and anterior (adenohypophysis)
•
The posterior pituitary is an extension of the
hypothalamus, and consists of neural tissue
– The hormones are synthesised in the
hypothalamus neurons.
– They are transported down the axons
and stored in vesicles in the axon ending
located in the posterior pituitary
– Nerve impulses travel down axons into
the posterior pituitary.
– This causes the release of the vesicles of
hormones into the blood stream at the posterior pituitary
Pituitary-Hypothalamic
relationships
• The anterior pituitary has no
direct neural connection to
the brain.
⟶ Blood from the
hypothalamus drains into
capillaries of the anterior
pituitary
⟶ releasing and releaseinhibiting hormones are
carried in the blood to the
anterior pituitary to
regulate hormone release
Learning outcome 4
List the hormones
secreted by the
pituitary gland, the
major actions and the
consequences of
hyposecretion and
hypersecretion
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Hormone
Secretion
Posterior Pituitary hormones
• The posterior pituitary secretes
two peptide hormones:
⟶ oxytocin (OT)
⟶ antidiuretic hormone (ADH);
also known as vasopressin
Oxytocin
• Stimulates uterine contractions
during childbirth by mobilizing Ca2+
through a second-messenger
system
• Also triggers milk ejection
(“letdown” reflex) in women
producing milk
• Plays a role in sexual function in
males and females
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Antidiuretic Hormone (ADH)
• Osmoreceptors in the hypothalamus
respond to changes in the solute
concentration of the blood plasma
• If solute concentration is high
⟶ Osmoreceptors depolarize and
transmit impulses to hypothalamic
neurons
⟶ ADH is synthesized and released,
inhibiting urine formation by the
kidneys
•
Anterior Pituitary
hormones
All are proteins
• All except GH activate cyclic AMP
second-messenger systems at their
targets
• TSH, ACTH, FSH, and LH are all tropic
hormones (regulate the secretory
action of other endocrine glands)
Growth Hormone (GH)
• Stimulates cell division targets bone and skeletal
muscle
• Promotes protein synthesis
and encourages use of fats
for fuel
• Most effects are mediated
indirectly by insulin-like
growth factors (IGFs)
http://www.youtube.com/watch?v=hLeBNyB1qKU
Thyroid-Stimulating Hormone
• Produced by the anterior pituitary
• Stimulates the normal
development and secretory
activity of the thyroid
• Regulation of TSH release
⟶ Stimulated by thyrotropin-releasing
hormone (TRH)
⟶ Inhibited by rising blood levels of thyroid
hormones that act on the pituitary and
hypothalamus
Adrenocorticotropic Hormone
(ACTH)
• Secreted by the anterior
pituitary
• Stimulates the adrenal cortex to
release cortisol and aldosterone
(to a lesser extent)
• Regulation of ACTH release
⟶ triggered by hypothalamic
corticotropin-releasing hormone
(CRH) in a daily rhythm
⟶ internal and external factors such
as fever, hypoglycaemia, and
stressors can alter the release of CRH
Gonadotropins
• Follicle-stimulating
hormone (FSH) and
luteinizing hormone (LH)
secreted by the anterior
pituitary
⟶ FSH stimulates gamete (egg
or sperm) production
⟶ LH promotes production of
gonadal hormones
Prolactin (PRL)
• Secreted by the anterior pituitary
• Stimulates milk production
• Regulation of PRL release
– Primarily controlled by prolactininhibiting hormone (PIH)
(dopamine)
• Blood levels rise toward the end of
pregnancy
• Suckling stimulates PRH release and
promotes continued milk production
Learning outcome 5
Describe the major actions of
the following hormones:
⟶ thyroid hormone
⟶ parathyroid hormone
⟶ insulin, glucagon
⟶ cortisol
⟶ aldosterone
⟶ adrenalin (epinephrine)
⟶ erythropoietin
7
hormone
s
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used with permission under license. These images and/or photos may not be copied or downloaded without permission from 123RF Limited.
Thyroid Gland
• This gland is located at the
front of the neck, below the
larynx.
