Download The Endocrine System

Chapter 8: The Endocrine System
Hormones...not just for teenagers
•You
have seen how aldosterone plays an important
part in water regulation
•Most of your body’s processes are regulated by
hormones – they are considered to be chemical
messengers
•The
collective set of these organs that secrete
hormones important for homeostatic regulation are
known as the ENDOCRINE SYSTEM
Efficient or Inefficient?
•If
you think about the fact that the blood is the
primary substance in our body that passes
through EVERY organ, it makes sense to use it
as a transportation vector
•Therefore, chemical signals, though inefficient
when compared to something like nervous
signals in the brain, are useful because very
distant parts of the body can communicate with
each other
•However, the complication is that hormones
must travel through the WHOLE body before
reaching its target organ
Delay
•This
means that most systems controlled by
hormones will have lag period of start up and
stopping
•They are also regulated by very tightly
monitored feedback loops in order to stop the
secretion of the hormone when it is no longer
needed
•Most feedback loops in the endocrine system
are negative
Basic set up
•If
you recall the first lecture on homeostasis, a given
organ in the endocrine system will follow a basic
feedback loop
•Diagrams are often used to describe the mechanism of
action for a given hormone
•Others
are named according to the hormone involved
•There are usually at least two organs involved; a
monitoring organ and a target organ, but axes may
include more than one organ
MONITORING
ORGAN
(-)
(+)
TARGET ORGAN
(+)
TARGET ORGAN
CHANGE
(+)
Types of hormones
•Hormones
can be classified into two types
based on their chemical structure:
•Based
on the chemical structures, each type of
hormone has slightly different mechanisms of
action and can affect cells through different
methods
Steroid hormones: welcome in
•Recall
lipids
that the cellular membrane is composed of mainly
•Therefore,
the release and absorption of steroid
hormones are less well regulated, since they have the
capacity to exit and enter cells freely
•If they can pass through cellular membranes, they can
pass through nuclear membranes as well; they are often
used to control DNA transcription for this reason
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Protein hormones: knock first
•Protein
hormones require more regulation
because they cannot easily pass through the
cellular membranes to affect cells
•Protein hormones must interact with cells via
receptors on the surface of the cell
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A noteworthy gland: the pituitary
gland
•One
particular gland that is of great
importance in the endocrine system is the
pituitary gland
•It is located at the base of your brain, just in
front of the brain stem
Nervous control
•The
pituitary gland is a good example of how
the brain directly links into the endocrine
system
•The brain not only helps monitor the state of
the body, but it also acts to synthesize and
release hormones that control other hormones
•Therefore, the brain itself is an important
endocrine organ
Hormone axes
The hormone axes we are going to study will include:
Glucagon:
Hypothalamic-Pituitary-Adrenal axis:
Hypothalamic-Pituitary-Thyroid axis:
Parathyroid:
Hypothalamic-pituitary-testicular axis:
Hypothalamic-pituitary-ovarian axis:
8.2 Glucose regulation
Glucose regulation
•Because
glucose is an essential molecule for
cellular survival, its regulation is important
•A specific concentration of glucose must be
kept in the blood in order for cells to access the
glucose they need to function
The pancreas
•Pancreatic
tissue contains many different types
of cells – each different type is responsible for
secreting different substances
•The cluster of cells that regulate blood sugar
form a structure known as the ISLETS OF
LANGERHANDS
Glucagon and insulin
•Insulin
and glucagon are antagonistic
hormones – they produce opposite effects and
work together to balance glucose levels
•Insulin acts on cells to increase their
permeability to glucose by binding to receptors
on the cell surface and helping increase the
activity of glucose transporters – this decreases
the level of glucose in the blood
Time lag
•Understanding
this axis helps us to understand the
importance of portion sizes in healthy eating
•After a meal, glucose floods your bloodstream – if
there is more glucose than can be used immediately
by cells, it will be put into storage by the liver, who,
due to insulin secretion, will be increasing its uptake
of glucose, and therefore, creating more glycogen
•Between meals, your “starvation mode” – glucagon
is secreted to break down the glycogen stored earlier
Diabetes
•Diabetes
occurs when insulin production or
response to insulin is compromised
•There are two types of diabetes:
•Type 1 Diabetes/Juvenile Diabetes/Diabetes
Mellitus:
•Type
2 Diabetes/Adult-Onset Diabetes:
Symptoms of Diabetes
•Diabetes
results:
•Increased urine production and therefore
increased loss of water
