Download The Endocrine System and Homeostasis

The Endocrine System
and Homeostasis
Worth a “looksee”!
Chemical Control Systems
“No cell operates in isolation”
Animals must respond to stimuli from inside and
outside the body
The nervous system and endocrine system work
together to control the various organs
 The nervous system reacts quickly to changes in
the environment
 The endocrine system reacts slower and maintains
control over a longer duration (ex. hours – years)
Locations of the Major Endocrine
Glands
* The small intestine and stomach have certain cells that
produce hormones that affect digestion
Importance of the Endocrine
System
Hormones:
chemical controls/regulators
 speed up or slow down certain bodily processes
 produced by cells in one part of the body
 affect cells in another part of the body
 Control: a) growth and development
b) maintenance of homeostasis

Glands are specialized organs for secreting
substances needed by the organism




Exocrine Glands:
Secrete substances that
are needed quickly
Secreted through small
ducts
Examples: salivary, sweat,
and milk glands
Endocrine Glands:
 Secrete hormones directly
into the bloodstream,
without the use of ducts
 Delivered by the
circulatory system
 Long-term control
 Examples: hypothalamus,
pituitary, pancreas
HORMONES
Classified according to their activation site
NONTARGET
HORMONES
Affect many cells
throughout the body
TARGET
HORMONES
Affect specific
cells
*only a small amount is necessary to alter cell metabolism
Chemical Structure of Hormones
Two major types of hormones:
1. Protein-type hormones
ex) insulin, adrenaline
2. Steroid hormones
ex) testosterone,
Protein-type Hormones






Chains of amino acids
Soluble in water
Combine with specific
receptor protein on the
plasma membranes of
target tissue cells =
recognizes target tissue
easily
Never actually enters the
cell
2nd messenger is activated
inside the cell to cause a
series of reactions to
perform the necessary
action
Ex) insulin, adrenaline
Steroid Hormones






Chemical structure = complex
rings of carbon, hydrogen, and
oxygen
Soluble in fat; Not in water
Actually pass through the
membranes of target cells
Combine with receptors in the
cytoplasm
The steroid-receptor complex
then moves into the nucleus
where it activates a gene that
produces the response
Ex) testosterone, estrogen
The Lock and Key Model
Hormones affect target cells by binding to specific receptor
molecules in the cell membrane
 This “unlocks” the cell’s response
 Many drugs work by mimicking a naturally occurring hormone.


If the drug causes the receptor to respond in the same way as the
naturally occurring substance, the drug is referred to as an agonist.
• Ex. Nicotine and morphine
• These drugs “pick the lock”

Other drugs work in the opposite way as antagonists. These drugs
bind to the receptor, but do not produce a response. Because the
drug prevents the receptor from binding to the normal hormone or
neurotransmitter, it has an inhibitory effect on the naturally
occurring substance.
• Ex. Caffeine and atropine.
• These drugs “jam the lock”
“Lock and Key”
The Pituitary – Hypothalamus
Relationship
Homeostasis is maintained by hormone
levels responding to internal and
external stimuli



The hypothalamus regulates
the pituitary gland by nerve
stimulation
The pituitary (aka Master
Gland) regulates endocrine
glands that secrete
chemicals that affect the
hypothalamus’ nerve activity
by negative feedback.
The pituitary releases
hormones which stimulate
other glands to release
hormones that either
increase or decrease cell
processes
Hypothalamus
Pituitary Gland
Other
Endocrine
Glands
The Pituitary Gland



Anterior lobe
Makes and stores its own
hormones
Releases these into the blood
to travel to target glands/cells
where secondary hormones will
be “mobilized” to alter cell
activities
Posterior lobe
Stores hormones
produced by the
Hypothalamus


1.
2.
Oxytocin –regulates uterine
muscle contraction and milk
production
ADH – regulates water
concentration in blood
*a 3rd lobe exists
(not discussed here)
Negative Feedback
 The rate of hormone secretion is not
constant
 determined by the needs of the animal at the
particular time
 The nervous system may cause a gland to
speed up or slow down due to a stimulus;

But it is usually another hormone that is the
messenger for the body activities to increase or
decrease
 Negative feedback maintains body
processes at the levels required by the
body
 These requirements may vary according
to the conditions of the body:


Negative feedback acts to decrease a hormone in
the blood if it increases beyond the level required
If the concentration is too low, negative feedback
will act to increase the level
Negative Feedback for Response to
Extreme Cold (stimulus)
Negative feedback in glucose
homeostasis
 A rise in blood glucose causes release of
insulin from beta cells the pancreas, promoting
glucose uptake in cells and storage as glycogen
in the liver.
 A fall in blood glucose stimulates alpha cells in
the pancreas to secrete glucagon, which causes
the liver to break down glycogen and release
glucose.
Control of Blood Glucose Level
1.
High concentration of glucose in blood after eating
2.
Liver and skeletal muscle stores some as glycogen
3.
Excess glucose is stored as fat
4.
When glucose levels in the bloodstream decrease,
glycogen is “mobilized” to increase levels as needed
5.
Once glycogen reserves are depleted, fat stores are
used to supply needs until the next meal can
replenish glucose levels in the bloodstream.
6.
If animal always has access to glucose and does not
need to use stored energy, animal gains weight
(mass)
Insulin and Uptake of Glucose
Glucose enters cells through the action of transport
proteins in the cell membrane.