• Follicular cells secrete
thyroxine (T4) and
triiodothyronine (T3),
collectively termed “thyroid
hormone” (TH)
• Other Thyroid cells produce
a second hormone
“calcitonin”
Thyroid Hormone
• Major metabolic hormone
• The effects of TH are:
⟶ stimulation of growth (in conjunction
with growth hormone)
⟶ development of the nervous system
in the foetus and infant
⟶ increased basal metabolic rate and
increased heat production
⟶ increased alertness, reflexes
TH is derived from the amino
acid tyrosine, and iodine (I)
Hypothalamus
Thyroid Hormone
TRH
Anterior pituitary
TSH
Negative feedback
regulation of TH release
Rising TH levels provide
negative feedback
inhibition on release of TSH
Thyroid gland
Thyroid
hormones
Target cells
Stimulates
Inhibits
Calcitonin
• Produced by parafollicular cells of
the thyroid gland
• Antagonist to parathyroid
hormone (PTH)
• Inhibits osteoclast activity and
release of Ca2+ from bone matrix
• Stimulates Ca2+ uptake and
incorporation into bone matrix
Parathyroid Glands
• Four to eight tiny glands embedded in the posterior aspect of the
thyroid gland
• secrete parathyroid hormone (PTH), a peptide hormone
• PTH—most important hormone in Ca2+ homeostasis
• Functions
⟶ Stimulates osteoclasts to digest
bone matrix
⟶ Enhances reabsorption of Ca2+ and
secretion of phosphate by the kidneys
⟶ Promotes activation of vitamin D
(by the kidneys); increases absorption
of Ca2+ by intestinal mucosa
Adrenal Glands
•
Paired, pyramid-shaped organs atop
the kidneys
•
Structurally and functionally, they are
two glands in one
– Adrenal medulla —nervous tissue;
part of the sympathetic nervous
system
– Adrenal cortex —three layers of
glandular tissue that synthesize and
secrete corticosteroids
Adrenal Glands
The cortex secretes corticosteroids (aldosterone and cortisol) and androgens (male sex hormones)
The medulla secretes catecholamines (adrenalin, noradrenalin)
Mineralocorticoids
• Mineralocorticoids regulate electrolytes
(primarily Na+ and K+) in ECF
⟶ Importance of Na+: affects ECF volume,
blood volume, blood pressure, levels of other
ions
⟶ Importance of K+: sets resting membrane
potential of cells
• Aldosterone is the most potent
mineralocorticoid (a steroid)
⟶ Stimulates Na+ reabsorption and water
retention by the kidneys
Glucocorticoids (Cortisol)
• Keep blood sugar levels relatively constant
• Maintain blood pressure by increasing the action of
vasoconstrictors
• Cortisol is the most significant glucocorticoid
⟶ Released in response to
ACTH, patterns of eating and
activity, and stress
⟶ Prime metabolic effect
is gluconeogenesis - formation
of glucose from fats and proteins
⟶ Promotes rises in blood glucose,
fatty acids, and amino acids
Aldosterone (i)
• Aldosterone regulates Na+ and K+
in blood and other extracellular
fluids (ECF)
– Importance of Na+: affects water
content of ECF thus affects blood
volume, blood pressure, blood
concentrations of other solutes
– Importance of K+: sets resting
membrane potential of cells
Aldosterone (ii)
Aldosterone acts at the kidney tubule to
– Stimulate Na+ reabsorption and water retention by the
kidneys
– Stimulate K+ secretion in kidney tubules and thus its removal
from the blood
Aldosterone secretion is increased by:
• falling blood pressure: thus Na+ is reabsorbed, water is passively
reabsorbed with it, and blood volume is maintained
• rising blood levels of K+ : excess K+ is secreted from the blood,
leaves the body in urine, and levels do not rise to adversely
affect excitable tissue such as nerve, cardiac muscle
Gonadocorticoids (Sex
Hormones)
• Most are androgens (male sex hormones) that
are converted to testosterone in tissue cells or
estrogens in females
• May contribute to
⟶ The onset of puberty
⟶ The appearance of secondary sex characteristics
⟶ Sex drive
http://www.youtube.com/watch?v=V08dWz5XNBA&feature=related
Adrenal Medulla
• cells in the adrenal medulla
secrete adrenalin (80%) and
noradrenalin (20%)
• These hormones cause
⟶ Blood glucose levels to rise
⟶ Blood vessels to constrict
⟶ The heart to beat faster
⟶ Blood to be diverted to the
brain, heart, and skeletal muscle
Stress
Pancreas
• Has both exocrine and endocrine cells
⟶ Exocrine produce an pancreatic juice for digestion
⟶ Pancreatic islets (islets of Langerhans) contain endocrine
cells:
⇉ Alpha () cells
produce glucagon
⇉ Beta () cells
produce insulin
Blood glucose regulation
Glucagon raises blood
glucose
Major target is the liver, where it promotes:
• Glycogenolysis: breakdown of
glycogen to glucose
• Gluconeogenesis: synthesis
of glucose from amino
acids and fatty acids
• Release of glucose to
the blood
Insulin lowers blood glucose
Effects of insulin
• Lowers blood glucose levels
• Enhances membrane transport of
glucose into fat and muscle cells
• Activates an enzyme cascade which
leads to:
⟶the oxidation of glucose for ATP
production
⟶ Polymerize glucose to form glycogen
⟶ Convert glucose to fat (particularly in
adipose tissue)
Stimulates glucose uptake by cells
Tissue cells
Insulin
Stimulates
Pancreas
glycogen
formation
Glucose
Blood
Glycogen
Liver
glucose
falls to
normal
range.
Stimulus
Blood
glucose level
Stimulus
Blood
Blood
glucose level
glucose
rises to
normal
range.
Glucose
Pancreas
Liver
Glycogen
Stimulates
glycogen
breakdown Glucagon
Figure 16.18
•
Other hormone-producing
tissues
Kidneys
– Erythropoietin stimulates production of red blood cells
• Adipose tissue (fat tissue)
– Leptin is involved in appetite control, and stimulates
increased energy expenditure
• Stomach, small intestine
– Hormones that regulate metabolic and digestive functions
• Heart
– Hormone that helps regulate blood volume, blood pressure