•Increased thirst
•Increased glucose secretion in urine and therefore
increased glucose levels in urine
Long term complications of diabetes
•Peripheral
neuropathy – numbness in the
extremeties
•There
is a thought that the heart is also directly
affected by the inability to regulate glucose
levels
Treatment
•Type
1 diabetes
•Type
2 diabetes
Gestational Diabetes
•Pregnant
mothers are also at risk of developing
gestational diabetes
•This results in an increased blood glucose level
that can affect the baby
8.3 Metabolic control
Metabolism
•Metabolism
refers to the sum total of all chemical
reactions that maintain a living organism
•It is most closely associated with those processes
that produce and store energy in the body which
would include, in simple terms, the making and
breaking of chemical bonds
Metabolic control
•The
BASAL METABOLIC RATE refers to the rate at
which an organism can process energy
•This rate refers to the amount of energy that the
body uses when at rest
•BMR
is different for everyone – this explains why
two people that have the same diet may have very
different body masses
•High BMR usually means that you burn more
calories per minute than someone with low BMR
Thyroid gland
•The
thyroid gland is a large endocrine gland on
its own, but situated on top of the thyroid are 4
individual glands that collectively make up the
parathyroid gland
•The thyroid gland secretes very powerful
steroid hormones that control metabolism
Thyroid gland
•Two
hormones, T3 and T4, are secreted by the
thyroid gland – collectively known as Thyroxine
•The “3” and “4” refer to the positions of iodine
which are bound to the molecule
HypothalamicPituitary-Thyroid
axis
TRH= Thyroid releasing
hormone
TSH = Thyroid
stimulating hormone
Thyroid disorders
•Because
iodine is an essential part of the T3 and T4
molecule, a lack of iodine can lead to the
underproduction of these hormones
•If T3 and T4 are not being secreted, the
hypothalamus will not receive negative feedback;
therefore, it will continually secrete TRH
•This will cause the anterior pituitary to continually
secrete TSH
•TSH stimulates the thyroid to secrete T3 and T4 –
and to do meet the constant demand, the thyroid
will increase in size (by increasing the number of
secreting cells)
•An
Hyper and Hypothyroidism
over or undersecretion of thyroxine can lead to a
change in BMR
•Hyperthyroidism refers to an oversecretion of thyroxine
which can lead to rapid weight loss, heightened state of
agitation and nervousness, difficulty focusing
•The
cause can be a tumour (for example, tumour blocks
the exit duct for the hormone) and it is common with
aging as cells begin to slow down
Calcium balance
•The
parathyroid glands, along with the thyroid
gland, work together to balance calcium levels
in the blood
•Parathyroid hormone (PTH) is secreted by the
parathyroid gland and is antagonistic to
Calcitonin, secreted by the thyroid gland
Calcitonin and PTH
•Low
levels of blood Ca stimulate the release of
PTH
•PTH increases blood Ca by stimulating the
activity of osteoclasts, bone cells that break
down bone
•This will lead to the release of Ca into the
blood stream
Problems with Ca balance
•Improper
Ca blood levels can affect bone and
muscle
•If Ca levels cannot be maintained at normal
values, this can affect the amount of available
Ca to bones for normal development
Human Growth Hormone
•HGH
is an important hormone required for
growth in young children
•Think carefully about what happens to appetite
during adolescence: growing children require
large amounts of energy in order to fuel
muscle, bone and neuronal development
Secretion
•HGH
is secreted by the anterior pituitary
•Its secretion is controlled by the hypothalamus
that secretes GHRH (growth hormone releasing
hormone)
•HGH acts on the liver to secrete hormones such as
IGF (Insulin like growth factor) that acts on muscles
to increase hypertrophy (increase in size of cells)
and hyperplasia (increase in number of cells) of
muscle
(-)
(-)
(-)
(-)
IGF
Actions
•GH
will also:
•
Mobilize fat stores by increasing the breakdown of fat
Stimulate the growth of internal organs
•
Stimulate immune system
•
Problems with GH
•Lack
of GH secretion has a much more severe
effect during childhood than adulthood
•Low GH levels during development will lead to
shorter statures and smaller builds in children
•This can be a cause of dwarfism –
•Too
much GH secretion during childhood can
result in the opposite – gigantism –
•Oversecretion
of GH in adulthood can cause
increased body mass and bone thickness which can
interfere with normal skeletal movements –
creating a condition called acromegaly
8.4 The stress response
The stress response
•Human
response to stress is a very important
evolutionary mechanism that has evolved to keep us
alive
•It is controlled by the hypothalamus
•Therefore
the first cues that a stress response is
necessary due to a threat of danger would be
processed through the brain, usually via visual
information (as primates are reliant heavily on vision
as a sense)
If you’re in danger, then what?