Insulin is believed to help glucose enter cells by bringing
more transport proteins to the plasma membrane from
inside the cell.
When insulin binds to a receptor, vesicles containing
transport proteins fuse with the cells plasma membrane,
providing more transporters to bring glucose into the cell
Insulin (produced in pancreas) causes excess glucose to enter liver
and muscle cells for storage as glycogen.
When glucose levels fall, less insulin is produced causing glycogen
to revert to glucose.
Diabetes Mellitus
 Too much glucose in the blood due to
regulatory problems (insulin is not getting its
job done for some reason) causes:





Excess glucose to enter urine
Water and salts move from body cells in to the
blood due to osmosis and diffusion
Kidneys work harder to remove excess water and
salts
Frequent urination/thirst are common symptoms
Long-term complications – stroke, blindness,
kidney failure, circulatory problems
Adrenal Glands
Located above kidneys; made of 2 glands in one shell
Adrenal Cortex
 Outer casing
 Regulated by
hormones
 Produces 3 different
types of hormones



Glucocorticoids
Mineralocorticoids
Sex hormones (few)
Adrenal Medulla
 Inner gland
 Regulated by the
nervous system
 Produces 2 hormones:


Epinephrine
(adrenaline)
Norepinephrine
(noradrenaline)
Aldosterone
Cortisol
Androgens
Adrenal Cortex

Using cholesterol as the starting material, the cells
of the adrenal cortex secrete a variety of steroid
hormones. These fall into three classes:




glucocorticoids (e.g., cortisol)
mineralocorticoids (e.g., aldosterone)
androgens (e.g., testosterone)
Production of all three classes is triggered by the
secretion of ACTH from the anterior lobe of the
pituitary.
These hormones achieve their effects by:
Travelling through the body in the blood.
1.

Entering from the blood into all cells
Binding to their receptor
2.
3.

The dimer migrates into the nucleus (if it did not form there).

The hormone-receptor dimer binds to specific hormone
response elements in DNA.

6.
a protein present in the cytoplasm and/or nucleus of "target" cells
The hormone-receptor complex binds to a second to form a
dimer.
4.
5.
Because they are so hydrophobic, they must be carried bound to a
serum globulin.
These are specific DNA sequences in the promoter of genes that will be
turned on (sometimes off) by the interaction.
Other transcription factors are recruited to the promoter and
gene transcription begins.
Glucocorticoids
The most abundant glucocorticoid is cortisol.
(also called hydrocortisone)
Raise the level of blood sugar (glucose).
2. Also have a potent anti-inflammatory effect on the
body (They depress the immune response).

For this reason glucocorticoids are widely used in
therapy:
1.
 to reduce the inflammatory destruction of rheumatoid
arthritis and other autoimmune diseases
 to prevent the rejection of transplanted organs
 to control asthma
Mineralocorticoids
 Affect mineral metabolism.
 The most important of them is the steroid
aldosterone.

Acts on the kidney promoting the reabsorption of
sodium ions (Na+) into the blood
Water follows the salt and this helps maintain normal
blood pressure.


Acts on sweat glands to reduce the loss of sodium
in perspiration
Acts on taste cells to increase the sensitivity of the
taste buds to sources of sodium
Androgens

The adrenal cortex secretes precursors to androgens
such as testosterone.

In sexually-mature males, this source is so much lower
than that of the testes that it is probably of little
physiological significance. However, excessive
production of adrenal androgens can cause premature
puberty in young boys.

In females, the adrenal cortex is a major source of
androgens. Their hypersecretion may produce a
masculine pattern of body hair and cessation of
menstruation.
Adrenal Medulla

The adrenal medulla consists of masses of neurons
that are part of the sympathetic branch of the
autonomic nervous system.

Instead of releasing their neurotransmitters at a
synapse, these neurons release them into the blood.

Thus, although part of the nervous system, the
adrenal medulla functions as an endocrine gland.

The adrenal medulla releases:

adrenaline (also called epinephrine) and

noradrenaline (also called norepinephrine)
Release of adrenaline and noradrenaline
is triggered by nervous stimulation in
response to physical or mental stress.
“Fight or Flight
Response”
Fight or Flight Response

When activated, the fight or flight response causes
a surge of adrenaline and other stress hormones to
pump through our body.

This surge is the force responsible for mothers
lifting cars off their trapped children and for
firemen heroically running into blazing houses to
save endangered victims.
Some of the effects are:
increase in the rate and strength of the heartbeat resulting in
increased blood pressure
 blood shunted from the skin and digestive system to the skeletal
muscles, coronary arteries, liver, and brain
 rise in blood sugar (increase energy)
 increased metabolic rate
 bronchi dilate (increase O2)
 pupils dilate (improved vision)
 hair stands on end ("gooseflesh" in humans)
 reduced pain sensation
 clotting time of the blood is reduced
 increased ACTH secretion from the anterior lobe of the
pituitary
 immune system mobilizes

All of these effects prepare the body to take
immediate and vigorous action.
We become prepared
— physically and psychologically —
for fight or flight.
When our fight or flight system is activated, we tend to perceive
everything in our environment as a possible threat to our survival.
By its very nature, the fight or flight system bypasses our rational
mind—where our more well thought out beliefs exist—and moves us
into "attack" mode.
Water Regulation

Antidiuretic hormone binds to receptors on cells in the
collecting ducts of the kidney and promotes reabsorption of
water back into the circulation. In the absence of antidiuretic
hormone, the collecting ducts are virtually impermeable to
water, and it flows out as urine.

Antidiuretic hormone stimulates water reabsorption by
stimulating insertion of "water channels" or aquaporins into the
membranes of kidney tubules. These channels transport solutefree water through tubular cells and back into blood, leading to
a decrease in plasma osmolarity and an increase osmolarity of
urine.