•To
understand the mechanism better, it is important
to understand what happens physiologically in times
of danger
•This group of symptoms is associated with what
biologists refer to as the “flight or fight” response:
The players: the adrenal gland
•Divided
•The
into two main parts:
medulla is innervated directly by the brain via
the spinal cord, and the cortex is affected by
hormones secreted by the anterior pituitary
•The adrenal gland secretes three hormones to
produce this group of responses:
Hormone
Secreted from
Function
Increase in:
Blood glucose
Heart rate
Breathing rate
Cellular metabolism
Constriction of blood vessels to redirect
blood flow to muscles and heart
Increase in:
Na+ and water retention in kidneys
Blood volume
Blood pressure
Increase in blood glucose
Conversion of fats and proteins into
glucose
Supression of inflammatory response in
immune system
See p. 382 from text
The players: hypothalamus and the
anterior pituitary gland
•The
brain identifies a stressful situation and
signals the adrenal cortex directly; this initiates
a short term stress response
•This
branch of the axis produces a more long
term stress response
SPINAL
CORD
(+)
(+)
BRAIN
HYPOTHALAMUS
(+)
ANTERIOR
PITUITARY
(+)
ADRENAL GLANDS
CORTEX
MEDULLA
GLUCOCORTICOIDS
ADRENALINE AND
NORADRENALINE
MINERALCORTICOIDS
The stress response and health
•It
is easy to see why stressful lives can lead to long
term health problems; remember that human society
has changed what our “stress” is
•Previously, our response to danger was physical, and
so the stress response prepares our bodies for that
•Now, due to changes in human society, most stress
responses are emotional or neurological – the final
exam, death in the family, the big deadline at work
•Someone who is consistently in a state of stress will
be creating physiological conditions where blood
pressure is pushed past normal
•This can stress the heart and lead to the increased
risk of cardiovascular disease throughout one’s
lifetime if coupled with poor lifestyle choices
8.5 Reproductive hormones
The hormones that make teens
famous
•Reproductive
hormones in males and females
are focused on regulating reproductive
processes
•In males: this affects the production of sperm
•In females: this affects the menstrual cycle
which regulates the release of ova from the
ovaraies each month
The male reproductive system
•The
main hormone responsible for regulating
the male reproductive system is testosterone
•Testosterone is secreted by the interstitial cells
which are found within the testicles
Male reproductive anatomy
•In
males, the urinary and reproductive systems
are linked
•Both sperm and urine exit out of the urethra
•The testicles contain a series of very coiled and
very thin tubes known as the seminiferous
tubules
Male reproductive anatomy cont.
•From
there, sperm exits the epididymis through
the vas deferens
•The vas deferens is connected to 3 major glands:
•Prostate gland:
•Cowper’s
•Seminal
gland:
vesicle:
Male reproductive hormones
•Testosterone
production and secretion is
controlled by a negative feedback loop that
includes the hypothalamus and the anterior
pituitary gland along with the testicles
•GnRH (gonadotropin releasing hormone) is
secreted by the hypothalamus
•FSH
acts on sertoli cells and LH acts on interstitial
cells (AKA: Leydig cells)
•LH signals interstitial cells to secrete testosterone
(Interstitial
cells)
Hypothalamic-Pituitary-Testicular axis
Testosterone’s other effects
•Testosterone
will increase hair growth, muscle
hypertrophy (helps males increase muscle size),
and increase aggression
•Testosterone secretion during puberty causes the
larynx to lengthen, thus deepening the male voice,
maturation of the penis and the development of
pubic hair
•Excess testosterone secretion can lead to increased
aggression
Female Reproductive anatomy
•In
women, the reproductive system is separated
from the urinary system
•This is an obvious adaptation as both systems need
to remain separate for the gestation of offspring
•The ovaries are the main hormone producing
structure (along, ofcourse, with the development
and storing of ova or eggs)
Horomone regulation
•Once
past puberty, females release one egg per month
(usually – though sometimes more than one can be
released)
•The 27 day cycle is based on the release of the egg, the
preparation of the uterine lining for pregnancy, and the
shedding of the lining if pregnancy doesn’t occur
Oogenesis
•Unlike
men, women are born with all their eggs
•The eggs are stored in the ovary within structures
called follicles
•Therefore,
older ova that have been exposed to
more radiation/chemicals are at higher risk of
having chromosomal anomalies that can lead to
congenital birth defects
The menstrual cycle
•Hormone
fluctuations are timed around the
release of the ova
•The menstrual cycle is timed from the start of
menses (period) when the uterine lining is shed
Follicular phase
•The
hypothalamus releases GnRH that stimulates
the anterior pituitary to release FSH (Follicle
stimulating hormone)
•The developing follicle then secretes estrogen
•The
peak of estrogen secretion at ovulation feeds
back positively on the anterior pituitary to secrete
LH (Luteinizing hormone)
Ovlulation
•When
the follicle is mature, a spike in LH (Luteinizing
homone) secreted by the Anterior pituitary stimluates
the follicle to rupture and release the ovum
•By
now, the uterine lining has thickened in case the
ovum is fertilized in the fallopian tube and is ready for
implantation when it reaches the uterus
Luteal phase
•The
developing Corpus Luteum secretes some
estrogen but mainly progesterone
•Progesterone helps to maintain the uterine
lining, keeping it ready for implantation
•This
prevents the maturation and release of
any further follicles
Pregnant or not?
•If
no fertilization occurs, the corpus luteum will
die due to the lack of FSH and LH
•Once the corpus luteum dies, production of
estrogen and progesterone stops
•This will remove the negative feedback on the
hypothalmus and anterior pituitary leading to
the secretion of FSH and LH to restart the cycle
See p. 398 for
Hypothalamic –
pituitary –
ovarian axis
•If
Pregnancy
the ovum is fertilized, the developing placenta
will secrete hormones to maintain itself
•hCG (human chorionic gonadotrophin) is secreted
to supress the regression of the corpus luteum –
this keeps it alive to prevent levels of progesterone
and estrogen from falling
•The detection of hCG is the basis of the pregnancy
test
•All
PMS
men make fun of it, and some women do – but
it is important to respect the effect of hormonal
fluctuations in women
•PMS
is used to describe the effect of these
hormones on the brain before menses when their
concentrations are high
•In some women, the brain’s response to the
hormones is so severe that it can stimulate severe
depression leading to suicidal tendencies or even
overt aggression
Menopause
•Women
between the ages of 46-54 will enter
into Menopause (yes, the name literally tells
you what is going on) where ovaries lose their
sensitivity to FSH and LH
•It
is interesting to note that menopause is rare
in most animal species
Women aren’t so much from Venus
and Men aren’t so much for Mars
•Testosterone,
progesterone and estrogen are
steroid based hormones that can be converted into
each other via common biochemical pathways
•It is worthwhile to note that women do have
testosterone and men do have estrogen – however,
the levels of these hormones are very small
Over or undersecretion
•Under
or oversecretion of any of testosterone in
women or estrogen in men can result in
masculinization or feminization of physical features
•These
hormones are very powerful and can cause
these effects even after puberty